commonly used for archaeological surveys; resistivity surveys are good for beginners, ..... survey found low readings at or near the buried remanent of an adobe.
Geophysical exploration for buried buildings This is a pre-publication version of an article that was later published in the journal, Historical Archaeology, 2006, volume 40, number 4, pages 27 - 50. That published article is available at: http://www.jstor.org/stable/25617382?seq=1#page_scan_tab_contents While the abstract is also at: https://sha.org/publications/sha-volume-indices/contents_abstracts_vol_40/#winterab This pre-publication version lacks some final copy-editing. The figures here have a higher resolution than in the publication, and some of them are now colored. This version includes bookmarks to the sections of the article and to the figures. There are also hyperlinks (with blue text) to the figures which follow the text. Furthermore, there are links (invisible) between the sites mentioned in the main text to their further details in the appendix. Finally, there are hyperlinks (also invisible) from the appendix back to the main text.
Geophysical Exploration for Buried Buildings Bruce W. Bevan Abstract Cellars are usually easy to detect, but wall foundations can be difficult to find. A scatter of rubble in the soil may reveal the former location of a building; buried buildings are most commonly detected this way. It appears that chemical changes to the soil near buildings may also allow the locations of those buildings to be estimated; perhaps these buildings may be located even if no artifacts or features remain. Auxiliary features, such as buried roads, paths, wells, or privies, may suggest that a building was once nearby. An appraisal of several dozen of my geophysical surveys suggests that they have had a 50 % chance of locating buildings. Introduction For historical archaeology, the most common application of geophysical exploration is a search for buildings whose remanents are now buried and no longer visible at the surface. Geophysics can be successful for these searches; this is because the remanents of buildings can cover a large area and form a thick deposit. The search for a building is more likely to be successful than a search for small features such as graves or privies. A summary of many different surveys is given here. Both successful and failed geophysical searches are described here; this may keep the conclusions from being affected by unusual conditions at a few sites. Therefore, this summary can suggest the reliability of surveys at a wide variety of sites. Geophysical surveys at domestic houses are emphasized here; this is only because these houses are most frequently sought. While few examples are included here, the findings should apply to many commercial, industrial, farming, religious, and military buildings. Almost all of the examples here are from the northeastern part of the U.S.; this is primarily due to the fact that archaeologists in this area applied geophysical surveys earlier and more frequently than archaeologists in other parts of the country. Known or anticipated differences in geophysical practices will be described for regions that have different soils and geology. While this summary is written primarily for archaeologists, an appendix includes technical details that may aid those individuals who do geophysical surveys. Tools for the Search Before a geophysical survey is attempted, simple procedures that may be successful should first be considered. Historical maps and photographs can give more accurate locations for former buildings than is possible with geophysics. The main disadvantage of this historical approach is that it does not allow an estimate of what remains underground. Also, maps and photos may only determine the relative locations of nearby buildings. This would be true if the modern landscape no longer has enough clues to the historical landscape; for example, the trees or buildings that are visible in a historical photograph may no longer be standing. While aerial photographs rarely reveal the locations of former buildings, it is easy and inexpensive to make an oblique photograph of an historic area from a Page 1
Tools for the search rented airplane. Other alternatives to geophysical surveys should also be considered. Shovel test pits and soil probing (McManamon 1984) can be cost-effective and they have a long history of successful searches for buildings. A tile probe (steel T-bar) is particularly suitable for detecting foundations at a depth of 1 ft or so. This exploration may only be practical where the soil is soft and contains few stones. A search with a metal detector (Connor and Scott 1998) should always be considered before a geophysical survey. A scan with a metal detector is both more economical and much faster than a typical geophysical survey; an initial scan with a metal detector may also reduce the area that needs to be explored with a geophysical survey. While metal may accumulate in the vicinity of historical buildings, concentrations of metal may also be found in areas of trash disposal. Geophysical surveys, also, cannot usually distinguish a midden from a building. With formal geophysical surveys, recorded measurements may be made along a series of straight lines that allow the mapping of a rectangular area. Informal or reconnaissance geophysical surveys are quick searches that are made without recording the measurements or perhaps their locations; this procedure is also called the method of free search (Smekalova et al. 1993:88). With these reconnaissance surveys, unusual readings are noted by watching the display on the geophysical instrument; the area where these readings are found can be explored in more detail. These informal surveys can speed the exploration of large areas, but they may be poor for defining the shape of buried features and they do not usually allow a detailed interpretation. The following paragraphs describe the advantages and disadvantages of several different geophysical instruments that could be applied to a search for buildings. Further comparisons of these techniques are in the publications by Clark (1996), Heimmer and De Vore (1995), or Bevan (1996a). Ground-penetrating radar detects a wide variety of underground features and allows an approximation of the cross-section of some features. A set of radar profiles that are recorded along parallel lines can generate rough three-dimensional views of buried features. Except for this 3D geometry, ground-penetrating radar is not very good for identifying the materials that are underground; metal may appear to be identical to wood, ceramic, or an air cavity. The radar's information on the relative depth of features is particularly important. As with other archaeological findings, greater depth can mean greater age; shallow features can be modern. Radar is usually the best instrument if an air-filled void is sought; underground chambers and tunnels can be detected. A ground-penetrating radar is composed of several components. The main part is the antenna that transmits and receives the radio pulse; this antenna is typically mounted in a box that is pulled or pushed along the ground like a sled. These antenna boxes typically range in size from 0.2 to 1 m in width. Smaller boxes are for antennas that operate at a higher frequency; this allows them to detect smaller and shallower features. The radar also includes electronics for the transmitter and receiver and the equipment for displaying and recording the radar echoes. Ground-penetrating radar is excellent for surveys on the coastal plain of the U.S. In Page 2
Tools for the search this area, the shallow, archaeological zone of the soil column is typically rather sandy, but the soil becomes increasingly clayey with greater depth; stones are not natural in this soil and the radar usually reveals little evidence of soil strata. All of these factors aid a radar survey. Stones and natural strata confuse the patterns on radar profiles. While clay attenuates the radar signal, a cellar or other pit that has been dug into clay that is below a sandy layer can be very apparent on a radar profile. While the eastern coast of the U.S. is generally excellent for a radar survey, much of the rest of this country is less suitable for radar. The silt of the Midwest can cause the profiling depth of the radar to be too shallow; this same problem may be found with saline soils in irrigated areas and with calcareous soils, such as those in the Southwest. Glacial and periglacial soils in the north can be so complex that small features cannot be isolated. The shelly sediments of Florida and the sands of the north-central U.S. can be excellent for radar surveys. The major difficulty with radar surveys is that the soil can attenuate the radar pulse so much that the depth of profiling is too shallow; while loam may allow a profiling depth of 3 m (10 ft), clay may limit the profiling depth to less than 0.5 m (1 ft). This attenuation factor can be estimated in the U.S. from special soil maps (Doolittle et al. 2003). Measurements of the electrical resistivity of the soil are also a good predictor of profiling depth with a radar. In addition to this attenuation factor, radar surveys are affected by the stratigraphic complexity of the soil; if the soil is naturally stony, small archaeological features may be masked on radar profiles by the echoes that are caused by clusters of stones. The roots of large trees (with trunk diameters greater than 1 ft or 0.3 m) cause a similar difficulty in the vicinity of those trees. On the coastal plain of the U. S., a magnetic survey is often complementary to a radar survey; the two surveys detect very different features and the findings have little unnecessary duplication. At any location, a magnetic survey should have high priority if iron artifacts or burned structures are sought. A magnetic survey will probably fail if there is magnetic rock in the ground; black igneous stone such as basalt or diabase usually makes it impossible to succeed with a magnetic survey. A geological map can reveal these igneous areas; it is conventional for them to be colored red on the map. Magnetic surveys are ideal in the search for kilns and furnaces. While a magnetometer will detect brick, it is not necessarily a good instrument for this search. Modern pipes, fences, and building roofs that contain iron are strongly detected by a magnetic survey and cause interference for those surveys. While a radar survey is impossible where there is brush at the surface, magnetic surveys can sometimes be done in brushy areas; this is because the magnetic sensor is carried above the surface of the soil and the sensor can be small enough that it can be pushed into a bush for a measurement. There are many different types of magnetometers, and these are described by technical terms such as fluxgate, proton, cesium, or Overhauser; each of these types of magnetometer can be suitable for archaeological surveys. With a magnetic survey, a magnetic sensor is carried around the area of interest, and point-by-point measurements of the earth’s magnetic field are recorded. At many historical sites, there is a good quantity of modern iron trash in the soil; the magnetic patterns of these unwanted artifacts can confuse the patterns from the archaeological features of interest. Page 3
Tools for the search Metallic objects are almost invisible to a resistivity survey. However, resistivity surveys can be impossible if there is a hard pavement at the surface. This is because each resistivity instrument has four metallic electrodes that must make a good electrical contact to the soil. These four electrodes can be arranged in many different patterns, called arrays, and all of these arrays (such as the Wenner, twin electrode, and pole-pole array) are suitable for archaeological surveys. During the operation of a resistivity meter, an electrical current is sent through the earth between one pair of the four electrodes; a voltage is measured at the other pair of electrodes. The ratio of this measured voltage to the applied current increases with the electrical resistivity of the soil. Resistivity instruments can be the least expensive of the instruments that are commonly used for archaeological surveys; resistivity surveys are good for beginners, for there are fewer errors to make as compared to other types of geophysical survey. While resistivity surveys are difficult or impossible if the soil is very hard, dry, or stony, these surveys can be done in a wider range of sites than other geophysical surveys, for a resistivity survey has relatively few restrictions on soil, geology, and electrical interference. Resistivity surveys are excellent for searching for organic trash pits in sandy soil and also for accumulations of stone in silt or clay; in both of these cases, there is a large difference in resistivity between the archaeological feature and the surrounding natural soil. Conductivity surveys detect many of the same features as do resistivity surveys, but conductivity surveys are much faster and easier to do than resistivity surveys. Unlike a resistivity survey, a conductivity survey strongly detects shallow metal objects (less than 0.2 m underground) that are the size of a tin can lid or larger. A conductivity instrument also readily detects buried wires and metallic pipes, and these can be common at historic sites. Finally, a conductivity instrument receives more interference from nearby electric wires than do other geophysical instruments. Conductivity meters are similar to one type of metal detector; these meters have a pair of coils of wire, called the transmitter and receiver coils, that are separated at a distance of 1 - 3 m. An oscillating magnetic field is generated at the transmitter coil; this passes through the soil and is detected at the receiver coil. The coupling of this magnetic field through the soil increases with the electrical conductivity of the soil. Conductivity is simply the reciprocal of resistivity; this is why conductivity and resistivity surveys detect so many of the same features. Geophysical surveys may be done of the same area with more that one geophysical instrument. This can increase the reliability of a survey, and also the number and variety of features that are detected, but the cost of the survey also increases. If one geophysical instrument is known to be excellent for detecting the features that are sought, there is little reason to try any other instrument. A compromise between cost and completeness can be considered: For example, one may apply three-quarters of the geophysical effort to the principal (most suitable) geophysical instrument, and the remaining time to the second best technique. A general limitation of geophysical surveys is that small features cannot be detected if they are deep. As a rule, features will be invisible if they are deeper than their size. For example, a wall foundation that has a width of 0.5 m cannot usually be detected if it is deeper Page 4
Indications: cellar that 0.5 m underground. With some features or artifacts (such as metal), radar, conductivity, and magnetic instruments can detect objects that are deeper than their size. Indications of Buried Buildings There are several different aspects of the underground remains of buildings that allow buildings to be detected. These include a cellar, a floor, a foundation, a scatter of debris, a fireplace, a change in soil chemistry, and auxiliary features (such as paths or pipes). These aspects will be discussed next, in that order. Cellar If a building once had a cellar, this will greatly aid the location of that building's remains. Cellars are usually easy to detect with a geophysical survey; this is primarily because they are larger than other features at historic sites. The debris within a cellar may also extend to a good depth; cellars are unusual in that there are many shallow features at archaeological sites, but there may be few deep ones. Cellars do not appear to be as common as a geophysicist would wish. A chaotic mixture of rubble within a filled cellar can be detected with most geophysical instruments. While a rubble-filled cellar allows a more certain detection of a building than any other aspect, some cellars remain invisible to a geophysical survey. Examples of both successes and failures follow. Each of my surveys is listed with the postal abbreviation of the site's state; this is followed by a sequence number that shows the order of the surveys in each state. Further details about these examples are included in an appendix. Figure 1 illustrates how a cellar may be revealed with a ground-penetrating radar. This is the cellar of the priest's house at the Chapel Field in St. Mary's City (MD04). Few other cellars have been revealed as clearly by geophysics as this one. The complex radar echoes in Figure 1 result from the reflection of the radar pulse from many different lenses of soil and debris within the cellar fill. The radar profile suggests that the cellar is 16 ft. (5 m) long; the profile also indicates that the rubble begins at a depth of 1 - 2 ft. (0.5 m). The profile does not provide reliable information about the depth to the bottom of the rubble. While this depth may be roughly 6 ft. (2 m), echoes extend to a much greater apparent depth (which is actually echo delay time); these repeated echoes are a reverberation of the radar pulse between lenses of different materials. If a cellar hole was filled recently, the modern debris in the fill likely contains a large quantity of metal, in particular, iron. The known cellar of an ice house at the Robinson Farmstead on the Manassas Civil War battlefield (VA10) was readily detected by magnetic and radar surveys because of the modern fill. The cellar of the Bullock House (VA14) on the Chancellorsville battlefield was also detected with radar, magnetic, and conductivity surveys because of the large quantity of metal that it contained. A magnetic survey at Fort Ellis in Montana clearly detected a root cellar with sides that were about 5 m (15 ft.) long (Weymouth 1996); this cellar contained some brick, and this was at least partly the reason why this feature was more magnetic than the surrounding soil. The larger a rubble-filled cellar is, the easier it is to detect. A small cellar, 1.5 by 3 ft. (0.5 by 1 m) in size, was detected with a radar survey at Fairfield (VA19) because the nearby Page 5
Indications: cellar soil strata showed few other complex features at a shallow depth. While the identification of a rubble-filled cellar is often reliable with a geophysical survey, it is not certain. Geophysical identification is more difficult than detection: Detection means that a geophysical instrument found a difference in the soil within an area; identification means that this difference was interpreted to be significant and that the underground feature could be named. Perhaps archaeological information has already suggested that certain types of features may be located in the area of survey; this information will increase the reliability of an identification. A radar profile across the cellar of the Master Armorer's Quarters at Springfield Armory (MA01) revealed a span with complex soil strata and another span with simple and planar strata; the natural soil was complex at this site, and the strata within the cellar fill were simple. Since it was anticipated that the cellar fill would have complex soil strata, the identification of the cellar was in error. Within the Gallows Green area of St. Mary's City (MD04), a radar survey reliably located complex strata at a depth of 3 ft. (1 m) in an area of 30 by 40 ft. (10 by 12 m); excavations revealed that this feature may be a sawing pit. This feature was clearly detected by geophysics, but it could not be identified. A magnetic survey at Saint-Gaudens National Historic Site (NH01) found large and deep masses of iron in an area where the cellar of the sculptor's studio was thought to have burned; while it is possible that the iron was from the armatures within clay models, the iron masses did not reliably define the location of the studio. Radar was not successful at this site because the stony soil caused the radar profiles to be complex everywhere. Geophysical surveys have also failed to detect cellars. A root cellar that was excavated near Benjamin Bannecker's cabin (MD03) contained large stones to a depth of 5 ft. (1.5 m), but this was not detected by a radar survey. The cabin was also not revealed by the radar, even though a quantity of stone was found near the foundation. These failures may have been due to three factors: The soil was rather conductive (limiting the profiling depth with the radar). Bedrock was perhaps at a depth of a few feet (about 1 m) and the stoniness of the soil caused the radar profiles to be complex. Finally, the spacing between the radar profiles was 10 ft. (3 m) in order to allow a large area to be explored; small features were lost between the profiles. At Johnson Hall (NY04), the area of a wooden cellar was adequately explored with a radar, but no trace of it was detected. The cause of this failure is unknown. It is generally the rubble within cellars that makes them distinctive to a geophysical survey. In a few cases, the detection of a cellar floor has suggested the location of a former building. Ground-penetrating radar is probably the only geophysical instrument that might be successful in this search. However, radar will likely fail at sites where natural soil strata are fairly horizontal and discontinuous. Geophysics will then give too many false alarms. A radar survey at the site of the Dawson House (DE01) could not identify the historically-important cellar floor that was later discovered by excavation. While this early floor was detected by the radar, natural soil strata in the area gave similar patterns at the same depth. Therefore, the fragment of the cellar that was detected could not be identified as a cultural feature. The U-shaped echo band in Figure 2 reveals the cellar floor of an early building at Page 6
Indications: floor Appomattox Manor (VA03); there is almost no hint of rubble within this cellar. This floor is about 14 ft. (4.3 m) long and 6 ft. (2 m) underground. The slight dip in the middle of the floor is exaggerated by the large compression (eleven-fold) of the horizontal scale of the radar profile. While the other profiles within this article show findings that are unusually clear, the echo from this cellar floor is typical of several other cellar floors that have been detected with a radar. On Jamestown Island, the cellar floors of some 17th-century buildings have been detected with radar surveys. One edge of Structure S-105 was first located by excavations that were made by the National Park Service (NPS) in the 1930's or 1950's. The unexcavated floor of the cellar of this building was clearly detected by the radar (VA06); rather little rubble was detected within the cellar. The contrasting fill within this cellar was also revealed by resistivity and conductivity surveys. On another part of Jamestown Island, a radar survey detected the cellar floor of an earth-fast building from the period of the James Fort (VA08). The radar revealed this cellar to be a broad basin; a geophysical estimate of the outer boundary of the cellar differed by 2 - 5 ft. (0.8 - 1.5 m) from the boundary that was determined by later excavation. Several other 17th-century cellar floors that are similar to this one at Jamestown were revealed by a radar survey at Westmoreland Berry Farm (VA20) on the Rappahannock River. At Mount Pleasant (VA18), southwest of Jamestown Island, two cellars were detected during a radar survey. While a thick zone of rubble revealed one cellar, the other was identified by a floor at a depth of 5 ft. (1.5 m). Geophysical surveys are valuable for an exploration below depressions that are visible at the surface. At Appomattox Court House (VA16), there is a depression next to the Peers House that is 2 ft. (0.6 m) deep and 30 ft. (10 m) in diameter. Neither radar nor conductivity surveys could find any suggestion of a cellar at this location. Excavations could not determine a cultural origin for the depression either. While cellars can be detected with magnetic, radar, conductivity, and resistivity surveys, they should also be detectable with a seismic refraction survey; seismic surveys are slow, but they are excellent for detecting the difference between natural soils (which can be compact and cemented) and fill soils (which are loose and porous). Floor Except at the base of cellars, floors are seldom detected by geophysical surveys. This appears to be due to the shallow depth of floors; they may have been broken up by rodents, plowing, or other landscaping. Any of the geophysical instruments are probably suitable for the search for the remains of shallow floors. The known buried floor of a modern building was readily identified with a radar survey at the site of the Dawson House (DE01). Figure 3 illustrates how a floor at Rosewell (VA11) was detected with a radar. The echo bands in the middle of the profile extend over a span of 24 ft. (7 m) and are probably caused by an abrupt change in underground materials at a depth of less than 3 ft. (1 m). Before the radar survey was done, the brick floor in a corner of this building had been discovered in an excavation; the geophysical survey simply defined the remainder of the building. Brick that is in the rather sandy soil at this site can be invisible to a radar survey; Page 7
Indications: foundation therefore the source of the radar echo could be a layer of fill that is below the brick floor. Clay or cinders are possible materials that could cause the strong echo and the reverberation of the radar pulse. A layer of bricks from a former structure could not be identified by either a magnetic or a radar survey at the Watt House (VA13) on the Richmond National Battlefield. Iron debris in the soil and iron in a nearby garage had a large effect on the magnetic survey. While complex echoes caused by tree roots affected the radar survey, it is possible that part of the floor was detected by the radar. A dairy barn at the Gilpin House at Brandywine (PA07) was known from historical photographs; this barn was constructed after 1910. A radar survey detected the floor of this former barn as a simple and planar interface. In Japan, a radar survey readily revealed the basin-like floors of early buildings (Imai et al. 1987); these floors were about 6 m (20 ft.) wide and 1 m (3 ft.) underground. There was a large contrast between the floors and the volcanic ash that covered them. The stone floor of a cabin was found to give high readings with a resistivity survey (Kvamme 2001); the stone foundations of the cabin were visible at the surface and had high resistivity also. Foundation Foundations have rarely been revealed by my geophysical surveys. This is because there is typically so much construction rubble around foundations that the foundation cannot be isolated within a complex pattern in the geophysical data. The linear extent of a foundation is an aid to its identification; this identification is more certain if either perpendicular or parallel lineaments are revealed by geophysics. Foundations are typically narrow and this makes the search more difficult. The builder's trench may be as detectable as the foundation itself. The chance of a successful geophysical search will increase where a foundation has a large cross-sectional area. Figure 4 illustrates a radar profile that crosses the foundation of the Great Brick Chapel at St. Mary's City (MD04). This brick foundation has a width of 3 ft. (1 m), and extends from a depth of 1 ft. to 4.5 ft. (0.3 to 1.5 m). As with previous illustrations, the radar did not detect the bottom of the foundation, and the profile suggests that it goes deeper than it actually does. The radar also detected the two upper corners of each foundation with separate peaks on the radar profile; this is rare, for most foundations are narrower and not flat on top. The foundation of this chapel had been exposed during excavations in 1938 and later those excavations were backfilled; this excavation may have aided the detection of this foundation, for it could have removed loose bricks from above the intact foundation. Incidentally, the echoes from three lead coffins that were later excavated are also shown in Figure 4; the geophysical survey could not identify these as either lead or coffins. The span between the two foundations is within the chapel and it is dense with graves; essentially none of these are revealed by the radar on this line. Little rubble was evident around the foundation of the Great Brick Chapel; this helped the radar survey. At the Original Phoenix Townsite (AZ01), foundations of stone and concrete were readily traced with a radar survey. This was apparently because the Page 8
Indications: foundation above-ground parts of the buildings were cleanly removed during demolition; there was little rubble in the fill to hide the foundations. A fire can aid the search for a foundation. The firing can magnetize the brick or perhaps the stone of a foundation; a magnetic survey can then be ideal. The Taylor House burned during the first stage of the Civil War siege of Petersburg (VA01); the brick foundation of the house was clearly defined by a magnetic survey. This building was unusual in how distinctly it was detected. The foundation walls were revealed by magnetic, radar, and resistivity surveys. The magnetic and radar surveys allowed estimates of the height of the cellar wall that remained; these surveys also agreed in their estimates of the orientation of the building. The radar survey detected the cellar floor. Resistivity, conductivity, and radar surveys revealed the fill within the cellar. The main house at the Robinson Farmstead on the Manassas battlefield (VA10) was destroyed in a 1993 fire; however, a magnetic survey found no distinctive anomalies in the area of the house. Neither radar nor magnetic surveys could detect three sandstone foundations that were found nearby during later excavations. A cinderblock foundation that was later excavated at this site was moderately magnetic; this foundation was not identified by the geophysical survey. Almost all brick is magnetic, even if it has not been burned in a house fire. While brick foundations can be detected by a magnetic survey, typical small foundations cause complex patterns and the foundation may not be delineated. Even the large foundation of the Great Brick Chapel (MD07) was detected as a lumpy and irregular pattern on a magnetic map; similar patterns are found in the magnetic maps of basalt walls (Barba et al. 1996). These complex patterns are caused by the random directions of the remanent magnetization of the brick or stone. On the coastal plain of the U.S., brick foundations can be difficult to detect with a radar or resistivity survey; this is because the electrical properties of brick can be very similar to the surrounding soil. At Mount Pleasant (VA18) and on Jamestown Island (VA08), brick foundations were invisible to the radar. Stone foundations may be detectable with any of the geophysical instruments. Interestingly, foundations of non-magnetic stone such as limestone or quartzite can be detected with a magnetic survey (Smekalova and Maslennikov 1993; Gaffney et al. 2000; Clay 2001). For this survey to succeed, the surrounding soil must be magnetic; these magnetic soils are typically found in areas that have limestone bedrock or river-deposited clay. For the greatest likelihood of success, the foundations should be large and shallow. Stone typically has a higher electrical resistivity than the surrounding soil; this allows it to be detected by a resistivity or conductivity survey. Possible stone foundations were traced with a resistivity survey in New York state (Kvamme 2003). High resistivity was found along the 4 by 7 m (12 by 20 ft.) rectangle of the underground foundation of a powder magazine at Fort Atkinson (Weymouth 1996) near Omaha, Nebraska. A magnetic survey at Fort Ellis (Weymouth 1996) near Bozeman, Montana, revealed intermittent parallel lines spaced by 20 m (60 ft.) that were caused by the foundations of a building. At the site of Ink's Tavern (PA08) on the Fort Necessity battlefield, a cellar hole containing stone rubble is visible. A magnetic survey of this area revealed broad anomalies with no archaeological significance; the intact stone walls that were buried could not be Page 9
Indications: debris scatter detected. The rock that is native to this area is sedimentary and non-magnetic; the cause of the geophysical failure is not known. A tile probe is particularly good for tracing the tops of foundations that are at a shallow depth. Aerial photography may also be suitable for locating shallow foundations. It appears that remanents of the foundations of 19th-century buildings were revealed at Fort McHenry (MD01) in aerial photos taken of a mown lawn. With the radar, foundations that are shallow may be more difficult to detect than deeper foundations. At Stenton Mansion (PA02), a radar survey detected the lower surfaces of schist foundations, rather than the tops, which were only 0.5 ft. (15 cm) underground. Linear foundations are much easier to identify with a geophysical survey than are separated piers of stone or brick. Post molds that mark the wooden pillars of buildings are even more difficult to detect and identify. At the site of Grant's cabin (VA02) near Petersburg, these post molds were much too small to be detected by a radar and magnetic survey. In addition to hard foundations of stone, brick, or concrete, some buildings may have had a simple wooden sill foundation or have been constructed with sod or adobe. Traces of these foundations may sometimes be detectable with geophysical surveys. At Fort Lowell (AZ02), a resistivity survey found low readings at or near the buried remanent of an adobe wall. Debris Scatter The aspect of a buried building that is most commonly detected by a geophysical survey is the rubble that overlies and surrounds the building. This rubble may contain stone, brick, metal, and other refuse. The area of rubble may be diffuse, and without sharp boundaries. The geophysical patterns of rubble may be identical to those caused by debris middens or by natural clusters of stones in the soil; therefore, these geophysical patterns do not have a high reliability for identifying former buildings. Any of the geophysical instruments can be suitable for detecting rubble. Ground-penetrating radar is particularly good for identifying cultural soils by their stratigraphic complexity and this instrument is excellent for locating structural debris. Figure 5 combines the findings of a radar, magnetic, and conductivity survey that detected three areas of rubble inside Fort Griswold (CT01), a stone fortification from the time of the Revolutionary War. The three dotted areas mark where complex soil strata were detected. Note that these areas are just blobs; they reveal no rectangular patterns that suggest foundations. This finding is typical. The geophysical evidence is that buildings were once located at the three areas that are dotted in Figure 5. Historical records and also excavations after the geophysical survey indicate that a blockhouse stood at the middle anomaly, while a barracks was at the southeast. Both of these structures were found to have rubble-filled cellars. The geophysical survey did not identify these as cellars; they simply appeared to be concentrations of debris. No cellar was found at the northeastern anomaly, and this area was disturbed by a barracks from World War II. Geophysical surveys have correctly suggested the locations of buildings by a concentration of debris at many sites. At Raritan Landing (NJ01), the radar was suitable Page 10
Indications: fireplace even though the soil was silty and therefore had a high attenuation for the radar pulse. At Peachfield (NJ03), the radar detected a midden-like feature that appears to be debris from an earlier building. At the Abraham Van Wyck House (NY05), magnetic, resistivity, and radar surveys each suggested a concentration of debris on the east side of a standing building; an historical photograph and prior excavations had already indicated that there was once an earlier structure there. At Valley Forge (PA03), stone rubble from an unknown 18th-century structure was detected by radar and resistivity surveys. At the site of the Widow Tapp House (VA05), a thin scatter of debris was noticeable with magnetic and conductivity instruments. At the Stevens House (VA07), excavations confirmed that a zone of debris (detected by a radar survey) marked the building that was sought; the analysis of an historical map agreed with this. At the site of the Academy Dwelling at Appomattox Court House (VA16), a cluster of magnetic anomalies appear to define the original extent of the former building. During a magnetic search for traces of the city of Sybaris (Ralph et al. 1968) in Italy, the roof tiles of destroyed buildings were readily detected. Geophysical surveys have also falsely suggested that regions of debris located former buildings. Around the Gilpin House on the Brandywine battlefield (PA07), a radar survey could not distinguish between zones of building rubble and lenses of other debris. Other surveys have also been failures: At Pluckemin (NJ02), the native granitic stone is magnetic and thick in the soil; neither magnetic nor radar surveys revealed the locations of buildings from a Revolutionary War encampment. At the Emlen Physick Estate (NJ04), the natural sandy soil at this coastal location was unexpectedly complex on radar profiles and the survey had little success in suggesting the former locations of buildings. At some sites, geophysical surveys have detected concentrations of debris that could be remanents of buildings; however, excavations that followed the geophysical surveys have sometimes revealed neither buildings nor a cause for the geophysical findings. This was the case at Salem Maritime National Historic Site (MA02), Long Hill (MD06), the Shapiro House at Strawbery Banke (NH02), the Robinson Farmstead on the Manassas battlefield (VA10), and the West House (VA22) on the Richmond Civil War battlefield. A resistivity survey at Fort Riley in Kansas revealed that some concrete foundations and accumulations of building debris had high resistivity (Hargrave et al. 2002); building debris was also detected as high values of resistivity. In Haiti, stone rubble from buildings was apparent as high resistivity (Shapiro 1984). The success of a geophysical search for rubble will increase where the soil itself has only a simple stratification (the soil should not be stony and not have been glaciated); success will also be better at sites with little modern debris (particularly metallic objects). Fireplace A fireplace and its chimney may contain more stone or brick than any other part of a house; therefore, the detection of chimney rubble or an intact fireplace base or hearth may provide a good indicator of the location of a house. Since a fireplace may be either in the middle of a house or at its outer edge, the detection of a fireplace alone may not define the exact location or extent of a house. On Jamestown Island (VA08), a radar survey revealed complex strata where Page 11
Indications: soil chemistry excavations later found a dense concentration of brick rubble from a fallen chimney. This survey did not detect an intact brick foundation that was nearby. At the site of the Fairview Cabin (VA15) on the Chancellorsville battlefield of the Civil War, one quadrant of the stone base of a fireplace was detected with a radar survey. Excavations could not determine why the rest of the fireplace was invisible; it could be due to a difference in the type of stone or perhaps due to changes in the soil above or below the stone. Some of the stone that is native to this area is magnetic, and a magnetic survey failed to detect any part of this fireplace or the cabin. At Fort Lennox, in Canada, a resistivity survey (Ralph 1969) revealed high readings at a pair of stone hearths within a former hospital. Geophysical surveys with radar, resistivity, and conductivity instruments all failed to detect the intact stone base of a chimney that was later excavated at the Academy Dwelling at Appomattox Court House (VA16). At New Windsor Cantonment (NY02), a magnetic survey detected the stones in three hearths that were within huts, but failed to detect three other hearths that were found by excavation. At this site, a radar survey was unsuccessful because of the natural stoniness of the soil. A magnetic survey at Valley Forge (PA03) was done over the burned earth of a hearth that had earlier been exposed by an excavation; this test showed that such a hearth could be detected by a magnetic survey. However, this hearth was not detected by an earlier survey because the magnetic pattern of the hearth was completely obscured by four modern iron tent pegs that were buried in the soil. While the cellar of the Bullock House (VA14) at Chancellorsville was readily detected with radar, magnetic, and conductivity surveys, the brick bases of three chimneys were invisible to each of the instruments. These fireplaces were only revealed by later excavations. Brick is moderately magnetic, but it is much less magnetic than iron or steel; a small amount of iron will readily mask the detection of nearby brick. When a cluster of bricks has been fired in place, it becomes about ten times more magnetic than it would be if it was not refired. Magnetic tests have been made on the bases of two fireplaces on Jamestown Island (VA21). These tests showed that the brick of these fireplaces was not remagnetized by the heat of the fires that burned there; it is possible that the earth below the base cooled the brick or that ash on the hearth insulated the brick so well that it did not become heated enough to be remagnetized. It is possible that the brick higher on the side of a fireplace may have been heated more, but this part of a fireplace is not preserved during destruction. Soil Chemistry The remanents of buildings that have been described above would all be visible to the eye of the excavator. While it has not been proven, it appears likely that geophysical surveys may be able to detect the former locations of buildings even where nothing unusual would be visible in an excavation, not even an increase in the spatial density of artifacts. Geophysical surveys can detect contrasts that are caused by cultural modification of the soil. This may be an increase in the soil's organic matter or porosity, or perhaps there is a greater quantity of cultural refuse in the soil. While chemical surveys have been successful Page 12
Indications: soil chemistry for this type of search, geophysical surveys are faster. Conductivity and resistivity instruments are likely to be the most suitable geophysical tools for this search. Ground-penetrating radar is the least suitable; radar is best at detecting abrupt soil interfaces, and these chemical boundaries are blurred and diffuse. The common type of magnetic survey is not usually appropriate for locating these soil contrasts either. While most magnetic instruments measure how iron-containing materials indirectly affect the Earth's magnetic field, an instrument that is called a magnetic susceptibility meter directly detects magnetic materials. Magnetic susceptibility measurements are excellent for defining cultural soils (Dalan and Banerjee 1998); these susceptibility readings may also benefit the analysis of the soils in shovel test pits. Geophysical evaluations of these soil contrasts may be confused by fertilization and landscaping, and also by middens. These geophysical searches are based on the assumption that the soil near and over a buried building may be more conductive or more magnetic than the natural soil in the area. An apparent example of this is shown in Figure 6; this is a conductivity profile that crosses the cellar of the Bullock House at the Chancellorsville battlefield (VA14). Two different conductivity instruments were used for this test; one explored to the shallow depth of 5 ft. (1.5 m), while the other explored to a depth of 20 ft. (6 m). Both instruments reveal an abrupt change where the cellar of the building was later found by excavation; these large changes were caused by modern metal-containing debris within the cellar fill. It is the gradual rise in the conductivity values for a distance of 100 ft. (30 m) from the building's location that is most important. While there could be a natural cause for this rise, the higher conductivity readings may also be due to organic debris that was once discarded in the vicinity of the house. A similar rise in conductivity was found around the nearby Fairview cabin (VA15), although part of that conductivity increase was probably caused by an accumulation of topsoil in the shallow basin at the cabin. At the Richmond National Battlefield, a geophysical search located two slave quarters (VA17) near the Crewes-Mettart House. A radar survey was not successful, but conductivity and magnetic susceptibility surveys detected the two buildings. The buildings could be identified on the geophysical maps because the relative locations of two geophysical patterns coincided with the separation of the buildings on an historical map. Figure 7 illustrates three lines of conductivity measurement. Excavations located one building near the left side of the figure, at N30; a brick pier was unearthed at this location, and the land in this area was never plowed. The second building was discovered near the middle of Figure 7; this area had been plowed, and only small fragments of brick and other artifacts were revealed in excavations. At a later date, another geophysical survey was done about 200 yards (200 m) east of this site on the Richmond National Battlefield, searching for former buildings near the West House (VA22). While radar, magnetic, and conductivity surveys gave estimates of where some buildings might have stood, no hints of buildings were found by later excavations. The geophysical patterns appear to be caused by natural features such as contrasts in sedimentation and filled-in gullies. A resistivity survey near the Provost Office at Harpers Ferry (WV01), detected an area of low readings where an historical map indicates that a stable once stood. A conductivity survey at Red Hill (VA23) found unusually high readings in the vicinity of standing buildings. Page 13
Indications: auxiliary features High readings of conductivity are equivalent to low readings of resistivity. A high conductivity anomaly within West's field at St. Mary's City (MD08) marked the probable location of Phillip West's house, which was occupied for a period of less than 20 years. However, other conductivity anomalies at this site appear to have been caused by geological effects, such as a changes in the thickness of soil over a layer of sand and gravel. A radar survey at Brookhaven (NY07) revealed a zone where the radar signal was attenuated; this area was centered on a visible depression. While excavations discovered that the soil at this location was unusually silty to a depth of about 4 ft. (1.2 m) , it is not known if a building once stood there. A conductivity survey in the vicinity of the Connor-Sweeney Cabin at Appomattox Court House (VA16) detected bands of high readings next to the standing cabin. These conductive bands continued along lines that suggested an extension of the cabin in one direction. However, excavations could find no structural evidence in the area of the possible extension. Therefore, it is not certain if soil changes by themselves will allow buried buildings to be reliably detected by geophysical surveys. Further investigations of this geophysical technique are necessary. Auxiliary Features Buried roads, paths, pipes, and wires may be aligned toward a house and therefore may allow the location of the house to be estimated. These long and linear features are often easy to detect with a geophysical survey. Privies and wells may be situated near a house, and are always important to find. However, these features are usually difficult to detect with geophysical exploration. While they extend to a good depth, their surface area is small. Paths quickly subside into grass-covered soil; they may also wander about a garden, without pointing to a house. If the buried path has a moderately thick and dense layer of gravel, cinders, or perhaps shell, it may be very apparent with a resistivity or radar survey. The left side of Figure 2 shows how a buried path was revealed at Appomattox Manor (VA03); the clues are a shallow and rather flat interface that is a few feet (1 m) wide and which extends as a ribbon for a good length. A similar interface that is deeper may instead be caused by the gravel-lined trench of a French drain; these features were detected during a radar survey at Mount Vernon (VA04). Gravel-bottomed trenches are also found in modern septic drain fields; these parallel bands were readily traced with a radar survey at the Bullock House (VA14) and the West House (VA22). If several parallel trenches are detected, the identification of a drain field is almost certain. Linear paths were identified with a radar survey at Monteplier Mansion (MD05) and at the Shapiro House at Strawbery Banke (NH02). At the Travis site (VA09) on Jamestown Island, a possible buried path was traced but this feature did not reveal the location of the large house that was sought. At Gunston Hall (VA12), a radar survey suggested that a buried path was three times wider than the path that is visible at the surface. At Long Hill (MD06) a buried shell path was not detected with a high resolution radar; other evidence also suggests that shell paths, particularly where they are thin, can be invisible to geophysics, at least in sandy soil. Aerial photography should always be considered during the search for paths; at Lemon Hill Mansion (PA01) in Philadelphia, the locations of paths were suggested Page 14
Indications: auxiliary features in aerial photographs. Buried roads may be detected with any of the geophysical instruments. Part of a road known from an historical map appears to have been delineated by a radar survey at Fairfield (VA19); irregular soil stratification was found along a band that was 10 ft. (3 m) wide. At Friendship Hill (PA06), the radar revealed that the underground part of a road was offset by 10 ft. (3 m) from the gravel band that is visible at the surface. At Fort Necessity (PA08), radar and resistivity surveys sought the location of the Braddock Road in a span where it is not visible. Neither survey found any hint of the road. Metal pipes and underground wires are usually traced by geophysical surveys much easier than is wished; they are typically modern and unimportant. They can also be so dense that other features are masked by the geophysical patterns of the utility lines; this finding is particularly true for magnetic and conductivity surveys. Buried utility lines are unusually dense at Mount Vernon (VA04). Magnetic surveys strongly detect iron pipes, while conductivity surveys can cause huge anomalies near metallic pipes and wires. While radar surveys detect these buried utility lines, nearby features can still be isolated. A resistivity survey detects little metal. Wooden or ceramic pipes are easiest to find with a radar. A brick-arched drain at Rosewell (VA11) was easily traced with a radar; a magnetic survey also detected this drain. A stone drain at Smith's Castle (CT02) was not detected by a radar survey, perhaps because of other stones and tree roots in the soil. At Johnson Hall (NY04), parallel lines of stone walls defined 18th-century drains; these double lines could partially be traced with a radar survey. The locations of two former privies at this site were revealed by the radar because of the complex patterns caused by the stone rubble. At Harpers Ferry (WV01), neither resistivity, conductivity, nor magnetic surveys detected two of three privies that were later excavated; the third privy had a large magnetic anomaly that the excavations could not explain. Perhaps privies have been detected by geophysical surveys at other sites, but none have been identifiable as privies from the geophysical data. In principle, the dripline adjacent to a former building may be detectable with a geophysical survey because of the greater sandiness of the soil along that line; in practice, other features should be much easier to detect. Wells are also rarely identified by geophysics. In one unusual case, a well could be positively identified because the dug shaft was filled with iron; this well was at the Petersburg battlefield (VA01), and a magnetic survey allowed the identification (a mass of iron that is vertical in the earth causes a unique magnetic anomaly). A radar survey may sometimes reveal the subsidence cone of fill soil around a former well. In principle, vertically-aligned echoes on a radar profile may reveal debris within the shaft of a well; even when reverberations of the radar signal are discounted, this pattern appears to be unreliable. If air-filled tunnels or vaults are sought, radar is the geophysical instrument that is most likely to have success. A known underground vault was readily detected at Stenton Mansion (PA02). An air-filled cistern was detected (although not identified) during a radar survey at Mount Vernon (VA04). A tunnel was revealed by the radar at the Evans-Mumbower Mill (PA10), even though this tunnel was filled with silt. Rodent dens cause clear and interesting radar echoes; they are readily confused with valuable archaeological features, such as refilled pits. It appears that the air voids of rodent dens caused strong radar echoes near the Page 15
Special considerations Crewes-Mettart House (VA17) at Malvern Hill, and at Fairfield (VA19). The air voids in rodent dens may last for decades after the animal has departed. Special Considerations Geophysical surveys are least likely to succeed in cities. Rubble from several generations of buildings is probably thick and dense over the earlier buildings that are sought. Electrical interference from wires, electrified trains, industries, and transmitters will usually be severe. With a radar, higher frequency antennas (for example, 300 MHz, rather than 200 MHz) may find less radio interference. A radar survey in downtown Philadelphia (PA04) failed because the conductive and complex soil severely limited the profiling depth. Identical problems were found with the radar at two locations where early Dutch buildings were sought in lower Manhattan (NY01, NY03). A radar survey at Rose Hill Manor (NY06) in the Bronx did not find a problem with profiling depth, but the soil strata were too complex for reliable interpretation. While resistivity surveys may detect only minor electrical interference and may not be severely affected by underground metallic debris, the thickness of the debris will probably cause resistivity surveys to fail also. Fill soil, even if it is homogeneous and contains no debris, can reduce the success of a geophysical survey. This is because the features are now deeper underground. For all instruments (except sometimes radar), features are easiest to detect where they are most shallow. Surveys may also be less successful at sites where buildings have been occupied into recent times; this is because there is likely to be a large quantity of metallic debris at these modern sites (Clay 2003) that may mask the detection of earlier structures. While farming contributes some metal to a site, modern occupation adds more metal. In general, the farming of an abandoned site appears to aid its geophysical evaluation; St. Mary’s City in Maryland is an example. Perhaps plowing homogenizes the topsoil and decreases the number of rodent dens. In addition to detecting buildings, geophysical surveys may also attempt to map entire towns or cities; then, the roads that define blocks of buildings may be most apparent. While some work with this goal has been done in North America (Hargrave et al. 2002; Kvamme 2003) much more work has been done in Europe (Gaffney et al. 2000). Early surveys of towns or cities were done in about 1968 by Elizabeth Ralph (University of Pennsylvania) at Elis, Greece, by Richard Linington (Lerici Foundation) at Metapontum, Italy, and by Irwin Scollar (Laboratory for Field Archaeology, Bonn) at Colonia Ulpia Trajana in Germany. Modern landscaping has an effect on geophysical surveys at many historical properties. Perhaps flowerbeds must not be walked through; if this is the case, there will be gaps in the data and the survey will be slower. Fences cause similar problems, although wooden fences may sometimes be dismantled. Beds of ivy or bark mulch may cover part of the earth; these materials appear to diminish the profiling depth of a radar. In principle, the burning of a building should aid in its location. Many artifacts and building materials could have been destroyed and are now buried with the building. Even better, the fire could have caused the burned materials to become much more magnetic (McPherron and Ralph 1970). The sites that are described here have not had significantly greater geophysical success as a result of burning. This is probably because radar surveys Page 16
Special considerations have generally worked well at these sites, and radar findings are not significantly improved by a fire. The effect of fires on foundations has already been described; other effects are mentioned here. At Franklin Mill (MD02) in Baltimore, amorphous magnetic anomalies were found inside some structures; a 1934 fire may have contributed to these patterns. The Prior Wright House at Appomattox Court House (VA16) was destroyed in a fire, but no trace of it was detectable with a magnetic survey. Benjamin Bannecker's cabin (MD03) burned in 1806, but it could not be detected with a radar survey; a magnetic survey was not tried because bedrock there is magnetic (gneiss or igneous intrusive stone). The remanents of small buildings, such as typical outbuildings, will always be more difficult to detect than larger buildings. Most of the buildings that are described here were moderately large. Near the Taylor House (VA01), on the Petersburg battlefield, excavations and historical maps indicate that there were 2 to 4 outbuildings in the area of survey; a smokehouse and a dairy might have been in this group. While some remanent of the smokehouse may have been detected by geophysics, none of the other buildings were revealed. Geophysical surveys rarely succeed in close proximity to standing buildings. The greatest amount of debris may be found in this area, and there may be a strong geophysical effect from the building itself. Brick and metal in the building cause the greatest problem. Bushes and utility lines are common close to buildings, and these complicate a survey; bushes limit access to the area and buried pipes and wires cause complex geophysical patterns. Geophysical surveys may be done inside buildings, perhaps searching for foundations below a cellar; again, metal within the building may have a large effect. Radar and resistivity surveys on the basement floors of two buildings in Philadelphia (PA09) were not successful. The radar was limited by conductive soils, and also by echoes from overhead ceilings; the resistivity measurements were limited by rubble in the soil and the area available for work. Radar profiles in the cellar of the Shapiro House (NH02) at Strawbery Banke detected an underground feature where a junction between two adjoined buildings was anticipated. Geophysical surveys may be done on the walls of standing buildings, perhaps searching for an air void behind a stone wall. Resistivity and radar surveys may be most suitable for these explorations. A seismic study of the stone columns of a Roman temple allowed estimates of the weathering of the stone (Cardarelli and de Nardis 2001). A geophysical evaluation was made in the U.S. Capitol (DC01), trying to find a procedure for detecting a silver plate below the foundation of the building. While a metal detector or a chemical exploration of the soil may have some success, normal geophysical procedures would not be suitable because of the difficult architectural geometry and a proximity to other metal. If a cornerstone is sought, then radar or conductivity surveys can be tried if a scan with a metal detector is not successful. The cast iron facade of an historic building in Philadelphia (PA05) was saved in a then-dry water reservoir. The location of the facade was lost after the reservoir was filled with soil. A magnetic survey found a clear anomaly; the analysis of this magnetic pattern suggested an iron mass whose quantity, dimensions, and depth were reasonable for the facade. Excavation found only incinerated trash. Page 17
Conclusions It may be valuable to find where no buildings were once located; perhaps a new visitors' center is planned for an historical property. This type of survey may be done almost the same as a search for buildings. An evaluation of the geophysical data may suggest locations where buried buildings and other cultural features are least likely. However, a geophysical survey may fail at this search also; the survey may suggest that no building was in the survey area, while excavations reveal one. Conclusions The aspect of a buried building that is most commonly detected by a geophysical survey is the scatter of demolition rubble at and near the location of the former building. The success of this survey will be lower where the soil is naturally stony; the stones can mimic the rubble of buildings. Rubble-filled cellars, or cellar floors without rubble, can give clear indications of a former building. Wall foundations are seldom delineated. The rectangular shape of a typical building is seldom revealed by a geophysical survey. This is because of the low spatial resolution of geophysical maps; the remains of the building are probably fragmentary and the geophysical patterns are confused by rubble. The examples here have suggested that my chance of successfully detecting a buried building has been about 50 % with a geophysical survey. This success rate is higher than that of a search for unmarked graves, which may be 30 %. Some factors must be considered that could cause this low rate of success. First, half of the surveys of mine that are evaluated here were done before 1990; instruments and analyses have improved since the early work. The major change is that geophysical instruments have become faster, and this allows surveys to be done with a closer measurement spacing; this means that it is now practical to detect smaller features. The second factor that must be considered is the economy of geophysical surveys. These surveys would have been more reliable if more time could have been devoted to them. These conclusions about the success of geophysics apply to the author's surveys but not necessarily to the surveys of others; the success of a geophysical survey will change with expertise and funding. The success of geophysical surveys may also be different in other parts of the country. While rates of success will differ with location and the geophysicist, one must not assume that buildings will be detected by geophysical surveys. While my chance of detecting a building is about 50 %, the likelihood that I have been able to identify a building with a geophysical survey is only about 25 %. Identification means that a geophysical pattern is reasonably and reliably interpreted as being caused by a building; identification does not mean that the owner or the date of a building could be determined by geophysics. Rectangular foundation lines and cellar floors provide the most certain identification of buildings. The detection of rubble allows only an uncertain identification of a building unless there is confirming evidence from history or archaeology. While it would be valuable if the reliability of identifying a building was higher, the geophysical detection of the possible locations of buildings allows a thoughtful and economical placement of archaeological excavations. Except for the surveys in cities, the examples here have been done in areas that were generally favorable for geophysical surveys. If a greater fraction of these surveys had been Page 18
Appendix: survey sites done in glaciated areas, or at locations with igneous rock, or where the soil was clayey or saline, or in cities, the success rate would have dropped. If smaller buildings were sought more frequently, the success rate would also have been lower. The examples here have illustrated sites that have been more suitable for radar surveys than normal; this is because many of the surveys were done on the coastal plain of the U.S. The rather sandy and weakly stratified soil provides excellent conditions for radar surveys. A major point has not been answered in this article. How cost-effective are geophysical surveys relative to alternative methods of exploration, such as excavation or shovel tests? The best evidence that geophysical exploration is cost-effective is that it continues to be applied to archaeology. Even if they are not cost-effective, geophysical surveys can aid archaeology by allowing a greater part of an archaeological site to remain preserved in the soil. The cause of the failure (and sometimes the success) of a geophysical survey is often unknown. The discipline of geophysics as applied to historical investigations would benefit if geophysical evaluations of the findings of excavations were possible. On-site tests in open excavations would be most valuable. These tests could determine if a feature encountered in an excavation was the entire source of the geophysical pattern. Measurements of the geophysical parameters of features in excavations will increase the reliability of future geophysical interpretations. Appendix A listing of sites and surveys that illustrate my searches for buildings is included here. This information allows the sites to be located. If a survey is considered for a site that is nearby, or at a location that has similar geology, soils, or archaeological features, the findings with the survey here may guide considerations for the neighboring site. Each listing starts with the state's abbreviation and a sequence number; this is followed by the site's location and the starting date of the survey. Next, the names of the individuals and organizations involved in the surveys are given; my thanks go to these people and groups for their interest in applying geophysical surveys. I particularly appreciate the many archaeologists who have sent me reports on the findings of their excavations that followed my surveys. The affiliations listed here apply to the time of the survey. For further information about a survey, or to request the report of the geophysical survey, please contact the archaeologist in charge of the project. Geophysical parameters and anomaly amplitudes are included in order to benefit geophysicists who may work in nearby areas or search for similar features. Electrical resistivity of the soil is an average through the archaeological zone; it has usually been determined from the analysis of resistivity soundings. Radar pulse velocity has been estimated by geometrical measurements of the hyperbolic echo arcs from underground objects. The magnetic moments of features have been estimated by an analysis of magnetic maps. Finally, cross-references are given at the end of each listing to the sections in the main text where the survey is described. These headings are abbreviated to one word: Cellar, Page 19
Appendix: survey sites Floor, Foundation, Debris (scatter), Fireplace, Soil (chemistry), Auxiliary (features), Special (considerations). The listing is in order of state names; the secondary sorting is by the date of survey. Arizona AZ01, Original Phoenix Townsite, Phoenix, 2 Sep 80. Survey for Lyle Stone (Archaeological Research Services). The geophysical survey was described by Bevan (1984). Foundation. AZ02, Fort Lowell, Tucson, 20 Apr 00. Survey for Steven De Vore (NPS). A pseudosection found a resistivity of about 60 ohm-m near the adobe wall, but 140 ohm-m outside the area of the wall. Foundation. Connecticut CT01, Fort Griswold, Groton, 3 May 84. Survey for Ricardo Elia (Office of Public Archaeology, Boston University). The survey and excavations are described by Elia (1985). The survey was done within the grassy interior of this fort; Figure 5 also shows features that were visible at the surface. A conductivity survey (Geonics EM31) found high values at the central structure, but low values at the southeastern structure; anomalies were 0.5 mS/m above or below a background of 3 mS/m. The line spacing of the radar and magnetic surveys was 5 ft. (1.5 m), while the spacing between conductivity lines was 10 ft. (3 m). Bedrock in the area is shallow, granitic, and moderately magnetic. Radar pulse velocity = 9.1 cm/ns. Debris. CT02, Smith's Castle, Cocumscussoc, near Wickford, 18 Jul 91. Survey for Patricia Rubertone (Brown University). Radar pulse velocity = 15.5 cm/ns; soil resistivity = 1800 ohm-m. Auxiliary. District of Columbia DC01, Capitol cornerstone, Washington, 13 Mar 90. Evaluation for Kara Schoenberger (Office of the Architect of the Capitol). Special. Delaware DE01, Dawson House site, Dover, 22 Jul 97. Survey for John Bedell and Charlie LeeDecker (Louis Berger). Excavations at this site are described by Bedell and Scharfenberger (2000) and Bedell (2001). Radar pulse velocity = 14 cm/ns; soil resistivity = 3000 ohm-m. Cellar. Massachusetts MA01, Springfield Armory, Springfield, 1 Jul 84. Survey for Dana Linck (NPS). The survey is summarized in Bevan (1996b). Cellar. MA02, Salem Maritime National Historic Site, Salem, 4 Apr 90. Survey for Michael Alterman (Louis Berger) (Alterman et al. 1995); funded by the NPS. Radar pulse velocity = 7.6 cm/ns; soil resistivity = 700 ohm-m. Debris.
Page 20
Appendix: survey sites Maryland MD01, Fort McHenry, Baltimore, 21 Aug 74. Photographs for Harry O'Bryant (NPS). Foundation. MD02, Franklin Mill, Dickeyville Historic District, Baltimore, 9 Nov 83. Survey for Elizabeth Comer and Charles Cheek (Office of the City Archaeologist, Baltimore). Site and excavations are described by Weber (1984). Special. MD03, Benjamin Bannecker cabin, near Ellicott City, 5 Sep 85. Survey for Kristen Peters, Robert Hurry, and Elizabeth Brown (Maryland Historical Trust). Site 18BA282. The soil was moderately conductive and magnetic (EM38 reading: 12 mS/m and 0.002 in SI units). Radar pulse velocity = 8.5 cm/ns. Cellar, Special. MD04, St. Mary's City, Chapel Field and Gallows Green, 15 May 89. Survey for Henry Miller (Historic St. Mary's City) (Chaney and Miller 1989). The radar profile in Figure 1 is along line W1145 in the archaeological coordinates of the site; the profile in Figure 4 is along line W1045. Electrical resistivity of the soil at Gallows Green is about 1000 ohm-m in the upper 1.5 m, but 40 ohm-m below that; radar pulse velocity = 8.5 cm/ns; predominant frequency of radar pulse = 315 MHz. Cellar, Foundation. MD05, Montpelier Mansion, Laurel, 6 Nov 89. Survey for Francine Bromberg (Engineering-Science). Late 18th-century residence of Thomas Snowden II. Soil resistivity = 180 ohm-m; radar pulse velocity = 7.6 cm/ns. Auxiliary. MD06, Long Hill, Wetipquin, 25 Apr 91. Survey for Michael Trostel and Mrs. Donald Graham; excavations by Hettie Ballweber (ACS Consultants) (Ballweber 1991). Radar pulse velocity = 10.7 cm/ns; soil resistivity = 1900 ohm-m. Debris, Auxiliary. MD07, St. Mary's City, Great Brick Chapel, 7 Apr 92. Survey for Henry Miller (Historic St. Mary's City). The magnetic survey is described in Bevan (1994). Foundation. MD08, West's field, St. Mary's City, 29 May 01. Survey for Henry Miller (Historic St. Mary's City); the survey and excavations described by Mitchell and Miller (2001). The EM38 conductivity anomaly at the probable location of Phillip West's house was 5.8 mS/m compared to a background of 4.5 mS/m. Soil. New Hampshire NH01, Saint-Gaudens National Historic Site, Cornish, 18 Sep 91. Survey for Jim Mueller (NPS); excavations by Joseph Balicki (John Milner) (Balicki 1991). The survey was described by Bevan (1996b). Soil resistivity = 750 ohm-m; radar pulse velocity = 10.7 cm/ns. Cellar. NH02, Shapiro House, Strawbery Banke, Portsmouth, 6 Jul 95. Survey for Martha Pinello (Strawbery Banke Museum). Radar pulse velocity = 7.7 cm/ns; soil resistivity = 150 ohm-m. Debris, Auxiliary, Special. New Jersey NJ01, Raritan Landing, Piscataway, 12 Jun 78. Survey for Joel Grossman (Rutgers Archaeological Survey Office). Warehouse from the 18th century. This survey was described by Grossman (1980). Debris. NJ02, Pluckemin, 6 Nov 79. Survey for John Seidel (Drew University). Winter Page 21
Appendix: survey sites encampment (1778-1779) and artillery park for General Knox. This site has been described by Seidel (1983). Debris. NJ03, Peachfield, near Mount Holly, 4 May 88. Survey for Elizabeth Heyl and Anna Louise Rudner (National Society of the Colonial Dames); excavations by Edward Larrabee and Susan Kardas (Historic Sites Research) (Kardas and Larrabee 1988). Soil resistivity = 400 ohm-m; radar pulse velocity = 8.2 cm/ns. Debris. NJ04, Emlen Physick Estate, Cape May, 20 Apr 94. Survey for Diane Cripps (Mid-Atlantic Center for the Arts). Radar pulse velocity = 10.1 cm/ns; soil resistivity = 550 ohm-m. Debris. New York NY01, near Fraunces Tavern, Broad and Pearl Streets, Manhattan, 14 Aug 79. Test for Cathryn Mish (NYC Landmarks Commission). The electrical resistivity of nearby soil was found to be 4 - 14 ohm-m. Special. NY02, New Windsor Cantonment, Vails Gate, 14 Sep 81. Survey for Charles Fisher (NY State Parks and Recreation). Area of the final winter encampment of the Continental Army in 1782-1783. The total magnetic moment of all of the fired stones in each of the hearths was about 0.2 Am2. This survey has been described by Sopko (1983). Fireplace. NY03, 100 Broadway, Manhattan, 19 Oct 83. Survey for Joel Grossman (Greenhouse Consultants). The electrical conductivity of the earth was generally over 50 mS/m (Geonics EM31) and the profiling depth with a radar was less than 1.2 m. Special. NY04, Johnson Hall, Johnstown, 21 May 84. Survey for Lois Feister (NY State Office of Parks, Recreation and Historic Preservation); she also provided a summary of excavation findings. Radar pulse velocity = 11.0 cm/ns; soil resistivity = 150 ohm-m. Cellar, Auxiliary. NY05, Abraham Van Wyck House, East Fishkill, 12 Jul 84. Survey for Roberta Wingerson (Cultural Resource Surveys). Building constructed in 1802. The electrical resistivity increased from 150 to 500 ohm-m at the location of the complex strata. A magnetic profile revealed 200 nT anomalies at the boundary of this feature (sensor height = 75 cm). Debris. NY06, Rose Hill Manor, Bronx, 16 Jan 85. Survey for Allan Gilbert (Fordham University). The profiling depth with the radar was adequate (about 1.5 m with a 315 MHz antenna). Special. NY07, Brookhaven site G, Long Island, 4 Oct 94. Survey for Mark LoRusso (Anthropology Survey, Cultural Education Center, New York State). Radar pulse velocity = 13.1 cm/ns; soil resistivity = 3000 ohm-m. Soil. Pennsylvania PA01, Lemon Hill Mansion, Fairmount Park, Philadelphia, 14 Jun 74. Photographs for Betty Cummin (Colonial Dames of America). Auxiliary. PA02, Stenton Mansion, Philadelphia, 12 Jul 75. Survey for Jeff Kenyon (University of Pennsylvania) and described in Bevan and Kenyon (1975). Foundation, Auxiliary. PA03, Valley Forge Historical Park, 4 Jan 78. Survey for the NPS and directed by Elizabeth Ralph (University of Pennsylvania). The magnetic moment of the hearth in a Page 22
Appendix: survey sites soldier's hut from the 1777-1778 encampment was 0.1 Am2; the hearth was a square about 1.2 m on a side. Survey with Diana Bermingham and Harold Spaulding (University of Pennsylvania). Information about the survey is available in Parrington (1979). The structure composed of stone rubble had a resistivity of 1000 ohm-m above a background of 250 ohm-m. Debris, Fireplace. PA04, Area F (between Front and Second Streets and north of Sansom), Philadelphia, 23 Mar 79. Survey for Brooke Blades, NPS. Soil resistivity = 15 - 45 ohm-m. Special. PA05, George's Hill, Fairmount Park, Philadelphia, 4 Oct 80. Survey for Craig Blakely (Philadelphia Historic Preservation Corporation). The building was once located in the 100 block of Arch Street. The survey was done with Diana Bermingham, Helen Schenck, and Michael Parrington (University of Pennsylvania). Magnetic interpretation suggested that about 30,000 kg of iron was buried within a depth span of 2 - 3 m. The estimated concentration of iron in the excavation was 10 - 50 %. Special. PA06, Friendship Hill, near Uniontown, 24 Sep 84. Survey for David Orr (NPS). Site was the home of Albert Gallatin and the buildings were constructed in 1789-1823. The soil resistivity was 1000 ohm-m in the upper 0.4 m and 350 ohm-m below; radar pulse velocity = 9.4 cm/ns. Auxiliary. PA07, Gilpin House, Brandywine Battlefield, Chadds Ford, 12 May 87. Survey for Stephen Warfel (State Museum of Pennsylvania); excavations by Mark McConaughy (1989). Radar pulse velocity = 8.8 cm/ns; soil resistivity = 150 ohm-m. Floor, Debris. PA08, Ink's Tavern site, Fort Necessity National Battlefield, near Uniontown, 27 May 88. Survey for David Orr (NPS); excavations by Kenneth Basalik (Cultural Heritage Research Services) (Basalik 1990). Foundation, Auxiliary. PA09, Merchants Exchange and Bishop White House, Philadelphia, 29 Jul 97. Survey for Paul Inashima (NPS). Soil resistivity = 10-300 ohm-m. Special. PA10, Evans-Mumbower Mill, Lower Gwynedd Township, 20 April 01. Survey for the Millbrook Society; survey coordinated by Gerald Ames. Radar pulse velocity = 7.6 cm/ns. Auxiliary. Virginia VA01, Taylor House, Petersburg National Battlefield, 19 Sep 79 (a larger survey started 23 Jan 91). Survey for David Orr (NPS). The total magnetic moment of the brick foundation was interpreted to be 41 Am2. The cellar fill was detected as high conductivity with an EM38 (5.5 mS/m compared to a background of 3 mS/m) and an EM31 (10.25 mS/m compared to a background of 7.5 mS/m). A resistivity survey detected part of the wall as low resistivity (225 ohm-m compared to a background of 375 ohm-m) and an EM38 susceptibility survey revealed the shallow part of the brick foundation as high susceptibility (1 ppt compared to a background of 0.5 ppt). Details are in Bevan (1996a) and Bevan et al. (1984). Foundation, Auxiliary, Special. VA02, Grant's cabin, City Point, Hopewell, 2 Nov 81. Survey for David Orr and Brooke Blades (NPS). The location of the cabin that was the residence of General Grant during the 1864 siege of Petersburg was sought (Bevan 1996b). Soil resistivity was 100 ohm-m below a depth of 0.15 m; radar pulse velocity = 11.3 cm/ns. Foundation. Page 23
Appendix: survey sites VA03, Eppes Mansion, City Point, Hopewell, 2 May 83. Survey for David Orr (NPS). The radar profile in Figure 2 is along line N3280 in a coordinate system set up by the NPS (Bevan 1996b). Soil resistivity = 100 ohm-m; radar pulse velocity = 10.1 cm/ns; predominant frequency of the radar pulse = 180 MHz. Cellar, Auxiliary. VA04, Mount Vernon, 17 Dec 84. Survey for Alain C. Outlaw (Virginia Historic Landmarks Commission) and sponsored by The Mount Vernon Ladies Association of the Union. The information about the cistern was from Esther C. White (Mount Vernon Ladies’ Association). The French drains caused flat echoes on the radar profiles at a depth of about 2 ft. (0.6 m); they were about 3 ft. (1 m) wide. Radar pulse velocity = 9.1 cm/ns; soil resistivity = 340 ohm-m. Auxiliary. VA05, Widow Tapp House, Fredericksburg and Spotsylvania National Military Park, 27 Apr 89. Survey for Wilson Greene and David Orr (NPS) (Bevan 1996b). Debris. VA06, Jamestown Island, settlement area, 17 Feb 93. Survey for Marley Brown III (Colonial Williamsburg Foundation) and the NPS. The cellar floor of structure S-105 was detected with an EM38 survey (as a conductivity low of 11 mS/m compared to a background of 14 mS/m); it was also partially detected as high resistivity (180 ohm-m compared to a background of 140 ohm-m) in a survey by Andrew Edwards, Greg Brown, Audrey Horning-Kossler, Fred Smith, Christina Adinolphi, Jeff Watts, Pegeen McLaughlin-Pullins, and Meredith Moodey. Radar pulse velocity = 7.9 cm/ns; soil resistivity = 190 ohm-m. Cellar. VA07, Stevens House, Fredericksburg, 9 Mar 93. Survey for David Orr and Noel Harrison (NPS) (Bevan 1996b). Debris. VA08, Jamestown Island (near church), 28 Jun 94. Survey for David Orr (NPS) and William Kelso (Jamestown Rediscovery); excavation findings from Eric Deetz (Jamestown Rediscovery). The brick rubble of a fallen chimney was part of structure 163. The cellar floor was part of a former building (pit 4, JR 158, structure 165). Radar pulse velocity = 11.3 cm/ns; soil resistivity = 400 ohm-m in upper 0.4 m and 180 ohm-m below. Cellar, Foundation, Fireplace. VA09, Travis site, Jamestown Island, 19 Jun 95. Survey for Jane Sundberg and Jim Haskett (NPS); shovel test pits by Dennis Blanton (College of William and Mary). Auxiliary. VA10, Robinson Farmstead, Manassas Battlefield, 25 Sep 95. Survey for Terry Klein (Greiner) and Stephen Potter (NPS); the survey and excavations are described by Parsons (2001). Soil resistivity = 100 ohm-m in the upper 0.6 m and 20 ohm-m deeper; radar pulse velocity = 9.1 cm/ns. Cellar, Foundation, Debris. VA11, Rosewell, near White Marsh, 7 Apr 97. Survey for the Rosewell Foundation; sponsored by the Garden Club of Virginia; survey coordinated by Nicholas Luccketti and Rudi Favretti. A summary of earlier excavations is given in Luccketti and Wood (1994). The radar profile in Figure 3 is along line W90; while the archaeological grid was applied, the N0E0 point was at the front door of the mansion. Soil resistivity = 160 ohm-m in upper 0.3 m and 40 ohm-m below; radar pulse velocity = 7.3 cm/ns; predominant frequency of radar pulse = 180 MHz. Floor, Auxiliary. VA12, Gunston Hall, Mason Neck, 5 Oct 98. Survey for Andrew Veech and Thomas Lainhoff (Gunston Hall Plantation); sponsored by the Garden Club of Virginia and Gunston Hall Plantation. Radar pulse velocity = 10.4 cm/ns; soil resistivity = 100 ohm-m. Auxiliary. Page 24
Appendix: survey sites VA13, Watt House, Old Cold Harbor, 12 Oct 99. Survey for Allen Cooper (NPS); excavations by Maggie Tyler (Sweet Briar College). Radar pulse velocity = 8.5 cm/ns; soil resistivity = 90 ohm-m within a depth span of 0.3 to 1 m but 400 ohm-m above and below that. Floor. VA14, Bullock House, Chancellorsville, 10 Apr 00. Survey for Robert Krick and Noel Harrison (NPS); excavations by Clarence Geier (James Madison University) (Geier and Sancomb 2000). The conductivity profiles in Figure 6 were along line N30 (grid east was parallel to Bullock Road, and point N150 E370 is at the middle of a wooden pole for a high voltage power line near the corner of Bullock Road and Highway 610). The measurement spacing with the conductivity instruments was 2.5 ft. (0.8 m) and the magnetic dipoles of the instruments were vertical; instrument heights were 0.3 m for the EM38 and 0.9 m for the EM31. The EM38 detected the metal fill as a high positive reading because the cellar was rather deep relative to the exploration depth of the instrument; the EM31 gave negative readings (truncated in the figure) because the metal appeared to be nearby to that instrument. Radar pulse velocity = 8.5 cm/ns; soil resistivity = 400 ohm-m. Cellar, Fireplace, Soil. VA15, Fairview cabin, Chancellorsville, 10 Apr 00. Survey for Robert Krick, Noel Harrison, and Eric Mink (NPS); excavations by Clarence Geier (James Madison University). Fireplace, Soil, Auxiliary. VA16, Appomattox Court House, 21 Sep 00. Survey for Allen Cooper and Joe Williams (NPS); excavations by Mark Kostro (Kostro 2002) and Marley Brown III (Colonial Williamsburg). Radar pulse velocity = 7.9 cm/ns; soil resistivity = 40 ohm-m. Cellar, Debris, Fireplace, Soil, Special. VA17, slave quarters, Crewes-Mettart House, Malvern Hill, 27 Nov 00. Survey for Donna Seifert (John Milner) and NPS; excavations by Joseph Balicki (John Milner). Radar pulse velocity = 8.8 cm/ns; soil resistivity = 270 ohm-m. The readings in Figure 7 were made with a Geonics EM38 conductivity meter with a measurement spacing of 1 ft (0.3 m) ; the magnetic dipoles were vertical and the base of the instrument was elevated by 0.2 m above the surface. Three lines of measurement are plotted in the figure (W37.5, W40, and W42.5 ft) in a grid set up by Milner. The conductivity is slightly elevated between the two buildings. Small undulations on the curves are caused primarily by electrical interference. The EM38 measured a magnetic susceptibility high of about 50 ppm at the location of the northern building. An historical map located the two buildings to an accuracy of about 50 ft (15 m). The spatial density of trees in the area was 181 per acre. Soil, Auxiliary. VA18, Mount Pleasant, near Surry, 4 Jan 01. Survey for Nicholas Luccketti (James River Institute for Archaeology). Radar pulse velocity = 10.1 cm/ns; soil resistivity = 300 ohm-m. Cellar, Foundation. VA19, Fairfield, near White Marsh, 8 Jan 01. Survey for David Brown (Gloucester Historical Society). Natural, geological strata were shallower, more evident, and were more complex than is usual at sites on the coastal plain. Radar pulse velocity = 9.8 cm/ns; soil resistivity = 200 ohm-m. Cellar, Auxiliary. VA20, Westmoreland Berry Farm, near Leedstown, 28 Mar 01. Survey for Nicholas Luccketti (James River Institute for Archaeology). Radar pulse velocity = 8.8 cm/ns; soil Page 25
References resistivity = 80 ohm-m. Cellar. VA21, Jamestown Island, 13 Nov 01. Survey for Eric Deetz (Jamestown Rediscovery). Survey at structure 165. Fireplace. VA22, West House, Malvern Hill, Richmond Battlefield, 10 Dec 01. Survey for Benjamin Ford (Rivanna Archaeological Consulting) and Allen Cooper (NPS). Soil resistivity = 130 ohm-m; radar pulse velocity = 10.1 cm/ns. Debris, Soil, Auxilliary. VA23, Red Hill, Brookneal, 19 Feb 02. Survey for Jon Kukla (Patrick Henry Memorial Foundation). EM38 conductivity survey found high conductivity (>10 mS/m compared to a background of 5 mS/m) in the vicinity of standing buildings; these high values were not caused by metal within the buildings. Radar pulse velocity = 7.6 cm/ns. Soil. West Virginia WV01, near Provost Office, Harpers Ferry, 24 Apr 89. Survey for Paul Shackel and Susan Frye (NPS). The survey is described in Shackel (1993) and Bevan (1996b). Soil, Auxiliary. References Alterman, Michael L., Bruce Bevan, and J. Lee Cox, Jr., 1995, Terrestrial and Marine Archeological Remote Sensing and Archeological Monitoring, Salem Maritime National Historic Site, Salem, Massachusetts. Report to National Park Service, Eastern Applied Archeology Center, Silver Spring, MD, from Cultural Resources Group, Louis Berger, East Orange, NJ. Balicki, Joseph, 1991, Management Report, Phase I Archeological Investigations at the Studio of the Caryatids, Saint-Gaudens National Historic Site, Cornish, New Hampshire. Report to National Park Service, Eastern Applied Archeology Center, Rockville, MD, from John Milner Associates, Alexandria, VA. Ballweber, Hettie L., 1991, Archaeological Investigations at Long Hill, Wicomico, Maryland. Report to Mrs Donald C. Graham, York, PA, from ACS Consultants, Columbia, MD. Barba, Luis, Karl Link, Agustin Ortiz, and Albert Hesse, 1996, Magnetic Study of Archaeological Stone Foundations at Loma Alta, Michoacan, Mexico. In Expanded Abstracts with Biographies, 1996 Technical Program, pp. 786-789. Society of Exploration Geophysicists, Tulsa, OK. Basalik, Kenneth J., 1990, Archaeological Survey, Fort Necessity National Battlefield Site, Farmington, Pennsylvania. Report to the National Park Service, Philadelphia, from Cultural Heritage Research Services, North Wales, PA. Bedell, John, 2001, Delaware Archaeology and the Revolutionary Eighteenth Century. Historical Archaeology, 35(4):83-104. Bedell, John, and Gerard P. Scharfenberger, 2000, Ordinary and Poor People in 18th-Century Delaware. Northeast Historical Archaeology, 29:23-48. Bevan, Bruce W., 1984, Looking Backward: Geophysical Location of Historic Structures. In The Scope of Historical Archaeology, David G. Orr and Daniel G. Crozier, editors, pp. 284 - 302. Temple University, Philadelphia, PA. Bevan, Bruce W., 1994, The Magnetic Anomaly of a Brick Foundation. Archaeological Page 26
References Prospection, 1(2):93-104. Bevan, Bruce W., 1996a, Geophysical Exploration for Archaeology. Geosight Technical Number 4. Geosight, Weems, VA. Bevan, Bruce W., 1996b, Geophysical Exploration in the U.S. National Parks. Northeast Historical Archaeology, 25:69-84. Bevan, Bruce, and Jeffrey Kenyon,1975, Ground-Penetrating Radar for Historical Archaeology. MASCA Newsletter, 11(2):2-7. Bevan, Bruce W., David G. Orr, and Brooke S. Blades, 1984, The Discovery of the Taylor House at the Petersburg National Battlefield. Historical Archaeology, 18(2):64-74. Cardarelli, Ettore, and Rita de Nardis, 2001, Seismic Refraction, Isotropic and Anisotropic Seismic Tomography on an Ancient Monument (Antonino and Faustina Temple AD 141). Geophysical Prospecting, 49(2):228-240. Chaney, Edward, and Henry M. Miller, 1989, Archaeological Reconnaissance and Testing at the Gallows Green Site (18ST1-112), St. Mary’s City, Maryland. Report to St. Mary’s College of Maryland, St. Mary’s City, MD, from Historic St. Mary’s City, St. Mary’s, MD. Clay, R. Berle, 2001, Complementary Geophysical Survey Techniques: Why Two Ways Are Always Better Than One. Southeastern Archaeology, 20(1):31-43. Clay, R. Berle, 2003, Conductivity (EM) Survey: A Survival Manual. Cultural Resource Analysts, Lexington, Kentucky. Clark, Anthony, 1996, Seeing Beneath the Soil. B. T. Batsford, London, England. Connor, Melissa, and Douglas D. Scott, 1998, Metal Detector Use in Archaeology: An Introduction. Historical Archaeology, 32(4):76-85. Dalan, Rinita A., and Subir K. Banerjee, 1998, Solving Archaeological Problems Using Techniques of Soil Magnetism. Geoarchaeology, 13(1):3-36. Doolittle, James A., Fred E. Minzenmayer, Sharon W. Waltman, and Ellis C. Benham, 2003, Ground Penetrating Radar Soil Suitability Maps. Journal of Environmental and Engineering Geophysics, 8(2):49-56. Elia, Ricardo J., 1985, The Fort on Groton Hill: An Archaeological Survey of Fort Griswold State Park in Groton, Connecticut. Report to the State of Connecticut, Department of Environmental Protection, Hartford, from the Office of Public Archaeology, Boston University, Boston. Gaffney, C. F., J. A. Gater, P. Linford, V. L. Gaffney, and R. White, 2000, Large-scale Systematic Fluxgate Gradiometry at the Roman City of Wroxeter. Archaeological Prospection, 7:81-99. Geier, Clarence R., and Kimberly Sancomb, 2000, Proposed House Structure Locations: Fairview and Bullock Archaeological Sites, Chancellorsville Battlefield, Fredericksburg and Spotsylvania National Military Park. Report to National Park Service, Fredericksburg and Spotsylvania National Military Park, Fredericksburg, VA, from the Department of Sociology and Anthropology, James Madison University, Harrisonburg, VA. Grossman, Joel W., 1980, Defining Boundaries and Targeting Excavation with Ground-Penetrating Radar: The Case of Raritan Landing. Environmental Impact Assessment Review, 1(2): 145-166. Page 27
References Hargrave, Michael L., Lewis E. Somers, Thomas K. Larson, Richard Shields, and John Dendy, 2002, The Role of Resistivity Survey in Historic Site Assessment and Management: An Example from Fort Riley, Kansas. Historical Archaeology, 36(4):89-110. Heimmer, Don H., and Steven L. De Vore, 1995, Near-Surface, High Resolution Geophysical Methods for Cultural Resource Management and Archeological Investigations. National Park Service, Denver, Colorado. Imai, Tsuneo, Toshihiko Sakayama, and Takashi Kanemori, 1987, Use of Ground-Probing Radar and Resistivity Surveys for Archaeological Investigations. Geophysics, 52(2):137-150. Kardas, S., and E. Larabee, 1988, Preliminary Archaeological Survey at Peachfield, 1988 Season. Report to the National Society of Colonial Dames of American in the State of New Jersey, Mount Holly, NJ, from Historic Sites Research, Princeton, NJ. Kostro, Mark, 2002, Archaeological Identification Study and Evaluation of Geophysical Prospecting at Appomattox Court House National Historical Park, Appomattox, Virginia. Report to National Park Service, Appomattox Court House National Historical Park, Appomattox, VA, from the Colonial Williamsburg Foundation, Williamsburg, VA. Kvamme, Kenneth L., 2001, Current Practices in Archaeogeophysics. In Earth Sciences and Archaeology, Paul Goldberg, Vance T. Holliday, and C. Reid Ferring, editors, pp. 353-384. Kluwer Academic, New York. Kvamme, Kenneth L., 2003, Geophysical Surveys as Landscape Archaeology. American Antiquity, 68(3):435-457. Luccketti, Nicholas, and Margaret Wood, 1994, Archaeology of Rosewell. In Discovering Rosewell, Rachel Most, editor, pp. 25-52. Rosewell Foundation, Gloucester, VA. Mitchell, Ruth M., and Henry M. Miller, 2001, Results of a Phase I and Phase II Archaeological Survey of the West’s Field Tract in St. Mary’s City, Maryland, a Preliminary Report. Report for St. Mary’s College of Maryland, St. Mary’s City, MD, from Historic St. Mary’s City, St. Mary’s City, MD. McConaughy, Mark A., 1989, The 1987 Archaeological Investigations Around the Gideon Gilpin House (36De84), Brandywine Battlefield Park, Delaware County, Pennsylvania. Manuscript, State Museum of Pennsylvania, Pennsylvania Historical and Museum Commission, Harrisburg, PA. McManamon, Francis P., 1984, Discovering Sites Unseen. In Advances in Archaeological Method and Theory, vol. 7, Michael B. Schiffer, editor, pp. 223-292. Academic Press, New York, NY. McPherron, Alan, and Elizabeth K. Ralph, 1970, Magnetometer Location of Neolithic Houses in Yugoslavia. Expedition, 12(2):10-17. Parrington, Michael, 1979, Geophysical and Aerial Prospecting Techniques at Valley Forge National Historical Park, Pennsylvania. Journal of Field Archaeology, 6(2):193-201. Parsons, Mia T. (editor), 2001, Archeological Investigations of the Robinson House Site 44PW288: A Free African-American Domestic Site Occupied from the 1840s to 1936. National Park Service, National Capital Region, Occasional Report No. 17, Washington, DC. Page 28
References Ralph, Elizabeth K., 1969, Archaeological Prospecting. Expedition, 11(2):14-21. Ralph, Elizabeth K., Frank Morrison, and Douglas P. O'Brien, 1968, Archaeological Surveying Utilizing a High-Sensitivity Difference Magnetometer. Geoexploration, 6:109-122. Seidel, John L., 1983, Archaeological Research at the 1778-79 Winter Cantonment of the Continental Artillery, Pluckemin, New Jersey. Northeast Historical Archaeology, 12:7-14. Shackel, Paul A. (editor), 1993, Interdisciplinary Investigations of Domestic Life in Government Block B: Perspectives on Harpers Ferry’s Armory and Commercial District. National Park Service, National Capital Region, Occasional Report No. 6, Washington, DC. Shapiro, Gary, 1984, A Soil Resistivity Survey of 16th-Century Puerto Real, Haiti. Journal of Field Archaeology, 11(1):101-110. Smekalova, T. N., and A. A. Maslennikov, 1993, Cadaster of Geophysical Maps of Bospor Sites in the Coast of Asov Sea. In Geophysical Exploration of Archaeological Sites, Andreas Vogel and Gregory N. Tsokas, editors, pp. 27-35. Vieweg Publishing, Braunschweig, Germany. Smekalova, T., O. Voss, N. Abrahamsen, 1993, Magnetic Investigation of Iron-Smelting Centres at Snorup, Denmark. Archaeologia Polona 31:83-103. Sopko, Joseph, 1983, Geophysical and Soil Chemical Investigations at New Windsor Cantonment. Northeast Historical Archaeology, 12:24-30. Weber, Carmen A., 1984, A Phase I / Phase II Archaeological Reconnaissance Survey for the Gwynns Falls Sewer Interceptor, Baltimore, Maryland. Report to the Baltimore City Department of Public Works, Baltimore, from the Baltimore Center for Urban Archaeology, Baltimore. Weymouth, John, 1996, Digs Without Digging: Exploring Archaeological Sites with Geophysical Techniques. Geotimes, 41(11):16-19. Bruce W. Bevan Geosight 356 Waddy Drive Weems, VA 22576-2004
Page 29
FIGURE 1. The radar profile of a debris-filled cellar. The depth to the bottom of the cellar cannot reliably be determined. The length of the profile is 45 ft. (14 m). St. Mary's City, priest's house at Chapel Field (MD04).
FIGURE 2. The radar profile of a cellar floor. It appears to be 0.5 ft. (0.15 m) deeper in the middle; the horizontal compression of the profile exaggerates the dip of the floor. Eppes Mansion at City Point, Hopewell (VA03).
FIGURE 3. The radar profile of a shallow floor. A large and abrupt contrast in the soil at or below the floor causes a reverberation: The series of echoes that appear deeper. Rosewell (VA11).
FIGURE 4. The radar profile of two foundations of a building. The tops of the foundation were readily detected, but not their bottoms. St. Mary's City, Great Brick Chapel (MD04).
FIGURE 5. The findings of a geophysical survey. The dotted areas locate where rubble was detected; cellars were later excavated at the features in the center and the southeast. Fort Griswold (CT01).
40 EM38, shallow EM31, deep
Apparent conductivity, mS/m
30
20
building
10
0 0
50
100
150 East coordinate, ft
200
250
300
FIGURE 6. Electrical conductivity across a debris-filled cellar. The gradual rise in the readings for a long distance suggests that there is a cultural modification of the soil around the building. Bullock House site, Chancellorsville battlefield (VA14).
20
Apparent conductivity, mS/m
EM38 conductivity profiles
10
building
0
-10 0
50
100
150 North coordinate, ft
200
250
300
FIGURE 7. Electrical conductivity at a former building. It is revealed by the slight rise in the middle of this graph, at N160. An excavation in this area located fragments of brick and other artifacts, but no intact part of the former structure. Extremely high and low readings are caused by metallic objects at a depth of less than 1 ft (0.3 m). Slave quarters, Malvern Hill (VA17).
