Detection of Underground Remains by Remote Sensing ... - IEEE Xplore

Detection of Underground Remains by Remote Sensing and Geophysics Peng Jia*, Yueping Nie

Guoding Song

Joint Laboratory of Remote Sensing Archaeology, Institute of Remote Sensing Applications CAS, Beijing, China *[email protected]

Department of Scientific History and Archaeology, Graduate University of Chinese Academy of Sciences, Beijing, China

Abstract—Shenmingpu Site, located in Henan Province in China, will be a part of the flooded areas during the magnificent SouthNorth Water Transfer Project. According to previous records and land surface investigations, there may be some remains of underground burial so it is urgent to conduct a rescue excavation. Due to a large size of the site, it will cost high expense, substantial labor and time if we totally rely on traditional methods to detect them. Both remote sensing and geophysics are powerful tools for detecting surface and underground archaeological information, especially when they are combined. Remote sensing is useful in large-scale observation and detection when the remains are buried not very deep and make the soil or vegetation covering them different from the surrounding in physical or chemical appearance, which makes discrepant features appear on satellite imageries; geophysics can detect the underground structural compositions of small-scale areas based on the differences of density or resistivity. Multi-temporal CBERS-02 images were used to find the anomaly in the large protected area because the winter wheat planted above underground burial matured later than the surrounding. For the anomalous area on the images, two geophysical prospecting methods, the natural electric field method and high-density resistivity method, were conducted to validate the anomaly. After an in-situ excavation on the chosen anomalous ground, a group of graves including brick-chamber graves and shaft graves were unearthed. Keywords- remote sensing; geophysics; underground burial; archaeology; grave

I.

INTRODUCTION

Shenmingpu Site sits in the north of Shenmingpu Village, which belongs to Xichuan County, Nanyang City, Henan Province in China, according to the administrative division. Topographically speaking, it is located on the mesa along the south bank of Danjiang River. The center coordinates of the site, recorded in the form of latitude and longitude, is 33°00′47″ (N), 111°17′47″ (E) and the elevation ranges from 158m to 163m. In 1976, for the first time, Shenmingpu Site was found in the Cultural Relics Investigation organized by Cultural Relic Management Committee in Xichuan County. In 1984, it was

Support Fund: Program “0680020080”

rechecked in the Cultural Relic General Survey of Henan Province. In 1985, it was formally placed on the list of Province-level Cultural Relic Protection Units. Through several traditional land surface investigations, it was estimated that the area of the site was approximately 20,000 square meters, but on the ground there are no reliable clue showing the locations of underground burial. A few separate remains of ceramics and bricks were found on the surface. According to them, it was concluded that the age of the remains may date back to Shang Dynasty, Zhou Dynasty and Han Dynasty, which means a great value for the researches of Chinese history. II.

REMOTE SENSING DETECTION

A. Remote Sensing in Archaeology Remote sensing is an effective tool to record the energy reflected and emitted from the ground objects in a broad region. The ability of recording the reflected energy shows that we could know the actual ground situation from recorded energy because different types of objects can reflect, absorb and transmit the incident radiation in each own proportions. It is useful when the underground remains are buried not very deep and make the soil or vegetation covering them different from the surrounding in physical or chemical appearance (porosity, weathering degree, water content, surface temperature, component, etc.), which makes discrepant features appear on satellite imageries directly or after a series of image processing sometimes. The feature of covering a broad region at one time leads it to be widely used to look for the remains of ancient magnificent projects on the images more easily, such as the Great Wall, the Beijing-Hangzhou Grand Canal, etc. Therefore, remote sensing has been introduced to the field of archaeology and is being progressively applied to more and more archaeological projects. B. Image Selection According to the principles of remote sensing in archaeology, ancient remains buried underground can make the soil or vegetation covering them discrepant from the surrounding due to the difference in moisture content, porosity, fertility, etc., which consequently leads to the discrepancy of growth and distribution of the wood and shrub, or the difference of height, density and color of the crop and

grass. All these discrepancies will make each type of objects appear on the images with its own characteristic, which is recognized as the vegetation features of underground remains and phenomenon. As the indicative plant for recognition based on different features on the images, herbaceous plant is better than woody plant because it looks fine-grained on the images during the vegetation growing time and this phenomenon will appear repeatedly each year. Cereal crop is quiet useful because if there are rammed earth, bricks, rubble or ancient roads under the land surface, the crops will be mature later than the surrounding during the harvest time. Based on the analysis above, the most ideal coverage on the ground is wheat because it not only belongs to herbaceous plant but also belongs to cereal crop. It is fortunate that it is winter wheat that was cultivated on the ground of Shenmingpu Site. Winter wheat is seeded in the autumn and harvested in the next summer. In addition, another strain of the wheat is the spring wheat, which is seeded in the spring and harvested in the same autumn. Through further investigations, the winter wheat on the site was seeded in the early October, began growing in about November or December, flourished in about March or April, got mature and was harvested in the late May or the early June. According to the detailed growth cycle and existing remote sensing images, we chose two scenes both from the CBERS-02 taken respectively on 12 June 2005 and 11 June 2006.

III.

GEOPHYSICAL DETECTION

A. Geophysics in Archaeology Geophysical prospecting is both a theory and a method that applies physical principles to prospect underground resources and research geological structures. Because non-destructive, it Figure 1. Shenmingpu Site (red-rimmed)

Figure 2. Study area (red-rimmed)

Figure 3. Shenmingpu Site (red-rimmed) Figure 4. Study area (red-rimmed)

TABLE I.

ELECTRIC PARAMETERS OF DIFFERENT LITHOLOGIES

is usually used as an advanced archaeological technology to directly detect underground structural compositions based on the discrepancy in physical appearance of different objects, such as density, resistivity, etc.

CBERS-02 is manufactured jointly in China and Brazil, which belongs to the first generation of transmission remote sensing satellite of earth resource. The multispectral camera taken with CBERS-02 can take images in five bands, that is, B1 (0.45~0.52μm), B2 (0.52~0.59μm), B3 (0.63~0.69μm), B4 (0.77~0.89μm), B5 (0.51~0.73μm), all with the 19.5m spatial resolution. C. Study Area Recognition Shenmingpu Site is in the small red-rimmed square area on Figure 1, which was taken on 12 June 2005, and the cyan oblique linear feature passing through the square represents the Danjiang River. The square on Figure 1 is magnified as Figure 2, on which the anomaly is found out through color stretching processing. It is possible that the wheat in the anomalous area got matured later than the surrounding, so it still existed when the surrounding have been harvested. Figure 3 was taken on 11 June 2006, and the red-rimmed square area on it was magnified as Figure 4. We found out the same anomaly in the same location again on Figure 4 as on Figure 2. Therefore, we chose the area marked by the red-rimmed polygon on both Figure 2 and Figure 4 as our study area, where geophysical prospecting will be conducted to validate the anomaly.

B. Method According to the terrain and geophysical condition of Shenmingpu Site, the natural electric field method and highdensity resistivity method were chosen. First of all, we used the natural electric field method to conduct a general investigation on the whole study area. Then, we carried out the high-density resistivity measurement to validate the anomalous areas in the course of general investigation. According to results of the geophysical field test, electric parameters of different lithologies are shown in TABLEⅠ. 1) Natural Electric Field Method This method is an alternating current exploration method. Based on the difference of the resistivity of underground rocks and minerals, on the ground we measure the changes of several electric field components in different frequencies produced by the geo-electromagnetic field, which is made use of as the source of working field, to study the electrical changes of underground geo-electric sections, in order to solve the geological problems.

Because of portable equipment, easy working methods, high efficiency, considerable information obtained and easily explainable data, this method is appropriate to general investigation. There are eight power levels for the equipment we used, one of which can be selected to work each time. According to the analysis of the strata in the sampling holes and the estimation for buried depth, we chose the power level which can explore underground about 0.8m~1.0m deep. 2) High-density Resistivity Method As for the principle, the same as the natural electric field method, this method is also an electric exploration method on a basis of the conductivity of rocks and minerals. Under the effect of the artificial electric field, the distribution of electric current passing through the underground reflects the distribution of geological objects. In this method, dense measurement points are distributed on the ground. A two-dimensional exploration process (i.e. vertical and horizontal) can be completed quickly and abundant geological information can be obtained from it. Practice has proven that because of high observation precision and reliable data obtained, it is a better method in engineering geology exploration, hydrogeology exploration and underground burial exploration, e.g. underground grave exploration. C. Survey According to the terrain, we evenly set up eight surveying lines covering the whole study area to conduct the general investigation (Figure 5). They were orderly named 1-1’, 22’…… 8-8’. After analyzing the investigation result, we added three lines, respectively next to the three surveying lines (2-2’, 3-3’, 4-4’) showing strong anomaly, to meet the need of detailed investigation. According to each locality, three added lines were respectively named Add 2-2’, Add 3-3’ and Add 44’. So there were eleven surveying lines in all evenly set up in the whole study area. For the purpose of comparing with the natural electric field method, we conducted the high-density resistivity exploration on three added lines. After balancing the degree of the anomaly and our project funding, we first decided to excavate the area on seven surveying lines (i.e. from 2-2’ to 5-5’). The data obtained through two methods were analyzed in the next section. IV.

GEOPHYSICAL DATA ANALYSIS

Figure 5. Distribution of eight surveying lines

The profile Add2-2’ is 112m long and the background value is 1.6mV; the maximum is 1.8mV and the minimum is 1.5mV. It is inferred that there are graves ( V=1.8mV) where the values respectively equal 24~28m, 36m and 48m in the horizontal axis (Figure 7). The profile 3-3’ is 120m long and the background value is 1.2mV; the maximum is 1.9mV and the minimum is 1.0mV. It is inferred that there are graves ( V=1.6~1.9mV) where the values respectively equal 32m, 48m and 64~68m in the horizontal axis (Figure 8). The profile Add3-3’ is 120m long and the background value is 1.6mV; the maximum is 2.0mV and the minimum is 1.4mV. It is inferred that there are graves ( V=1.8~2.0mV) where the values respectively equal 40m and 92m in the horizontal axis (Figure 9). The profile 4-4’ is 130m long and the background value is 1.2mV; the max is 1.6mV and the min is 0.6mV. It is inferred that there are graves ( V=1.5~1.6mV) where the values respectively equal 20m, 44m and 68m in the horizontal axis (Figure 10). The profile Add4-4’ is 130m long and the background value is 1.5mV; the maximum is 1.8mV and the minimum is 1.5mV. It is inferred that there are graves ( V=1.8mV) where the values respectively equal 48m, 72~76m and 92m in the horizontal axis (Figure 11). The profile 5-5’ is 140m long and the background value is 1.2mV; the maximum is 1.4mV and the minimum is 0.9mV. It is inferred that there are graves ( V=1.3~1.4mV) where the values respectively equal 52m, 60m and 76m in the horizontal axis (Figure 12).

Figure 6. The values along the profile 2-2’

A. Result of Natural Electric Field Method The profile 2-2’ is 112m long and the background value (normal value) is 1.5mV; the maximum is 1.7mV and the minimum is 1.4mV. It is inferred that there are graves ( V=1.7mV) where the distance value equals 88m in the horizontal axis (Figure 6).

Figure 7. The values along the profile Add2-2’

B. Isograms All the data obtained on the area to be excavated were plotted in the form of the isograms (Figure 13). Figure 14 is the partly magnified image of Figure 13. It is inferred that there are graves where the isograms with high values (1.5~1.7mV) are closed, e.g. 1.65mV circle and 1.7mV circle in the upper of Figure 14, not where the isograms are only dense, e.g. numerous circles with the values between 1.2~1.3mV in the lower of Figure 14. Figure 8. The values along the profile 3-3’



Therefore, through analyzing the isograms (Figure 13), we can infer that there are graves where the values respectively equal 26~30m, 34~36m and 46~52m in the horizontal axis for the profile between 2-2’ and Add2-2’; where the value equals 66~72m in the horizontal axis for the profile between Add2-2’ and Add3-3’; where the value equals 20~22m in the horizontal axis for the profile between 4-4’ and Add4-4’; where the values respectively equal 46~48m and 74~78m in the horizontal axis for the profile between Add4-4’ and 5-5’. C. Result of High-density Resistivity Method The profile Add2-2’ is 100m long and it is found that the resistivity value is high (ρ=30~53Ω·m) within the depth of 1.2m. It is inferred that there are graves where the distance values respectively equal 16m, 23m, 30m, 55m, 62m, 82m and 87m in the horizontal axis (Figure 15). Under the depth of 1.5m, it scatters silty clay, silt, silty-fine sand and pebble. The profile Add3-3’ is 100m long and it is found that the resistivity value is high (ρ=24~70Ω·m) within the depth of 1.2m. It is inferred that there are graves where the values respectively equal 22m, 28m, 40m, 62m, 85m and 95m in the horizontal axis (Figure 16). Under the depth of 1.5m, it scatters silty clay, silt, silty-fine sand and pebble. The profile Add4-4’ is 100m long and it is found that the resistivity value is high (ρ=26~155Ω·m) within the depth of 1.2m. It is inferred that there are graves where the values respectively equal 26m, 30m, 48m and 97m in the horizontal axis (Figure 17). Under the depth of 1.5m, it scatters silty clay, silt, silty-fine sand and pebble. D. Comparison In this subsection we call the natural electric field method “Method 1” and call the high-density resistivity method “Method 2” for conciseness.


For the profile Add2-2’, the anomaly (22~28m) detected through Method 1 is basically consistent with the anomaly (23m, 30m) detected through Method 2. For the profile Add3-3’, the anomaly (40m, 92m) detected through Method 1 is basically consistent with the anomaly (40m, 95m) detected through Method 2. For the profile Add4-4’, the anomaly (48m, 92m) detected through Method 1 is basically consistent with the anomaly (48m, 97m) detected through Method 2.


V.

Figure 13. The isograms of the study area

IN-SITU EXCAVATION

A. Strata Analysis The stratum of the site can be divided into four layers from the top to the bottom: the first (top) layer is the modern cultivated soil layer; the second layer is the modern disturbed soil layers; the third layer is the cultural layer of Han Dynasty; the fourth layer is the cultural deposit of Eastern Zhou Period; and under the fourth layer is the immature soil layer. B. Excavation Site According to the terrain we plotted nine square areas to be excavated on seven surveying lines and each square area was 10×10 square meters. After excavating, the aerial photo of the excavation scene was taken by the low-altitude unmanned aerial photography system (Figure 18).

Figure 14. The partly magnified image of Figure 13

C. Unearthed Remains Up to now, there were 26 brick-chamber graves unearthed. It is inferred that they could entirely date back to Eastern Han Period or later except one dating back to about Tang Dynasty. In addition, there were 39 shaft graves unearthed, among which ten graves probably dated back to Qing Dynasty and the others dated back to a period of time spanning from the late Spring and Autumn Period to the middle Western Han Period. Because the ground was leveled up many times and the bricks were ever taken away for other purposes by local residents, most brick-chamber tombs have been severely damaged. Compared with brick-chamber graves, shaft graves remained better.




Figure 18. The excavation scene

VI.

SUMMARY

① From the excavation result we can see that the anomalous area is not large. It is difficult to see such a small anomaly clearly on the images without high-resolution, so under the permission of the project fund, we will try to buy some images with higher resolution next time, such as SPOT, ALOS, etc. ② The difference of electric potential energy in electric field between silty clay and grave is the key to the natural electric field prospecting. ③ The difference of resistivity between silty clay and grave is the key to the high-density resistivity prospecting. ④ The GPS equipment was useful to find the corresponding locations at the spot according to the locations on the images. ⑤ Advanced technical methods, which are advocated in modern archeology field, played an important role in the detection stage of the whole project. The remains of underground burial, though shallow, are buried underground and as for the proportion, they just covered a rather small portion of the whole site, so the research in this paper belongs to weak underground information detection based primarily on remote sensing and geophysics methods.

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