Geophysical study of the Peinan Archaeological Site, Taiwan

Journal of Applied Geophysics 89 (2013) 1–10

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Geophysical study of the Peinan Archaeological Site, Taiwan Lun-Tao Tong a,⁎, Kun-Hsiu Lee b, Chang-Keng Yeh b, c, Yan-Tsong Hwang a, Jeng-Ming Chien a a b c

Industrial Technology Research Institute, Bldg. 24, 195, Sec. 4, Chung Hsing Rd., Chutung, Hsinchu, 31040, Taiwan, ROC National Museum of Prehistory, No. 1, Museum Rd., Taitung, 95060, Taiwan, ROC Department of Anthropology, National Taiwan University, No. 1, Sec. 4, Roosevelt Rd., Taipei, 10617, Taiwan, ROC

a r t i c l e

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Article history: Received 14 May 2012 Accepted 5 November 2012 Available online 17 November 2012 Keywords: Peinan archaeological site Prehistoric village Electromagnetic Magnetic Ground-penetrating radar Electrical resistivity tomography

a b s t r a c t The Peinan archaeological site is the most intact Neolithic village with slate coffin burial complexes in Taiwan. However, the area that potentially contains significant ancient remains is covered by dense vegetation. No reliable data show the distribution of the ancient village, and no geophysical investigation has been performed at this site. To evaluate various geophysical methods under the geological setting and surface condition of the site, the physical properties of the remains were measured and four geophysical methods involving magnetic, electromagnetic (EM), electrical resistivity tomography (ERT), and ground-penetrating radar (GPR) were tested along three parallel profiles. The results imply that the EM and magnetic methods are much cost-effective and suitable for investigating the entire area. GPR and ERT methods can provide high resolution subsurface image, which are much suitable for subsequently detail investigation. The EM and magnetic surveys were thus conducted over the entire Peinan Cultural Park to understand the distribution of the ancient building remains at the Peinan site. The results of this study were verified by subsequent excavations, which indicate that the EM survey was successful in delineating the majority of the ancient village because the basements of building are highly resistive in comparison to the background sediment. The results of this investigation suggest that the ancient village was broadly distributed over the eastern part of the Peinan Culture Park and extended to the southeast. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The Peinan archaeological site is the site with most complete residential condition and information in the archaeological history of Taiwan (NMP, 2012). As shown in Fig. 1, it is located in southeastern Taiwan in the southeastern foothills of Peinan Mountain and on the terrace of the Peinan River. The entire area hosts recent alluvial sediments. Carbon-14 dating shows that the Peinan site is approximately 2300 to 5300 years old (Lien, 1991). The earliest records of the Peinan site are by Ryuzo Torii, a Japanese anthropologist, who took two photographs in 1896 of the stone pillars on the ground surface at the Peinan village in Taitung. At that time, there were numerous slate pillars erected on the ground surface. By 1945, Japanese scholars conducted a small-scale pit at the location indicated in Fig. 1 and inferred that there used to be an ancient tribe at Peinan (Lee, 2009; Sung, 1992). Today, only one stone pillar (Fig. 1) remains at the surface and it have become a famous symbol of the Peinan site. In 1980, more than 1500 slate stone coffins and over 20,000 stone and pottery artifacts were unearthed during excavation at the Peinan train station near the eastern Peinan site (Fig. 1). A team of salvage archaeologists from National Taiwan University was invited by the Taitung County Government to lead the archaeological preservation project. ⁎ Corresponding author. Tel.: +886 3 5915449; fax: +886 3 5820017. E-mail address: [email protected] (L.-T. Tong). 0926-9851/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jappgeo.2012.11.004

From 1980 to 1988, exceptionally rich remains were excavated during a 13-stage archaeological excavation (Lien and Sung, 2006; Sung and Lien, 2004). After years promotion, the Peinan Cultural Park, an outdoor museum (Fig. 1), was opened to the public in 1997. Research (Lee, 2009; Lien, 1991; Lien and Sung, 2006; Sung, 1992; Sung and Lien, 2004) has shown that the area potentially rich in ancient remains is encircled by a white-dashed line as shown in Fig. 1. This area is so-called the Peinan site, the study area of this study, and is protected by the Cultural Heritage Preservation Act. However, the area that has been excavated at the Peinan site is relatively small (Lee, 2009), and much of the area that may contain ancient remains is covered by dense vegetation. The goal of the archaeological research at the Peinan site is to meticulously analysis the form and distribution of underground prehistoric building structures and to understand the relationship between the ancient buildings and the sedimentary/weathering process in the Peinan area. Within the given surface condition, it is a challenge to map the area of ancient remains and locate the spots for subsequently archaeological study. The onsite exhibition located at the southeastern corner of the Peinan cultural park (Fig. 1) acts as the window of understanding the spatial distribution of ancient building and the physical properties of the archaeological targets. Fig. 2 shows the sketch of the archaeological features observed at the onsite exhibition, and Fig. 3 shows the photographs taken at this site. The basements of dwellings at this site were constructed with slate or schist slabs or boulders (Fig. 3a) and were

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L.-T. Tong et al. / Journal of Applied Geophysics 89 (2013) 1–10

Fig. 1. Location of the Peinan archaeological site. The survey area of this study is encircled by white-dashed line. The location of the first small-scale pit in 1945 is indicated by a white circle symbol. The stone pillar remains on the surface is a famous landmark of the Peinan site and also named as crescent-shape stone pillar.

constructed 0.5 m to 1.0 m below the surface (Sung, 1992). The floors were constructed of flat slates (Fig. 3b). A large amount of pottery shards and stone tools were buried 0.3 m to 1.0 m below the surface, and intact jades and ceramics were found inside slate coffins that were buried 1.62 m to 3.03 m below the surface (Lien and Sung, 2006; Sung and Lien, 2004). The lessons learned from the onsite exhibition show that the ancient building has the property of alignment and consist of large amount of slate boulders, which imply that physical contrast will be sufficient to be detected with geophysical survey. Observations of the onsite exhibition implies that the ancient village might extent to north and south, however, no reliable data show the boundary of the ancient village. Geophysical techniques are nondestructive and efficient for archaeological surveys. They have proven very successful in mapping subsurface archaeological targets (Fiore and Chianese, 2008; Tsokas et al., 1994; Vafidis et al., 2005). It is essential to improve the understanding of the distribution of the ancient village with geophysical survey before planning of archaeological study. However, no geophysical investigation has been previously conducted

at the Peinan site. The purpose of this study is to evaluate various geophysical methods at the site and to improve the understanding of the distribution of the ancient village. To understand the contrast between archaeological targets and surrounding sediment, the physical properties of the archaeological targets and background sediment at the Peinan site were measured. Four geophysical methods involving electromagnetic (EM), magnetic, electrical resistivity tomography (ERT), and ground-penetrating radar (GPR) were tested to evaluate which are suitable for mapping subsurface ancient building within the given geology and vegetation (Fig. 1) conditions. A geophysical survey covering the entire Peinan Cultural Park was conducted to improve the understanding of the distribution of the ancient village. 2. Measurements of physical properties Geophysical surveying can be successful if there is a sufficient contrast between archaeological remains and the surrounding sediment.


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Fig. 2. The sketch of the archaeological features observed at the onsite exhibition. Original drawing is provided by the National Museum of Prehistory.

To evaluate whether the physical contrast is large enough to be resolved, the resistivities, magnetic susceptibilities, and dielectric constants of the archaeological targets and background sediment were measured at the onsite exhibition.

2.1. Resistivity The MiniOhm resistivity meter, manufactured by OYO, Japan, was used to measure the in-situ resistivities of the background sediment, the slate boulder, and slabs. A Wenner array with 7 cm electrode spacing was used to perform the resistivity measurements. Thirty-five measurements were made, and the results show that the slate/schist has the highest resistivity (over 1000 Ωm), while the resistivity of the surrounding alluvium is less than 500 Ωm. This implies that the basements of prehistoric building have high resistivities in contrast to the surrounding sediment. 2.2. Magnetic susceptibility The KT-9 magnetic susceptibility meter, manufactured by Geomatrix, UK, was used to measure the in-situ magnetic susceptibility of the slate boulder, jade articles, and pottery which were collected from the onsite exhibition. Sixty measurements were made and the results show that the magnetic susceptibilities of the slate, jade, and pottery are approximately 0.0003, 0.002, and 0.003 SI units, respectively. Although the magnetic susceptibility of the jade is high, the total magnetic influence may be weak due to its presence only in small amounts and its deep burial. The magnetic susceptibility of the pottery is about one order of magnitude higher than that of the slate. However, the Koenigsberger ratio of the pottery measured in the laboratory ranges between 1.33 and 7.51, which suggests that remnant magnetism of pottery cannot be ignored. The magnetic signal from pottery is increased or reduced when the directions of induced magnetism and remnant magnetism are consenting or opposing. Thus, the magnetic anomalies caused by pottery may be strong or weak due to the influence of remnant magnetism. 2.3. Dielectric constant

Fig. 3. Photographs taken at the onsite exhibition. (a) Wall structure, floor and slate menhir. (b) Wall and floor of the basement of an ancient house.

The approximate dielectric constant of the material can be calculated using k = (C/V) 2, where k is the dielectric constant, C is the speed of

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Fig. 4. Photograph of the test line. Plants along the survey line were cut, however the dense vegetation on both sides of the survey line can still be observed.

light in a vacuum, and V is the radar velocity of the material. The dielectric constant of the material can be estimated if the radar velocity of the material is known. A steel rod was inserted horizontally into the ground

from a pit wall at a depth of 0.35 m. The radar velocity of the ground can be calculated as the depth of the steel rod divided by the one-way travel time of the radar signal emitted by a 500 MHz antenna from the surface

Fig. 5. Geophysical results of the test profiles. (a) Analytic signal of the total magnetic field. (b) Ground resistivity of each profile obtained using EM. (c) Resistivity section obtained using ERT. (d) Radar images obtained using GPR. G1 and G2 denote the notable reflection anomalies. P represents the house basement and H represents the zone that was later filled with sediments.


to the top of the buried steel rod. The dielectric constants of the background sediment were calculated to be between 6 and 8. Based on the same concept described above, the radar velocity of the slate can be calculated as the thickness of the slate sample divided by the one-way travel time of the radar signal emitted by a 900 MHz antenna across the slate boulder collected on the surface, onsite exhibition and in the pits. The dielectric constants were measured for ten slate boulders. The average calculated dielectric constant of slate is 4.26, which is much lower than the dielectric constant of the background sediment. This suggests that positive reflections in the radar image should occur at the interface between the slate and surrounding sediment. Based on the physical properties measured for the archaeological targets and background sediment, the physical contrasts between ancient remains and surrounding sediment is high. Thus, the distribution of subsurface remains can be obtained from geophysical surveying. In particular, the reflections from the flat slate and the hyperbolic diffraction from the slate boulders of the house basements should be very helpful in identifying archaeological targets. 3. Test survey along parallel profiles Three parallel profiles crossing the suspected boundary of the ancient village that was suggested by archaeologists were used to evaluate the performance of various geophysical methods. The plants along the survey line were cut (Fig. 4) before conducting the test. Three parallel profiles were labeled as L1, L2, and L3 as shown in Figs. 1 and 4. The length of each profile is 304 m, and the spacing between profiles is 2 m. The EM and magnetic measurements were conducted along each profile with a spacing of 2 m, GPR was conducted along L2, and ERT was conducted at particular distances along L2 based on the results of the EM measurements. Fig. 5 shows the results of each method, aligned by distance for easy comparison.

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3.2. EM method The EM method had been widely applied for mapping subsurface objects (Rodrigues et al., 2009; Simpson et al., 2009). It utilizes a transmitting coil and a receiving coil. The apparent conductivity of the ground within a given depth can be estimated from the ratio between the secondary field and primary field, where the primary field is the magnetic signal emitted by the transmitting coil and the secondary field is the magnetic signal generated by the eddy current. The eddy current is induced by the primary field and is dependent on the subsurface conductivity. The Geonics EM-31 instrument with vertical co-planer configuration was used to perform the EM survey. The maximum exploration depth is approximately 3 m (Hauck et al., 2001), and it is most sensitive to the material near the surface and sensitivity decreases with depth. The spike removal was applied to eliminate unusual data before preparing the final map. As shown in Fig. 5b, the variation in ground resistivity at distances of less than 130 m is small, but the variation increases at distances greater than 130 m, which implies that the boundary of the ancient village may be at a distance of 130 m. Southeast of the boundary, the ground resistivity is greater than 1,000 Ωm, which may be related to the ancient housing area. The increased resistivity could be caused by undulating basements that are buried at shallow depths, as confirmed in onsite exhibition (Figs. 2 and 3).

3.1. Magnetic method The magnetic method is widely applied to mapping buried archaeological targets (Drahor, 2006; Drahor et al., 2008; Fiore and Chianese, 2008) due to its quick and easy operation. Magnetic surveys measure slight variations in the earth's magnetic field intensity caused by magnetic objects in the subsurface. It was first used in the 1950s and has been widely applied in archaeological surveys (Wynn, 1986). Typically, the total magnetic intensity or vertical magnetic gradient is measured to map variations in the geomagnetic field produced by shallowly buried magnetic objects. The distribution of buried magnetic objects can then be obtained after proper data processing. The Geometrics G-858 magnetometer with a single sensor kept 30 cm above the ground was used to perform the magnetic measurement in this study. The operator took-off all the iron belongings when conducting the measurement to reduce the influence of magnetic reading. The interval between stations is 2 m. The geomagnetic intensity recorded at a stationary station nearby the Peinan site was used for diurnal correction after manual spike removal. We did not apply IGRF (International Geomagnetic Reference Field) correction because the length of this profile is short, so that, the variation of IGRF can be omitted. Fig. 5a shows the analytical signal for the total magnetic field. Analytical signal is widely used in mapping subsurface magnetic objects (Büyüksaraç et al., 2006; Jeng et al., 2003; Oruç, 2010; Salem et al., 2002). As shown in Fig. 5a, high-amplitude anomalies at distances of 132 m, 228 m, 266 m, and 306 m are associated with the location of stony ridge and are unlikely to be caused by subsurface remains. However, the anomaly between 132 m and 228 m has the property of medium magnitude and scattered pattern which may be caused by buried pottery shards.

Fig. 6. Radar images of line L2g. (a) Original radar image after distance normalization. (b) radar image after horizontal and vertical band-pass filtering. Location map shown in Fig. 5.

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3.3. ERT method The ERT method has proven to be successful in a variety of engineering, environmental, and archaeological applications (Domenico et al., 2006; Kampke, 1999). Modern ERT surveys involve multiple electrodes with equal spacing along the survey line. The current and potential electrodes are automatically switched by the instrument, following a specified electrode configuration. The resolution of the resistivity image constructed from ERT can be improved with dense data coverage and the use of advanced inversion algorithms. The Super STING multi-channel resistivity meter and a Wenner array with 1 m spacing were used to conduct the ERT measurements in this study. ERT measurements were conducted at distances of 100 m to 205 m, which covers the possible boundary of the ancient village suggested from the EM measurements (Fig. 5b). The manual spike removal was applied to eliminate unusual data before inversion. The RES2DINV software (Loke and Barker, 1996) was used to perform the finite-element 2D inversion with incorporating tomography. Fig. 5c shows the resistivity image after 2D inversion. The RMS error is 5.4% after ten iterations. As shown in Fig. 5c, the depth to the top of the high-resistivity layer is approximately 3 m at distances less than 130 m, but the disordered and highly resistive anomalies associated with the basements of ancient building are present at shallow depths for distances greater than 130 m, which agrees with the interpretation of the EM measurements. 3.4. GPR method The GPR method has been widely employed at archaeological sites (Porsani et al., 2010; Rodrigues et al., 2009; Shaaban et al., 2009)

because of its capability to explore the subsurface at a high resolution. It was invented in the 1920s and has been widely applied in shallow engineering (Porsani et al., 2012; Tong, 1993). The method involves a transmitting antenna and a receiving antenna. The transmitting antenna emits a repeating high-frequency electromagnetic pulse. The signals are reflected back to the surface and recorded by the receiving antenna. A high-resolution subsurface image is then constructed from the reflected radar signals. The GSSI SIR-10 and a 500 MHz radar antenna were used to perform the GPR survey in this study, and the measurements were not continuous due to the appearance of a stony ridge and step-like terrain. The continuous survey mode was used to record the radar signal. Based on the dielectric constant of the surrounding sediment obtained previously, the record length of the radar trace is set to be 60 ns which the penetrating depth is about 3 m. One hundred traces can be recorded every second at the transmitting rate of 49.6 KHz. The RADAN software developed by GSSI was used for editing and processing. The radar image with 50 traces per meter was obtained after distance normalization. Both horizontal and vertical band-pass filtering were applied to the portion of radar image below first arrival to improve the S/N ratio. Fig. 6 shows the radar image before and after band-pass filtering. High- and low-frequency noises are removed. Numerous and obvious diffractions can be observed at a two-way travel time of 20–40 ns which imply that the diffractions come from the boulders of the house basement, which is coincident with the observation at the onsite exhibition (Figs. 2 and 3a). The diffraction signal is useful for identifying underground point reflector. Thus, we did not apply migration to the radar image to remain the diffractions shown on the radar image for interpretation.

Fig. 7. (a) Radar image of line L2a. (b) Radar image of line L2b. Location map shown in Fig. 5.


As shown in Fig. 5d, there is a continuous reflection on the radar images of line L2a at a two-way travel time of 12–16 ns that deepens to the southeast, as shown in line L2b. No obvious diffraction pattern can be found above this reflection, and the variation in ground resistivity obtained from the EM measurements (Fig. 5b) at the same location of line L2a and L2b is small. Obvious diffraction patterns can be found at distances greater than 100 m, and they can be categorized into two zones (P and H), as shown in Fig. 5d. The P zone consists of obvious high-amplitude diffractions, which might be related to the reflections from the stone in the basement of a house. The H zone consists of weak sub-horizontal reflections and scattered diffractions, which might be associated with the interior of a house or a street that was later filled with sediments. There are two notable reflection anomalies, labeled G1 and G2 in Fig. 5d, that were not resolved in the EM measurements (Fig. 5b) owing to their small sizes. Fig. 7 shows the radar images of line L2a and L2b, which contain G1 and G2. The dielectric constant of the sediment measured previously was used to convert the two-way traveling time into depth on the radar image. As shown in Fig. 7, the topsoil contains a weak reflection attributed to the uniform sediment, and the gravel layer contains numerous small, weak diffractions. The dashed black line in Fig. 7 denotes the bottom of the gravel layer, which produces a strong reflection. At distances of 15 m and 22 m, just below the bottom of the gravel layer, a set of diffractions and strong flat

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reflections are observed (Fig. 7a). This implies that the diffraction and the flat reflection relate to stone structures and toppled slate, respectively. A pit-like anomaly is also observed at distances of 61 m to 63 m in Fig. 7b. There are some diffractions and inclined reflections at the bottom of the pit-like anomaly. No obvious reflections are observed in the pit, which implies that it may have been refilled by soil. This suggests that the pit-like anomaly may relate to a grave containing a slate coffin. 3.5. Brief summary Based on the information gathered from the test measurements, distances less than 130 m may be host to undisturbed sediment. Distances greater than 130 m might be associated with the location of the ancient building. The test measurements show that the EM and magnetic surveys can be performed by one operator which are cost-effective and were thus suggested to survey the entire areas as shown in Fig. 1 to obtain a preliminary understanding of the distribution of the ancient village. GPR can be used to image detailed subsurface structures that cannot be obtained from EM or magnetic surveys, and it is therefore suitable for surveying specified areas in detail. The ERT method can also provide useful information through resistivity. However, ERT survey needs more than 3 workers to spread the cables and dig the electrodes. The

Fig. 8. Color-shaded relief map of the ground resistivity obtained from the EM survey. The aerial photograph is used as the base map. The white line shown in the middle of the map denotes the ground resistivity of profile L2 obtained from the EM survey. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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ERT method will be suggested as a source of supplemental information owing to its high cost of data acquisition.

4. EM and magnetic surveys A geophysical survey in the Peinan site using a regular grid would be very difficult due to the dense vegetation (Fig. 4). Thus, both EM and magnetic surveys were conducted at an irregular spacing and at the same location. The survey area is extended to northern and southeastern of the area of the Peinan Cultural Park. A total of 3781 stations were acquired and the average spacing between stations was 5 to 10 m. The location of each station was obtained using a global positing system (GPS) surveying instrument (Leica ATX1230) based on the virtual base station real-time kinematic technique with an accuracy of 1–5 cm. After spike removal, Fig. 8 shows the color-shaded relief map of ground resistivity obtained from the EM survey. The ground resistivity obtained from EM measurement along profile L2 is overlain on the resistivity map. As shown in Fig. 8, there is an obvious high-resistivity zone (HRZ) that is sub-linear in the eastern part of the survey area. This implies that the HRZ may be caused by a large number of slate boulders buried in shallow subsurface, which could be related to the basements of ancient building. Thus, the boundary of the ancient village can be associated with the boundary of the HRZ. As shown in Fig. 8, a medium-resistivity zone (MRZ) with a mottled texture and relatively

lower resistivity compared to the HRZ is northwest of the HRZ, which shows that the slate boulders are distributed locally or are deeply buried in the MRZ. The distributions of the HRZ and MRZ suggest that the ancient village was broadly distributed over the eastern part of the Peinan Culture Park and extended to the southeast. The magnetic inclination at the Peinan site is about 33 degrees, thus the TMI (total magnetic intensity) grid is not suitable for direct interpretation. Following spike removal, diurnal correction, and IGRF correction, Fig. 9 shows the analytical signal of the total magnetic field, which is useful for interpreting the locations of subsurface magnetic objects. A highly magnetic zone (HMZ) is encircled by a dashed black line in the northern part of the survey area. The HMZ has a mottled texture that might be associated with broadly distributed pottery shards. The MRZ (Fig. 8) is roughly coincident with the HMZ (Fig. 9), which implies that they may be linked to the secondary distributions of the ancient remains.

5. Test pits of excavation Four test pits were undertaken to confirm the results from the geophysical surveys. Their locations are shown in Figs. 8 and 9. Fig. 10 shows photographs taken at the test pits. The Pit A1 (Fig. 10a) is located in the HRZ, as shown in Fig. 8. The depth to the top of the stone structure is only approximately 30 cm,

Fig. 9. Color-shaded relief map of the analytic signal of the total magnetic field. The aerial photograph is used as the base map. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)


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Fig. 10. Four photographs showing pit results with the EM and magnetic surveys. (a) Pit A1. (b) Pit B1. (c) Pit B2. (d) Pit B3.

which is coincident with the interpretation of the HRZ. Two obvious rounded stone structures and a trench are shown in the central part of this photograph. The Pit B1 (Fig. 10b) is located on the interpreted boundary of the ancient village (Fig. 8). A stone structure was found at a depth of 50 cm and it deepens to the southeast, which is consistent with the interpretation of the boundary of the ancient village suggested from the EM survey (Fig. 8). The Pit B2 (Fig. 10c) is located in the MRZ, near anomaly G1 in GPR line L2a (Fig. 7a) and next to the HMZ, as shown in Figs. 8 and 9. A portion of a manmade stone structure and many ceramics were found below the gravel layer (Fig. 10c), which is consistent with the interpretation of the GPR, EM, and magnetic surveys. The Pit B3 (Fig. 10d) is located in a low-resistivity zone (Fig. 8) and in the HMZ (Fig. 9). Some pottery was found between depths of 1.5 m and 1.8 m, and no stone structures were found above 2.5 m depth in this pit. 6. Conclusions Complete archaeological excavation of the Peinan site is impractical and may be impossible because the site is protected by the Cultural Heritage Preservation Act. Careful selection of excavation priorities is essential for future archaeological research at the Peinan site. Based on the measured physical properties of the archaeological targets at the Peinan site, the contrast between archaeological targets and background sediment is high enough to be detected by geophysical surveys. Based on the tests performed in this study, the EM and magnetic methods are suitable for large-scale investigation of the Peinan site, and the GPR and ERT methods are more suitable for detailed investigation.

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