10th Asian Regional Conference of IAEG (2015)

Resistivity profiles and foundation structure of Central Tower inBayon Temple, Angkor ThomKoichi NAKAGAWA(1), Yoshinori IWASAKI(2) Makoto ARAYA(3)Shunsuke YAMADA(3), Ichita SHIMODA(4), Takeshi NAKAGAWA(3)and Tomofumi KOYAMA(4)(1)Faculty of Science, Osaka City University,
E-mail:knaka@sci.osaka-cu.ac.jp(2)Geo Research Institute, (3)Waseda University,(4)University of Tsukuba, (5)Kansai University
Abstract
Electric exploration survey was conducted to study the foundation structure of theCentral Tower of stone masonry in the Bayon Temple, Angkor Thom. Masonry structures
were constructed upon manmade filled terrace with three different levels. The Central
Tower of 31m in height stands upon the top level of the terrace of 11m in height from theground. Several borings which consist of vertical, oblique and horizontal drillings under thecentral tower area preceded the electric study to grasp the foundation features. Past studies
in this area have clarified the outline of the underground structure. By those results, it isknown that the filled terrace of the tower foundation consists of compacted soil, artificiallysundried laterite and floor sandstone above natural Holocene ground. The laterite block ismore rigid mechanically than the compacted soils or the original soils in the area.
Four poles type measurement technique for the electrical survey was adopted atseveral sites. The measurement lines were set on the upper terrace radially and circularly tothe axis of the Central Tower.
A unit spacing for the electrodes was 1.5 meter. A piece of aluminum foil with
conductive paste was applied to surfaces of the floor stone as an electrode. Magnitude of theresistivity obtained from all measurement ranges in common values from order of 10 to1000 ohm meter. The resistivity profiles were carried out by the inversion in terms of thefinite element method.
The higher resistivity parts were corresponding to the laterite blocks. The resistivity
profiles and the boring core correlation were rather consistent. Those results lead ahypothesis that a donut-shaped laterite mass with about 36 meters in outer diameter, 28
meters in inner diameter and 6 meters in thickness should exist beneath the sub-tower andgallery zone surrounding the central tower.
One of the present objects is to confirm the feature of the existing laterite ringstructure. The profile along the circular measurement line above the laterite ring suggeststhat the distribution of laterite block is not uniform shape and the laterite blocks lack underthe east part of the sub-tower and the gallery. This result may give important information tothe discussion on the stability problem to restore the foundation mound supporting
considerable loads of the stone towers.
Keywords: Electrical survey, Resistivity, Foundation structure, Bayon Temple

1.

Introduction

The most distinctive feature of the temple is a uniquearchitectural display of giant stone carvings of faces
on the many towers which jut out from the upperterrace and cluster around its central peak (fig.1).

The Bayon located in 10km north of Siem Reap isrenowned as one of the most famous Angkor temples.

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10th Asian Regional Conference of IAEG (2015)(a)Fig.1 General Plan and Section of Bayon
Those monuments are in half ruined condition withsuffering from the deformation of the foundation andthe damage by weathering. Accordingly, JSA(Japanese Government Team for SafeguardingAngkor) has been engaged in the conservation andrestoration work for the Angkor Thom.
Seven drillings with one vertical, two oblique andfour horizontal axes were performed inside the innergallery in 2011 to 2012 to examine the geotechnicalconditions of the ground (Fig, 2). Those drillings
suggest an extent of laterite mass with thickness of
six meter beneath flagstones that were laid on thesurface.
Electrical survey was recognized as one of theuseful tools to study underground structure in Angkor.
Some electrical surveys in this area have been carriedout (Sugimoto et al., 2007, Nakagawa et al., 2012). In
the previous studies, an applicability of electricalsurvey to on the flagstone nondestructively and ageneral pattern of the resistivity distribution under theBayon temple were examined. The results from thosestudies showed that a significant electricalprospecting on the flagstone becomes adequately
possible by adapting sheet type electrode to stone
surface and the laterite block has a higher resistivity
than that of natural soil or compacted soil. In addition,a ring-shape structure of the laterite in the foundation
surrounding the central tower was assumed fromconsidering the correlation between the resistivity
profile and drilling core data. In the present study, theelectrical prospecting was planned to clarify theunderground structure, especially, the configurationof the laterite ring body which is important to discusson the stability of the foundation.
2.

(b)Fig. 2 Plan (a) and North-South section (b) of the areaof tower cluster and gallery, and the distribution oflaterite bodies estimated by the directive drillingcorrelation (Red segments show lateritic part of thecore).

Fig. 3 Electrode array of typical four poles technique(a) Wenner’s technique (b) Eltran’s technique
Symbol; I: electrodes to measure the current, E:
electrodes to measure the potential
those techniques is shown in Fig. 3. The Wenner's
technique is adequate to extract the continuity
componentoftheundergroundstructure.
Contrastively, the Eltran's technique is adequate toextract discontinuous component. The resistivity
profiles were analyzed by inversion in terms of thefinite element method. The minimum interval of theelectrodes, that is, unit spacing is 1.5 meter. Numberof electrode is twenty four and the length for ameasurement line, therefore, is 34.5 meter.
Locations of these survey lines are shown in Fig. 4.
In order to inspect the continuity and configuration of
the laterite mass, survey lines were set across or along

Measurement

Four poles type measurement techniques called
Wenner's and Eltran's techniques were adopted at theall measurement sites. Electrode array pattern of210th Asian Regional Conference of IAEG (2015)

the top terrace where the laterite block was confirmed
its existence beneath the flagstone by the drilling
survey.
Electrodes of aluminum foil with conductive paste
were used. Conductive paste was made by wheat
flour with saline water. This electrode setting must be
useful to decrease the contact resistance between
electrode and stone surface.
3.

Results

The resistivity profiles were obtained from theanalysis of apparent resistivity by inversion in termsof the finite element method. Resistivity profiles
obtained are shown in Fig. 5, 6 and 7. Fig.7 shows
an example of different image by the result fromdifferent prospecting methods. All images in Fig. 6Fig.4 Layout of survey lines

(a)

(b)

(c)

Fig. 5 Resistivity profiles determined from the different measurement methods(a): Wenner’s technique, (b): Eltran’s technique, (c): Hybrid technique intergrading them3

10th Asian Regional Conference of IAEG (2015)

(a)

(b)

(c)

Fig. 6 Resistivity profile (a): Line-1, (b): Line-2, (c): Line3

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10th Asian Regional Conference of IAEG (2015)

E(a)

W

S

(b)

W

N

(c)

Fig. 7 Resistivity profile (a): Line-1, (b): Line-2, (c): Line3
The location of each profile is shown in Fig. 8.

and Fig.7 are of the profiles analyzed from
integrating with both methods of Wenner and Eltran.
The magnitude of resistivity ranges from severaltens to about two thousand ohm･meter
4. Discussions
There is a slight difference between profiles
obtained from Wenner’s and Eltran’s techniques asshown in Fig. 7. The difference of absolute
resistivity, however, is not so large, not more than afactor of two or three. A comparison of the present
resistivity profiles with the distribution of laterite
block of N-S section is shown in Fig. 9. As shown in
the figure, absolute value of the resistivity in thenorthern section is lower than the southern section. Itmay relate to humidity of the ground depending with
degree of sunshine. The correspondence between
distributions of the resistivity and the laterite mass,

Fig. 8 Detailed layout of the survey line 55

10th Asian Regional Conference of IAEG (2015)

Fig. 9 Comparison between the geological section and the resistivity profile under upperterrace of the Bayon temple

Fig. 10 A bird's eye view of the schematic
resistivity profile image beneath the gallery
surrounding the central tower obtained fromanalysis of the Wenner’s arrayFig。11 Resistivity profiles of the N-S and E-Wlines (from Nakagawa et al., 2012)
not a true circle but an ellipse so that layout of thewhole monuments is non symmetric to the axis ofcentral tower and the eastern buildings are grantwith front gate. Accordingly, present measurement
line may put out from the elliptic ring of laterite.

however, seems to be rather well. The laterite mass
corresponds to relatively higher resistivity parts inthe sections. The pattern of resistivity distribution in
the N-S and E-W sections of the previous study(Fig.11 Nakagawa et al.) and Fig.9 suggested that
underlain laterite mass is shaped into a ring structure.
The present study performed to search the truthfulshape of the laterite ring. The relating result isshown in Fig. 10 as a bird’s eye image. As shown in
the figure, a higher resistivity layer which seems to
be laterite is underlain uniformly except the east
segment of the region. It is possible that the ring is

5. Conclusion
Electrical survey was conducted on the flagstoneover the gallery surrounding the central tower ofBayon. Measurement lines were developed radiallyand circularly to the axis of the central tower. Those
sites are underlain the laterite blocks. Resultsobtained by inversion analysis as follows;610th Asian Regional Conference of IAEG (2015)

References(1) The practical Electrical prospecting ispossible on the flagstone nondestructively.
(2) The correspondence between the resistivity
profile obtained and the geologic structurefrom drilling data is rather consistent.
(3) Resistivity of the laterite block ranges orderof 102 to 103 ohm･meter which is larger thanthe other kinds of soil at the site.
(4) The higher resistivity layer which is inferred
to corresponding to the laterite block was notcontinuous uniformly in the section along
circular measurement line on the gallery.
(5) There is a possible that the laterite ring
structure is not circular, but elliptical.

Nakagawa, K., Shimoda, I., Yamada, S., Kanai, R.,
Ogawa, Y., Ohta, (Yasuda) Y., Fukuda, M., Jomori,A., Takahashi T. and Iwasaki Y. (2012): Resistivity
profiles and foundation structure of Central Tower,
Bayon in Angkor, Central Asian GeotechnicalSymposium (IVth CAGS), Geoengineering forconstruction and conservation of cultural heritageand historical sites, Samarkand, pp333-338, ISBN9789 94 31 1199 8Iwasaki, Y., Fukuda, M., Haraguchi, T., Kitamura, A.
Ide, Y., Tokunaga, T. and Mogi, K. (2013):
Structure of Platform Mound of Central Tower
Based upon Boring Information, JASA 2013 Report,
39p

Acknowledgment
A part of this work was supported financially by
JSPS
AKENHI
Grant
Number26303010(Grants-in-Aid for Scientific Research, Japan Society
for the Promotion of Science; Principal Investigator:
Tomofumi. Koyama).

Sugimoto, Y., Yamada, N., Tokunaga, T., Mohamed,N., Mogi, K., Ohnishi, K. and Iwasaki, Y.
(2008): ”Geophysical Prospecting Survey at theBayon Complex” Annual Technical Report on theSurvey of Angkor Monument, 2008, Safeguardingof Bayon Temple of Angkor Thom, JASA,
published by JSA, Tokyo, pp.96-107

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