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Fig. 1: Phimai temple

From the history of remote sensing, the first aerial photographs of an archaeological site (Stonehenge) were taken from a war balloon by Lieutenant P. H. Sharpe in the early 1900s. Remote sensing technology could be used to assist in archaeological work to see the big picture from the bird’s eye view of the archaeological sites, to look for some key of the sites. For example, aerial photographs and satellite images could be used to identify the boundary, wall, or water reservoir of some archaeological sites, which are difficult to identify from the ground. This is the basic advantage of remote sensing information to archaeological study.

A more exciting outcome is from the fact that rocks, soils and vegetation look different at these longer wavelengths and because a lot of materials appear to show more contrast at longer wavelengths, they’re easier to isolate and detect. This is the advantage that comes when we use information from different wavelengths. The belief in the potential of space imaging was really revealed in 1981,


Fig. 2 The map shows the location of the study area (Phimai) and Ankor Wat

when NASA launched an imaging system called SIR-A on the Space Shuttle. Unlike other optical sensors, which used reflected sunlight to make an image, SIR-A sent out its own radar signal and then record to the return signal. Archaeologist Farouk El-Baz had asked NASA to fly SIR-A over the eastern Sahara desert. No one was quite prepared for the images that came back. The Sahara is the driest place on earth right now and SIR-A was able to penetrate the sand and reveal an ancient landscape below that. These previous studies of using remote sensing data, especially radar images, are the inspiration of this study in Thailand.


Fig. 3 An Aerial photograph of Phimai temple area (1954)

AirSAR
AirSAR is the airborne SAR system that has been operated by NASA. In short, the wavelengths of AirSAR are: P band (0.45 Ghz, 67 cm) L band (1.26 Ghz, 23 cm) and C band (5.31 Ghz, 5.7 cm). PACRIM 96 is the AirSAR mission that was flown in various countries in Asia. AirSAR flight illustrated the advantage of using SAR data for various applications such as coastal study and oceanography, geology, forestry, and archaeology. The advantage of AirSAR data can be used for the exploration of archaeology sites as demonstrated in this document. By analysing the combination of P, L and C bands data, we can identify human activities 1200 years ago.


Fig. 4 Different polarisations could provide different information.

Study Area and Other Information Available
The study area is in the region of Phimai area, Nakhorn Ratchasima province, Thailand. For this study, AirSAR data had been provided directly from JPL. In addition to AirSAR data, we used the following information:

• Aerial Photograph (1954)
• Historical information about Phimai temple and the surrounding area.

Methodology


Fig. 5 Colour composite shows the baray area in yellow box in the lower part of the image

As illustrated in Fig. 4, different polarisations could provide different information. The difference can provide additional information that could be used to identify undiscovered human activities. By integrating all of these information, we found an undiscovered water reservoir or “baray” in the local language. We have been trying with various techniques to identify the manmade structures surrounding the Phimai temple ] to identify any unknown human activities that may be hidden to ordinary observation. After many combinations of methods and procedures with integration of all available information, we found an unknown ancient water reservoir, at this moment the biggest in Thailand. The width is around 900 metres and the length around 2000 metres. In addition, at the centre of this water reservoir, we found a mound that is an archaeological site similar to the temple of west Mebon in Ankor, which is also situated at the centre of the western baray of Ankor.



Fig. 6 Photographs of the mound in the centre of the ancient baray (a) and the edge of the baray (b)

Further Study
This is just the start of a new approach to archaeological study, that integrates information from all the techniques available to restore, preserve, and discover any unknown remains of archaeological importance. We believe that the new technology will save time, budget, and human resource to discover unknown archaeological sites.