Array GPR “Yakumo” and its application to archaeological survey and environmental studies

We have investigated effective ways of data acquisition and sampling for Ground Penetrating Radar (GPR). 2D GPR profiles obtained from one-dimensional GPR surveys show good vertical profiles, which are 2-dimentional (1-D Horizontal position-depth) image. However, 3-dimentional images (2-D Horizontal position-depth) obtained from 2-dimentional surveys, have poor quality, if the antenna position is not precise. One of the approaches to solve this problem is “3DGPR”, which uses a GPR antenna equipped with a high-precision positing system such as iGPS, which is a laser positioning system. 3DGPR system uses high density data acquisition for accurate 3-dimentional imaging by interpolation of data sets. We have demonstrated successful subsurface 3-dimentinal images by this approach, however, we think this approach needs too much time for data acquisition. Then we have developed an array GPR system, which is named “Yakumo”. Yakumo has 8 transmitting antenna and 8 receiving antennas, and is a stepped frequency continuous wave (SF-CW) radar operating at between 10MHz-1.5GHz. The width of the radar system is 2m, and it can be pulled by 1-2 persons. All the transmitters and receivers can be switched sequentially, and we obtain 8×8=64 channels at each data acquisition. The system can reach 7km/hour for 1cm data acquisition interval along the survey line. We have developed a synthetic aperture radar (SAR) processing using these data sets. This is a multi-static radar, having 8 different antenna spacing for each imaging point. Using this multi-static radar system, we could also estimate the vertical profile of subsurface material by using Common Midpoint (CMP) signal processing. In the CMP analysis, we fix a subsurface reflection point, and using the transmitter and receiver pairs, we take correlation between the measured reflection time and the theoretical travel time by assuming the wave velocity. Then we can estimate the vertical profile of the velocity, and it can be converted into soli water content or the dielectric constant. A massive number of archaeological surveys are being planned in Northeast Japan because of the relocating residential areas from coast to inland as a part of rehabilitation. In order to contribute to efficient and urgent surveys, an array GPR system suited for large-scale measurements, which is expected to accelerate these surveys. We have demonstrated the operation of “Yakumo” at several locations, which include archaeological survey and buried pipe location. One of the examples of GPR profile obtained by Yakumo is shown in Figs.5 and 6. Fig.5 shows the vertical profile and Fig.6 shows the horizontal profile of GPR data sets, which were acquired in Yuriage beach, south of Sendai, shown in Fig.4. More than 15,000 people were killed by the East Japan Earthquake and Tsunami in March 11, 2011 in Japan, and still more than 1,000 people are still missing. Yuriage is a small fisher port and village, and more than 40% of population was killed by Tsunami. The area is vast, and we started a survey area of 100m by 50m. It took about 2 hours to acquire the GPR data. We cannot see the discontinuity of the horizontal slice data, which are acquired by 2m width strip. This means that the quality of the data is quite uniform. We could detect many buried objects in this survey, and the circle in Figs 5 and 6 show one object. We dug out the detected object and found it is a wood piece, which is about 1.5 m long and 30cm by 50cm cross section. It must be a horizontal bar used in a wooden house. It was found about 1m deep in sand. We think this wooden piece was a part of broken house attacked by Tsunami, and immediately after the Tsunami, it was buried in the sand. The height of the Tsunami in this area reached about 10m, and probably sand layer of several meter were completely moved by the Tsunami force. We are investigating more sophisticated signal processing using the multi-static GPR data sets acquired by Yakumo.