Integrated archaeological exploration using geophysical methods at Futenma area, Okinawa prefecture, Japan

Abstract

The archaeology and history of Okinawa are receiving a great deal of public attention as the Okinawans attempt to define their cultural and political identity in the closing decade of the 20th century. In an attempt to explore some of the old ruins in the area of Futenma, an integrated geophysical survey had been conducted in the form of two and three dimensional geoelectrical resistivity, surface seismic wave and seismic reflection surveys. The interpretation results of geoelectric survey had identified three-layer structure, which consists of superficial low-resistivity layer that indicates filling of Shimajiri maaji followed by high-resistivity layer that indicates Ryukyu limestone, and low-resistivity layer that indicates Shimajiri mudstone group. Also clear-cut reflectors of seismic reflection lines with good continuity are identified. These reflectors considered to reveal the surface of Ryukyu limestone of archaeological remains. The result of the surface wave survey identified two-layer structure that consists of superficial layer of surface wave velocity (0.3 km/s) overlies a relatively high velocity layer (0.5 km/s). The low velocity layer is estimated to indicate filling of Shimajiri maaji, while the high velocity layer correlates Ryukyu limestone formation.From the results it was predicted that subsurface structures of ruins influence the result of exploration even if their sizes are not large. Therefore, characteristic patterns due to a certain ruin structure can be distinguished, by making comparison with the excavation findings. The soil covering Ryukyu limestone is relatively thin; however, ruin structures naturally exist in the sedimentation soil above the limestone. It is highly probable that, as we trace back to older time, the land surface would be closer in shape to the upper surface of Ryukyu limestone. Furthermore, the survey results showed that Ryukyu limestone itself is considered to be divided into fresh zone and weathered zone, which possibly has been erroded and is providing channels for the ground water flow including a doline.According to the results we obtained in this work, even though, geoelectrical resistivity technique is stand-alone for archaeological prospection, its integration with seismic methods, mainly Surface seismic wave, survey give successful results.