Toshiyuki Yokota

Seismic reflector imaging by prestack time migration in the Kakkonda geothermal field, Japan

We show that a prestack migration method improves the S/N ratio of seismic reflection profiling in the Kakkonda geothermal field where seismic reflection data are of poor quality. We use non-iterative prestack time migration (PSTM), which does not require multiple iterations to determine the velocity structure for prestack time migration. The optimum constant migration velocity can be determined at each image point from a migration velocity analysis based on primary diffraction patterns. Our results delineate a strong reflector beneath a zone of high seismicity. According to the correspondence between the fracture distribution, the distribution of microearthquakes, and geothermal structure, this reflector is interpreted to be a zone of low-angle fractures saturated with hydrothermal fluids, and to be strongly controlled by the geothermal structure.

Repeated seismic reflection measurements in the Kakkonda geothermal field

Temporal variations in seismic reflection responses apparently caused by changes within a geothermal reservoir have been detected in the Kakkonda field, where production wells were shut in prior to annual power plant maintenance. Three seismic surveys were carried out during a 12-day period spanning the shut-in period. Receivers were deployed without replanting throughout the surveys, providing good data acquisition repeatability. We applied prestack time migration (PSTM) to each of the three seismic data sets and calculated the cross-correlation coefficients between PSTM sections. Our results indicate that the reservoir changes associated with shut-in are large enough to be seismically detectable. The region over which the seismic response changed corresponds to the zone of geothermal fluid flow paths inferred from reservoir temperatures and geochemical data. We have also compared our cross-correlation maps with the epicenters of micro-earthquakes, which are inferred to indicate the existence of fractured zones. Dense regions of micro-earthquake activity lie within the seismically identified zone of changes in reservoir properties. We demonstrate the feasibility of repeated seismic surveys in providing substantial improvements in temporal resolution during geothermal reservoir monitoring.

A 3D ground penetrating radar imaging of the heavy rainfall-induced deformation around a river levee: A case study of Ara River, Saitama, Japan

This paper describes a three-dimensional ground penetrating radar (GPR) survey carried out around a levee of the Ara River in Saitama, Japan, where deformation of the ground was observed after heavy rainfall associated with the typhoon of September 2007. The high-density 3D GPR survey was conducted as a series of closely adjacent four directional sets of 2D surveys at an area surrounding vertical cracks on the paved road caused by deformations induced by heavy rain. The survey directions of the 2D surveys were 0, 90, 45, and –45 degrees with respect to the paved road and the intervals between lines were less than 0.5 m. The 3D subsurface structure was accurately imaged by the result of data processing using Kirchhoff-type 3D migration. As a result, locations and vertical continuities of the heavy rainfall induced cracks in the paved road were clearly imaged. This will be a great help in considering the generation mechanisms of the cracks. Moreover, the current risk of a secondary disaster was found to be low, as no air-filled cavities were detected by the 3D GPR survey.

A seismic survey at the region near the mouth of Fuji River, Shizuoka Pref., Japan

It is very important to know the subsurface structure and depositional environment from the coastal to the shallow sea region, when studying groundwater flow. However, when we acquire geophysical survey data, since the data acquisition methods of both land and marine cannot successfully acquire the data at the domain from the coastal to the shallow sea regions, such regions are often left as blank of geophysical surveys. Furthermore, in our country, such domains are generally highly developed and even setting up geophysical survey lines are difficult. Therefore, we are investigating the geophysical survey methods appropriate for the surveys beneath the coastal to the shallow sea region. We are also developing the evaluation methods for such regions. Therefore, we conducted a seismic reflection survey to image the subsurface structure of coastal to the shallow sea region of the mouth of the Fuji River, Shizuoka, Japan where the Fujikawa-kako fault group exists, and checked applicability of the technique. As a result, we obtained subsurface structure down to the 5000 m.