2018 Volume 40 Issue 1 Pages 45-75
The recent geophysical exploration method, such as airborne gravity gradient survey, is progressed tremendously. The measurement over wide area at short intervals in the short period of time leads to hope for the efficient tools of the geothermal resources assessments. The commonly applied methods for the surface-layer density estimation and its application to surfacelayer correction of the acquired gravity gradient data is based on one kind of surface-layer density, which may not always be reflecting the frequently variable surface-layer density especially in the geothermal prospecting areas, therefore, the corrected gravity gradient and gravity would be inadequate. The new approach, namely the horizontal gravity gradient stack (HGGS), is proposed for the assessment of surface-layer density, and the method is applied with the filtering of the horizontal gravity gradient data (Data). The comparison between the HGGS filtered acquired Data and the equivalent calculated response of the DEM based surface-layer is analyzed for estimating the surface-layer density by the moving window correlation (MWC) methods. Its application to the actually acquired Data around the Ogiri geothermal plant in the western Japan is analyzed, and its results fit adequately well with the existing fractured zones and hydrothermal alteration zones as the low surface-layer density area, and the possible prominent distributions of intrusions and lava deposits as the high surface-layer density area. The surface-layer density corrected vertical gravity gradient and vertical gravity is calculated for comparison with the seismic sections. Some discrepancies of both data lead us a suspicion how deep the gravity gradient data is reflecting the geological anomalies.
