Seismic Tomography Measurements in a Volcanic Geology Area Accompanied with Vertical Fractures

Abstract

After a preliminary experiment, a full scale crosswell tomography survey has been carried out in the Tanna Basin in the central Japan where the Tanna Fault system has been fairly well defined through eh extensive surface and subsurface geological investigation. The velocity tomogram obtained by the travel time inversion of the crosswell data strongly suggests that the fault system is much more complicated than the geological model so far proposed. A multi offset VSP images which have been reconstructed by using a part of the tomography data show a geological structure consistent with the velocity tomogram. In the course of the time pick of the first arrival, three component geophone data have been carefully studied so that the signal to noise ratio could be maximized by the coordinate rotation. It has been verified through this study that detailed structures can be detected even in volcanic areas by a careful application of the tomography technique.