PS変換波を利用した反射法地震探査の基礎実験

摘要

Shear (S-) wave velocity structure is one of the key elements for the estimation of strong ground motion generated by earthquakes. Reflection seismic survey is one of the best technologies-to image the underground structure and the velocity. Generally, S-wave reflection surveys are conducted aiming at shallow structures, because there are limitations for the energy propagation. For a deeper structure survey, the reflection survey using PS converted-wave is supposed to be superior to the one using S-S wave, because the source can be stronger and the attenuation is smaller. To confirm the superiority of PS converted-wave reflection survey, we conducted two feasibility surveys using 9-component acquisition system, and compared PS converted-wave reflection results with P-wave and SH-wave reflection results. The survey areas are in an alluvial plain (Line 1) and a Pleistocene upland (Line 2) in Ibaraki, Japan. The source was a minivib T15000. It can be used for both P-wave and S-wave source by changing the vibrating direction. We first processed P-wave reflection data and SH-wave reflection data. Then processed PS converted-wave data. For Line 1 we estimated complex S-wave static corrections, up to 200 ms difference for some locations. We applied the SH-wave static correction values to PS converted-wave data processing as receiver domain static corrections in both lines. The complex S-wave static corrections of Line 1 make the PS converted-wave processing difficult to get a good image of the reflections. The two experimental surveys result as follows. The P-wave time sections are in good quality, and we can see almost flat shallow reflections and weak undulations at deep section in both cases, whose two-way traveltime are about 0.9s and 0.55s respectively. The SH-wave time sections show reflectors in shallower part, but in the deeper part, they are in poor quality and hardly imaged. According to the velocity analysis result of P and SH-wave reflection data, average Vp/Vs at the undulation reflections are 2.9 and 3.9 respectively. Using these values, three time sections for each line are correlated for the undulation reflections. For the deeper part, the PS converted-wave sections give better results than the SH-wave sections. These results suggest PS converted-wave can be used to extract deep S-wave velocity information.