音波検層波形記録を用いた地震波減衰解析

抄録

Velocity is valuable information for finding the methane hydrate-bearing sediments. Only use of velocity for finding MH-bearing sediments is not enough and recent surveys have shown that the presence of MH in sediments has significant influence on seismic attenuation. Seismic attenuation can be conveniently separated into the intrinsic attenuation and apparent attenuation. Intrinsic attenuation is a measure of the amount of energy that is irreversibly removed from a seismic wave as it propagates through an inelastic medium due to energy conversion mechanisms. In contrast, apparent attenuation is due to elastic scattering from velocity and density heterogeneities, and commonly is referred to as scattering attenuation. The intrinsic attenuation is important in characterizing the physical properties of subsurface formations. However, it is generally difficult to distinguish between intrinsic attenuation and apparent attenuation.
The purpose of this study is to isolate the scattering attenuation and source-formation radiation coupling from the total attenuation derived from sonic waveform data obtained in MH-bearing sediments at the Nankai Trough area.
Firstly, we applied the median frequency shift method and spectral ratio method for sonic waveform data to estimate attenuation and compared their results. Results showed that at the Nankai trough, median frequency shift is better method for estimating attenuation.
Secondly, we estimated attenuation due to source-formation radiation coupling and scattering attenuation to isolate intrinsic attenuation. In this paper, we assume that the apparent attenuation includes only both the scattering attenuation and source-formation radiation coupling. Both source-formation radiation coupling and scattering attenuation showed moderate correlation at the MH-bearing layers, but the sizes of the heterogeneities showed that we need the data with higher resolution for precise estimation of scattering attenuation.