S-wave attenuation estimation from walkaway vertical seismic profiling data in methane hydrate-bearing sediments at Nankai Trough, Japan

Abstract

Previous studies have demonstrated that the presence of methane hydrates (MHs) causes high S-wave attenuation (1/Q) at sonic logging frequencies. However, there are extremely few examples with which to estimate S-wave attenuation at seismic frequencies for MH reservoirs because of the low signal-to-noise ratio (SNR) of S-wave events. To elucidate the mechanisms responsible for such attenuation, it is important to obtain S-wave attenuation at various frequencies. We improve the SNR by horizontally stacking traces after linear moveout with fine-tuning over the offset range. Then we select better-quality S-wave events observed in methane hydrate-bearing sediment (MHBS). The better quality is possibly due to better coupling between the geophones and MHBS. Finally, we estimate S-wave intrinsic attenuation at a frequency range from 30 to 100 Hz from walkaway vertical seismic profiling (w-VSP) data. To isolate intrinsic attenuation from total attenuation, scattering attenuation is estimated using one-dimensional synthetic data from sonic velocity and density logs. The estimated intrinsic attenuation ranges from 0.34 to 0.39. By comparing our results with previously published results on S-wave attenuation at sonic frequencies of 0.5–1 kHz, we attribute a possible S-wave attenuation mechanism to the viscous friction due to the elastic contrast between hydrate and sand grains.