Frequency-dependent P-wave attenuation in hydrate-bearing sediments: a rock physics study at Nankai Trough, Japan

Most sonic logging data acquired in hydrate-bearing zones have shown increasing velocity of P and S waves accompanied by high attenuation of P and S waves. In the eastern Nankai Trough, the attenuation values of P and S wave estimated in the sonic logging frequency range (10-20 kHz for P waves and 0.5-1 kHz for S waves) also show a significant increase in hydrate-bearing sediments. However, P-wave attenuation values from zero-offset vertical seismic profiling (VSP) data (30-110 Hz) indicate a lower attenuation compared to the values in the sonic logging frequency range at the same well locations, which implies frequency-dependent P-wave attenuation in hydrate-bearing sediments. In order to elucidate the attenuation mechanisms responsible for this frequency-dependent P-wave attenuation in hydrate-bearing sediments, we apply two different rock physics models that have recently been developed to consider the squirt flow in porous/ microporous hydrate and the interaction between sand and hydrate grains. The predicted attenuation values are compared with those derived from field sonic logging and VSP data. Finally, we infer that this frequency-dependent P-wave attenuation may be due to the squirt flow caused by the combined effect of the degree of hydrate saturation and two permeable systems (one is between sand grains and the other is between hydrate grains), and/ or due to the squirt flow caused by fluid inclusions with different aspect ratios in a microporous hydrate.