Evaluation and modeling of velocity dispersion and frequency-dependent attenuation in gas hydrate-bearing sediments

Abstract

Wave velocity and attenuation of gas hydrate-bearing samples are often extrapolated from ultrasonic to lower seismic/sonic frequencies, but the impact of measurement frequency on these properties is rarely studied. This study evaluates velocity dispersion and frequency-dependent attenuation in gas hydrate-bearing sediments (GHBS) by comparing vertical seismic profile (VSP) and sonic logging data from the Nankai Trough and Mallik field. We also employ rock physics modeling to simulate the velocity dispersion and frequency-dependent attenuation at both sites. The results show strong velocity dispersion and frequency-dependent attenuation in the Nankai Trough, attributed to thin, low-saturation hydrate intervals. In contrast, the Mallik field exhibits weak dispersion and frequency dependence, likely due to thick, highly saturated hydrate layers. A compilation of global hydrate reservoir attenuations indicates frequency dependence, with a peak in the sonic logging range, likely due to pore-scale hydrate effects. These findings emphasize the necessity of considering the effect of measurement frequency when performing time-to-depth conversion for seismic data based on the sonic velocity at higher frequencies, particularly for thin and lowly saturated hydrate layers, thereby improving the accuracy of hydrate reservoir characterization.