THE VELOCITY DISPERSION AND ATTENUATION OF MARINE HYDRATE‐BEARING SEDIMENTS

AbstractP‐wave and S‐wave velocity will increase and the attenuation will vary when the concentration of gas hydrate increases. The analysis of velocity dispersion and attenuation for hydrate‐bearing sediments (GHBS) would contribute to the estimate of gas hydrate concentration. Based on effective medium theory (EMT), we study the nonlinear variation feature of P‐wave and S‐wave velocity for marine unconsolidated hydrate‐bearing sediments. Moreover, we use BISQ model to replace Gassmann equation in the EMT, and research the velocity dispersion and attenuation of hydrate‐bearing sediments in the full frequency band. Based on this model, the velocity and attenuation always increase with the increasing amount of gas hydrate, and the rock porosity and clay content doesn't make any differences to the attenuation. After the numeral modeling, we apply the sonic logging (20 kHz) and VSP (100 Hz) data from Ocean drilling Program (ODP) leg 164 in obtaining the concentration of gas hydrate stability zone (GHSZ). In the application, the average hydrate concentration of GHSZ from site 995 in ODP leg 164 is about 5%∼7%, and corresponds to the pressure core sample (PCS) data, Helgerud et al.'s research conclusions and the prediction data from neural network (NN). Due to the velocity dispersion, the estimated hydrate concentration from VSP data would be lower than the estimated results from sonic logging data. The prediction results of three hydrate Sites (SH2, SH3 and SH7) from Shenhu area also coincide with the PCS, South China Sea. Moreover, based on the peak frequency method, the effective seismic quality factors (Q) of the BSR are estimated from the inline prestack seismic gathers. The Q‐values suggest that the effective saturation of gas hydrate estimated by this model fluctuates between 15%∼30%. The results of numeral modeling and applications indicate the velocity dispersion and attenuation of GHBS always vary with the concentration of hydrate. The study of velocity dispersion and attenuation feature for GHBS in the full frequency band would contribute to the estimate of gas hydrate concentration.