The velocity dispersion and attenuation of marine hydrate-bearing sediments
ZHANG Ru-Wei;LI Hong-Qi;WEN Pen-Fei;ZHANG Bao-Jin;China University of Petroleum (Beijing),State Key Laboratory of Petroleum Resource and Prospecting;Key Laboratory of Marine Mineral Resources,Ministry of Land and Resources,Guangzhou marine Geological Survey;
P-wave and S-wave velocity will increase when the concentration of gas hydrate increases,and the attenuation will vary too.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 gashydrate,and the rock porosity and clay content doesn′t make any differences to the attenuation.After the numeral modeling,we apply the sonic logging(20kHz)and VSP(100Hz)data from Ocean drilling Program(ODP)leg 164 to obtain the concentration of gas hydrate stability zone(GHSZ).In the application,the average hydrate concentration of GHSZ from hole 995 in ODP leg 164 is about 5% ~7%,consistent with the pressure core sample(PCA)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 is lower than the estimated results from sonic logging data.The prediction results of three hydrate stations(SH2,SH3 and SH7)from Shenhu area also coincide with the PCA,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 the effective saturation of gas hydrate estimated by this model fluctuates between 15%~30%.The results of numeral modeling and applications indicate that 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.
【Fund】： 国家重点基础研究发展计划(2009CB219505);; 国家高技术研究发展计划(2013AA092501)联合资助
【CateGory Index】： P631
【CateGory Index】： P631