Seismic vis-a-vis sonic attenuation in gas hydrate bearing sediments of Krishna-Godavari basin, Eastern Margin of India

Abstract

Seismic velocities in the sediments containing gas hydrates show a marked increase compared to the background, intuitively implying that there would be a reduction in the seismic attenuation (Q−1) of such sediments. However, the attenuation measurements carried out in the sonic frequency range from various gas hydrate provinces show that there is a notable increase in seismic attenuation within the gas hydrate layers and the results obtained are counter-intuitive. In this work we try to compare the attenuation derived by applying frequency shift method to the multichannel seismic (MCS) and sonic data sets at the same location in the Krishna-Godavari (KG) basin. The role of complex geology in attenuating the seismic signal is also studied by generating synthetic seismic data for different geological models and by computing the corresponding attenuation. It has been found that the Q−1 obtained from the field seismic data compares well with the Q−1 computed for a simple layered geological model. The results indicate that the Q−1 (0.0029) from field seismic data is ∼4.3 times lower than Q−1 (0.0123–0.0125) obtained from the sonic data, implying that the thickness of the gas hydrate layer in the KG basin is not sufficient enough to average the bulk properties. Our results also indicate that there could be substantial contribution of the pore scale interaction to the observed attenuation.