Azimuthal anisotropy from OBS data in Mahanadi offshore India: A case study

Abstract

Multi-component ocean bottom seismometer (OBS) survey carried out along with the multi-channel seismic survey for gas hydrate exploration in the Mahanadi offshore, India has shown some interesting observations in seismic waveform data. The fast (S1) and slow (S2) axes of propagation are seen in the radial azimuthal gathers of the OBS data, while amplitude nulls and amplitude highs are seen in transverse azimuthal gathers, indicating S-wave splitting. These two features are diagnostic of the existent anisotropy, which are modeled by generating full waveform synthetic seismograms. We interpret the occurrence of anisotropy to be due to the presence of fractures. The density of fracture set and the orientation are delineated by full wave modeling of the OBS data and the strike of this fracture set (~130°N) is delineated from the variation in the P-wave amplitude with azimuth. A qualitative match between the synthetic and the observed data is obtained for a near vertical fracture (angle of about 85°). The seismic image obtained from the high resolution multi-channel (MCS) data correlate well with the OBS results. From the joint interpretation of the OBS and MCS data, we identify a fracture zone that causes an azimuthally varying anisotropy within the sedimentary layers. These zones perhaps act as conduits for the supply of free gas to hydrate bearing layers.