Azimuthal anisotropy from OBS observations in Mahanadi offshore, India

Abstract

We have carried out an ocean bottom seismometer (OBS) survey in a grid along with multichannel seismic survey for gas hydrate exploration in the Mahanadi offshore, India. Here, we report on some interesting observations in seismic waveform data and their interpretations. These include sudden amplitude dimming in the multichannel data that is azimuth- and space-dependent and a clear manifestation of seismic anisotropy in the region. We observe significant patterns of shear wave splitting in the azimuthal gathers in the OBS data, clearly isolating the fast (S1) and slow (S2) axes of propagation in the radial azimuthal gathers. Further, amplitude nulls and amplitude maxima are observed in the transverse azimuthal gathers. These two features are diagnostic of the existence and orientation of anisotropy which is also modeled by generating full waveform synthetic seismograms. We interpret the occurrence of anisotropy to be due to the presence of fractures. The strike of this fracture set is inferred to be ∼130° from the S1 and S2 orientation and variation in the P-wave amplitude with azimuth. The density of fracture network is estimated by full wave modeling of the OBS data. A good match between the synthetic and observed data is noticed for a near vertical fracture (dip angle of about 85°). The seismic image obtained from the 2D high-resolution multichannel profiles correlate well with the OBS results. Based on these analyses, we are able to delineate a fracture zone, which is linked to the near vertical faulting in the gas hydrate layers.