Detection of Seismic Anisotropy Using Ocean Bottom Seismometers: A Case Study from the Accretionary Prism Off Southwest Taiwan

Abstract:

A multicomponent ocean-bottom seismometer data set was collected by National Central University, Taiwan in the accreationary prism off southwestern Taiwan in 2013 and 2014, respectively. The OBS contains four component receivers, including a three component 4.5 Hz geophone unit containing three orthogonal components and a hydrophone. GI-gun shots located at 1 mile and 1.5 miles radius from the OBS, with spacing approximately 40 m along the sail line. The OBS recorded data at a sampling rate of 250 Hz and from a shot pattern that gave good azimuthal coverage around the OBS. Based on P and P-S converted waves recorded between the direct and multiple arrivals, this experiment targeted the top few hundred meters of sediment in the study area. Synthetic seismograms were calculated from a model representative of the sediment sequence at this site indicating that converted amplitudes are dominated by P to S mode-converted waves generated on reflection. After preliminary processing, including a static correction, the data were optimally rotated to radial (R) and transverse (T) components. The principal technique used to detect the anisotropy was azimuthal stacking of the radial and transverse horizontal geophone components. The R component shows azimuthal variation of traveltime indicating variation of velocity with azimuth; the corresponding T component shows azimuthal variation of amplitude and phase. From the radial component azimuthal gather and mode-converted wave amplitude variation for the first few layers and determined corresponding anisotropy parameter and VP/VS values. We attribute the observed azimuthal anisotropy to the presence of microcracks and grain boundary orientation due to stress since fracture at this depth is not likely to occur.