[S-SS03] Seismological advances in the ocean

[SSS03-P08] Space-time monitoring of seafloor shear-wave velocity changes using seismic ambient noise

*Peng Guo¹, Erdinc Saygin¹, Brian L N Kennett² (1.The Commonwealth Scientific and Industrial Research Organisation (CSIRO), 2.Research School of Earth Sciences, The Australian National University)

Keywords:seismic ambient noise, time-lapse monitoring, seafloor , Scholte wave, shear-wave velocity

We use seafloor seismic ambient noise recorded by dense ocean bottom nodes (OBNs) at the Gorgon gas field, Western Australia, to compute time-lapse seafloor models of shear-wave velocity. The extracted hourly cross-correlation (CC) functions in the frequency band 0.1 – 1 Hz contain mainly Scholte waves with very high signal to noise ratio. By comparing the time-lapsed and reference (stacked) Scholte waves, we observe temporal velocity variations (dv/v) of the order of 0.1% with a peak velocity change of 0.8% averaged from all station pairs. The analysis uses moving-window cross-spectral analysis with the assumption of a spatially homogeneous dv/v. With a high-resolution reference model from full waveform inversion of Scholte waves, we present an elastic wave equation based double-difference inversion (EW-DD) method to map time varying dv/v in the heterogeneous subsurface, using arrival time differences between the reference and time-lapsed Scholte waves. The time-lapse seafloor shear-wave velocity models reveal increasing and decreasing patterns of shear-wave velocity in agreement with those from the conventional analysis. The velocity variations exhibit a ~24-hour cycling pattern, which appears to be inversely correlated with sea level height, possibly associated with dilatant effects for porous, low-velocity shallow seafloor and rising pore pressure with higher sea level. This study demonstrates the feasibility of using dense passive seismic surveys for quantitative monitoring of subsurface property changes in the horizontal and depth domain.