Stress-induced seismic azimuthal anisotropy in the upper crust across the North West Shelf, Australia

Abstract

Seismic azimuthal anisotropy (SAA) is observed in many areas of the Earth, and fast polarized directions (φ) have been mapped using earthquake surface waves and teleseismic S wave splitting over large regional and tectonic scales. Higher-resolution petroleum exploration data, including 3-D seismic surveys and dipole shear logs, often exhibit azimuthal anisotropy; however, we are unaware of any published studies mapping SAA from exploration-scale data to study large-scale regional and tectonic SAA trends. The physical mechanisms causing SAA in earthquake data are a subject of great interest; comparing regional φ trends to the higher-resolution exploration trends may help understand these mechanisms. We present a SAA analysis using seismic exploration data across the North West Shelf (NWS) of Australia. We map the fast polarization directions φ from 34 dipole shear logs and two 3-D seismic surveys and compare them to in situ maximum horizontal stress orientations (σH). Our results show that the φ and σH trends correlate across a geographical area spanning almost 2000 km and are similar to published results in the region from earthquake seismology. These results suggest that differential horizontal stress is the likely mechanism causing SAA at a wide range of spatial scales on the NWS of Australia. We also show that SAA is not observed at exploration scales in some areas of the NWS and propose a lithology-dependent stress sensitivity model in which SAA is strongest in clean, unconsolidated quartz-dominated sandstones and weaker or nonexistent in well-consolidated cemented rocks and shale-dominated sediments.