Full-waveform inversion as a tool to predict fault zone acoustic properties

Abstract

Understanding the physical properties of fault zones is essential for various subsurface applications, including carbon capture and geologic storage, geothermal energy and seismic hazard assessment. Although three-dimensional seismic reflection data can image the geometries of faults in the sub-surface, it does not provide any direct information on the physical properties of fault zones. We currently cannot use seismic reflection data to infer directly which faults may be leaking or sealing and are reliant instead on shale-gauge ratio type calculations, which are fraught with uncertainties. In this paper, we propose that full-waveform inversion P-wave velocity models can be used to extract information on fault zone acoustic properties directly, which may be a proxy for subsurface fault transmissibility. In this study, we use high-quality post-stack depth–migrated seismic reflection and full-waveform inversion velocity data to investigate the characteristics of fault zones in the Samson Dome in the SW Barents Sea. We analyse the variance attribute of the post-stack depth migrated and full-waveform inversion volumes, revealing linear features that consistently appear in both datasets. These features correspond to locations of rapid velocity changes and seismic trace distortions, which we interpret as faults. These observations demonstrate the capability of full-waveform inversion to recover fault zone velocity structures. Our findings also reveal the natural heterogeneity and complexity of fault zones, with varying P-wave velocity anomalies within the studied fault network and along individual faults. Our results indicate a correlation between P-wave velocity anomalies within fault zones and the modern-day stress orientation. Faults with high P-wave velocity are the ones that are perpendicular to the present-day maximum horizontal stress orientation and are likely under compression. Faults with lower P-wave velocity are the ones more parallel to the present-day maximum horizontal stress orientation and are likely in extension. We propose that these P-wave velocity anomalies may indicate differences in how ‘open’ and fluid filled the fault zones are (i.e. faults in extension are more open, more fluid filled and have lower V_P) and therefore may provide a promising proxy for fault transmissibility.