Enhancing illumination in DAS-FWI by combining strain-rate and pseudo-pressure data: A comparative study

ABSTRACT Distributed acoustic sensing (DAS) offers dense spatial sampling and cost-effective seismic data acquisition by measuring strain or strain rate along fiber-optic cables. Integrating DAS with full waveform inversion (FWI) has the potential for high-resolution subsurface monitoring; however, the directivity and wave-mode sensitivity of strain-rate data differ fundamentally from conventional seismic measurements, complicating its integration with conventional FWI formulations. Although combining DAS with traditional point sensors may improve inversion accuracy, this is often infeasible in practical scenarios. Hence, understanding the differences between direct strain-rate inversion and inversion using data converted to conventional seismic components is crucial for optimizing DAS-based workflows. Direct strain-rate FWI and pseudo-pressure FWI converted from the same DAS measurements were systematically compared within a 2D acoustic medium and multi-offset vertical seismic profiling settings. The results demonstrated that standard pressure FWI assuming ideal conversion from DAS data outperforms strain-rate FWI, offering broader illumination and more accurate velocity estimations. The difference arises from the directive nature of DAS data and its gradient errors near receiver locations. Pseudo-pressure FWI retains the broad sensitivity of pressure data but suffers from conversion errors for quasi-perpendicular arrivals, limiting its accuracy. A hybrid approach was proposed that sequentially inverts strain-rate and pseudo-pressure data sets to exploit their complementary strengths. This combined method achieves enhanced resolution and improved deep-structure imaging compared to using either data set alone. These findings highlight the potential of DAS-based FWI for high-resolution monitoring and emphasize the importance of carefully designed workflows, especially when only DAS measurements are available.