Crosswell imaging using distributed acoustic sensing: A hydrogeophysics case study

ABSTRACT Over the last decade, distributed acoustic sensing (DAS) has become a popular tool for high-density seismic acquisition. DAS has been successfully applied to imaging and monitoring of a wide range of processes using surface and borehole fiber-optic arrays. However, its application in a crosswell setting has been extremely limited due to the high noise level observed in crosswell DAS data. In this study, one of the first successful crosswell DAS surveys was conducted using a custom piezoelectric source and a single-mode linear fiber-optic array, targeting hydrogeophysical attributes of a shallow site in Houston, TX. The use of a source with an improved low-frequency response enabled substantial improvements in signal-to-noise levels. The observed crosswell and single-well waveforms and amplitude responses were effectively captured using numerical models. Using a velocity model obtained through travel-time tomography, a regional aquifer was identified as a high-velocity zone, and this interpretation was confirmed using wireline well logs. Timelapse analysis demonstrated that the source was highly repeatable and stable over time, a key prerequisite for longer-term monitoring efforts. Because fiber is typically installed behind casing, unlike traditional seismic receivers, crosswell DAS only requires one source well to conduct a survey. These factors suggest that crosswell DAS has the potential to become a cost-effective and accurate tool for subsurface imaging and process monitoring.