DAS and its Coupling for VSP Applications Using Wireline Cable

Distributed acoustic sensing (DAS) acquisitions in borehole seismic surveys are limited by coupling issues when the fibre is deployed using wireline cable. This deployment method is very cost-effective because the sensing cable is simply loosely lowered down the borehole, but data is usually acquired with a low signal-to-noise ratio (SNR). The motivation of this work is to understand the circumstances that enable good coupling and therefore data acquisition with high SNR. We first analyse DAS datasets of two field trials where lowering more cable down the borehole led to an increase in SNR starting from the bottom of the well. Next, we use numerical methods to analyse the movement of the cable during this deployment method and show that lowering more cable down the borehole evolves as helical bending of the cable. Because of this bending, the cable comes into direct contact with the borehole wall, which appears to be critical for acquiring high-quality data. Furthermore, our numerical analysis enables quantifying the amount of extra cable required to achieve clear signal detection throughout the well.