Vertical seismic profiling using distributed acoustic sensing in Weisweiler, Germany

The Weisweiler region in North Rhine-Westphalia, Germany, offers promising potential for geothermal energy use. The respective geology in the Carboniferous and Devonian is rather unexplored, so seismological and geological exploration of the region was started recently. To create a shallow seismic velocity profile with high depth resolution that can serve as a basis for further research, we used a fiber optic cable in a 500 m deep exploration well in the center of the area to perform a vertical seismic profiling (VSP) campaign.

A VSP campaign with four dropped weight shot points was carried out in which we recorded the deformation along the cable caused by the seismic waves with a distributed acoustic sensing (DAS) interrogator. With a spatial resolution of approximately 0.8 m along the cable, we were able to resolve a total of 618 depth-averaged measurement points over the 500 m borehole depth. The gauge length was set to 10 m.

Interval velocity profiles were determined by manually detecting wave arrivals. P wave velocities were compared to sonic log velocities. In some depth sections, shear wave arrivals could be identified and shear wave velocity profiles and v_P/v_S ratios could be derived.

We show the measurement setup and processing steps, as well as the processed data, and present the resulting velocity profiles in comparison to previously available data sets of the region. Here, the measurement methodology also reveals its limitations, as the strain per measurement point in DAS is measured over a depth range of one gauge length, which limits the depth resolution. Nevertheless, the results correspond very well with previously known geological models and also coincide with the sonic log, while supplementing previous findings with a S wave velocity profile and a v_P/v_S ratio.