3D Imaging from Vertical DAS Fiber at Brady’s Natural Laboratory

Abstract

Faults play an important role in geothermal fluid transport and can also present a contrast in acoustic impedance such that seismic methods can approximately locate their presence in the subsurface. In March 2016, a continuous active seismic survey collected 184 three-component shots while a vertical DAS cable was in place 150 to 280 meters below surface. First forward modeling was performed to understand the volume of illumination given the location of the vertical DAS cable and the geometry of the vibe shot. Reverse time migration (RTM), a prestack depth migration was used. The modeling domain is 1560m by 915m by 1500m in x, y and z respectively. Two horizontal reflectors that extended through the entire modeling domain at depths 700m and 1,100m respectively were used demonstrate how much of the image could be accounted for the illumination and not migration artifacts, using the most current velocity model for the area (Matzel et al., 2017). It was confirmed given approximately 30 meters of reliable fiber in the well and the geometry of the vibe shots, a 300 by 100 by 500m (depth) volume can successfully recover the 2 horizontal reflectors. Next, imaging of the current fault model of Brady’s (Siler and Faulds, 2013) was performed to analyze if certain fault dips and strikes would be detectable given the shot geometry. The final reflectivity model indicated faults passing through the well and as far away as 1km could be imaged with well-processed data, assuming the velocity model well represents the true subsurface. Finally, the real data was migrated to analyze the resolving power of the DAS cable. Some shots present stronger P to S reflections than P-P . Thus, a P to S migration was performed additionally using the Vp for the downgoing waves and Vs for the upgoing.