Time-Lapse Seismic Imaging of Hydraulic Fracturing at the Sanford Underground Research Facility (SURF)

Time-lapse seismic imaging was performed using cross hole seismic tomography during hydraulic fracturing experiments on the 4850 Level (1.5 kilometers below the surface) of the Sanford Underground Research Facility (SURF) in Lead, South Dakota, USA. The test site, part of the permeability [k] and Induced Seismicity Management for Energy Technologies (KISMET) project, consisted of a central borehole used for fracture stimulation with seismic monitoring equipment installed in adjacent boreholes. Seismic data were analyzed for P- and S-wave travel-time variations smaller than the original sampling rate of the equipment using seismogram interpolation. Travel-time variations were observed for both P- and S-waves, and seismic tomograms prepared from the cross-hole data correlated well with pressure cycles during fracture stimulation. Time-lapse tomographic models successfully delineated the fracture imaged at 1 meter below the stimulation zone. Time-lapse tomograms displayed two notable trends that were used to infer fracturing-related processes when compared with interval pressure data from the stimulation zone: (1) a transition from fracture growth (propagation) to stabilization (open and closing) and (2) possible permanent deformation. Results from this study are applicable to other Enhanced Geothermal System (EGS) fracture stimulation projects as well as subsequent work conducted on the 4100 Level of SURF.