Composition and Structure of the EGS Collab Test Bed 1 Based Upon Electrical Resistivity Tomography, Core Compositions, and Wireline Logging

The EGS Collab project drilled eight 60 m long boreholes at the Sanford Underground Research Facility (SURF) at a depth of 1.5 km to support rock fracturing and stimulation investigations. The test bed was in a complexly folded metasedimentary carbonate mica phyllite with a NW-SE structural grain. Monitoring included electrical resistivity tomography (ERT) that shows the presence of a well-defined electrical conductivity high that defines a fold structure occupying much of the test bed with an orientation similar to the local structure. The nature of this folded conductive unit was evaluated using a range of methods characterizing the rock and geophysical properties. These included core examination, high resolution XRF analyses on selected core intervals, and wireline geophysical logs. Electrical resistivity profiles from the wireline geophysical surveys are consistent with the electrical resistivity tomographic three-dimensional images, and the XRF profiles show elevated levels of sulfur in the vicinity of the interpreted conductivity anomalies. The compositional and wireline logging data are consistent with interpretations from the ERT data. Based upon these observations the best candidate for the source of the high electrical conductivities is the presence of significant amounts of sulfides within the unit making up the folds.