Mapping A Near Surface Variable Geologic Regime Using An Integrated Geophysical Approach

An integrated geophysical approach involving seismic, electromagnetic, and electrical methods was employed to<br>map fluvial, colluvial and bedrock geology, to delineate bedrock channels, and to determine fracture and joint<br>orientations that may influence migration of petroleum hydrocarbons at the Glenrock Oil Seep. Both P (primary)-<br>wave and S (shear)-wave seismic refraction techniques were used to map the bedrock surface topography, bedrock<br>minima, stratigraphic boundaries, and possible structure. S-wave data were preferred because of better vertical<br>resolution due to the combination of slower velocities and lower frequency wave train. Azimuthal resistivity/IP<br>(induced polarization) and azimuthal electromagnetics were used to determine fracture orientations and<br>groundwater flow directions. Terrain conductivity was used to map the fluvial sedimentary sequences (e.g.,<br>paleochannel and overbank deposits) in the North Platte River floodplain. Conductivity measurements were also<br>used to estimate bedrock depth and to assist in the placement and recording parameters of the azimuthal<br>soundings.<br>The geophysical investigation indicated that groundwater flow pathways were controlled by the fluvial<br>paleochannels and bedrock erosional features. Primary groundwater flow direction in the bedrock and collwial<br>sediments was determined from the azimuthal measurements and confirmed by drilling to be N20-4OW along the<br>measured strike of the bedrock and joint orientations. Joint/fracture orientations were measured at N20-4OW and NlO-<br>30E from the azimuthal data and confirmed from measurements at a bedrock outcrop south of the site. The bedrock has<br>an apparent NlOE anisotropy in the seismic velocity profiles on the old refinery property that closely match that of<br>measured joint/fracture orientations and may have a minor effect on groundwater flow.