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

Abstract

An integrated geophysical approach involving seismic, electromagnetic, and electrical methods was employed to map fluvial, colluvial and bedrock geology, to delineate bedrock channels, and to determine fracture and joint orientations that may influence migration of petroleum hydrocarbons at the Glenrock Oil Seep. Both P (primary)-wave and S (shear)-wave seismic refraction techniques were used to map the bedrock surface topography, bedrock minima, stratigraphic boundaries, and possible structure. S-wave data were preferred because of better vertical resolution due to the combination of slower velocities and lower frequency wave train. Azimuthal resistivity/EP (induced polarization) and azimuthal electromagnetics were used to determine fracture orientations and groundwater flow directions. Terrain conductivity was used to map the fluvial sedimentary sequences (e.g., paleochannel and overbank deposits) in the North Platte River floodplain. Conductivity measurements were also used to estimate bedrock depth and to assist in the placement and recording parameters of the azimuthal soundings. The geophysical investigation indicated that groundwater flow pathways were controlled by the fluvial paleochannels and bedrock erosional features. Primary groundwater flow direction in the bedrock and colluvial sediments was determined from the azimuthal measurements and confirmed by drilling to be N20-40W along the measured strike of the bedrock and joint orientations. Joint/fracture orientationsmore » were measured at N20-40W and N10-30E from the azimuthal data and confirmed from measurements at a bedrock outcrop south of the site. The bedrock has an apparent N10E anisotropy in the seismic velocity profiles on the old refinery property that closely match that of measured joint/fracture orientations and may have a minor effect on groundwater flow.« less