Surface geophysical methods were found to be valuable for refining knowledge of the geohydrology of Melton Valley, an area used for burial of low-level radioactive waste at the Oak Ridge Reservation in Tennessee. The valley is characterized by locally complex geologic structures in lithologies of interbedded shale and limestone. Radionuclides have been transported away from the burial areas by groundwater along paths that are, in part, controlled by geologic structures and rock type. Direct current resistivity soundings were used to determine the depth to bedrock and to aid in the delineation of subsurface stratigraphy. Depth to bedrock, as indicated by an increase in resistivity in the interpreted geoelectric layering, was compared to auger-hole data at five sites, where bedrock depths ranged from 1 to 14 ft and were within 3 ft at four sites. The subsurface contact between shale and limestone was indicated by an increase in model calculated resistivity from < 100 to > 150 ohm-meters at four different sites. Terrain conductivity profiles were used to aid in mapping surficial geologic contacts between shale and limestone units. Terrain conductivity for shale at 33-ft coil separation was generally greater than 15 millimhos/m and ranged from about 10 to 40 millimhos/m. Conductivity for limestone was generally < 15 millimhos/m and ranged from 5 to 25 millimhos/m. Azimuthal conductivity surveys indicated that conductivity was generally greatest when the transmitter and receiver coils were oriented parallel to strike. This anisotropy in terrain conductivity shows the need to keep the coil orientation consistent throughout the length of a profile. (Author 's abstract)
