Applications of Roll-Along Electrical Resistivity Surveying in Conjunction with Other Geophysical Methods for Engineering and Environmental Site Characterization
Roll-along electrical resistivity surveying was used with seismic refraction, magnetometer and gravity surveying in geophysical characterization of sites with a specific environmental or engineering problem. Three examples are presented where resistivity surveying provided vital constraints on acquisition and interpretation of other data in chaotic terrane.
A commercially resistivity meter was used with prototype equipment designed, assembled, and tested at Virginia Tech. The equipment included a multiconductor cable consisting of interchangeable segments and a circuit allowing selection of numerous electrode configurations.
The Sinking Creek Landfill, a 10-acre site, was used for disposal of municipal waste in the early 1970’s. Roll-along resistivity proved to be the most useful geophysical tool in ascertain its internal structure. Wenner configuration resistivity data, sensitive to both conductive leachate and ferrous metals, showed trenches within the landfill displayed in profile. Magnetic field measurements revealed anomalies over some trenches suggesting a method for discriminating between ferrous metal and leachate. Results of a resistivity survey can help planners of a refraction survey avoid low velocity “blind” layers.
The Mid County Landfill borrow area is a 26 acre site situated within the Max Meadows Breccia and used for cover material for an adjacent landfill, The engineering problems were to measure the volume of rippable material, but travel time data were somewhat ambiguous. The refraction data interpreted using a) conventional 3-layer analysis b) horizontal 3-layer analysis of single shots, and c)continuous velocity gradient analysis of single end shots were compared with auger refusal depth. The single end horizontal analysis matched auger refusal depths most closely. Roll-along resistivity pseudo-sections made along the refraction lines proved to be effective for qualitatively imaging pinnacles and megaclasts.
Excavation of fill material from a 75 acre river terrace in Pembroke exposed an antiform cut by high angle, near surface faults. Geophysical characterization was undertaken to determine the thickness of the alluvial deposit, and the relationship of the faults with structures in the underlying bedrock. Seismic refraction showed the terrace was as much as 134 feet thick. Resistivity pseudosections revealed a resistivity anomaly associated with the graben could be detected for a horizontal distance of several hundred feet. A gravity gradient paralleling the resistivity anomaly extends the feature more than 1000 ft from the exposed structure. Tenuous evidence of a bedrock escarpment beneath the near surface structure is found in a combination of seismic refraction, gravity, and electrical resistivity data.
Roll-along resistivity has proved to be key to geophysical interpretation of these three areas. Images displayed on pseudosections reveal lateral inhomogeneity more clearly than could be discerned from seismic, gravity and magnetic data. Roll-along resistivity data can provide information for efficient siting of additional geophysical studies.