Of all the geophysical services that HGI provides, electrical resistivity profiling is conducted most often and in many scenarios to solve a wide variety of problems. We have used this geophysical survey method to map the geology associated with underground mine explosions, define depth to bedrock, localize contaminant sources, aid in mineral extraction, find open voids and karstic features, find leaking containers, and map weaknesses in underground tunnels. Resistivity profiling is easily one of the most versatile geophysical survey methods, with substantial case studies and validation to reduce ambiguity and uncertainty in its interpretation.
Resistivity profiling is easily one of the most versatile geophysical survey methods.
Electrical resistivity profiling is simply a two-dimensional resistivity method, where an image representing a slice of the earth is created and electrical property contrasts are used to interpret differing geological or hydrogeological conditions. We use color contouring to visually identify these conditions. Two-dimensional resistivity profiling can also be used to increase understanding of borehole data or help locate a new borehole in electrically unusual areas by spatially extending information away from the hole.
HGI has a significant number of validated case studies, greatly reducing ambiguity and uncertainty when interpreting the meaning behind the data.
The figure below shows an example of geoelectrical profiling at the Hanford Site in eastern Washington. The profile was acquired over an historical disposal trench, where about one million gallons of nitrate waste, laden with radioactive elements, was disposed to the ground. The data were collected using the SuperSting R8 with a pole-pole array and 6 foot electrode separation, and were modeled with RES2DINVx64. The data show a strong correlation between the resistivity low and nitrate high from well C4191, at depths ranging from 50 to 150 feet below ground surface.
The successful application of resistivity profiling depends on the ease by which the data can be collected, the magnitude of contrasts in electrical properties, the depth to the target, and the size of the target. Surface conditions that are free of vegetation, in remote areas, and with flat terrain make ideal candidates for acquiring resistivity data; however, HGI has collected data in dense forests with steep terrain, and in the midst of urbanization with great success. The sensitivity and resolution of the resistivity profile decreases with depth and deeper targets need to be larger in order to be accurately resolved. The example below shows different sized targets at different depths and their relative resolution. In the figure, the conductive target at 750m along the line shows up at approximately 150ft deep and is likely due to a clay-gouged fault. One of the great advantages of this type of geophysical survey is the nonintrusive nature of the methodology. In the example below all data were acquired from the surface, and a significant amount of detail is realized before any drilling commences.