By Moira Poje – hydroGEOPHYSICS Geoscientist
From site planning and geologic mapping to environmental/metallurgical characterization and monitoring, industries need flexible technologies to obtain information about the subsurface. Like a subsurface “camera,” electrical resistivity is a geophysical technique that captures an image of the subsurface without excavation or boring. This article briefly describes the electrical resistivity method and discusses how hydroGEOPHYSICS, Inc. (HGI) has applied electrical resistivity to environmental, mining, and engineering problem solving.
Electrical resistivity can be used to address industry needs with non-invasive imaging that provides a “snapshot” of the subsurface.
Electrical resistivity can help identify subsurface targets when a surficial image isn’t enough or when broad site information needs to be obtained on a budget. Secondary drilling or sampling can then be more accurately sited and used to verify and extrapolate information across an investigation site. Resistivity mapping has spatial, temporal and environmental flexibility, so the data collection can be customized to fit the needs of many projects.
As a general example for how the method works – when fluid is present in the ground, there is a contrast in electrical properties between the native material and the intruding fluid. Electrical resistivity measurements can detect these contrasts and identify hydrologic anomalies in the subsurface. Below you can see an example where various targets in a heap leach pad were identified by a single resistivity survey:
Method Acquisition and Display
A resistivity survey consists of a combined transmitter and receiver computer, a passive multi-core cable and electrodes to make contact with the ground. These electrodes can either be made of porous ceramic pots or steel stakes. They can be deployed in a variety of configurations: across the surface of a site, down boreholes, or distributed across a body of solution. The electrode setup and data collection sequence is optimized for each survey depending on the target of the investigation.
To capture the resistivity image, electric current flows into the ground via a pair of electrodes (transmitting dipole) and is measured at a second pair of electrodes (receiving dipole) at some distance away from the first pair. The received voltage is a measure of the electric potential (voltage) of the subsurface at depth. Data are collected by the transmitter and receiver computer, which is programmed to switch between electrode pairs automatically. The method is safe and requires minimal disruption to daily activity of active work sites.
The distance between the transmitting dipole and receiving dipole determine the depth of data collection, such that greater distance yields a deeper geophysical map of the subsurface. The survey design and viewing depth are tailored to meet the needs of the site investigation.
In the photo above, you can see a standard resistivity setup with the SuperSting R8 system (Advanced Geosciences, Inc.).
For the final presentation, the voltage measurements are inverted and color contoured into earth snapshots to interpret different hydrogeological conditions at depth. Typically, electrodes are deployed along a transect ranging from a few meters to kilometers in length, such that a two-dimensional cross-section of resistivity can be interpreted, though 3D surveys and resulting imagery are also an option.
Surveys can also be collected as one-dimensional vertical electrical soundings (VES) – see below for an 11-mile cross section (18,000 m) through a basin in the southwestern United States, collected by hydroGEOPHYSICS with the VES technique and presented as a two-dimensional profile. The data can be used to identify geologic structures such as fault blocks, bedrock, and clay lenses.
Surface conditions that are free of vegetation, in remote areas, and with flat terrain make ideal candidates for acquiring resistivity data; however, data can also be acquired in dense forests, steep terrain, or in the midst of urbanization with great success. Limitations on depth of investigation for resistivity mapping can be related to cultural noise in the area, electrode spacing, high subsurface resistivity, and total transect length. Resolving limitations often falls into the time and budget categories, but intelligent survey design will mitigate most constraints.
The successful application of resistivity profiling can be affected by the ease of data collection, magnitude of contrasts in electrical properties, depth to the target, and the size of the target.
During secondary resource extraction on mine sites, high pressure injection can be used to recover the remaining resources in previously extracted ore. The fluid is injected at high pressure and high flow rate, but without tracers to determine the location of the fluid. An electrical resistivity survey can characterize the movement of this plume of fluid in four dimensions. The results of the resistivity mapping can be used to identify future targets and evaluate the extent of fluid movement throughout the crushed ore. When surveys are monitored in time, a series of crosscutting transects can be inverted in three dimensions. These three dimensional geophysical “snapshots” can be displayed as a time-lapse sequence to track the fluid flow through a medium, like the below linked animation of a sulfuric acid injection into a copper oxide ore body that HGI monitored using electrical resistivity geophysics:
The robust design of a resistivity system services a diversity of projects, from monitoring dynamic environments, to detecting leaks in HDPE liners, to tracking environmental contaminants…
HGI also successfully uses the electrical resistivity method for leak detection of HDPE liners in ponds, landfills, and heap leach pads, whether bare, double- , or single-lined. The flexible geophysical survey design with resistivity mapping accommodates liners filled with consolidated rock, unconsolidated soils, or solutions with varying pH and conductivity. Pond liner leak detection is based on the principle that electric current flows more readily through liner leaks rather than the HDPE liner. Additionally, the method allows for leak detection without the survey personnel ever entering the pond itself. In the map below you can see a birds-eye view of leaks located in a pond liner displayed as electrical resistivity contours.
On a short or long term scale, resistivity mapping can aid in tracking contaminants as they spread through an environment. Identifying contaminant location and movement can assist in environmental remediation planning and clean up. HydroGEOPHYSICS used the electrical resistivity method to survey an old mine to assess the integrity of the fill cover over the mining area. A sample of results from that survey is shown below:
HGI participated in a project to support Panama Canal expansion; electrical resistivity data were collected by boat (floating array) to characterize material at the bottom of the canal. The electrical resistivity method helped differentiate the unconsolidated material from bedrock. Identifying material properties can help reduce excavation costs by providing key information to select the proper canal excavation method (ex. dredging or blasting) for the rock type at each location. The resistivity survey results correlate well with an ancient river channel that flowed across the current location of the Panama Canal. The river channel maps as more resistive (warmer colors) compared to the harder bedrock material.
The simple and robust design of the resistivity system makes it an essential tool for addressing many subsurface challenges. By capturing images of the ground beneath your feet, electrical resistivity becomes a valuable target recognition tool. To read about more applications of the electrical resistivity method, visit HGI’s website.
About the Author: Moira Poje | hydroGEOPHYSICS
Moira Poje is a Staff Geoscientist with hydroGEOPHYSICS, Inc. She works on contaminant delineation, void detection projects for mine reclamation, subsurface fluid flow mapping, and depth-to-bedrock delineation via seismic, gravity, and electrical methods.
Moira has presented at the AGU Annual Meeting, Arizona Hydrological Society Annual Meeting, CUR REU symposium, and published in EEGS’s magazine FastTIMES. Moira serves on the Board of Directors for the Environmental and Engineering Geophysical Society (EEGS). Moira holds a B.A. in Geophysics and Planetary Science from Boston University.
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