Electrical Resistivity Tomography

Yes, it’s a mouthful! Electrical Resistivity Tomography (ERT) is a non-evasive geophysical method of exploring what is underground. A physical characteristic of any material is conductivity or how easily electricity flows through it. Resistivity is conductivity’s opposite, meaning the degree to which it resists the flow of electrical currents. A good conductor of electricity, like metals such as copper or pyrite, have a low resistivity. But a poor conductor, such as silica sand or limestone, resists the electricity. ERT uses a series of electrodes to send electrical currents through the earth and records how that current passes through the materials in the ground.  

ERT is by no means a new technology, with origins dating back to the early 1900’s. Historically, a four-electrode array was utilized for ERT surveys. The process was extremely slow, labor intensive, and provided only one-dimensional data points. By the 1980’s, multi-electrode arrays were being developed. Then by the early 1990’s, those multi-electrode arrays were paired with computer automation of data collection. Coupled with inversion modeling software, ERT surveys became more efficient than ever, allowing for 2D and even 3D analysis. 

ERT has archeological, environmental, and geotechnical applications in addition to mineral and aggregate exploration. It is used to locate groundwater, identify leaking dams, detect caves and sinkholes, and has multiple marine applications. Westward utilizes this technology to evaluate potential aggregate reserves, identify mining obstacles to assist in mine planning, to isolate high probability drilling targets for in-depth exploration, estimate overburden thicknesses and delineate saline/fresh groundwater interfaces. ERT surveys are particularly helpful in areas with significant faulting in hard rock and in sand and gravel deposits from braided river systems.

Our geology team at Westward has used ERT in several locations to evaluate potential sand and gravel reserves under windrows from historic mining operations, locate caprock on a salt dome, and define formation contacts in a quartzite and nepheline syenite.

Related to the above graphic, Westward was able to distinguish between areas that had been previously mined, then backfilled, from areas that had been mined and backfilled but had additional mineable material under the fill.  The image above is the result of a 2D survey spanning about a 400-foot transect with imaging depth to 100-feet below the surface of the earth. Four distinct layers are visible: blues represent more conductive materials (clays) while greens, yellows and reds represent more resistive materials (sand and gravel). At the surface, the soil horizon can be seen as a thin blue line ranging from a few inches to a few feet thick. Below it is the target sand and gravel unit in the greens, yellows, and reds. And lastly, the image shows a thick clay layer up to 50-feet thick, which is sitting on top of a bedrock/caliche deposit of weathered limestone.

Rivers can be strong erosional forces over time (think Grand Canyon). As they flow, they slowly chip away at the earth around them. At one point in time, the river in the area shown in the profile below flowed much slower in the green area, depositing more clay in the sand and gravel deposit section.  In contrast, the red area shows a zone of cleaner sand and gravel, which has a higher flow velocity. The blue represents clay materials and is defined here as overburden at the top, and the lower confining unit on the bottom.   

With recent advances in ERT, the technology has become incredibly useful for exploring and assessing what is below the surface of the earth without clearing, digging or drilling. It saves money by indicating where the deposit is, or isn’t, which allows for more precise targeting of drilling locations. It provides critical data for ongoing and future projects, whether it be exploration, development or remediation efforts.  Contact us at general@westwardenv.com if you have any questions or would like to learn more.