Electrical Resistivity Tomography (ERT)

About Electrical Resistivity Tomography (ERT)

The Electrical Resistivity Tomography (ERT) method is one of the longest-established electrical resistivity methods for geophysical investigations. The basis of the method is that current is injected into the ground through electrodes, a resulting potential is measured which allows for the calculation of resistance and hence resistivity.

Variations in electrical resistivity typically correlate with variations in lithology, water saturation, fluid conductivity, porosity and permeability, which may be used to map stratigraphic units, geological structures, sinkholes, fractures and groundwater.

The ERT method is often used in conjunction with other reconnaissance geophysical methods, such as electromagnetic mapping or ground-penetrating radar (GPR), to help further constrain any anomalous areas. It also complements other profile mapping techniques such as seismic refraction and MASW.

Applications of Electrical Resistivity Tomography (ERT)

ERT is a very versatile geophysical method with many potential applications. In general, ERT is used to map variations in subsurface resistivity measurements, which in turn can be related to changes in subsurface lithology, water saturation, fluid conductivity, porosity and permeability.

The main applications of ERT are in engineering and environmental geophysics, although the method is also used in hydrogeology, archaeology, mineral exploration and crime scene investigation. Common applications for resistivity data include:

Identifying areas of potential instability or slope failure
Mapping the depth to bedrock
Delineating aquifers and aquitards
Detecting leaks from landfill sites and hazardous waste disposal sites
Mapping the extent of contamination plumes in soil and groundwater
Locating buried channels, dikes, or other structures that may impede groundwater flow
Locating areas of potential sinkhole collapse
Studying the effects of dewatering or other fluid injection/extraction processes on subsurface fractures

Benefits of Electrical Resistivity Tomography (ERT)

There are many benefits to using ERT over other geophysical methods. These include:

Non-invasive - ERT is a completely non-destructive method that has no effect on the environment.
Repeatable - Resistivity measurements can be made at any time and under any conditions, making it ideal for long-term monitoring applications.
Versatile - ERT can be used in a wide range of different environments, from deep mines to shallow wetlands.
Flexible - ERT surveys can be tailored to meet the specific needs of the project. For example, the electrode spacing and array configuration can be varied to achieve the desired depth of investigation and resolution.
Cost-effective ERT is a relatively low-cost geophysical method, especially when compared to invasive methods.

Performing an Electrical Resistivity Tomography Survey

Resistivity survey imaging involves measuring a series of readings along a profile with the current electrodes at a set spacing. The resistivity data can be collected using a variety of electrode array configurations, the choice being selected by the geophysicist on-site to meet the objectives of the survey.

Electrical resistivity imaging provides a complete data set from very close to the surface to a considerable depth. The depth of investigation and the resolution achieved are dependent on the electrode spacing and electrode configuration used. Longer spreads will achieve a greater depth, trading resolution to do so.

The apparent resistivity values are affected by a number of factors and it is important, therefore, that the geophysicist considers all relevant information when interpreting resistivity data. Different materials have different resistivity values, which means that the interpretation of ERT profiles requires an understanding of the geology and hydrogeology in the area under investigation.

High resistivity values indicate a low conductivity and are usually associated with materials such as air, sand or gravel. A low resistivity value indicates a high conductivity and is usually associated with materials such as clay or water.

Electrical Resistivity Tomography (ERT) FAQs

What does ERT measure?

ERT determines the electrical resistivity of material through which an electric current is passed. The resistivity is a measure of the material's ability to oppose the flow of electric current.

How does ERT work?

When a current is passed through the ground via electrodes inserted in the ground, variations in sub-surface resistivity will alter the flow of current within the ground. This will affect the distribution of electric potential at the surface, which can be measured by the electrodes.

Is ERT the only electrical method?

No. There are a number of different electrical methods, each with its own advantages and disadvantages. ERT is just one of many electrical methods that can be used to investigate the subsurface electrical properties. For example, electromagnetic mapping (EM) is another electrical method that uses electromagnetic waves instead of direct current to measure sub-surface resistivity.

Vertical Electrical Sounding (VES) is a geophysical method used to investigate the subsurface electrical resistivity structure in one dimension (1D). It involves injecting an electrical current into the ground through two current electrodes and measuring the resulting voltage difference between two potential electrodes.

How is ERT data collected?

Data is collected by measuring a series of readings along a profile with current electrodes at a set spacing. The selected electrode spread is interfaced to a computer-controlled multi-core cable system connected to a resistivity meter.