What is Electrical Resistivity Tomography (ERT)?
Electrical Resistivity Tomography (ERT), also known as electrical resistivity imaging (ERI), is a geophysical method used to determine the electrical resistivity distribution of the subsurface. By measuring resistivity variations, it is possible to generate a detailed resistivity profile of the underground environment. This technique is widely used in geotechnical engineering, environmental and geological investigations due to its effectiveness in mapping subsurface materials.
ERT works by injecting an electrical current into the ground using a pair of current electrodes, while potential electrodes measure the resulting voltage difference. The resistivity values derived from these measurements offer insights into the subsurface electrical properties.
How Does Electrical Resistivity Tomography Work?
The fundamental principle of ERT is based on Ohm’s Law, which relates electrical current, voltage, and resistance. Current electrodes inject the electric current into the ground, while potential electrodes record the voltage response at various locations. By measuring variations in subsurface resistivity, the method can detect low resistivity zones, such as clay layers, and high resistivity zones, such as dry sandy soil or most rock types.
The data collected is processed to produce a two-dimensional or three-dimensional resistivity map of the subsurface. Advanced data inversion techniques are employed to convert raw data into useful resistivity models, revealing the subsurface structure and electrical properties of materials.
Common Applications of ERT in Geophysical Surveys
image source: https://en.wikipedia.org/wiki/Electrical_resistivity_tomography#/media/File:2D_inversion_of_electrical_resistivity_data.png
The versatility of ERT makes it an useful method for subsurface characterisation across various industries. In mineral exploration, for example, ERT can be used to map subsurface resistivity and identify mineral deposits. In environmental investigations, it is used for locating contamination plumes and mapping the extents of landfill. It often works best when there is a strong contrast between the electrical properties of the subsurface material of interest at the site, e.g. where electrically conductive clays overlie electrically resistive bedrock or where an electrically conductive contaminant flows through an electrically resistive aquifer material.
Equipment and Setup for ERT Surveys
Electrical Resistivity Tomography (ERT) surveys require specialised equipment, including a resistivity meter, electrodes, and connecting cables. Electrodes are arranged in specific electrode array configurations, such as Wenner, Schlumberger, dipole-dipole, and pole-dipole, each offering distinct advantages depending on the survey’s objectives. The electrodes inject an electrical current into the ground, and the potential electrodes measure the resulting voltage differences. The spacing of the electrodes and overall array length plays a critical role in determining the spatial resolution of the survey and maximum investigation depth, respectively. With larger arrays and wider spacings utilised for deeper investigations and tighter electrode spacings for high resolution, shallow investigations. These measurements are then used to calculate the apparent resistivity section. The apparent resistivities are then used to determine the modelled “true” resistivity section by an iterative mathematical algorithm.
In environments where electrode contact may be poor, materials like water, saltwater or bentonite clay are often used to enhance electrical conductivity and improve the accuracy of resistivity measurements.
Advantages of Electrical Resistivity Tomography
ERT’s key advantages are is its non-invasive nature, relatively high rate of field production and largely automated data processing software. ERT can provide detailed subsurface information over large areas with minimal disruption to the site, in a reasonable timeframe. The method’s ability to produce high-resolution resistivity profiles enables geophysicists to create realistic models of subsurface conditions, aiding in everything from geotechnical engineering to environmental investigations.
Challenges and Limitations of ERT
Despite its many advantages, ERT does have some limitations. It is sensitive to noise from external sources, such as metallic structures, steel fences or buried cables, which can interfere with resistivity readings. Highly resistive ground or dry soil may also pose challenges, limiting current penetration and affecting data accuracy.
The method relies on a measurable electrical contrast between the materials of interest of the site. This is not always the case. It follows that the suitability of ERT for a given project should be assessed by an experienced Geophysicist on a site-by-site basis.
The interpretation of resistivity measurements often requires the use of inversion algorithms to model subsurface features. Without adequate supporting data, such as borehole logs or other intrusive tests, resistivity models can be ambiguous, especially in areas with complex subsurface conditions.
Innovative Uses and Future Prospects
As ERT technology advances, its applications continue to expand. Time-lapse ERT imaging, for example, is being used to detect reservoir leaks. Techniques like cross-hole ERT, where electrodes are placed in boreholes, offer higher resolution at greater depths.
Conclusion
Electrical Resistivity Tomography (ERT) is a key geophysical method for understanding subsurface conditions, providing valuable insights for various sectors. Integrated into our geophysical consulting services, ERT supports consultants, engineers, and project managers in making informed decisions. With the coexpertise of our delivery partners, Geophysicists, geotechnical engineers and environmental scientists, we deliver reliable data and interpretations to address complex site challenges efficiently.
