What is ERT?
Electrical Resistivity Tomography (ERT) is a near-surface geophysical method that uses direct current to measure the earth's resistivity. The current is injected into the subsurface through steel electrodes installed 10-20 cm into the ground (marked C in the illustration), and the apparent resistivity distribution along a profile or area is measured (electrodes marked P). Using data processing and inverse modelling a two- or three-dimensional resistivity model of the subsurface can be derived.
From resistivity to underground structure
Subsurface resistivity is highly varying and is mainly governed by the sediments and rock types (lithology) in the area. ERT thus enables us to distinguish different materials, ranging from low resistivity clay to high resistivity hard rock. Chargeability, inferred from Induced Polarization (IP) measurements, is a complimentary physical parameter, which is useful for brownfield investigations, mineral prospection or groundwater-related issues.
A general estimate for depth of investigation is 10-20% of the array length, depending on the Earth resistivity structure. Our current instrument is capable of reaching a maximum depth of 70 m.
- Bedrock depth/sediment mapping: In combination with boreholes or geotechnical logs (CPT, total soundings, RPS) the depth to bedrock can be mapped continuously and sedimentary infill distinguished.
- Quick clay: The salt content and thereby the conductivity of quick clay is normally lower than for non-sensitive clay. ERT can thus distinguish non-sensitive clay from sensitive clay.
- Geo-hazards/rock quality: Areas prone to rock slides are characterized by zones of fractured and incompetent rock. These zones are mostly clay and/or water-filled pockets which makes them good targets for ERT surveys. The same applies to rock quality in general.
- Archaeological artefacts: Old structures of timber, stones or bricks appear as small scale resistivity anomalies and can be distinguished if buried in highly conducting sediments, such as clay.
- Environmental risk assessment: With ERT, landfills can be mapped and delineated. In combination with IP, brownfield investigations can be conducted to characterize the subsurface contamination and its extent.
Geological materials exhibit a wide resistivity range, even within the same lithology. Therefore, a thorough interpretation is crucial and experienced geophysicists and geologists are needed to deliver high-quality results.
- Noise-free and non-invasive: Digging, large-scale drilling or blasting are not required. Even indoor surveys can be performed.
- Very versatile: All walkable terrain can be assessed with ERT, and measurements can be performed on almost all types of ground and in almost all weather conditions.
- Economic and efficient: 400 m to 1000 m can be conducted per day, depending on resolution, topography and site conditions.
- Rapid and robust processing: Preliminary results can be assessed in the field.
- Detailed: As opposed to point-source information such as drillings, ERT provides continuous and detailed lateral information.
Despite its many applications, ERT cannot completely substitute traditional geotechnical investigations. The data processing algorithms tend to poorly image sharp geological interfaces. Nevertheless, an ERT-inferred interface can be calibrated with just a few boreholes, and the total amount of required boreholes can be drastically reduced.
One should be aware that mapping is based solely on physical properties, and materials of similar resistivity (e.g. clay and shale) will be poorly resolved. In case the subsurface and geology have an especially complex geometry, information based on one single profile may not be sufficient, and a grid of profiles or 3D surveying is recommended. Snow and ice are highly resistive. For this reason, it is in general recommended to undertake ERT fieldwork during warmer periods.
Equipment at NGI
NGI uses a combined ERT and time-domain IP system, an ABEM Terrameter LS (4 x 20 electrode cables, max array length 400m with 12-channel recording unit acquiring resistivity, IP windows and raw time series for advanced IP processing). It is robust, waterproof and designed for reliable operation in harsh environments.
Research and Development
- Derive rock quality from ERT data
- Improve NGI's toolbox for geo-integration of ERT with other types of geophysics (AEM, GPR, seismic), as well as drill-holes and laboratory data
- Inversion and visualization of 3D data
- Advanced IP acquisition & processing for environmental mapping
Relevant Equipment and Software
ERT Related Services
- ERT data acquisition (down to 70m depth), processing and interpretation
- Feasibility studies and consulting for end users
- Integrating geophysical and geotechnical data