ERT & IP

Measuring the subsurface resistivity distribution is one of the most sophisticated geophysical technologies and can be used for various applications using today's advanced and robust Electrical Resistivity Tomography (ERT) systems.

During an ERT survey an electrical current is injected into the ground through steel electrodes while the created potential field is sampled via several potential electrodes in parallel. The method's depth of investigation is defined by the maximum distance between the current (C) and potential (P) electrodes, while the resolution is defined by the electrode spacing. A general estimate for depth of investigation is 10-20% of the array length depending on the Earth resistivity structure.

NGI's state-of-the-art expertise

High resolution quick clay mapping
Mapping of sediment thickness (bedrock depth) by integrating ERT and sparse conventional soundings.
Rock quality mapping
Integration of ERT results with field- and lab-data

As a rule, the greater the porosity and concentration of saline elements in the pore water will result in a rock or sediment with greater conductivity. There is no general correlation of the lithology with resistivity, but a broad classification is possible. Moraine sediments (gravel, sand, tills) are resistive to poorly conductive (50-10000 ohm-m) while clays are highly conductive (5-100 ohm-m). In sedimentary areas, conductivity depends on clay content, porosity, dissolved mineral content, and water saturation. ERT is in most cases successful in imaging basement topography due to the good resistivity contrasts between sediments and bedrocks. Leaching in marine clay reduces the salt content in the pore water and therefore increases its electrical resistivity. ERT can then image pockets of quick clay in intact marine clay thanks to the resistivity contrast between the two (see examples below).

Induced polarization (IP) is a phenomenon known from mineral exploration. The current in the ground (introduced by the ERT/IP system) charges materials in the pore spaces (sulfides, clay, hydrocarbons ...) and thus the measured voltage does not drop to zero right after switching off the electrode current. The voltage decay after turn-off contains information about these chargeable materials and can be interpreted e.g. towards hydrocarbon contamination / accumulation.

Equipment at NGI
NGI has recently acquired a combined Earth Resistivity Tomography and time domain Induced Polarization 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

Assess the efficiency of detailed 3D ERT surveys.
Improve NGI's toolbox for geo-integration of ERT, drill-holes, laboratory data, R-CPT, seismic etc.
Visualization of 3D data
Advanced IP acquisition & processing for environmental mapping

Relevant Equipment and Software