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FACT SHEET - ELECTRICAL RESISTIVITY PROFILING RUDEN AS provides a wide range of geophysical services, mainly related to water, geological and environmental prospecting as well as mapping of natural resources. One of our main geophysical services is Electrical Resistivity Profiling (ERT). Overview Resistivity profiling method is a non-invasive tool for subsurface geological, geophysical and hydrological exploration. It is capable of depict the underground resistivity distribution in 2D 2.5D or 3D without the need for digging or drilling. The measured apparent resistivity of the subsurface formations is the combined value of the rock and the pore-filling material. Changes in the subsurface conditions can be seen if there is a contrast between these combined values (either because of the change in the pore-filling material or the host rock). During electrical resistivity profiling direct current is injected into the ground and the induced electric potential is measured by electrodes planted into the ground. In modern equipment that have switching capability the current and potential electrodes are chosen automatically from numerous preinstalled electrodes, accelerating the measuring procedure very effectively. Electrical resistivity imaging is a very effective tool for shallow to medium depth underground exploration in: Water prospecting o Determine groundwater level o Locate underground water aquifers o Locate underground formations that are suitable aquifers o Locate underground aquitard (clay) layers o Differentiate between fresh and salt water reservoirs o Determine fresh to salt water surface Environmental studies o Delineate contaminant plumes (e.g. oil spill or landfill leachate) o Assess quick clay hazard o Environmental baseline studies and monitoring of dams, landfills, mines o Monitoring changes in groundwater level and flow (induced by construction work or any kind of intervention to natural processes) Structural and lithological surveying o Determine depth to bedrock o Fault and dyke investigation o Exploration of sedimentary layering – differentiation by grain size Ore prospecting Cavity and cave exploration Inverted resistivity distribution of a 2D profile 2D resistivity measurement layout and electrode connection to the main cable RUDEN AS Moloveien 2. N-1628 Engelsviken NORWAY Inverted resistivity distribution of a 3D profile Tel: +47 69 36 41 41 Cell: +47 97 69 85 12 Email: [email protected] Methodology An electrical resistivity profiling survey is composed of sometimes several thousands of single measurements (called quadripoles). In a single quadripole 4 electrodes are used. 2 electrodes are used for injecting current into the ground while the other two are used to measure the induced voltage difference between them (in multichannel equipment several potential electrode pairs can be used simultaneously for potential measurements). electrodes. According to these variations the apparent resistivity values can be drawn into a pseudosection. Pseudosections give a quick glance how resistive the ground is, but has to be treated with suspicion as it is not the true resistivity. Design of a 3D resistivity survey (apparent resistivity will be measured for every red point) Quadripole Depending on the actual spacing and arrangement of the current and potential electrode pairs different types of arrays exist: Wenner Schlumberger Dipole-dipole Pole-dipole Pole-pole and many more The length of an electrical resistivity profile is not limited by the maximum electrodes that a certain equipment can handle at a same time. In various modern equipment the rolling technique is built in. At rolling the first set of electrodes (connected to the same cable) are moved to the end of the cable layout and the new measurement starts. However, only the new quadripoles – includes at least one electrode from the moved cable – will be measured. The rolling concept Electrode configuration of some array types The measured apparent resistivity is then the product of an array dependent parameter (k, geometric factor) and the measured resistance between the potential electrodes. This value is not the true underground resistivity, but the resistivity of a homogeneous half space that would give the same reading for the same array as the actual measurement. Measuring several quadripoles in a row gives many apparent resistivity values that correspond to different parts of the measured profile depending on the spacing between the RUDEN AS Moloveien 2. N-1628 Engelsviken NORWAY To obtain the true resistivity values the pseudosection has to be inverted. The inversion is an iterative process where a resistivity model of the subsurface is built up and its response to the measurement process is calculated. If the actual measurement values and the calculated model response are closer than a threshold value the iteration stops and the last resistivity model will be the result of the inversion. 2D inversion in the RES2DINV software Tel: +47 69 36 41 41 Cell: +47 97 69 85 12 Email: [email protected]