Prior to construction and infrastructure projects, it may be necessary to characterise landscape features to assess their suitability for design purposes. Resistivity imaging is often employed due to its ability to penetrate up to depths of 30m+.
Resistivity Imaging is often used to:
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Measure bedrock & water table depths
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Detect sinkholes & hidden voids
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Map buried alluvial channels
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Profile landslip geometry
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Characterise fracture zones & discontinuities
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Define landfill sites and leachate contamination
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Locate abandoned mineshafts and mine workings
The Process:
Resistivity imaging techniques employ a line of electrodes temporarily inserted in the topsoil through which electrical current is made to flow. The voltage is converted into a resistivity value. This represents average ground resistivity between the electrodes. Different materials have different resistances, which are linked to moisture content, or porosity. Hard, dense features such as rock will give a relatively high resistance while features such as ditches, which retain moisture, give a lower response. This means that bedrock, fissures and other geological features can be mapped.
Above : Resistivity imaging survey sampling sequence.
Depth probes provide models of vertical variations in ground resistivity using an expanding electrode array offset from a central reference point. Depth penetration increases with wider electrode separation. This provides a one dimensional layered resistivity model. Composite sections are produced by interpolating between depth probes at regular intervals along a survey line. Electrical Resistivity omography (ERT) (also known as Resistivity Imaging), is an advanced development of the method. Enhanced data quality and resolution provide continuous two-dimensional resistivity models. Fifty or more electrodes are set out in an evenly spaced array, and connected to a computer-controlled resistivity meter.
Unit electrode spacing is determined by parameters that include profile length, desired resolution and targeted depth penetration. A switching unit takes a series of constant separation readings along the length of the electrode array. The separation between sampled electrodes is then widened to increase the effective depth penetration and the procedure is repeated, as shown above. Readings are taken down to 20 levels of increasing depth range. This means that we can assess larger deeper and shallower smaller features.
Above : Assessing slope stability by resistivity imaging.
The Deliverable:
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Advanced data processing using specialist inversion software removes distortions caused by the effects of electrode geometry. It produces a high-resolution image of the variations in ground resistivity with depth. The model is contoured using a colour scale to produce a two-dimensional cross-sectional model of ground resistivity. Our geophysicists use the final resistivity imaging section to provide clients with a detailed interpretation of the ground conditions on site.
Above : Topographically corrected Resistivity Imaging over long barrow.
Survey Methodologies
Detailed Magnetic Survey
Soil Self-Potential
Electrical Resistivity Tomography (ERT)
Soil Thermal-Resistivity Testing
Electromagnetic Ground Conductivity
Electromagnetic Location (EML)
Ground Penetrating Radar (GPR)
