Prior to construction and infrastructure projects, it may be necessary to characterise landscape features to assess their suitability for design purposes. Resistivity imaging if often employed due to its ability to penetrate up to depths of 30m+.
Resistivity Imaging is often use to:
Measure bedrock & water table depths
Detect sinkholes & hidden voids
Map buried alluvial channels
Profile landslip geometry
Characterise fracture zones & discontinuities
Define landfill sites and leachate contamination
Locate abandoned mineshafts and mine workings
The resistivity imaging technique employs a line of metal stakes (electrodes) temporarily inserted in the topsoil through which electrical current is made to flow. The voltage is converted into a resistivity value representing average ground resistivity between the electrodes. Different materials have different resistances which are linked to moisture content, and therefore porosity. Hard, dense features such as rock will give a relatively high resistance while features such as a ditch, which retain moisture, give a relatively low response. This means that bedrock, fissures and other geological trends can be mapped.
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, providing a one dimensional layered resistivity model. Composite sections are produced by interpolating between depth probes at regular intervals along a survey line. Resistivity Imaging also known as resistivity tomography, 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 a regularly spaced array, connected to a computer-controlled resistivity meter via multi-core cables.
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 in the figure above. Readings are taken down to 20 levels of increasing depth range. This means that we can assess larger deeper and shallower smaller features, as shown in the examples below.
Advanced data processing using specialist inversion software removes distortions caused by the effects of electrode geometry, to produce 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.