RESISTIVITY

Constant Separation Resistivity and Resistivity Imaging Surveys:-

Resistivity is a measurement of the sub-surface made by assessing the potential drops between electrodes in a variety of configurations, depending upon the type of survey.

Surveys can be completed using a 4 electrode frame and the survey completed by walking the grid. The resulting data will give data from a fixed depth range governed by the electrode spacing.

Requirements for deeper surveys to provide engineering and environmental data can often benefit from making use of the latest development in resistivity surveying.
Resistivity imaging uses an array of electrodes to measure apparent ground resistance. The system uses a set of up to 256 electrodes under computer control to develop a pseudo-section using combinations of electrode readings.

The photograph shows the system in the field. The control unit is located in the vehicle in the distance.
Resistivity imaging to locate deeply covered mineworkings

The data are split into levels representing depth intervals and powerful modelling software is used to convert the data into calculated apparent resistivity depth sections. Depths of study up to 30m are not uncommon. This is a very powerful means by which to image the sub-surface over large sites for engineering projects.

Below is a resistivity section taken to determine the boundary of a landfill site. Higher resistivity (reds) capping materials overlie low resistivity waste materials with the boundary on the right. The presence of low resistivity (greens) near the surface at the boundary suggests that the cap is admitting pore fluids.

Resistivity tomography used to locate landfill boundaries and show capping deficiencies

Applications :-

Locate mineshafts, pipelines, foundations, voids.
Depth to rockhead, overburden, water table.
Locate pollution plumes, storage tanks, buried waste.
Identify solution features
Locate landfill / infill sites
Rapid pipeline / road route surveys.
Archaeology

Benefits :-

Output to client as sections and/or plans
Wide depth range
Easily correlated with borehole data
Ideal for covering large sites where a number of vertical profiles assist interpretation.