RESISTIVITY

Constant Separation Resistivity and Electrical Resistivity Imaging Tomography Surveys:-

Electrical resistivity imaging, resistivity tomography, RIS, ERI or ERT surveys provide measurements of the sub-surface by assessing the potential drops between electrodes in a variety of configurations, depending upon the type of survey.

Resistivity surveys can be completed using a 4 electrode frame and the survey completed by walking the grid. The resulting data will give apparent resisitivty data from a fixed depth range governed by the electrode spacing usually 0.25 to 1m.

Requirements for deeper surveys to provide engineering and environmental geophysics data can often benefit from making use of the latest development in resistivity surveying.
Electrical Resistivity Imaging or Tomography surveys use an array of electrodes to measure apparent ground resistivity. 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 resistivity imaging system in the field. The control unit is located in the vehicle in the distance.
Resistivity imaging surveys to locate quarry and landfill boundaries

The resistivity tomography survey 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 and on occasion 100m. This is a very powerful means by which to image the sub-surface over large sites for engineering or environmental projects.

Below is a resistivity imaging survey section collected to determine the internal situation within a closed landfill site. Note the low resistivity wastes (blue) over higher resitivity (reds) bedrock. Internal structures such as cell bunds and areas of drier waste are also visible.

Investigation of landfill sites using resistivity imaging tomography surveys can reveal depth and type of waste plus internal bunds.

Applications :-

Locate mineshafts, pipelines, foundations, voids.
Soil resistivity / earth testing for wind farm projects.
Depth to rockhead, overburden, water table, peat thickness.
Locate pollution plumes, buried waste.
Identify solution features
Locate landfill / infill sites
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.