Electrical resistivity site assessments

The method that sections the ground. Where a project needs to know what the geology actually does with depth — not what a boring 200 ft away suggested — resistivity imaging is usually the answer.

A steel electrode driven into the ground in a harvested crop field with a lead clipped to it, the cable running back across the stubble to a resistivity meter standing open further along the line. — An electrode driven and clipped to the lead. Resistivity is the one method here that needs real electrical contact with the ground — which is what makes frozen ground, dry gravel and pavement the conditions that rule it out.

What it measures

Current is injected through a pair of electrodes and the resulting potential difference is measured across many others. Repeated automatically through hundreds of electrode combinations along a line, and inverted, the result is a cross-section of resistivity — effectively a picture of the ground beneath the line.

Resistivity responds strongly to porosity, saturation, clay content and dissolved solids. That makes it the natural method for the questions that turn on water and weathering: depth to competent rock, the position of the water table, whether a clay layer is continuous, where a limestone is sound and where it has been dissolved.

A resistivity meter open on the ground between two coils of red electrode cable, its receiver panel and terminals visible, set in the stubble of a harvested field. — The meter at the centre of the spread. Current goes out through one pair of electrodes, the potential difference comes back across another, and it is the ratio between them that gets inverted into a section.

Depth is bought with length

The single thing worth knowing before commissioning a resistivity survey is that depth of investigation is roughly a fifth to a sixth of the line length. A 100 ft depth needs something like a 500 – 600 ft line laid out on the ground.

That governs feasibility more than anything else. On a constrained urban plot it may rule the method out, and seismic refraction or TDEM becomes the better route. On open ground it is rarely a problem. We work this out before quoting, not after arriving.

Electrode cable running away from a resistivity meter along a tramline through a harvested field, the line continuing over a rise toward a water tower on the horizon. — The spread running away up the tramline. Reaching a hundred feet down means several hundred feet of cable on the ground, which is why open sites suit this method and constrained ones often rule it out before anything else does.

Karst, voids and the water table

An air-filled void is highly resistive; a clay-filled one is highly conductive. Both stand out against sound limestone, which is why resistivity is a first-choice method on karst terrain — and why it pairs so well with microgravity, which answers the same question through a completely independent physical property. Where a project cannot afford a false negative, running both is the defensible approach.

What it will not tell you

Resistivity images are smooth. We set out what that means for an interpretation, and the number that tells you whether to believe one, in our note on reading an inverted section. Inversion produces the simplest model that fits the data, so a sharp boundary appears as a gradient and a small target may be smeared into a broad low. Absolute depths carry real uncertainty unless tied to a boring or a second method. It also needs electrode contact — on pavement, frozen ground or dry gravel that requires effort, and sometimes rules the method out.

Equipment and capability

2D resistivity tomography
3D resistivity arrays
Vertical electrical sounding
Induced polarization where the target warrants
Karst and void investigation
Depth to bedrock and water table
Contaminant plume delineation

What you receive

Inverted resistivity sections with a stated model misfit, interpreted horizons drawn on, depth uncertainty given honestly, and where boreholes exist, the tie between the two shown rather than asserted.

Ask before you commission

Tell us the target, the ground and the constraint and we will say whether this method answers it, whether it needs a second method alongside, or whether something else entirely is the better route. That conversation is free and it is usually short.

(844) 300-6GPR sales@geosearches.com

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