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03 / Electrical
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.
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.
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.
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. 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.