Quick answer
Electrical resistivity tomography, or ERT, estimates how strongly different parts of the ground resist the flow of electrical current. The result is an inverted resistivity model, not a direct underground photograph. It can help identify contrasts and patterns worth investigating, but the same resistivity response can have several causes. Geological observations, boreholes, samples, water levels, laboratory results, or monitoring may therefore be needed before a project team assigns a material, groundwater, environmental, or engineering meaning.
What the topic means
An ERT survey places a line or grid of electrodes at the ground surface. Selected electrodes introduce a controlled current while other electrodes measure voltage differences. Those measurements are combined into apparent-resistivity data. Inversion software then searches for a subsurface resistivity distribution that can reproduce the observed data within the adopted model assumptions.
Resistivity is a physical property influenced by several factors, including pore-water content and chemistry, clay content, porosity, weathering, fractures, temperature, mineralization, and cultural materials. ERT is consequently useful for detecting electrical contrasts; it does not identify a geological unit or engineering property by name on its own.
When it may be relevant
- Screening lateral or vertical changes between limited boreholes, exposures, or wells.
- Investigating possible weathering, fracture, karst, seepage, saline-water, fill, or contamination-related contrasts.
- Helping position confirmatory boreholes, test pits, sampling points, or monitoring installations.
- Comparing how an electrically responsive condition changes through time, provided repeat-survey controls are suitable.
Useful information and inputs
- The decision to be supported and the physical target that may create an electrical contrast.
- Required depth, lateral coverage, line access, surface condition, slope, and safe electrode-placement areas.
- Existing geology, boreholes, wells, water levels, utilities, earthworks, laboratory data, and previous geophysics.
- Coordinate reference, topography, line direction, chainage convention, and known electrical interference.
- Any required time-lapse comparison, including rainfall, pumping, injection, tidal, or construction context.
How the method or assessment generally works
A survey design is selected for the target, site geometry, expected depth, and required resolution. Field work records electrode positions, array and spacing, instrument settings, contact resistance, repeated measurements, rejected readings, cultural interference, topography, and acquisition conditions. The data are checked before inversion. The inversion mesh, topographic treatment, regularization, constraints, iterations, error measure, and stopping criteria are documented because they affect the appearance and resolution of the model.
Interpretation should begin with the geometry of resistive and conductive zones, data coverage, sensitivity or depth-of-investigation information, and residual behavior. Geological or engineering explanations are added only after comparison with independent evidence and plausible alternatives.
Typical outputs
- Survey layout and line-orientation plan.
- Acquisition and data-quality summary.
- Measured and calculated apparent-resistivity comparisons where available.
- Inverted resistivity section or volume with units, topography, scale, and coverage limitations.
- Interpretive overlays showing bounded zones, alternative explanations, and confirmation priorities.
How the outputs should be interpreted
Read an ERT model as a smoothed, resolution-limited estimate of subsurface electrical structure. A low-resistivity zone means the model requires a more conductive response there; it does not automatically mean groundwater, clay, contamination, or weak ground. A high-resistivity zone means a more resistive response; it does not automatically mean competent dry rock, a void, or a specific lithology. The same colors on different sections are comparable only when units, numerical limits, processing, geometry, and model conditions are compatible.
QA/QC and evidence checks
- Confirm line identity, direction, stationing, electrode spacing, coordinates, elevation reference, and units.
- Review reciprocal or repeat readings, contact problems, outliers, current levels, cultural noise, and excluded data.
- Inspect observed-versus-calculated response and residual patterns instead of relying only on one global error value.
- Record the inversion software and version, mesh, constraints, regularization, topography, and sensitivity or coverage basis.
- Check whether interpreted features occur within defensible coverage and whether independent evidence supports or contradicts them.
Limitations and common misunderstandings
ERT inversion is non-unique: more than one resistivity distribution can explain similar surface measurements. Resolution normally decreases with depth and away from well-constrained parts of the array. Surface access, electrode coupling, three-dimensional geology beside a two-dimensional line, buried metal, fences, utilities, power sources, and topography can affect the data. A visually smooth model or low fitting error does not prove that the geological explanation is correct.
What may be needed for confirmation
Confirmation depends on the decision. It may include geological mapping, boreholes, test pits, core or soil descriptions, laboratory testing, groundwater-level monitoring, pumping tests, water-quality sampling, construction exposure mapping, repeat geophysics, or a second method that responds to a different physical property.
What to prepare before contacting HydroGeo
- A short statement of the decision, target, preferred coverage, and project stage.
- A site plan with accessible survey corridors and known utilities or restricted areas.
- Available borehole, well, geological, survey, laboratory, monitoring, and design records.
- Known ground, electrical-noise, traffic, vegetation, slope, and safety constraints.
Project-planning reference
Evidence table and decision graph
Use these source-derived summaries to organize an enquiry and identify useful records. They are general guidance, not project data or a substitute for site-specific professional review.
ERT evidence and interpretation matrix
| Evidence stage | What is available | What it supports | What it does not establish |
|---|---|---|---|
| Field measurement | Injected current, measured voltage, electrode geometry and position | Apparent-resistivity observations with survey geometry | A direct material or groundwater identity |
| Data QA/QC | Contact resistance, repeats, reciprocals, rejected readings and interference notes | Whether readings are sufficiently controlled for modelling | That the final geological explanation is correct |
| Inversion | A model whose calculated response is compared with observations | A smoothed estimate of subsurface resistivity distribution | A unique underground arrangement |
| Interpretation | Resistive and conductive patterns compared with site evidence | Testable geological, groundwater, environmental or engineering hypotheses | Confirmation without boreholes, samples, water levels or other evidence |
On a small screen, swipe the table sideways to review every column.
Source-derived relationship
From surface readings to a bounded decision
A four-stage pathway from electrode measurements through quality control and inversion to evidence-checked interpretation.
- Surface readingsCurrent, voltage, array, coordinates and topography
- Controlled datasetRepeats, noise, coupling and exclusions reviewed
- Resistivity modelInverted response with coverage and sensitivity limits
- Project hypothesisCompared with geology, boreholes, samples and monitoring
Source basis and use boundary
This matrix explains the evidence chain. It cannot prescribe electrode spacing, array, depth, inversion settings or a geological meaning without the site objective and field conditions.
- U.S. EPA — Electrical ResistivityElectrical measurements, apparent resistivity, inversion, depth and resolution controls, non-uniqueness, and material-property influences.
- USGS — Application of Surface Geophysics to Ground-Water InvestigationsRole of surface geophysical measurements in groundwater investigation and the need to relate physical-property responses to hydrogeologic evidence.
Primary sources reviewed 2 September 2026. Recheck the linked source and applicable project criteria before relying on current requirements.