SUBSURFACE ELECTRICAL IMAGING
2D Electrical Resistivity Tomography (ERT)
Investigate lateral and vertical changes in ground resistivity for engineering and environmental decisions, with survey design, inversion quality and geological uncertainty kept in view.
ENGINEERING OVERVIEW
What 2D ERT reveals
2D Electrical Resistivity Tomography (ERT), also called electrical resistivity imaging, estimates how electrical resistivity varies beneath a survey line. Surface electrodes transmit current and measure voltage; a set of apparent-resistivity observations is inverted to produce a model of subsurface electrical properties. The result is an interpretation, not a direct photograph of rock, groundwater or contamination.
Where boreholes are sparse, a single ground profile may overlook lateral changes. ERT can help screen contrasts along an excavation, slope, buried channel or groundwater investigation corridor and guide where intrusive checks would be most useful. It is valuable when the engineering question is about electrical contrasts, not when the client expects a unique geological answer from resistivity alone.
MEASUREMENT & INTERPRETATION
What the method actually tells you
Electrical response, not lithology
Resistivity depends on pore-water chemistry, saturation, clay content, temperature and connected void space as well as material type. Similar resistivity values can represent different ground conditions.
Apparent versus inverted resistivity
Field readings are apparent-resistivity measurements. An inversion algorithm seeks an earth model that explains them under selected regularisation and error assumptions.
Line-based investigation
A single 2D line assumes limited out-of-plane variation. Nearby utilities, off-line conductive bodies and three-dimensional structures can distort the interpreted section.
Ground-truth integration
Boreholes, sampling, groundwater monitoring and local geology are used to test competing interpretations rather than simply assigning rock names to colours.
PRACTICAL SURVEY FLOW
From site question to defensible interpretation
Define the target
Agree the likely depth range, lateral extent, expected resistivity contrast and the engineering decision that would follow.
Design electrode geometry
Choose array type, spacing and line coverage around access, buried services, surface condition and target resolution.
Acquire and quality-check
Record coordinates, contact resistance, injected current, repeat/reciprocal readings and sources of electrical interference.
Invert and cross-check
Review misfit, data coverage, sensitivity and alternative models; compare with boreholes and other survey methods before making an engineering conclusion.
WHERE IT ADDS VALUE
Engineering applications and use cases
Groundwater and seepage screening
Map resistivity contrasts potentially related to saturated or conductive zones, subject to groundwater chemistry and geological context.
Excavation and foundation planning
Explore lateral changes in weathered rock, fill or soil conditions to help target boreholes and reduce avoidable uncertainty.
Environmental and karst investigations
Investigate candidate anomalies consistent with leachate, voided ground or altered strata, with intrusive confirmation where decisions are consequential.
MEASUREMENT BOUNDARIES
Limitations and conditions of use
Interpretation quality depends as much on survey suitability and independent evidence as on processing software.
- Conductive clay, saline groundwater and contamination may give overlapping signatures; electrical measurements alone cannot reliably distinguish them.
- Electrode access, paved surfaces, buried metal, fences and poor contact can reduce data quality.
- Resolution generally deteriorates with depth and is uneven across the model; a smooth inverted boundary need not represent a sharp geological contact.
- A 2D assumption may be inappropriate where ground conditions change strongly across the survey line.
METHOD SELECTION
How to choose the right approach
ERT versus VES
ERT is better suited to investigating variation along a line; vertical electrical sounding is usually more appropriate for a primarily layered, locally one-dimensional question.
ERT versus borehole normal resistivity
ERT measures a surface array response across a volume; normal-resistivity logging measures the formation response around a fluid-filled borehole.
ERT versus seismic refraction
ERT responds to conductivity contrasts, while refraction responds to wave-velocity structure. Agreement can strengthen a ground model; disagreement can be diagnostically useful.
Use 2D ERT when a project needs an interpretable spatial screening tool and when the target is likely to generate a measurable electrical contrast. For foundation founding levels, cavities, permeability or contaminant presence, combine it with independent information and targeted verification.
ENGINEERING ASSURANCE
Field quality, interpretation and review
A traceable project record should retain the following information where applicable:
- Electrode positions, datum, array sequence and cable continuity
- Contact resistance checks, reciprocal or repeated measurements where practical
- Removed observations documented with reasons; raw readings preserved
- Inversion settings, model fit and sensitivity or depth-of-investigation assessment
PROJECT DELIVERABLES
What a useful technical report should contain
01 · Project output
Georeferenced survey line layout and electrode schedule
02 · Project output
Raw and quality-screened apparent resistivity data
03 · Project output
Interpreted resistivity model with colour scale, assumptions and limitations
04 · Project output
Anomaly descriptions, correlation with ground investigation and recommended follow-up
COMMON QUESTIONS
Frequently asked questions
Does a low-resistivity anomaly prove groundwater?
Can ERT find a cavity accurately?
How deep can a 2D ERT survey investigate?
TECHNICAL REFERENCES
Further technical reading
- US EPA — Electrical Resistivity
- USGS — ERI survey pre-modelling and limits
- USGS — Instrument response and inversion comparison
References describe the underlying methods and recommended data-quality principles; interpretation for any particular site must address its own evidence and constraints.
PROJECT ENQUIRIES
Discuss your ground investigation challenge.
GEOOE · GEOORIGIN ENGINEERING LIMITED
Share the site location, investigation objective, expected depth range, available boreholes and decisions the results need to support. We can discuss appropriate technical methods, limitations and project-specific collaboration.