What is Resistivity Surveying?丨Resistivity Surveying Techniques

Overview：Resistivity surveying techniques help engineers investigate subsurface conditions by measuring electrical responses to injected current. This guide explains how an Electrical Resistivity Survey works, compares common electrode arrays, and outlines field procedures, data inversion, and applications. Learn how Resistivity Surveying Techniques support groundwater exploration, geotechnical investigations, environmental assessment, and mineral exploration through reliable survey design and geological interpretation.

Ⅰ. What Are Resistivity Surveying Techniques?

Resistivity surveying techniques are geophysical methods that measure how strongly subsurface materials resist electrical current. Engineers use these measurements to investigate geological structures, groundwater conditions, soil properties, and underground anomalies.

An Electrical Resistivity Survey introduces electrical current into the ground through current electrodes. Separate potential electrodes measure the resulting voltage difference. The measured response helps estimate the apparent resistivity of the subsurface.

These methods do not directly identify rock types or groundwater quality. Instead, they map electrical contrasts and use geological information to interpret their possible causes.

1. Definition of Electrical Resistivity Surveying

An electrical resistivity survey measures voltage and current through a controlled electrode arrangement to calculate apparent resistivity. Engineers interpret these measurements to identify subsurface variations in electrical properties.

The method relies on galvanic coupling between metal electrodes and the ground. Its performance depends on electrode contact, subsurface conductivity, survey geometry, and measurement quality.

2. What Does Electrical Resistivity Reveal?

Electrical resistivity reflects the combined influence of several geological and environmental factors.

• Pore-fluid content: Water in connected pores often increases electrical conductivity.
• Fluid chemistry: Dissolved ions can lower resistivity.
• Lithology: Clay-rich and mineralized materials may exhibit different electrical responses from clean, dry sediments.
• Porosity and saturation: Connected pore spaces influence current pathways.
• Fractures: Water-filled fractures may create measurable electrical contrasts.
• Temperature: Electrical conductivity can change with temperature.

No single resistivity value uniquely identifies a geological material. Different materials can produce overlapping responses.

3. Electrical Resistivity, Conductivity, and Resistance

Electrical resistance describes how strongly a particular object opposes current flow. Resistivity is an intrinsic electrical property that accounts for geometry. Conductivity is the reciprocal of resistivity.

The basic relationships are:

$$R=\frac{V}{I}$$R=IV​

$$\rho=\frac{1}{\sigma}$$ρ=σ1​

Where:

• $R$R = electrical resistance, in ohms.
• $V$V = measured voltage, in volts.
• $I$I = injected current, in amperes.
• $\rho$ρ = electrical resistivity, in ohm-meters.
• $\sigma$σ = electrical conductivity, in siemens per meter.

A field instrument measures current and voltage. Survey geometry then converts the measured resistance into apparent resistivity.

Ⅱ. How Does an Electrical Resistivity Survey Work?

An electrical resistivity survey injects current into the ground and measures the resulting potential difference. The electrode geometry determines the geometric factor used to calculate apparent resistivity.

1. The Four-Electrode Measurement Principle

A conventional four-electrode configuration uses two current electrodes and two potential electrodes.

• A and B: Current electrodes inject electrical current.
• M and N: Potential electrodes measure voltage.
• Resistivity meter: Controls current injection and records electrical measurements.

The instrument calculates resistance from the voltage-to-current ratio. The geometric factor converts this resistance into apparent resistivity.

For a general four-electrode arrangement:

$$\rho_a=K\frac{\Delta V}{I}$$ρa​=KIΔV​

Here, $K$K is the geometric factor determined by electrode positions. The apparent resistivity $\rho_a$ρa​ represents the response of a hypothetical homogeneous half-space that would produce the measured voltage-to-current ratio.

2. Why Is Apparent Resistivity Not True Resistivity?

Real geological formations are rarely homogeneous. They contain layers, fractures, varying moisture levels, and different mineral compositions.

Consequently, the current spreads through a complex three-dimensional volume. The measured voltage reflects the combined influence of materials throughout that volume.

Apparent resistivity is therefore a survey response, not a direct measurement of one specific underground layer.

3. What Controls the Investigation Depth?

Investigation depth depends on several factors:

• Electrode spacing and array geometry.
• Total survey-line length.
• Ground resistivity and conductivity contrasts.
• Signal strength and instrument capability.
• Noise levels and electrode contact.
• Survey design and inversion sensitivity.

Larger electrode separations generally increase sensitivity to deeper structures. However, increasing spacing does not guarantee a proportional increase in reliable investigation depth.

A site-specific sensitivity or depth-of-investigation assessment is preferable to using a fixed depth-to-spacing ratio.

Ⅲ. Main Resistivity Survey Methods

Resistivity surveying includes several acquisition strategies. The appropriate method depends on whether the project targets vertical layering, lateral changes, or three-dimensional structures.

1. Vertical Electrical Sounding (VES)

Vertical Electrical Sounding investigates changes in apparent resistivity with increasing electrode separation around a fixed survey center.

The method commonly uses Schlumberger or Wenner-type configurations. Engineers expand the current-electrode spacing to increase sensitivity to deeper formations.

VES is particularly useful when the subsurface can reasonably be approximated by horizontal layers.

Typical applications include:

• Groundwater exploration in layered sedimentary settings.
• Estimation of overburden thickness.
• Preliminary bedrock-depth assessment.
• Regional hydrogeological investigations.

VES has limited ability to resolve strong lateral heterogeneity. A one-dimensional interpretation may become unreliable near faults, irregular bedrock surfaces, or isolated conductive bodies.

2. Electrical Resistivity Profiling

Electrical resistivity profiling investigates lateral changes in electrical properties along a survey line.

The electrode configuration and spacing remain consistent while the array moves along the profile. Measurements reveal variations associated with geological boundaries, fractures, and other electrical contrasts.

Profiling is useful for:

• Mapping lateral geological changes.
• Identifying potential fracture zones.
• Tracing conductive or resistive anomalies.
• Selecting locations for more detailed investigations.

The method provides limited vertical information compared with a properly designed imaging survey.

3. Electrical Resistivity Tomography (ERT)

Electrical Resistivity Tomography combines multiple electrode configurations and measurement levels to produce a two-dimensional or three-dimensional resistivity model.

A multielectrode system automates measurements across a predefined electrode layout. Inversion software then estimates a subsurface resistivity distribution that fits the observations.

ERT is suitable for targets with significant lateral and vertical variation.

Common applications include:

• Geological and geotechnical investigations.
• Groundwater and aquifer characterization.
• Environmental contamination assessment.
• Karst and cavity investigations.
• Fracture-zone mapping.
• Engineering site characterization.

ERT results represent an interpreted electrical model. They do not independently establish lithology, groundwater yield, or contaminant identity.

4. Two-Dimensional and Three-Dimensional Resistivity Imaging

Two-dimensional ERT generally assumes that the subsurface varies mainly along the survey line and with depth. It also assumes limited variation perpendicular to the line.

Three-dimensional ERT uses measurements collected across a surface grid or another three-dimensional electrode arrangement.

Feature	2D ERT	3D ERT
Acquisition layout	Linear electrode profile	Surface grid or 3D layout
Model output	Resistivity cross-section	Resistivity volume
Best suited to	Predominantly profile-oriented structures	Complex, multidirectional structures
Field requirements	Usually fewer electrodes and simpler deployment	More extensive electrode coverage
Main limitation	May miss off-line anomalies	Higher acquisition and processing demands

The choice between 2D and 3D depends on geological complexity, target geometry, available space, and project objectives.

Ⅳ. Electrode Arrays: Configuration and Selection

Electrode arrays control the spatial sensitivity of resistivity measurements. Each arrangement offers different trade-offs in signal strength, depth sensitivity, and resolution.

1. Wenner Array

The Wenner array uses four electrodes with equal spacing $a$a.

The two current electrodes occupy the outer positions. The two potential electrodes sit between them.

For a homogeneous half-space, the geometric factor is:

$$K=2\pi a$$K=2πa

Therefore:

$$\rho_a=2\pi a\frac{\Delta V}{I}$$ρa​=2πaIΔV​

The Wenner array generally provides strong voltage signals and useful sensitivity to vertical changes in layered ground.

Typical applications: Layered geology, shallow environmental surveys, and investigations where stable signal strength is important.

2. Schlumberger Array

The Schlumberger array places the potential electrodes relatively close to the center. The current electrodes move outward as the survey progresses.

For a symmetric arrangement, its geometric factor is:

$$K=\pi\frac{AB^2-MN^2}{4MN}$$K=π4MNAB2−MN2​

Here, $AB$AB is the full current-electrode separation, and $MN$MN is the full potential-electrode separation.

The array is widely used for vertical electrical sounding. It can extend the current-electrode separation without moving the potential electrodes at every measurement step.

Typical applications: Layered subsurface investigations, overburden assessment, and preliminary groundwater studies.

3. Dipole-Dipole Array

The dipole-dipole array uses one current dipole and one potential dipole. The separation between the dipoles increases through successive measurement levels.

For equal dipole lengths $a$a and a separation factor $n$n, a common geometric-factor expression is:

$$K=\pi a n(n+1)(n+2)$$K=πan(n+1)(n+2)

The exact expression depends on the electrode geometry and sign convention.

Dipole-dipole surveys are often sensitive to lateral resistivity changes. They can help delineate steep geological boundaries and localized anomalies.

However, voltage signals may become weak at larger separation factors. Noise control and reliable electrode contact are particularly important.

4. Pole-Dipole Array

The pole-dipole array uses one current electrode within the survey area and a remote current electrode. A potential dipole measures the resulting voltage difference.

This configuration can provide useful coverage and sensitivity to lateral geological variations.

Its performance depends on the remote electrode location and the actual subsurface conditions. Survey designers should evaluate possible directional bias and verify remote-electrode effects.

5. Wenner-Schlumberger and Hybrid Arrays

The Wenner-Schlumberger array combines characteristics of Wenner and Schlumberger configurations. It can provide a practical balance between vertical and lateral sensitivity.

Hybrid acquisition uses more than one array during the same survey. This approach can improve data coverage and reduce dependence on one sensitivity pattern.

However, combining arrays does not automatically improve model accuracy. The final result still depends on measurement quality, survey geometry, inversion settings, and geological constraints.

6. Electrode Array Comparison

Array	Main Sensitivity	Typical Strength	Main Limitation
Wenner	Layered and vertical variations	Strong signals in many field conditions	May provide less detail on some lateral boundaries
Schlumberger	Vertical layering	Efficient for sounding	Limited for strongly heterogeneous structures
Dipole-Dipole	Lateral changes	Useful for steep boundaries and localized anomalies	Signal may weaken at larger separation factors
Pole-Dipole	Mixed sensitivity	Flexible profile coverage	Requires a suitable remote electrode
Wenner-Schlumberger	Mixed vertical and lateral sensitivity	Balanced imaging response	Performance depends on target geometry
Hybrid arrays	Multiple sensitivity patterns	Broader data coverage	Greater acquisition and processing complexity

No array is universally optimal. Selection should reflect the target, geology, site access, and noise environment.

Ⅴ. Resistivity Survey Equipment and System Selection

A resistivity survey system includes current injection, voltage measurement, electrodes, cables, switching hardware, power supply, and data management.

1. Main Equipment Components

A typical system includes:

• Resistivity meter: Controls current and measures voltage.
• Electrodes: Establish electrical contact with the ground.
• Multielectrode cables: Connect electrodes to the instrument.
• Switching unit: Selects electrode combinations automatically.
• Power supply: Provides energy for current injection.
• Control software: Defines acquisition sequences and stores measurements.
• Positioning equipment: Records electrode locations and survey geometry.

System configuration should match the required current output, measurement precision, electrode count, field conditions, and project workflow.

2. How to Choose a Resistivity Meter

Selection Criterion	What to Evaluate	Why It Matters
Current output	Available current range and voltage compliance	Influences signal generation in resistive ground
Voltage measurement	Resolution, accuracy, and noise rejection	Determines the ability to record weak signals
Electrode capacity	Supported electrode count and switching options	Affects coverage and acquisition efficiency
Array support	Wenner, Schlumberger, dipole-dipole, and custom sequences	Must match the survey design
Contact diagnostics	Contact-resistance measurement and warning functions	Helps identify unreliable electrode connections
Data handling	Export formats, metadata, and software compatibility	Supports traceable processing
Environmental protection	Operating temperature, ingress protection, and ruggedness	Matters in demanding field conditions
Power and logistics	Battery capacity, weight, and cable configuration	Affects field deployment and productivity

Actual instrument specifications should be checked against the manufacturer’s technical documentation. A high current rating alone does not guarantee better data.

3. When Is a Multichannel or Multielectrode System Useful?

Multielectrode systems automate switching across many electrodes. They reduce repetitive manual repositioning and support dense measurement sequences.

They are particularly useful for 2D and 3D ERT projects. They can also improve acquisition consistency when the survey design requires numerous electrode combinations.

However, automation cannot compensate for poor electrode contact, unsuitable array geometry, or incorrect survey positioning.

Ⅵ. Resistivity Survey Field Procedures

A reliable survey begins with a clear geological objective and a practical field design.

1. Define the Investigation Objective

Before mobilization, define:

• The geological or engineering target.
• The approximate target depth and dimensions.
• The expected electrical contrast.
• The required spatial resolution.
• The available survey area and access constraints.
• The supporting geological and borehole information.

These factors determine whether VES, profiling, 2D ERT, or 3D ERT is appropriate.

2. Conduct a Site Reconnaissance

Inspect the survey area for terrain, vegetation, buried utilities, fences, pipelines, and other potential sources of interference.

Review geological maps, borehole logs, previous geophysical results, and available groundwater data.

Identify practical electrode positions and assess whether the planned survey line can extend beyond the target area.

3. Design Electrode Spacing and Survey Length

Electrode spacing influences near-surface resolution and depth sensitivity.

Smaller spacing generally improves the ability to resolve shallow features. Larger spacing increases sensitivity to deeper structures but may reduce the ability to distinguish small targets.

The survey line should extend far enough to provide useful sensitivity beneath and around the target.

A numerical forward model or sensitivity analysis can help evaluate the planned geometry before field acquisition.

4. Install Electrodes and Check Contact

Place electrodes at the planned locations. Establish reliable electrical contact with the ground.

In dry or coarse materials, contact resistance may be high. Depending on site conditions, operators may need to improve electrode-ground contact using suitable methods.

Record any electrode positions that cannot be installed as planned. Avoid undocumented changes to the acquisition geometry.

5. Configure the Instrument and Acquire Data

Set the array sequence, electrode spacing, current parameters, stacking settings, and measurement schedule.

Start with a test sequence to evaluate signal quality and contact conditions.

During acquisition, monitor:

• Injected current and measured voltage.
• Contact-resistance warnings.
• Repeatability of measurements.
• Instrument error flags.
• External electrical interference.
• Electrode and cable integrity.

Repeat questionable measurements when necessary. Record field conditions that may affect interpretation.

6. Perform Quality Control

Quality control should identify unreliable readings before inversion.

Useful checks include:

• Reciprocal measurements where supported by the acquisition design.
• Repeat readings at selected electrode combinations.
• Inspection of apparent-resistivity pseudosections.
• Review of contact resistance and instrument diagnostics.
• Identification of outliers and spatially inconsistent data.
• Verification of electrode coordinates and topography.

Data rejection thresholds should reflect instrument performance, site conditions, and project requirements. There is no universal threshold suitable for every survey.

Ⅶ. Data Processing, Inversion, and Interpretation

Resistivity interpretation transforms measured electrical responses into a geological model. The process requires both numerical analysis and geological judgment.

1. Data Preparation

Before inversion, verify the measurement files and survey geometry.

Check electrode coordinates, array definitions, topographic information, current values, voltage readings, and measurement quality.

Remove or flag unreliable observations using documented criteria. Preserve the original dataset so that processing decisions remain traceable.

2. Apparent Resistivity Pseudosections

A pseudosection displays apparent-resistivity measurements using a conventional plotting position related to the electrode geometry.

It helps identify broad patterns, outliers, and potential anomalies.

However, a pseudosection is not a true geological cross-section. Its plotting depth does not directly equal the depth of a specific geological boundary.

3. Inversion and Model Construction

ERT inversion seeks a resistivity model whose predicted measurements fit the observed data.

The process generally includes:

1. Defining a numerical model and mesh.
2. Calculating predicted electrical responses.
3. Comparing predictions with field measurements.
4. Updating model resistivity values.
5. Repeating the calculation until the chosen stopping criteria are met.

Inversion may use smoothness constraints, robust norms, or other regularization strategies.

The selected method should reflect the expected geological structure and the character of the data.

4. Model Validation

A low data misfit does not prove that the recovered model is geologically correct.

Several different resistivity distributions may produce similar observations. This is a fundamental non-uniqueness problem.

Model validation should consider:

• Data fit and residual patterns.
• Sensitivity and depth-of-investigation analysis.
• Alternative inversion settings.
• Geological plausibility.
• Borehole and surface observations.
• Independent geophysical information.

Interpretations should distinguish well-resolved structures from poorly constrained features.

5. From Resistivity Models to Geological Interpretation

Electrical resistivity values should be interpreted in context.

Observed Pattern	Possible Geological Explanation	Recommended Verification
Low-resistivity zone	Clay-rich material, saline water, conductive contamination, or mineralization	Borehole logs, water chemistry, and geological mapping
High-resistivity zone	Dry coarse sediments, competent rock, air-filled voids, or low-porosity material	Drilling, geological mapping, and complementary geophysics
Linear resistivity contrast	Fault, fracture zone, lithological boundary, or moisture contrast	Structural mapping and cross-line surveys
Localized conductive anomaly	Saturated zone, conductive fill, or buried feature	Targeted investigation and independent evidence
Layered resistivity pattern	Changes in sediment type, saturation, or bedrock depth	VES, borehole data, and regional geology

These associations are interpretive possibilities, not unique diagnostic rules.

Ⅷ. Applications of Resistivity Surveying Techniques

Resistivity surveying supports a broad range of geological, environmental, and engineering investigations.

1. Groundwater Exploration

Resistivity surveys can help identify formations that may contain groundwater and delineate potential aquifer boundaries.

Conductive zones may reflect saturated sediments, clay-rich layers, or saline groundwater. Resistive zones may represent dry materials or competent bedrock.

An electrical anomaly alone cannot establish groundwater yield or potability. Drilling, pumping tests, and water-quality analysis remain important for confirmation.

2. Geotechnical Investigation

ERT can help characterize subsurface variability before construction.

Typical targets include:

• Depth to bedrock.
• Weathered rock thickness.
• Fracture zones.
• Possible karst features.
• Variations in fill and natural ground.
• Potentially weak or saturated zones.

Results can guide borehole placement and help refine the geological model. They should not replace required geotechnical testing.

3. Environmental Site Assessment

Electrical resistivity methods can map contrasts associated with groundwater contamination, saline intrusion, and changes in subsurface fluid content.

Conductive anomalies may indicate increased dissolved-ion concentrations. However, natural clay and lithological changes can produce similar responses.

Environmental interpretations should integrate site history, groundwater sampling, and chemical analysis.

4. Mineral Exploration

Resistivity surveys can help map geological structures, alteration zones, and electrical contrasts associated with mineralized systems.

Some mineral deposits produce conductive responses. Others may be resistive or exhibit limited contrast against surrounding rocks.

Where disseminated metallic minerals are a target, induced polarization (IP) can provide complementary information about polarization properties.

5. Karst and Underground Cavity Detection

Resistivity imaging can help identify contrasts associated with cavities, fractured zones, and variable saturation.

Air-filled cavities may produce resistive anomalies, while water-filled or clay-filled features may produce conductive responses.

Detection depends on target dimensions, burial depth, geological background, electrode spacing, and survey geometry. Independent confirmation is recommended before engineering decisions.

6. Infrastructure and Environmental Monitoring

Repeated resistivity surveys can track changes in subsurface electrical properties over time.

Potential applications include:

• Monitoring infiltration and water movement.
• Assessing changes in soil saturation.
• Observing saline intrusion.
• Evaluating remediation-related changes.
• Investigating seepage around embankments.

Time-lapse comparisons require consistent acquisition geometry and careful control of seasonal and environmental variations.

Ⅸ. Common Challenges and Limitations

Resistivity surveying provides indirect information. Several factors can limit resolution and interpretation reliability.

1. Geological Ambiguity

Different geological materials may share similar resistivity values.

A low-resistivity anomaly may represent clay, saline water, or conductive mineralization. Geological context is essential for distinguishing these possibilities.

2. Poor Electrode Contact

High contact resistance can reduce current injection and weaken measured voltage signals.

Dry ground, coarse gravel, frozen soil, and highly resistive surface materials may complicate electrode installation.

Contact diagnostics and appropriate field procedures help manage this issue.

3. Electrical Noise

Power lines, buried infrastructure, stray currents, and other electrical sources may interfere with measurements.

Survey planning should identify potential noise sources. Acquisition settings and measurement repetition should be adapted to actual field conditions.

4. Resolution and Depth Trade-Off

Greater investigation depth generally requires broader electrode spacing and survey coverage.

This can reduce sensitivity to small shallow targets. A survey designed for deep bedrock may not resolve thin near-surface layers adequately.

5. Three-Dimensional Effects

A 2D inversion assumes limited variation perpendicular to the profile.

Strong off-line structures can distort the resulting cross-section. Cross-lines or 3D acquisition may be necessary when geology changes significantly in multiple directions.

Ⅹ. How to Choose the Right Resistivity Survey Technique

The selection process should begin with the target geometry and expected geology.

1. Technique Selection Matrix

Survey Objective	Suggested Technique	Key Consideration
Identify vertical layering	VES	Suitability of the layered-earth assumption
Map lateral changes along a line	Resistivity profiling	Electrode spacing and profile coverage
Image complex subsurface structures	2D ERT	Target alignment and off-line effects
Investigate multidirectional anomalies	3D ERT	Grid coverage and acquisition resources
Map steep boundaries	Dipole-dipole or hybrid ERT	Signal quality and lateral sensitivity
Investigate groundwater potential	VES or ERT	Geological context and borehole validation
Assess possible conductive mineralization	Resistivity combined with IP	Mineral properties and geological setting
Monitor changing subsurface conditions	Time-lapse ERT	Repeatability and environmental controls

2. Practical Survey Design Checklist

Before starting a project, confirm the following:

• The target depth and dimensions are defined.
• The survey method matches the geological setting.
• Electrode spacing supports the required resolution.
• Survey length provides adequate target coverage.
• Site access and electrode installation are feasible.
• Instrument specifications match the planned acquisition.
• Electrical noise and buried infrastructure have been considered.
• Data quality-control procedures are documented.
• Geological and borehole information is available where possible.
• Interpretation limitations will be communicated in the final report.

3. What Should a Professional Survey Report Include?

A complete report should include:

• Project objectives and geological background.
• Survey layout, electrode coordinates, and topography.
• Instrument configuration and acquisition parameters.
• Electrode array and measurement sequence.
• Data quality-control procedures.
• Apparent-resistivity pseudosections.
• Inversion settings and model-fit information.
• Resistivity sections or three-dimensional models.
• Geological interpretation and uncertainty discussion.
• Recommendations for verification or additional investigation.

A transparent report allows engineers to distinguish measured evidence from geological interpretation.

Ⅺ. Conclusion

Resistivity surveying techniques provide a flexible way to investigate subsurface electrical contrasts. VES, profiling, and ERT serve different purposes, while electrode arrays influence sensitivity, signal strength, and spatial resolution.

Reliable results depend on sound survey design, appropriate equipment, careful field procedures, robust inversion, and geological verification.

For engineering and environmental projects, resistivity data are most useful when integrated with boreholes, geological mapping, and other independent evidence.

Related Articles

Title	Core Content	URL
Geophysical Methods – Electrical Resistivity Tomography (ERT)	ERT operating principles, electrode configurations, field acquisition, and system capabilities.	https://geotechcn.net/service/geophysical-methods-ert-guide/
What Is Electrical Resistivity Tomography (ERT)?	ERT fundamentals, electrode arrays, survey workflow, and subsurface imaging.	https://geotechcn.net/service/what-is-electrical-resistivity-tomography-ert/
ERT Geophysics: Principles, Arrays & Field Applications	ERT principles, array geometry, and geophysical field applications.	https://geotechcn.net/service/ert-geophysics-principles/
What Are Resistivity Meters?	Resistivity meter fundamentals, measurement principles, and instrument applications.	https://geotechcn.net/service/what-are-resistivity-meters/
Geophysical Methods: Induced Polarization (IP)	Induced polarization principles, mineral exploration, and complementary electrical survey methods.	https://geotechcn.net/service/what-is-induced-polarization-ip-method-a-comprehensive-technical-guide/

Reference Sources

Title	Core Content	URL
Electrical Resistivity — U.S. Environmental Protection Agency	Electrical resistivity principles, apparent resistivity, electrode arrays, investigation depth, and environmental applications.	https://www.epa.gov/environmental-geophysics/electrical-resistivity
Electrical Methods — U.S. Environmental Protection Agency	Overview of electrical geophysical methods, electrical properties of earth materials, and interpretation limitations.	https://www.epa.gov/environmental-geophysics/electrical-methods
ASTM D6431-25: Standard Guide for Using the Direct Current Resistivity Method for Geophysical Site Investigation	Guidance on DC resistivity methods, field procedures, and subsurface investigation.	https://store.astm.org/d6431-25.html
Induced Polarization (IP) and Complex Resistivity — U.S. Environmental Protection Agency	IP principles and complementary applications for mineral exploration and subsurface characterization.	https://www.epa.gov/environmental-geophysics/induced-polarization-ip-and-complex-resistivity
The Application, Technique, and Theory of Gish-Rooney Instruments, Methods, and Interpretation in Electrical Resistivity Measurements — U.S. Geological Survey	Historical technical report on electrical resistivity measurement techniques and interpretation.	https://pubs.usgs.gov/publication/ofr5899

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