Electrical Resistivity Tomography: A Complete Technical Guide

TIPS：Electrical Resistivity Tomography (ERT) is a non-invasive geophysical technique that maps subsurface resistivity by injecting direct current through electrode arrays. This technical guide explains how an ERT System works—from Ohm’s Law fundamentals and electrode configurations to 2D/3D data inversion—helping geophysicists understand resistivity distribution for groundwater, mineral, and engineering projects.

Ⅰ. What Is Electrical Resistivity Tomography?

Geophysical surveys often face a critical challenge. Drilling is expensive and provides only point data. Surface geophysics fills this gap. Electrical Resistivity Tomography (ERT) stands out as the most versatile shallow-imaging tool.

ERT measures how strongly subsurface materials resist electric current. It creates 2D or 3D images of resistivity distribution. These images reveal geology, hydrology, and contamination without excavation. Modern ERT System hardware collects thousands of measurements automatically. A multi-electrode cable with 48 to 120 take-outs spans the survey line. The resistivity meter switches current and potential electrode pairs in seconds.

Resistivity values range from less than 1 Ω·m for saline water to over 10,000 Ω·m for dry granite. This broad dynamic range makes ERT applicable across diverse environments. Clay, water, ore minerals, and bedrock each display unique signatures.

---

Ⅱ. The Physics Behind ERT

1. Ohm’s Law and Apparent Resistivity

ERT relies on a simple physical law. Current flows between two electrodes (A and B). The system measures voltage at two other electrodes (M and N). Apparent resistivity is calculated using the geometric factor K:

ρs = K × (ΔV / I)

K depends only on electrode spacing. This formula works for uniform ground. Real geology is layered and heterogeneous. Apparent resistivity blends contributions from all materials in the current path. Inversion software later separates these mixed signals into true resistivity values.

2. Factors Controlling Rock Resistivity

Several properties govern how rocks conduct electricity. Pore fluid chemistry dominates in most sedimentary environments. Saline water lowers resistivity dramatically. Fresh water raises it. Clay minerals and metallic sulfides conduct current through their crystal structures. This makes them special cases.

Porosity and saturation also matter. Dry sand has high resistivity. The same sand saturated with fresh water drops to 100–500 Ω·m. Fractured bedrock shows lower values than intact rock. Weathering, faulting, and organic contamination all alter resistivity signatures.

Material	Typical Resistivity (Ω·m)	Geological Meaning
Clay / Silt	1 – 50	Low-porosity or saturated fine grains
Sand (fresh water)	50 – 500	Aquifer material
Sand (saline water)	< 10	Saltwater intrusion
Granite (fresh)	1,000 – 10,000	Intact bedrock
Ore minerals	0.1 – 10	Metallic sulfides

---

Ⅲ. ERT System Hardware Components

1. Resistivity Meter

The meter is the system brain. It generates stable direct current and records microvolt-level potentials. Modern units output 1–2A maximum current. Input impedance exceeds 10 MΩ to avoid loading effects. The Geotechcn ERT System includes automatic gain ranging. This prevents signal saturation in conductive clays.

2. Multi-Electrode Cable

Cables contain 24 to 120 electrode take-outs. Spacing ranges from 1 to 10 meters. The cable connects to a switching unit. The unit selects active electrode pairs automatically. Waterproof connectors and ruggedized conductors withstand tropical rain and desert dust.

3. Electrodes

Stainless steel stakes provide durable ground contact. Standard length is 30–60 cm. In rocky terrain, longer electrodes or conductive gel improve contact. Poor grounding is the leading cause of data noise.

4. Inversion Software

Software converts apparent resistivity into true geological models. It uses least-squares or smoothness-constrained algorithms. The output is a color-coded cross-section. Blue zones show conductive material. Red zones indicate resistive rock.

---

Ⅳ. Survey Configurations and Data Collection

1. Electrode Arrays

ERT uses several standard geometries. Each array trades resolution against depth and signal strength.

Wenner Array: All electrodes are equally spaced. It offers excellent vertical resolution and strong signals. Best for layered geology like sedimentary basins.

Schlumberger Array: Potential electrodes remain close together. Current electrodes spread wider. This array penetrates deeper while maintaining good resolution. Preferred for groundwater aquifer mapping.

Dipole-Dipole Array: Electrode pairs are separated. This array detects steep structures like faults and dykes. Signal strength drops with depth, so it suits shallow targets.

2. 2D vs 3D Surveys

2D surveys use a single cable line. They produce a vertical cross-section beneath the profile. This covers most engineering and groundwater projects.

3D surveys require multiple parallel lines or grid arrays. They create volumetric resistivity models. 3D Electrical Resistivity Tomography is essential for complex ore bodies, cave systems, and contaminant plumes. Data volume increases tenfold. Processing time extends from hours to days.

3. Field Workflow

A standard 2D survey follows four steps. First, mark electrode positions with GPS. Second, hammer electrodes and connect cables. Third, check contact resistance with the ERT System diagnostic tool. Fourth, run automated acquisition. A 48-channel survey with 2-meter spacing collects 900+ data points in 30–60 minutes.

---

Ⅴ. Applications of Electrical Resistivity Tomography

1. Groundwater Exploration

ERT maps aquifers and water table depth. Freshwater zones show moderate resistivity. Contaminated or saline water drops below 5 Ω·m. Clay layers act as aquitards with very low values. In arid regions, ERT identifies drilling targets without expensive dry holes.

2. Mineral Exploration

Disseminated sulfide ores create low-resistivity anomalies. When combined with Induced Polarization (IP), ERT distinguishes ore from barren conductive zones. The Geotechcn ERT System supports IP data acquisition. This dual-function capability reduces field mobilization costs.

3. Engineering and Environmental

Engineers use ERT to detect subsurface voids, evaluate dam integrity, and monitor landslides. Long-term monitoring systems track seasonal saturation changes. For contaminated sites, ERT maps pollutant plumes and landfill boundaries.

4. Geothermal and Oilfield

ERT identifies fracture networks and thermal fluid pathways. In oilfields, time-lapse surveys track water flooding fronts. The global geophysical equipment market is growing rapidly. Mining, water, and energy sectors drive demand for reliable ERT System solutions.

---

Ⅵ. Advantages and Limitations

1. Key Strengths

ERT is non-invasive. It requires no drilling or excavation. Data density is high. Modern systems collect thousands of points per day. The method resolves lateral variations better than Vertical Electrical Sounding (VES). 3D capability provides volumetric imaging for complex geology.

2. Practical Limitations

ERT needs galvanic contact. Rocky or paved surfaces prevent electrode insertion. Depth is limited by array length. Typical maximum penetration is one-third to one-fifth of the total spread. Cultural noise from power lines and pipes can distort data. Advanced processing or alternative arrays often resolve these issues.

Aspect	ERT Capability	Typical Limit
Depth	10 – 200 m	Array length / 5
Resolution	0.5 – 5 m	Electrode spacing / 2
Speed	1–2 km/day	Terrain dependent
Surface	Soil, grass	Asphalt, rock (difficult)

---

Ⅶ. Conclusion

Electrical Resistivity Tomography is a foundational geophysical method. It delivers high-resolution subsurface images for groundwater, mineral, and engineering projects. An ERT System combines field-hardened hardware with powerful inversion software. Understanding electrode arrays, resistivity principles, and application limits ensures reliable survey design.

Geotechcn provides scalable Electrical Resistivity Tomography solutions from 48-channel portable units to 120-channel 3D systems. Our equipment serves projects across Southeast Asia, Africa, the Middle East, and South America. Learn more professional geophysical solutions at www.geotechcn.net.

Reference

Reference Content	Source URL
ERT Technical Overview (IntechOpen)	https://www.intechopen.com/chapters/64562
ERT Applications in Groundwater Exploration	https://link.springer.com/article/10.1007/s43832-026-00362-x
Resistivity Meter Market Trends	https://www.lucintel.com/resistivity-meter-market.aspx
ERT Applications at Archaeological Sites	https://www.mdpi.com/2072-4292/16/3/519
ERT Principles and Electrode Configurations	https://www.frontiersin.org/journals/water/articles/10.

FAQ