An ERT survey (Electrical Resistivity Tomography) is one of the most useful tools in engineering geophysics. It produces a continuous picture of the subsurface along a line, showing where soil changes to rock, where groundwater sits and where weak or fractured zones hide between boreholes.
ERT full form and the basic idea
ERT stands for Electrical Resistivity Tomography. Different materials resist electric current differently: dry rock is highly resistive, while clay and saturated or saline ground conduct current easily. By injecting a small current into the ground and measuring the voltage at many electrode combinations, we can work out how resistivity varies with depth and distance.
The ERT survey method in the field
- Layout: a line of steel electrodes is placed at equal spacing, typically a few metres apart, and connected by a multi-core cable.
- Acquisition: a multi-electrode resistivity meter automatically switches through hundreds or thousands of electrode combinations (arrays such as Wenner, Schlumberger or dipole-dipole).
- Roll-along: for long lines, cable sections are moved forward to extend the profile.
- Topography: electrode positions and levels are surveyed with GNSS so the section follows the real ground surface.
- Inversion: software converts the measured values into a 2D (or 3D) resistivity model of the ground.
The depth of investigation depends mainly on the total spread length. As a rough guide, it is around one-fifth to one-sixth of the line length, so longer lines see deeper.
Electrical resistivity survey for groundwater
The most common search in India is the electrical resistivity survey for groundwater. ERT helps identify weathered and fractured rock zones, buried channels and aquifers that are likely to yield water, so a borehole is drilled where it has the best chance of success. Compared with a single-point vertical electrical sounding (VES), ERT shows how these zones change sideways, which reduces the risk of drilling a dry hole a few metres from a good one.
Results still need interpretation by an experienced geophysicist and, ideally, confirmation with nearby borehole or well data, because clay and saline water can both look conductive.
Engineering uses of ERT
- Foundations: mapping depth to rock and soft pockets between boreholes for buildings, bridges and industrial plants.
- Tunnels and metro alignments: identifying fault zones, weathered rock and water-bearing zones ahead of excavation.
- Dams and embankments: detecting seepage paths and weak zones.
- Landslide-prone slopes: locating slip surfaces and saturated layers (see our landslide protection work).
- Earthing design: soil resistivity values for substations and transmission towers.
- Environmental: mapping leachate plumes and contamination.
Relevant standard
In India, geological exploration by the electrical resistivity method is covered by IS 15736. Internationally, practitioners also follow ASTM guidance for surface resistivity methods. A good report states the array, electrode spacing, line length and inversion parameters so the work can be checked.
What affects the cost of an ERT survey?
- Total line length and number of profiles.
- Required depth (longer spreads, more cable and more time).
- Terrain, vegetation and access.
- Whether 3D coverage or time-lapse monitoring is needed.
- Integration with boreholes, seismic or GPR data in the report.
What you receive
A typical deliverable includes a location plan, colour resistivity sections with depth and chainage, an interpreted section showing likely soil, weathered rock, fresh rock and water-bearing zones, and clear recommendations such as where to drill or where foundations need special attention.
ERT is part of our wider geophysical survey service, often combined with MASW and borehole investigation for a complete ground model.
Frequently asked questions
What is the full form of ERT survey?
ERT stands for Electrical Resistivity Tomography, a geophysical method that maps how electrical resistivity varies below the ground surface in 2D or 3D.
How deep can an ERT survey see?
The depth of investigation is roughly one-fifth to one-sixth of the total electrode spread length, so a longer line investigates deeper. The array type and ground conditions also matter.
Can an ERT survey guarantee groundwater?
No geophysical method can guarantee water. ERT greatly improves the chance of success by locating fractured and weathered zones and aquifers, and the result should be interpreted with local geology and well data.
Which is better for groundwater, VES or ERT?
VES gives a one-dimensional sounding at a single point. ERT gives a continuous section along a line, which shows lateral changes and is usually more reliable for choosing a drilling point.
How long does an ERT survey take?
A single profile can often be acquired in a few hours. Processing, interpretation and reporting usually take a few days depending on the number of lines.
Talk to our team
Need an ERT survey for groundwater, foundations or a tunnel alignment? Call +91 98718 04288, email contact@geowingseng.com or send your site details and we will reply within one working day.
