Geophysical Survey Methods: GPR, ERT and Seismic Compared

What is a geophysical survey?

A geophysical survey maps what lies below ground by measuring a physical property from the surface, such as radar reflection, current flow or sound speed. The differences become a picture of the subsurface. It does not dig; it shows where things change, and a borehole or pit confirms what the change is.

Every method relies on contrast. A plastic pipe in dry sand reflects radar, saline groundwater conducts current, and hard rock under soft soil carries waves faster. Without contrast, a target stays invisible to that method, which is why the choice of method comes before the choice of contractor. ASTM publishes a whole guide, D6429, on selecting surface geophysical survey methods for a given question¹.

How do the main geophysical survey methods compare?

GPR gives the sharpest picture of the top few metres, while resistivity reaches deeper with less detail. Seismic methods read stiffness and rock depth, and electromagnetic induction sweeps large areas quickly for conductive features.

Depth figures are typical ranges from published guidance, not promises; site conditions decide the real figure.² For locating live cables specifically, a signal locator and GPR work best together; read EM Cable Locator vs GPR for that pairing.

Where does GPR fit among the methods?

Ground penetrating radar is the high-resolution, shallow method: it sends short pulses into the ground and records echoes from anything with different electrical properties. It is the workhorse for underground utilities, concrete scanning and cavity checks, so most urban survey requests in India end up as radar work.

Lower-frequency antennas see deeper but blur small targets, and higher ones resolve detail but fade quickly. Published guidance cites a 100 MHz antenna reaching about 18 m in clean dry sand, against under 2 m at 1,000 MHz in the same material². Clay and wet fill absorb far faster, so Indian cities on alluvium often give 1.5–3 m. The principle is explained under GPR full form, and real radar images sit in our GPR Radargram Library guide.

Radar is also the quickest way to find loose fill and air gaps under roads and slabs. That is often the real question behind a "soil anomaly" request; see void detection for how that work is scoped.

What do electrical resistivity methods tell you?

Resistivity methods inject current through electrodes and measure the resulting voltage. Clay, saline water and leachate conduct well, while dry sand, gravel and solid rock resist. A long electrode line inverted into a cross-section becomes electrical resistivity tomography.

ERT suits groundwater studies, depth to bedrock, landfill boundaries and karst, and the four-electrode Wenner version measures soil resistivity for earthing design at substations and towers. Resolution is roughly the electrode spacing near the surface and coarsens with depth, so it will not find a 100 mm pipe. Guidance also warns that 2D sections can mislead where ground changes sharply in three dimensions².

When are seismic refraction and MASW used?

Seismic refraction and MASW answer stiffness and rock questions: how deep the rock lies, whether it can be ripped, and how stiff the soil is with depth. Both use a line of geophones and a hammer or weight drop.

• Seismic refraction times first-arriving waves bent along faster layers. It cannot see a slow layer beneath a fast one, and the spread must be four to five times the target depth.
• MASW builds a shear-wave velocity profile for soil stiffness, ground improvement checks and foundation design.

Traffic and machinery noise slow city-centre seismic work, so night or weekend shifts are common.

Electromagnetic and magnetic methods

Electromagnetic induction instruments are carried across a site without touching the ground, recording conductivity changes and buried metal. They suit a first sweep of a large plot for old tanks or contamination, before GPR or trial pits examine the anomalies. Magnetometers do a similar job for iron and steel, and both are upset by fences, vehicles and overhead lines.

GPR vs ERT and the rest: which method answers your question?

Start from the question, not the instrument. Shallow and small calls for GPR, deep and broad for resistivity or seismic, and anything that matters for civil engineering design needs a borehole as well.

• Pipes and cables before digging: GPR with an electromagnetic locator, to avoid accidental utility strikes and service disruptions.
• A void under a road or slab: GPR, confirmed by a probe or core.
• Water table or aquifer: resistivity.
• Rock depth or soil stiffness: seismic refraction, MASW or resistivity, checked by boreholes with standard penetration tests.

How is a geophysical survey planned and reported?

A sound job defines a clear question, checks existing records, tests the method on a short trial line and designs line spacing so the smallest target is crossed. Processing and interpretation follow, then trial pits, cores or vacuum excavation confirm selected anomalies. The report gives each finding a quality level and states what the method could not see, so the utility maps and ground models reduce risks on construction projects. A blank GPR area might mean no services, or a signal that died at 1 m, and the reader needs to know which.

What does a geophysical survey cost with Mezux Touch?

Our GPR survey is ₹24 per metre standard and ₹32 per metre with an L-section drawing, GST extra. The minimum order is ₹50,000 in Delhi NCR and ₹1,00,000 elsewhere in India. Orders over 100 km receive 25% off the whole value, over 500 km 40%, and over 1,000 km 50%.

Mezux Touch runs 58 company-owned GPR systems and two rotary drilling rigs. Geotechnical drilling is ₹1,500 per metre including SPT, with a ₹1,00,000 minimum per site, and laboratory tests are charged per schedule. Where the question needs resistivity or seismic work, we say so at scoping instead of forcing radar onto it. NCR crews reach site the same day or within 1 working day. Elsewhere the job goes to our colleague living in that city, who begins immediately or one working day later. The quote carries a travel or mobilisation line only where one applies. Describe the problem, not only the method, and get a price for this work for your project.

Sources

1. ASTM D6429-23, Standard Guide for Selecting Surface Geophysical Methods. store.astm.org
2. Interstate Technology & Regulatory Council (ITRC), Advanced Site Characterization Tools, Section 5: Surface Geophysics. asct-1.itrcweb.org

Frequently asked questions

What is the difference between GPR and ERT?

GPR records radar reflections and gives fine detail in the top few metres. ERT measures electrical resistance and reaches deeper, but with much coarser detail.

Which geophysical method is best for groundwater?

Resistivity methods are the usual first choice, because water-bearing layers and their salinity change resistance strongly.

Can GPR detect bedrock?

In dry, resistive ground it can show shallow rock; in clayey or wet ground the signal often fades before reaching it, so seismic or resistivity methods are safer.

How deep can GPR see in Indian soils?

Often 1.5–3 m in clayey or wet urban ground, more in dry sand. For the factors that decide depth, read more on GPR Depth Accuracy.

Does a geophysical survey replace boreholes?

No, it shows where conditions change between boreholes and reduces how many are needed, but boreholes confirm what the materials are.

Is a geophysical survey safe for existing structures?

Yes, the methods are non-destructive, and seismic sources are small hammer blows or weight drops.

Written by Saniya Parveen · Technically reviewed by Rahul Rai · Updated 8 November 2026