Geophysical Methods Compared: Seismic, Resistivity and GPR

Geophysical methods measure the subsurface without digging, but because each method measures a different physical property, each answers a different question. Seismic methods see ground stiffness, electrical resistivity sees water and clay content, and ground penetrating radar (GPR) sees shallow objects and voids. Choosing well starts with answering “what am I looking for, and how deep?”. This guide compares the most common methods side by side.

Seismic refraction: where does bedrock start?

A vibration is generated at the surface and the time it takes to refract along layer boundaries and return is measured. The result is P-wave velocity by depth. It is used to estimate depth to bedrock, fill thickness and how hard excavation will be (rippability). Its limit: it works well where velocity increases with depth, but it can miss a slower layer beneath a faster one.

MASW and Vs30: site class for seismic design

Multichannel analysis of surface waves (MASW) gives shear-wave velocity (Vs) by depth. The average shear-wave velocity of the top 30 metres is called Vs30 and is used by many seismic codes to assign the site class. That is why much of the geophysics requested for building projects is MASW or a similar shear-wave method. Which methods are accepted depends on the local code.

Electrical resistivity (ERT): water, clay and voids

Current is injected into the ground through electrodes and voltage is measured; the result is a two-dimensional section showing how strongly the ground resists electricity. Groundwater and clay show low resistivity, dry sand and rock high. It is used for groundwater exploration, landslide slip surfaces, karst voids and contaminant spread. Investigation depth grows with the length of the electrode array; as a rough rule it is about one fifth of the array length.

Ground penetrating radar (GPR): shallow and detailed

GPR sends high-frequency electromagnetic waves and records the reflections. It gives the highest resolution for rebar in concrete, pipes and cables in the ground, shallow voids and archaeological features. Depth depends on antenna frequency and ground: high frequency gives centimetre detail in concrete but is shallow, while low frequency reaches a few metres. Clayey and saline ground attenuates the signal quickly, so GPR is limited there.

Microtremor (H/V): the ground’s natural frequency

A single station records ambient vibrations and the horizontal-to-vertical spectral ratio is calculated. The result is the dominant frequency of the site, used to judge whether it coincides with the building’s own frequency. It is quick and cheap and suits mapping large areas; on its own it does not give layer velocities, so it is usually combined with MASW.

Which method should I choose?

A short match by question: seismic refraction for depth to bedrock and excavation difficulty; MASW for seismic site class (with microtremor if needed); electrical resistivity for groundwater, clay and voids; GPR for pipes, cables, rebar and shallow voids. On complex sites, combining two methods lets one clarify what the other leaves uncertain. Geophysics does not replace boreholes: a borehole is a point but certain, geophysics is continuous but indirect, and the most reliable result comes from interpreting both together.

Common mistakes

1) Choosing the method by price: if the cheap method answers the wrong question, the survey is wasted. 2) Not stating the target depth: array length and antenna frequency are chosen from it. 3) Expecting GPR to work in clay: the signal can die within a few tens of centimetres. 4) Interpreting geophysics without boreholes: velocity or resistivity values alone do not identify the soil type with certainty. 5) Not agreeing the report format up front: parameters required by the code (such as Vs30) must appear explicitly in the report.

Frequently Asked Questions