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Methods

Geophysical methods use physical principles — such as seismic wave propagation, electrical conductivity, electromagnetic response, and gravity — to characterize subsurface conditions without excavation.
Each method provides a different type of information, depth of investigation, and resolution. In practice, several methods are often combined to build a reliable subsurface model.

Below is an overview of the most widely used methods in applied geophysics, their strengths, limitations, and common use-cases.

Ground-Penetrating Radar (GPR)

GPR transmits short electromagnetic pulses into the ground and records reflections from subsurface contrasts. It is one of the fastest and most detailed near-surface imaging techniques.

What it’s used for:

  • Locating utilities, pipes, cables, and buried infrastructure

  • Inspecting concrete, pavement, bridges, tunnels

  • Mapping archaeological structures and graves

  • Detecting voids, cavities, and sinkholes

  • Evaluating road layers and construction quality

Strengths:

  • High resolution

  • Rapid data collection

  • Non-destructive

Limitations:

  • Reduced performance in clay-rich or highly conductive soils

  • Shallow penetration compared to other methods

Geoelectrical Methods (ERT / Resistivity / IP)

Electrical resistivity tomography (ERT) measures the electrical resistance of the ground by injecting current and measuring the resulting voltage differences.

What it’s used for:

  • Groundwater mapping and aquifer characterization

  • Soil and bedrock profiling

  • Detection of contamination plumes

  • Locating cavities and weak zones

  • Monitoring moisture changes

Strengths:

  • Good depth penetration

  • Works well in conductive materials

  • Flexible 2D/3D configurations

Limitations:

  • Slower acquisition than EM or GPR

  • Requires electrode contact with the ground

Electromagnetic Induction (EM)

EM methods measure variations in soil conductivity using time-varying electromagnetic fields.

What it’s used for:

  • Rapid mapping of soil properties

  • Detecting buried objects and waste

  • Salinity and contamination studies

  • Agricultural soil assessment

  • UXO screening

Strengths:

  • Fast, non-invasive, no electrodes

  • Works well for large-area surveys

  • Effective in conductive soils where GPR struggles

Limitations:

  • Lower resolution than GPR

  • Conductivity only — no direct depth imaging

Seismology (Active & Passive)

Seismic methods measure how seismic waves travel through the ground. Different wave types reveal different physical properties.

Active Seismic (surface methods)

Used for high-resolution near-surface imaging.
Applications: bedrock depth, rippability, geotechnical site characterization, void detection.

Passive Seismic (ambient noise, HVSR)

Uses natural seismic energy.
Applications: estimating sediment thickness, resonance frequency of sites, microzonation.

Strengths:

  • Works at both shallow and large depths

  • Provides mechanical property data (Vs, Vp)

  • Essential for engineering and earthquake studies

Limitations:

  • More equipment and setup

  • Requires skilled interpretation

Magnetics

Measures variations in the Earth’s magnetic field caused by differences in subsurface geology or metallic materials.

What it’s used for:

  • Mapping bedrock structures

  • Mineral exploration

  • Archaeological investigations

  • UXO detection

  • Mapping buried ferrous objects

Strengths:

  • Very fast

  • Works well over large areas

  • Sensitive to ferrous materials

Limitations:

  • Non-ferrous targets are not detectable

  • Urban areas may contain magnetic noise

Gravimetry

Measures small variations in gravitational acceleration caused by density contrasts.

What it’s used for:

  • Mapping bedrock topography

  • Karst/void detection

  • Basin depth mapping

  • Large-scale geological structures

Strengths:

  • Sensitive to deep structures

  • Passive method — no energy input

  • Effective in areas where other methods fail

Limitations:

  • Lower resolution

  • Slow and requires precise measurements

Metal Detection

Metal detectors use electromagnetic induction to locate metallic targets near the surface.

What it’s used for:

  • Archaeology

  • Utility locating

  • Forensics

  • UXO screening

  • Engineering site surveys

Strengths:

  • Simple and fast

  • Great for locating metallic objects

Limitations:

  • No depth imaging

  • Only detects metal

XRF (X-Ray Fluorescence)

XRF is a non-destructive technique for determining the chemical composition of soils and materials using X-ray excitation.

What it’s used for:

  • Soil chemistry and contamination measurements

  • Material identification

  • Archaeological material analysis

  • Industry and mining exploration

Strengths:

  • Immediate on-site results

  • High accuracy for elemental composition

Limitations:

  • Not a structural imaging method

  • Requires calibration and careful interpretation

Method Selection Guide

When choosing a method, consider:

Soil type: (e.g., GPR in sand; ERT in clay)
Depth required: (GPR shallow; ERT/Seismic deeper)
Target type: (void, metal, groundwater, layer boundaries)
Resolution needed: (GPR highest resolution; EM fastest large-scale mapping)
Survey environment: (urban noise, accessibility, terrain)

A successful geophysical investigation often combines multiple methods to increase confidence and reduce ambiguity.