Geophysical methods are non-invasive techniques used to investigate subsurface conditions by measuring physical properties of soil and rock.
Geophysical methods provide continuous spatial coverage and are a cost-effective complement to borehole investigations.
What Are Geophysical Methods?
Geophysical methods measure contrasts in physical properties (electrical conductivity, seismic velocity, density, magnetic susceptibility) to infer subsurface geology, groundwater conditions, and hidden features. They are particularly valuable for:
- Site characterisation — mapping stratigraphy and bedrock depth
- Void and cavity detection — identifying sinkholes, mines, tunnels
- Groundwater investigations — aquifer mapping, salinity plumes
- Environmental assessments — contaminant plume delineation
- Infrastructure projects — tunnel and pipeline alignment studies
Common Geotechnical Geophysical Methods
Electrical Resistivity Tomography (ERT)
ERT measures the electrical resistivity of subsurface materials by injecting current through electrodes and measuring the resulting potential differences.
| Material | Typical Resistivity (Ω·m) |
|---|---|
| Clay (saturated) | 1–20 |
| Silt | 10–100 |
| Sand (saturated) | 50–500 |
| Sand (dry) | 500–5,000 |
| Gravel | 100–1,000 |
| Fresh rock | 1,000–10,000+ |
| Weathered rock | 100–1,000 |
| Groundwater (fresh) | 10–100 |
| Groundwater (saline) | 0.2–1 |
Applications:
- Bedrock depth profiling
- Groundwater exploration and salinity mapping
- Landslide investigation
- Cavity and void detection
- Contaminant plume delineation
- Archaeological surveys
Array Types:
| Array | Resolution | Depth | Best For |
|---|---|---|---|
| Wenner | Moderate | 0.17× spread length | Vertical changes, noise-free |
| Schlumberger | Good | 0.2× spread length | Horizontal layers |
| Dipole-dipole | Highest | 0.25× spread length | Lateral variations, voids |
| Wenner-Schlumberger | Good | 0.2× spread length | General-purpose |
Ground Penetrating Radar (GPR)
GPR transmits high-frequency electromagnetic pulses into the ground and records reflections from subsurface interfaces.
| Material | Typical Penetration | Resolution |
|---|---|---|
| Dry sand / gravel | 10–20 m | 0.1–0.5 m |
| Wet sand | 3–10 m | 0.1–0.3 m |
| Clay (dry) | 3–5 m | 0.1–0.3 m |
| Clay (wet) | 1–3 m | 0.05–0.1 m |
| Rock (competent) | 10–30 m | 0.1–0.5 m |
| Concrete | 0.5–1 m | 0.01–0.05 m |
Antenna Frequencies:
| Frequency | Depth | Resolution | Application |
|---|---|---|---|
| 25–100 MHz | Deep (10–30 m) | Low | Geological structures |
| 200–400 MHz | Medium (5–10 m) | Moderate | Utility location, stratigraphy |
| 500–1,000 MHz | Shallow (1–5 m) | High | Concrete, roads, shallow services |
| 1,500–2,600 MHz | Very shallow (< 1 m) | Very high | Pavement evaluation, rebar location |
Applications:
- Utility detection (pipes, cables)
- Concrete scanning (post-tension cables, rebar)
- Void and cavity detection
- Archaeological surveys
- Pavement thickness assessment
- Underground storage tank location
Electromagnetic (EM) Surveys
EM techniques measure ground conductivity using induced electromagnetic fields, without requiring ground contact.
| Method | Depth | Application |
|---|---|---|
| Frequency Domain (FDEM) | 0–10 m (variable by coil spacing) | Rapid mapping of near-surface conductivity |
| Time Domain (TDEM) | 10–100+ m | Deep groundwater and mineral exploration |
| Very Low Frequency (VLF) | 10–50 m | Geological structure mapping |
Applications:
- Salinity mapping
- Groundwater exploration
- Soil conductivity mapping
- Landfill leachate detection
- Archaeological surveys
- Rapid site screening
Seismic Refraction
Measures the travel time of compressional (P) waves refracted through subsurface layers to determine layer thickness and seismic velocity.
Applications:
- Bedrock depth determination
- Rippability assessment (excavation method selection)
- Landslide investigation
- Dam and embankment assessment
- Tunnel alignment studies
Seismic Surface Wave Methods (MASW, ReMi)
Measures the dispersion of surface waves (Rayleigh waves) to develop a shear wave velocity (Vs) profile.
| Method | Depth | Application |
|---|---|---|
| MASW (Multichannel Analysis of Surface Waves) | 1–30 m | Vs30 for seismic site class, soil stiffness |
| ReMi (Refraction Microtremor) | 10–100+ m | Deep Vs profiling, passive method |
Applications:
- Seismic site classification (Vs30 for AS 1170.4)
- Liquefaction assessment
- Ground stiffness mapping
- Compaction quality control
Downhole Geophysical Logging
Wireline logging involves lowering probes into a borehole to measure physical properties at depth.
| Log Type | Measurement | Application |
|---|---|---|
| Natural Gamma | Natural radioactivity of formation | Lithology identification, clay content |
| P-Wave Sonic | Compressional wave velocity | Formation stiffness, geomechanical properties |
| S-Wave Sonic | Shear wave velocity | Vs profile, Poisson's ratio |
| Full Waveform Sonic | Full seismic waveform | Fracture detection, permeability zones |
| Fluid Temperature | Borehole fluid temperature | Groundwater flow zones |
| Fluid Conductivity | Fluid electrical conductivity | Saline intrusion, water quality |
| Caliper | Borehole diameter | Cavity detection, washout zones |
| Optical / Acoustic Televiewer | Borehole wall imagery | Fracture orientation, bedding planes |
Survey Planning
Selection Guide
| Objective | Primary Method | Complementary Method |
|---|---|---|
| Bedrock depth | Seismic refraction | ERT |
| Cavity detection | GPR | ERT, microgravity |
| Groundwater / salinity | EM, ERT | Seismic |
| Landslide investigation | ERT | Seismic refraction, MASW |
| Rippability assessment | Seismic refraction | MASW |
| Utility location | GPR | EM |
| Contaminated land | ERT | EM |
| Seismic site class (Vs30) | MASW, ReMi | Downhole seismic |
Survey Design Considerations
| Factor | Impact |
|---|---|
| Line length / spread | Determines investigation depth (typically 4–5× target depth) |
| Station spacing | Determines resolution (closer spacing = higher resolution) |
| Site access | ERT and seismic require surface access for full array |
| Surface conditions | Asphalt, concrete, and hardstand affect GPR and resistivity coupling |
| Electromagnetic noise | Power lines, fences, pipelines affect EM and GPR |
| Target size vs depth | Deeper targets need wider arrays = lower resolution |
Australian Standards
| Standard | Title |
|---|---|
| AS 1726-2017 | Geotechnical site investigations (geophysical methods) |
| AS 1170.4 | Structural design actions — Earthquake loads (Vs30 for site class) |
| ASTM D5777 | Standard guide for using seismic refraction |
| ASTM D6429 | Standard guide for surface geophysics |
| ASTM D6431 | Standard guide for electrical resistivity |
| AS 4326 | Site preparation in landslide-prone areas |