VEXRAD • Deep Subsurface Intelligence
How Deep Can Geophysics See?
There is no single maximum depth for geophysics. The useful question is: at what depth can a particular method detect and resolve the geological feature you care about?
Depth is not one number
Penetration, detection and resolution are different things.
Penetration
How far useful physical energy or field sensitivity extends into the subsurface.
Detection
Whether a target creates a response distinguishable from background geology and noise.
Resolution
Whether the data can distinguish the target’s depth, geometry or boundaries at the scale needed for the decision.
Electromagnetic penetration
How deep can EM detect?
Electromagnetic energy is attenuated as it travels through conductive earth materials. In simplified conditions this is often described using skin depth: the depth at which field amplitude has fallen to about 1/e of its surface value.
A commonly used approximation for a conductive half-space is:
δ = skin depth (m) • σ = conductivity (S/m) • f = frequency (Hz)
Lower frequency → generally deeper penetration
Reducing frequency increases skin depth, all else equal, but commonly reduces spatial resolution.
Higher conductivity → generally shallower penetration
Conductive overburden, saline fluids, clays and conductive formations can strongly attenuate EM energy.
Skin depth is a physical approximation, not a promise that a target at that depth will be detected or geologically resolved.
Method matters
Different methods investigate very different depth scales.
These descriptions are intentionally indicative. Actual depth varies with geology, equipment, geometry, processing and survey objective.
| Method | Indicative depth context | Main depth controls | Primary role |
|---|---|---|---|
| GPR | Usually shallow; tens of metres possible in favourable low-loss materials | Conductivity, frequency, water/clay content, antenna | High-resolution shallow imaging |
| FDEM | Shallow to moderate; configuration dependent | Frequency, coil spacing, conductivity | Conductivity mapping |
| TDEM / TEM | Commonly tens to hundreds of metres | Loop size, time window, conductivity contrast | Resistivity/conductivity sounding |
| AMT / MT | Hundreds of metres to kilometres; long-period MT can investigate crustal scales | Period/frequency, resistivity structure, spacing, data quality | Deep electrical structure |
| Seismic | From shallow engineering scale to many kilometres with suitable design | Source, geometry, velocity contrasts, noise, processing | Structural/velocity imaging |
| ADR / VEXRAD | Project-specific; assessed against target depth, geology and evidence | Target response, host geology, acquisition, signal quality, processing and validation | Depth-referenced pre-drill subsurface intelligence |
The deep-target problem
Why “Can you see to 3 km?” is not enough.
What is the target?
A thick conductive unit, narrow vein, alteration zone, fracture system and lithological boundary are very different geophysical problems.
What surrounds it?
Host rock, conductive cover, fluids and structural complexity can help or hinder detection and interpretation.
What decision is required?
Detecting an anomaly, estimating a boundary and predicting lithology at a drill location require different evidence.
Where VEXRAD fits
Deep investigation should be framed as a testable geological question.
VEXRAD projects can be designed to investigate subsurface intervals extending to kilometre scale, but target depth alone does not establish detectability. VEXRAD evaluates target geometry and geology, expected physical-property response, acquisition conditions, signal quality, available calibration information and independent evidence.
A better feasibility question
Bring us the target, not just the depth.
Useful information
- Target depth and expected thickness
- Commodity, reservoir or geological objective
- Known host lithology and cover
- Existing drilling and geophysics
- Expected physical-property contrast
- Location and access constraints
What VEXRAD can assess
- Whether the geological question is sufficiently defined
- Whether ADR is potentially suitable
- What evidence should be integrated
- Whether calibration/control locations exist
- What a pilot could test
- How success should be validated
Frequently asked questions
Deep geophysics FAQs
How deep can geophysics see?
There is no universal depth limit. Useful depth depends on the method, geology, survey design, target and required resolution.
How deep can electromagnetic geophysics detect?
EM depth varies strongly with frequency, conductivity and system geometry. Lower frequencies and more resistive ground generally favour greater penetration; conductive materials increase attenuation.
What is skin depth in geophysics?
Skin depth measures EM attenuation: in a simplified medium, it is the depth at which field amplitude decreases to approximately 1/e of its surface value. It is not a guaranteed target-detection depth.
Does deeper penetration mean better imaging?
No. Depth and resolution often trade off. Deep sensitivity does not guarantee sufficient resolution for a small or subtle target.
Can geophysics replace drilling?
No. Geophysics can reduce uncertainty and help prioritise targets, while drilling provides direct subsurface evidence.
Can VEXRAD investigate kilometre-scale targets?
VEXRAD can assess kilometre-scale targets, but suitability is project-specific and depends on target geometry, geological setting, signal quality and available validation evidence.
Have a difficult deep target?
Bring us one deep target.
Tell us what you are trying to find, how deep you expect it to be and what evidence you already have. Start with a Project Fit Check before committing to a larger programme.