Application Guide
Geophysical Equipment for Groundwater & Aquifer Characterisation
Electrical and electromagnetic methods map aquifers, clay barriers, bedrock and saltwater intrusion — siting boreholes and building groundwater models with far fewer dry holes.
Groundwater geophysics works because water content, salinity and clay content all change the electrical resistivity of the subsurface. Clean sands and gravels with fresh water are relatively resistive; clays and saline water are conductive. Mapping resistivity in 2D or 3D therefore maps the hydrogeology.
Surface methods (ERT and TEM) give the spatial picture between and beyond boreholes; borehole logging ties the geophysics to measured lithology and water quality.
Methods Compared
| Method | Detects | Typical Depth | Limitations |
|---|---|---|---|
| Electrical resistivity tomography (ERT) | Aquifer geometry, clay layers, fractures, contamination | Roughly one-fifth of the electrode spread length — typically tens to 100+ m | Needs good electrode contact; long spreads for deep targets |
| Transient EM (TEM / TDEM) | Resistivity layering, saltwater interface, depth to bedrock | Tens to several hundred metres | Lower near-surface resolution; affected by metal infrastructure |
| Frequency-domain EM conductivity | Shallow salinity, clay distribution, plumes | Typically up to ~60 m depending on coil spacing | Qualitative depth information |
| Borehole logging | Lithology, water quality, flow zones, well construction | Full borehole depth | Requires an open or PVC-cased borehole |
Depths are typical field values — actual performance depends on ground conditions, target size and survey design.
ERT or TEM?
ERT gives the best resolution for the top tens of metres and works well along profiles and in 3D grids. TEM needs no electrode contact, works on dry or resistive surfaces, and reaches far deeper with a modest loop — ideal for deep aquifers, saltwater intrusion and depth-to-bedrock. Many groundwater programmes run both and invert them jointly.
Borehole siting
For water-supply drilling, a few ERT profiles or TEM soundings across the target area typically pay for themselves many times over by avoiding unproductive boreholes. Vertical electrical soundings (VES) with a single-channel meter remain a low-cost option for rural water programmes.
Recommended Instruments
A selection from our catalog. We source many more models — tell us what you need.
Which Condition Tier?
Resistivity and TEM systems are long-lived instruments. Absolute Certified and Refurbished units are popular with consultants and NGOs; classic single-channel meters such as the Terrameter SAS series are widely sourced Refurbished for borehole-siting programmes.
See the condition tier guide for what New, Refurbished, Absolute Certified and Surplus each include, or the industries overview for related sectors.
Frequently Asked Questions
How deep can resistivity surveys detect groundwater?
ERT depth of investigation is roughly one-fifth of the total electrode spread length, so a 400 m spread investigates to around 80 m. For greater depths, TEM is usually more practical.
Can geophysics find saltwater intrusion?
Yes — saline water is strongly conductive, so ERT, TEM and EM conductivity surveys map the fresh/saltwater interface very effectively.
Is a used Terrameter a good choice?
Yes, for many projects. ABEM Terrameter LS and SAS-series instruments are robust and well supported, and are available Refurbished or Absolute Certified with a calibration check.
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