Hydrogeological mapping tells us what the geology should look like based on regional knowledge and surface observations. Geophysical investigation tells us what the subsurface actually looks like beneath a specific site — without drilling a single hole. It is the bridge between surface interpretation and borehole targeting, and in competent hands, it dramatically improves drilling success rates.
The Role of Geophysics in Borehole Siting
Geophysical surveys measure physical properties of subsurface materials — their electrical behaviour, acoustic properties, density, or magnetic character — and use variations in those properties to infer what formations lie below. The goal is to identify fracture zones, aquifer layers, geological contacts, and depth to bedrock: the features that determine where groundwater is most likely to be found in usable quantities.
No geophysical method provides a direct image of water. Instead, each method detects proxies — physical signatures that correlate with water-bearing conditions. Used in combination, and interpreted alongside the hydrogeological survey, they provide a powerful basis for drilling decisions.
Electrical Resistivity Tomography (ERT)
Electrical resistivity is the most widely used geophysical technique in groundwater exploration, and for good reason: it is relatively affordable, field-portable, and directly sensitive to the presence of water.
The method works by injecting electrical current into the ground through electrodes and measuring how the current distributes through the subsurface. Water-saturated sediments and fractured rock conduct electricity more readily than dry or competent rock, producing distinct low-resistivity anomalies. Clay layers, saline water, and fresh groundwater each produce characteristic resistivity signatures that a trained interpreter can distinguish.
In vertical electrical sounding (VES), measurements are taken at progressively larger electrode spacings to build a one-dimensional profile of resistivity with depth. In 2D ERT profiling, electrodes are laid out in a long array and data is collected along the line to produce a two-dimensional cross-section of the subsurface — far more informative for identifying the lateral extent of aquifer zones and fault structures.
Seismic Methods
Seismic surveys use artificially generated sound waves — typically produced by a sledgehammer blow or a small explosive charge — and measure how those waves travel through the ground and return to the surface. Different geological materials transmit seismic waves at different velocities, allowing boundaries between formations to be mapped.
Seismic refraction is the most common technique in groundwater work. It is particularly effective at mapping the depth to bedrock, identifying weathered zones above hard rock (which often host significant groundwater), and locating buried valleys filled with alluvial sediments. It works best where subsurface layers increase in velocity with depth — a condition met in most hard-rock terrains.
Seismic reflection, more commonly associated with oil and gas exploration, can also be applied in deep or complex aquifer settings, though its cost is rarely justified for standard borehole projects.
Electromagnetic (EM) Surveys
Electromagnetic methods measure the ground’s response to oscillating magnetic fields rather than injected electrical current. They are particularly effective at rapid reconnaissance — covering large areas quickly to identify zones of interest before more detailed resistivity work is done. Time-domain EM (TDEM) is useful for deeper investigations, while frequency-domain EM suits shallower targets.
Magnetic and Gravity Surveys
In hard-rock environments, aeromagnetic and ground magnetic surveys can identify dykes and intrusive bodies that act as barriers to groundwater flow, as well as linear fracture zones along which groundwater preferentially accumulates. Gravity surveys can assist in mapping the depth and geometry of sedimentary basins.
Interpreting and Integrating Results
Geophysical data is only as useful as its interpretation. Raw resistivity or seismic data must be processed, modelled, and interpreted in the context of the regional geology, the hydrogeological survey findings, and any available borehole logs from nearby sites. Anomalies that look promising in isolation may be misleading without this context.
The best practice is to use multiple complementary methods, cross-validate results, and present interpreted profiles with a clear statement of confidence and uncertainty. The output — a set of recommended drilling locations with predicted depths and target formations — forms the direct basis for the drilling programme.
