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.
Site Selection Criteria
Geophysical surveys may identify several candidate locations with good groundwater potential. Site selection is the process of choosing among them — and that choice involves far more than hydrogeology alone. A productive aquifer beneath unsuitable ground is worthless if the site cannot be safely accessed, legally used, or practically developed. Site selection is where hydrogeological science meets engineering, law, environment, and community.
Hydrogeological Suitability
The starting point remains the subsurface. The preferred site should overlie the most promising aquifer target — the zone of highest predicted yield, appropriate depth, and acceptable water quality risk. Where multiple sites have similar hydrogeological scores, the other criteria below become decisive.
Depth to water is a key consideration. Shallower aquifers are cheaper to drill and pump, but may be more vulnerable to surface contamination. Deeper confined aquifers offer better natural protection but higher drilling costs and greater uncertainty.
Distance from Contamination Sources
Groundwater contamination is largely irreversible once established. Siting criteria therefore place strict minimum separation distances between boreholes and potential pollution sources. Common regulatory standards require:
- 30–50 metres from pit latrines, septic tanks, and soakaway pits
- 50–100 metres from animal enclosures and manure storage
- 100+ metres from fuel storage, industrial sites, and waste dumps
- Upslope or upgradient positioning relative to contamination sources where possible
These distances are minimums — greater separation is always preferable where land availability permits.
Accessibility for Drilling Equipment
Modern drilling rigs are heavy, large machines. They require firm, reasonably level ground capable of bearing loads of several tonnes. Access tracks must be wide enough and structurally sound enough to support the rig and its support vehicles. Sites that require extensive road construction add significantly to project costs.
Seasonal accessibility also matters. In areas with pronounced wet seasons, sites that are accessible in the dry season may become impassable during drilling if timing is not planned carefully.
Proximity to the Point of Use
Water must be delivered from the borehole to wherever it is needed. The greater the distance, the more extensive — and expensive — the reticulation infrastructure required. Siting a borehole as close as practical to the primary demand point reduces pipe lengths, pumping energy, and infrastructure maintenance costs.
In community water supply projects, this also affects who the borehole serves most directly, which can have social and equity implications that require careful handling.
Land Tenure & Legal Access
A borehole cannot be drilled on land to which the project developer does not have legal access. This seems obvious, but land tenure disputes are among the most common causes of borehole project delays and failures — particularly in peri-urban and rural settings where land rights may be informal, contested, or overlapping.
Before committing to a site, it is essential to verify ownership or occupancy rights, secure any necessary easements or wayleaves for pipelines, and confirm that the landowner's consent is documented in a form that will remain enforceable after the borehole is commissioned.
Community Acceptance & Social Factors
In development and humanitarian contexts especially, community buy-in is not optional. A borehole sited without adequate consultation may face resistance, interference, or neglect — all of which undermine its long-term functionality. Community members often have valuable local knowledge about seasonal flooding, soil conditions, and historical water sources that can improve siting decisions.
Effective community engagement during site selection builds ownership, supports future maintenance, and prevents the all-too-common outcome of a technically successful borehole that nobody uses or cares for.
Site selection is an exercise in multi-criteria decision-making. The best site is the one that optimally balances groundwater potential, safety, accessibility, legal clarity, and community acceptance. Documenting the rationale for site selection — including why alternative sites were rejected — is good professional practice and provides a clear audit trail for regulators, funders, and future project managers.