Why a Hydrogeological Survey Matters Before Drilling
Drilling a borehole without a prior hydrogeological survey is a gamble — and across Kenya, the consequences of a misplaced or wrongly-depth borehole are significant. A dry or low-yield borehole still costs the full drilling bill, and many cannot be rehabilitated. A survey costs a small fraction of the drilling investment but dramatically changes the probability of hitting a productive aquifer at the right depth.
Groundwater in Kenya does not occur in uniform underground lakes. It is found in fractures in hard rock (basement complex), intergranular pores in sediments, and vesicular zones in volcanic formations — all of which have highly variable spatial distribution. Geophysical surveys measure the subsurface electrical resistivity to identify where fractures, saturated zones, and impermeable boundaries lie, giving us a map of the most likely productive zones before a metre of ground is broken.
We recommend a survey for all commercial, institutional, agricultural, and community boreholes — and for domestic boreholes in areas with known difficult hydrogeology. Our survey reports give you a ranked list of siting recommendations with estimated depth and anticipated yield, so you can make an informed drilling decision.
Hydrogeological Survey Services We Offer
Vertical Electrical Sounding (VES)
VES is the most widely used geophysical method for borehole siting in Kenya. Electrodes are laid in a line at progressively increasing spacings, and the apparent resistivity measured at each configuration is interpreted to produce a 1D model of subsurface layering — showing depth, thickness, and resistivity (as a proxy for water content) of each geological layer. A VES survey on a standard plot takes 2–4 hours and results are processed the same day.
Electrical Resistivity Tomography (ERT)
ERT produces a 2D cross-section (pseudo-section) of subsurface resistivity along a survey line — far more detail than VES. This method is particularly valuable for tracing faults and fracture zones (which are the primary groundwater targets in Kenya's hard rock basement terrain), mapping buried river channels, and identifying the precise lateral extent of a productive zone. ERT surveys use multi-electrode arrays (typically 28–72 electrodes) and can cover 100–500m profiles.
Borehole Siting & Location Report
Following geophysical data collection and interpretation, we issue a formal written borehole siting report with GPS coordinates of the recommended drilling site(s), estimated depth to water table, expected aquifer type, anticipated yield range, recommended drilling diameter and casing depth, and a ranked priority order where multiple survey points were assessed. The report is WRA-compliant and can be attached to your drilling permit application.
Hydrogeological Mapping
For large projects — farms, tea estates, irrigation schemes, municipal water supply, and NGO community water programs — we conduct area-wide hydrogeological mapping using existing geological maps, satellite imagery, remote sensing lineament analysis, and multi-point VES/ERT grids. The output is a site-specific hydrogeological map identifying the most productive groundwater zones, aquifer boundaries, and optimal siting areas across the entire property.
Aquifer Yield Estimation
Based on geophysical interpretation, regional aquifer data, and comparison with nearby borehole records, we provide estimated yield ranges for your recommended drilling site. These are indicative estimates (not guarantees) but provide a basis for pump sizing, system design, and feasibility assessment before drilling commences. Post-drilling pumping tests are required to establish reliable design yields.
Pre-Drilling Risk Assessment
A written pre-drilling risk assessment evaluating hydrogeological risk factors for the proposed drilling site — including proximity to contamination sources (latrines, fuel storage, livestock, surface runoff), aquifer vulnerability, and expected water quality based on regional geochemistry. This assessment supports both drilling permit applications and project due diligence for loan or grant-funded water projects.
What Happens During a Hydrogeological Survey?
Our survey process is systematic and minimally disruptive to your site — no excavation, no heavy equipment, no ground disturbance.
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1Desk Study
Before the field visit, our hydrogeologists review existing geological maps, published aquifer data, satellite imagery, and records of nearby boreholes (where available from WRA) to inform the survey design and identify target zones.
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2Field Reconnaissance
A site visit to assess surface geology, topography, drainage patterns, and potential survey lines. Surface expressions of fractures and lineaments (ridges, valleys, vegetation patterns) are identified and GPS-logged.
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3Geophysical Data Acquisition
VES or ERT measurements are taken at planned survey points using a Terrameter or ABEM resistivity meter. For VES, this typically takes 2–4 hours per sounding point. ERT profiles require 4–8 hours per 100–200m line depending on electrode spacing and terrain.
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4Data Inversion & Modelling
Field data is processed using resistivity inversion software (IPI2WIN for VES, RES2DINV for ERT) to produce subsurface resistivity models. Results are correlated with geological expectations and any available nearby borehole lithological logs.
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5Interpretation & Siting Recommendation
Quantitative interpretation of resistivity models identifies potential aquifer zones, their depths, and estimated saturated thicknesses. The optimal drilling site is selected and GPS coordinates are marked in the field.
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6Written Survey Report
A full technical report is issued within 3–5 working days of the field survey, including methodology, field data tables, resistivity models, geological interpretation, drilling site coordinates, recommended depth, and anticipated yield class.
Kenya's Groundwater Geology — Why Siting Matters
Hard Rock Basement Areas
Much of Central Kenya, Eastern, and parts of Nairobi county overlie metamorphic basement complex where groundwater is confined to fractures, joints, and fault zones. Finding these features without geophysics is luck-dependent — success rates without surveys can be as low as 40–50%.
Rift Valley Volcanics
The Rift Valley region has complex aquifer geometry in volcanic formations — some volcanic horizons are highly productive (vesicular basalts, pyroclastics) while adjacent formations are impermeable. ERT surveys are valuable in this terrain to trace productive horizons laterally.
Sedimentary Basins
Coastal Kenya, the Lamu Basin, and parts of the Tana River area have sedimentary aquifers with more predictable geometry — but depth to water and saline intrusion are key uncertainties that geophysical surveys can identify before drilling.
Alluvial Aquifers
River floodplains and valley floors in many parts of Kenya have shallow alluvial aquifers that are productive and relatively low cost to develop — but with higher contamination risk. VES mapping helps define the depth and lateral extent of these alluvial deposits.
Hydrogeological Survey FAQs
It is not always legally required, but we strongly recommend it for any borehole where failure would be costly — commercial, agricultural, institutional, or community boreholes. For domestic boreholes in areas where many nearby boreholes have been successful (known productive geology, consistent water table), a survey may not be essential. For hard rock basement areas, the survey significantly improves success probability and is almost always a worthwhile investment relative to the drilling cost.
VES stands for Vertical Electrical Sounding. It works by injecting an electrical current into the ground through two current electrodes and measuring the resulting voltage drop across two potential electrodes. By progressively increasing the electrode spacing, deeper layers are sampled. The apparent resistivity at each spacing is calculated and the resulting data curve is mathematically inverted to fit a layered subsurface model — showing depth, layer thickness, and resistivity (which correlates with lithology and water content) for each layer.
No geophysical survey can guarantee groundwater. All geophysical methods are indirect — they measure physical properties (electrical resistivity) that are interpreted as indicators of subsurface conditions. Saturated rock and clay can have similar resistivity signatures; weathered unproductive rock can appear promising. What the survey does is significantly increase the probability of drilling in geologically favourable locations — improving success rates from ~50% to 80–90% in basement complex terrain based on our field experience and broader industry data.
Our hydrogeological survey report includes: project and site description; regional and local geology; survey methodology and equipment used; field data tables; resistivity model curves and cross-sections; geological interpretation of resistivity models; recommended borehole siting point(s) with GPS coordinates; estimated depth to water table and total recommended depth; anticipated aquifer type; estimated yield class (low/medium/high); and recommended casing and completion design. For WRA permitting purposes, the report is formatted to meet their technical requirements.
A standard VES survey for a single domestic or commercial borehole site — assessing 3–5 potential drilling points — typically takes one full day in the field. An ERT survey for a single long profile takes 4–8 hours. Large-area hydrogeological mapping projects for farms or community schemes may take 2–5 days of fieldwork. The written report is issued 3–5 working days after fieldwork. Total lead time from booking to report delivery is typically 7–10 days.