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Most of what geologists, engineers, and water developers need to know lies underground, out of sight. Geophysical surveys solve this problem by measuring physical properties of the Earth, such as electrical resistivity, magnetism, density, and seismic velocity, from the surface or from the air. Variations in these properties reveal buried rock units, faults, ore bodies, aquifers, and voids without the cost of drilling everywhere.
What Is a Geophysical Survey?
A geophysical survey is a non-invasive investigation that maps variations in subsurface physical properties. The measurements are processed and interpreted to produce profiles, maps, and models. Geophysics does not provide direct proof of what lies below, but when calibrated with geology and drilling, it narrows uncertainty and reduces the number of holes needed.
Main Geophysical Methods
Electrical resistivity (VES and ERT). Current is injected into the ground and the resulting voltage is measured. Resistivity depends on rock type, porosity, water content, and water salinity. It is the workhorse of groundwater exploration and is also used for engineering and mineral investigations.
Induced polarisation (IP). Measures the ground’s ability to store electrical charge. It is sensitive to disseminated sulphides and some clays, which makes it useful for metallic mineral exploration.
Magnetic surveys. Measure small variations in the Earth’s magnetic field caused by magnetic minerals. They map lithology, faults, dykes, and iron-rich formations, and are widely used in regional mineral and structural mapping.
Gravity surveys. Measure variations in gravitational acceleration caused by density contrasts. They help map sedimentary basin depth, intrusive bodies, and large structures, and are used in geothermal studies.
Electromagnetic (EM) methods. Use time-varying fields to detect conductive materials such as sulphides, saline groundwater, and clay layers. Time-domain and frequency-domain systems exist, deployed on the ground or from aircraft.
Magnetotellurics (MT). Uses natural electromagnetic signals to image resistivity to great depth. It is a key tool in geothermal exploration, where it can map conductive clay caps over hot reservoirs.
Seismic methods. Seismic refraction estimates the depth to bedrock and weathering thickness, while seismic reflection images layered structure. Both are used in engineering, groundwater, and energy studies.
Ground-penetrating radar (GPR). Uses radar pulses for shallow high-resolution imaging, such as utilities, voids, and shallow stratigraphy.
Radiometric surveys. Measure natural gamma radiation from potassium, uranium, and thorium. They help map lithology and alteration.
Applications Across Sectors
Groundwater. Resistivity methods locate fractures, weathered zones, and aquifers, and guide borehole siting.
Mineral exploration. Magnetics, IP, EM, radiometrics, and gravity help locate structures, alteration, and sulphide mineralisation, and focus drilling.
Geothermal. MT, gravity, magnetics, and seismic monitoring map heat sources, clay caps, and permeable structures in Rift-related systems.
Geotechnical and civil engineering. Seismic refraction and resistivity measure depth to bedrock, rippability, weak zones, and cavities for roads, dams, and buildings.
Environmental studies. Contaminant plumes, landfill boundaries, and saline intrusion can be traced using resistivity and EM.
Archaeology and infrastructure. GPR and magnetics locate buried features and utilities.
Planning a Survey
Effective geophysics begins with a clear question. The choice of method depends on the target’s physical contrast with its surroundings, the depth of interest, the terrain, and the level of noise. Planning includes selecting survey lines, station spacing, equipment, and processing steps. Pilot tests are useful for confirming that the chosen method can detect the target.
From Data to Interpretation
Raw measurements must be corrected for noise, drift, and topography. Processing may include filtering, gridding, and inversion, which converts measurements into a model of subsurface properties. Interpretation then links these models to geology and drilling. Because different materials can produce similar responses, geophysical results are inherently non-unique. For example, low resistivity could indicate fresh fractured rock with water, clay, or saline water. Integrating geology, multiple methods, and borehole data resolves such ambiguity.
Challenges in Kenyan Conditions
- Variable terrain and vegetation can complicate line layout and access.
- Electrical noise from power lines, fences, pipelines, and buildings affects resistivity and EM data.
- Poor ground contact in dry, stony soils may require special electrode treatment.
- Complex volcanic geology can produce rapid lateral changes that demand tighter station spacing.
- Cultural interference near towns and farms requires careful planning.
Quality Control
Good practice includes calibrating instruments, repeating readings, keeping field logs, recording coordinates with GPS, and checking data quality in the field. Reports should state limitations clearly rather than overstating certainty.
Value for Money
Geophysics is typically far cheaper than drilling. Even a modest survey can prevent drilling in unfavourable ground, direct holes to better targets, and provide a basis for engineering design. The best results come when geophysics is treated as part of an integrated programme, not as a stand-alone solution.
Conclusion
Geophysical surveys extend the reach of geological understanding below the surface. Used correctly, they improve success rates in water, minerals, geothermal, and engineering projects across Kenya.
Innercore Geoconsultants Services Limited offers resistivity, magnetic, and integrated geophysical surveys, with interpretation tied to geology and project goals. Contact us to match the right method to your target.
