Why Geophysics Matters in East Africa
East Africa's geological diversity, from the Archaean cratons of Tanzania and Uganda through the Proterozoic mobile belts to the Cenozoic rift volcanics. creates a correspondingly diverse geophysical signature. Mineralisation types range from BIF-hosted gold (highly magnetic, gravity-anomalous) to sediment-hosted copper-cobalt (IP-responsive), carbonatite-hosted REE (low-density, magnetic) and volcanogenic massive sulphide (VMS) systems (highly conductive, IP-chargeable). Selecting the appropriate geophysical method for the deposit type and geological environment is the first decision in programme design.
Airborne Geophysical Surveys
High-Resolution Aeromagnetics
Airborne magnetic surveys, flown with fixed-wing aircraft or helicopters towing a caesium-vapour magnetometer at line spacings of 50–500 m, are the most widely applied first-pass exploration tool in East Africa. Magnetic anomalies map the distribution of magnetite-bearing rocks, including BIF, mafic-ultramafic intrusions, and kimberlites. providing structural context and direct targeting information. Total magnetic intensity (TMI), reduced to pole (RTP), and derivative products (tilt angle, analytic signal, vertical derivative) are standard deliverables.
National survey data for Tanzania (acquired by the Geological Survey of Tanzania with JICA assistance) covers most of the country at 400–1,000 m line spacing and is available to licence holders. Private infill surveys at 50–100 m provide a 4–10× resolution improvement for advanced project areas.
Airborne Radiometrics
Gamma-ray spectrometry (measuring K, Th and U concentrations in the top 0.3 m of soil) is typically flown simultaneously with magnetics. Radiometric data assists with geological mapping. identifying granitic rocks (high K, Th), mafic rocks (low K, Th, U) and identifying alteration zones associated with potassic (high K), carbonate-replacement (high Th) or silica-flooding (low K) mineralisation styles.
Airborne Electromagnetics (AEM)
Frequency-domain and time-domain AEM systems detect conductive bodies such as massive sulphides, graphitic sediments and saline groundwater, in East Africa, AEM has seen limited application historically due to cost and logistics, but helicopter TDEM systems (e.g. VTEM, SkyTEM) are increasingly used for VMS-targeting in the Mozambique Belt and for base-metal exploration in Uganda and Rwanda.
Ground Geophysical Methods
Induced Polarisation (IP) and Resistivity
IP surveying detects disseminated sulphide mineralisation through its frequency-dependent electrical response (chargeability), in the East African context, IP is the preferred method for:
- Detecting sulphide halos around gold mineralisation (pyrite, arsenopyrite) below the oxide cap
- Mapping copper-cobalt sulphide mineralisation in Proterozoic sedimentary belts
- Delineating VMS-style massive sulphide bodies in greenstone belt settings
Dipole-dipole, pole-dipole and gradient array configurations are deployed depending on target depth, terrain and programme objectives. Typical investigation depths for pole-dipole at n=8 arrays are 200–400 m. 3D IP inversion using software such as VOXLER or UBC-GIF provides volumetric images of chargeable bodies.
Gravity Surveys
Gravity surveying detects density contrasts between mineralised bodies and their host rocks, in East Africa, gravity is particularly useful for:
- Mapping BIF (denser) against felsic host rocks
- Detecting mafic-ultramafic intrusions (nickel, PGM targets)
- Identifying greenstone belt keel geometry and depth to basement
- Kimberlite targeting in areas with thin Kalahari cover
Bouguer anomaly maps, combined with aeromagnetics, provide a powerful structural framework for any exploration project.
Transient Electromagnetic (TDEM)
Ground TDEM (loop-source or coincident-loop configurations) is used to detect moderately to highly conductive bodies at depths of 50–400 m. It is effective for massive sulphide and graphitic shear zone targets and can be deployed rapidly in difficult terrain where IP cable layouts are impractical.
Borehole Geophysics
Once drilling has commenced, downhole geophysical logging provides additional constraints that are not available from core alone. Standard suites include: natural gamma, resistivity and IP, magnetic susceptibility, density (sonic), and optical/acoustic televiewer for structural orientation. Downhole EM surveys can extend the effective "seeing distance" of a drillhole significantly, detecting off-hole conductors within 100 m of the borehole that may represent parallel or en-échelon massive sulphide lenses.