Geophysical surveys are among the most powerful tools available to mineral exploration geologists. By measuring physical properties of the subsurface — such as magnetism, electrical conductivity, density, and seismic velocity — geophysical methods allow exploration teams to identify potential mineral deposits without the need for extensive and expensive drilling. For mining investors and exploration companies operating in Uganda, understanding these methods is essential for planning cost-effective and successful exploration programs.
At ALOM Mining & Geohydro Services, our experienced geophysicists select the most appropriate survey methods based on the geological context, exploration objectives, and target minerals. This guide provides a comprehensive overview of the main geophysical survey methods used in mineral exploration.
What is a Geophysical Survey?
A geophysical survey is a systematic measurement of the physical properties of the earth's subsurface to identify geological structures, rock types, and potential mineral deposits. Unlike geological mapping, which examines surface exposures, geophysical surveys can detect features and anomalies hidden beneath the surface — sometimes at depths of hundreds of metres.
Geophysical surveys are non-invasive and relatively cost-effective compared to drilling. They provide broad coverage of large areas and help exploration teams focus drilling efforts on the most promising targets, significantly reducing overall exploration costs and risk.
Why Geophysical Surveys Are Essential in Mineral Exploration
Geophysical surveys play a critical role at multiple stages of the mineral exploration process:
- Target generation: Identifying areas of interest over large exploration license areas
- Target refinement: Narrowing down drill targets based on anomaly characteristics
- Geological understanding: Mapping subsurface structures, faults, and lithological contacts
- Depth estimation: Determining the depth and geometry of mineralized bodies
- Risk reduction: Minimizing the risk of drilling non-prospective targets
- Cost efficiency: Covering large areas at a fraction of the cost of drilling
In Uganda's diverse geological terrain — which includes Precambrian basement rocks, greenstone belts, rift valley structures, and volcanic formations — geophysical surveys are particularly valuable for mapping subsurface geology where outcrop exposure may be limited.
Common Geophysical Survey Methods
Magnetic Surveys
Magnetic surveys measure variations in the Earth's magnetic field caused by differences in the magnetic susceptibility of subsurface rocks. Rocks containing magnetic minerals — particularly magnetite and pyrrhotite — produce detectable magnetic anomalies.
Applications in mineral exploration:
- Mapping geological structures and lithological contacts
- Identifying iron ore deposits (which are strongly magnetic)
- Detecting alteration zones associated with gold mineralization
- Mapping intrusive bodies that may host mineral deposits
How it works: A magnetometer is carried along survey lines, either on foot (ground magnetic survey) or by aircraft (aeromagnetic survey). The instrument measures the total magnetic field intensity at each measurement point. After processing and correcting for diurnal variations and regional field, the data reveals magnetic anomalies related to subsurface geology.
Magnetic surveys are one of the most widely used geophysical methods in Uganda, particularly for gold exploration in greenstone belt terrains like those in Kitgum and Mubende districts.
Gravity Surveys
Gravity surveys measure small variations in the Earth's gravitational field caused by differences in rock density. Dense rocks (like massive sulphide ore bodies) produce positive gravity anomalies, while less dense rocks (like sedimentary basins) produce negative anomalies.
Applications:
- Detecting massive sulphide deposits
- Mapping geological structures and basin architecture
- Identifying dense intrusive bodies
- Regional geological mapping
Gravity surveys are particularly useful for distinguishing different rock types and mapping large-scale geological structures. They are often used in conjunction with magnetic surveys to provide a more complete picture of the subsurface.
Electrical Resistivity Surveys
Electrical resistivity surveys measure how easily electrical current flows through subsurface materials. Different rock types, minerals, and groundwater conditions have different resistivities, allowing geophysicists to map subsurface geology.
Applications:
- Mapping sulphide mineralization (which is typically conductive)
- Identifying weathered zones and saprolite profiles
- Groundwater exploration and aquifer mapping
- Environmental investigations
How it works: Electrical current is injected into the ground through metal electrodes, and the resulting voltage is measured at other electrodes. The apparent resistivity is calculated from the current and voltage measurements. By varying electrode spacing and position, a 2D or 3D image of subsurface resistivity can be constructed.
This method is particularly valuable in Uganda for both mineral exploration and groundwater development.
Induced Polarization (IP) Surveys
Induced polarization is closely related to electrical resistivity but measures the ability of subsurface materials to store electrical charge. Disseminated sulphide minerals are excellent IP targets because they produce strong chargeability anomalies.
Applications:
- Detecting disseminated sulphide mineralization (gold, copper, zinc, nickel)
- Distinguishing between massive and disseminated sulphides
- Mapping alteration zones associated with porphyry copper and epithermal gold deposits
- Complementing resistivity data for improved geological interpretation
IP surveys are one of the most effective methods for gold exploration, as gold is often associated with disseminated pyrite and other sulphide minerals. ALOM's geophysicists have successfully deployed IP surveys across multiple exploration projects in Uganda.
Electromagnetic (EM) Surveys
Electromagnetic surveys use the principle of electromagnetic induction to detect conductive bodies in the subsurface. A transmitter generates a time-varying electromagnetic field, which induces eddy currents in conductive materials. These currents generate a secondary electromagnetic field that is measured by a receiver.
Applications:
- Detecting massive sulphide deposits (which are highly conductive)
- Mapping conductive graphite zones
- Identifying conductive alteration zones
- Groundwater mapping
Types of EM surveys:
- Time-domain EM (TDEM): Measures the decay of the secondary field after the transmitter is turned off. Better for deeper targets and distinguishing conductors at different depths.
- Frequency-domain EM (FDEM): Measures the secondary field while the transmitter is operating at specific frequencies. Better for shallow, high-resolution mapping.
EM methods are particularly relevant in Uganda for graphite exploration, as graphite is highly conductive and produces strong EM responses.
Seismic Surveys
Seismic surveys measure the propagation of elastic waves (sound waves) through the subsurface. Different rock types transmit seismic waves at different velocities, allowing geophysicists to map geological boundaries and structures.
Applications:
- Mapping deep geological structures
- Identifying fault zones and geological contacts
- Determining overburden thickness
- Engineering site investigations
Types:
- Seismic refraction: Measures first-arrival times of refracted waves. Used for shallow depth investigations and determining overburden thickness.
- Seismic reflection: Measures reflected waves from geological boundaries. Used for deeper investigations and detailed structural mapping.
Seismic surveys are less commonly used in mineral exploration compared to other methods but are valuable for specific applications, particularly in rift valley settings like those found in western Uganda.
How to Choose the Right Geophysical Method
Selecting the appropriate geophysical method depends on several factors:
-
Target mineral and deposit type: Different minerals have different physical properties. Magnetic surveys are ideal for iron ore, while IP surveys are better for disseminated gold-bearing sulphides.
-
Geological context: The local geology determines which methods will be most effective. In areas with thick laterite cover (common in tropical Uganda), some methods may be more effective than others.
-
Depth of investigation: Different methods have different depth capabilities. EM methods may be limited to the upper few hundred metres, while gravity and magnetic surveys can detect deeper features.
-
Resolution requirements: The level of detail needed influences the choice of method and survey design. Detailed target delineation requires closer line spacing and more precise instruments.
-
Budget and timeframe: Some methods are more expensive and time-consuming than others. Ground magnetic surveys are relatively quick and affordable, while detailed IP surveys require more time and equipment.
-
Combination approach: In practice, the most effective exploration programs use multiple geophysical methods in combination. This integrated approach provides more robust geological models and reduces ambiguity in interpretation.
Geophysical Surveys in Uganda's Geological Context
Uganda's geological diversity makes it an excellent candidate for geophysical exploration. The country's geology includes:
-
Precambrian basement: The oldest rocks in Uganda, hosting greenstone belts with gold and base metal potential. Magnetic and IP surveys are particularly effective in these terrains.
-
Rift valley structures: The Albertine Rift in western Uganda creates complex geological structures that can be mapped using gravity and seismic methods.
-
Volcanic formations: The volcanic regions in eastern Uganda (Mount Elgon) and western Uganda (Rwenzori) create distinct geophysical signatures.
-
Laterite cover: Much of Uganda is covered by tropical weathering profiles (laterite), which can obscure surface geology. Geophysical methods are essential for "seeing through" this cover to the underlying bedrock geology.
ALOM has conducted ground geophysical surveys in Kabarole District and across other regions of Uganda, using integrated geophysical approaches tailored to local geological conditions.
Combining Geophysical Data with Geological Mapping
The most effective mineral exploration programs integrate geophysical data with geological mapping, geochemical sampling, and remote sensing data. At ALOM, our approach includes:
- Desktop study and data compilation: Reviewing existing geological and geophysical data before fieldwork
- Geological mapping: Ground-truthing and mapping surface geology
- Geophysical surveys: Collecting systematic geophysical data along planned survey lines
- Data processing and interpretation: Processing raw data and creating geophysical maps and models
- Integrated interpretation: Combining all datasets to generate exploration targets
- Drill target selection: Identifying the highest-priority targets for exploration drilling
This integrated workflow ensures that every dollar spent on exploration generates maximum geological understanding and value.
ALOM's Geophysical Survey Capabilities
ALOM Mining & Geohydro Services provides comprehensive geophysical survey services as part of our mineral exploration offerings. Our capabilities include:
- Ground magnetic surveys
- Electrical resistivity tomography (ERT)
- Induced polarization (IP) surveys
- Electromagnetic surveys
- Data processing, modelling, and interpretation
- Integrated geological-geophysical interpretation
- Drill target generation and prioritization
Our team of experienced geophysicists selects the most appropriate methods for each project based on the exploration objectives, geological context, and target mineralization style. We have a proven track record of successful geophysical surveys across multiple districts in Uganda.
Frequently Asked Questions
What is the difference between ground and airborne geophysical surveys?
Ground surveys involve carrying instruments along planned survey lines on foot or by vehicle. They provide higher resolution data but cover smaller areas. Airborne surveys use instruments mounted on aircraft or drones to cover large areas quickly at lower resolution. The choice depends on the exploration stage and area size.
How much do geophysical surveys cost in Uganda?
Costs vary depending on the method, area size, line spacing, and terrain. Ground magnetic surveys are generally the most affordable, while detailed IP and EM surveys cost more. ALOM provides tailored quotations based on specific project requirements. Contact us for a site-specific estimate.
Can geophysical surveys directly detect gold?
Gold itself cannot be directly detected by geophysical methods because it occurs in very small quantities. However, gold is often associated with sulphide minerals (pyrite, arsenopyrite) and specific geological structures that do produce detectable geophysical anomalies. IP surveys are particularly effective for detecting gold-associated sulphide mineralization.