Geophysical surveys have become indispensable in mining and mineral exploration. They offer a noninvasive means to explore subsurface composition and locate mineral and ore deposits. This article examines geophysical surveys and highlights their types and benefits in mining.
Image Credit: Lumiere Media/Shutterstock.com
What are Geophysical Surveys?
Geophysical surveys measure the physical properties of the Earth's surface or subsurface to detect and map variations or anomalies. These surveys are based on the principle that every element has distinct physical properties, such as density, magnetic susceptibility, or electrical conductivity. Geophysicists can identify hidden geological structures, ores, or minerals without extensive excavation by measuring these properties.
In mining, these surveys are invaluable in the early stages to locate potential ore environments and mineralized zones and in later stages to delineate ore bodies and estimate reserves. Their cost-effectiveness and ability to quickly cover large areas make them essential for preliminary and detailed exploration efforts.1
Types of Geophysical Surveys
Geophysical survey techniques can be classified as either passive or active.
Passive methods
Passive geophysical survey techniques measure naturally occurring fields or phenomena without introducing artificial signals. These methods include:
Magnetometry
Magnetic minerals, particularly iron-containing ones like magnetite and pyrrhotite, influence the Earth's magnetic field, leading to uniform variations.
Magnetometry, or magnetic surveys, detect these subsurface magnetic properties by identifying local disturbances or anomalies in Earth's magnetic field.
These surveys typically occur in two stages: initially, magnetic intensity is measured along set intervals to create a magnetic profile, followed by a more detailed survey over a grid pattern in areas of interest to refine the analysis.
Gravity surveys
Gravity surveying measures variations in the Earth's gravitational field resulting from differences in subsurface material densities. While the changes in gravity are extremely small – often measured in parts per billion of the Earth's total gravitational field – they can provide valuable information about subsurface structures and composition. These surveys are commonly used in oil, gas, mineral, and geological mapping.2,3
Active methods
Active geophysical survey techniques involve introducing artificial signals into the ground and measuring the response. These methods include:
Electromagnetic conductivity (EM)
Electromagnetic surveys differ from magnetic surveys as they induce an electromagnetic current into the ground rather than relying on naturally high-intensity magnetic minerals. This makes EM surveys suitable for exploring numerous metallic minerals, including rare-earth elements like cobalt.
The EM survey system consists of a transmitter coil that generates a primary electromagnetic field and a receiver coil that measures the secondary field induced in the ground. The magnitude of this secondary field reflects the material's conductivity and helps identify the types of materials and their distribution.
Electrical resistivity
Electrical resistivity surveys enable imaging of the Earth's subsurface on land and underwater, providing 2D sections or 3D volumes of rock.
The technique involves injecting an electrical current into the ground through electrodes and measuring the resulting voltage distribution at the surface. As each mineral has a unique electrical resistivity, the variations in voltage enable the characterization of its composition.
This method is particularly useful in mineral exploration for identifying metallic ore bodies due to the conductive properties of various metal-bearing minerals.
Induced polarization (IP)
IP is another electrical survey method that measures the Earth's capacity to store and release electrical charge, similar to a capacitor. It involves injecting an alternating current into the ground and measuring the voltage decay or phase shift when the current is switched off.
This method is crucial for detecting clay and chargeable minerals in the subsurface. It provides valuable information for mineral exploration, particularly for identifying sulfide minerals that host gold, silver, and copper, which generate strong IP signatures.
Seismic surveys
Seismic methods involve generating acoustic waves and measuring their propagation through the Earth. The waves are typically generated by a controlled source such as explosives, a weight drop, or a vibrator truck. As these waves travel through the Earth, they are reflected and refracted at boundaries between materials with different acoustic properties.
The method provides high-resolution images of subsurface minerals and structures and is the primary tool in oil and gas exploration.2,3
Equipment Used in Geophysical Surveys
The equipment used in geophysical surveys varies depending on the specific technique employed, site conditions, and the type and composition of the targeted materials.
- Magnetometers: These devices measure the strength and direction of magnetic fields to detect magnetic anomalies in the subsurface. They can be handheld for detailed ground surveys or mounted on aircraft for large-scale aerial surveys.
- Ground-penetrating radar (GPR) systems: GPR uses radar pulses sent into the ground to detect reflected signals, revealing subsurface features and structures.
- Seismic equipment: This equipment generates seismic waves using explosives or vibrator trucks, detects them with geophones, and records the data to map subsurface structures based on wave propagation.
- Gravimeters: These highly sensitive instruments measure minute variations in the Earth's gravitational field to reveal variations in subsurface density and structures.
- Radiometric detectors: These sensors measure radiation from radioactive elements like cesium, thorium, and potassium in rocks and soils to assess their concentrations and identify mineral deposits.2,4
What are the Benefits of Geophysical Surveys in Mining?
Geophysical surveys offer non-destructive exploration, enabling subsurface investigation without extensive drilling or excavation, reducing environmental impact and cost. They also reduce financial risk by offering preliminary data that helps optimize the placement of costly exploratory wells and boreholes, leading to more detailed intrusive investigations, comprehensive site information, and overall project cost savings.
These surveys are time-efficient, provide rapid sampling of entire sites, and enhance safety by identifying subsurface hazards or structures before excavation, improving safety in deep mining projects.5
Conclusion
Geophysical surveying has revolutionized the mining industry by offering a non-invasive and cost-effective approach to exploring subsurface structures and mineral deposits. These diverse techniques help mining companies make more informed decisions and target resources precisely, leading to reduced financial risk and safer operations.
As technology advances, geophysical surveys will be increasingly crucial in uncovering deeper and more complex deposits while promoting responsible and sustainable resource extraction.
Continue Reading: What is Mineral Prospecting and Exploration?
References and Further Reading
- Moitra, A. K., Bhattacharya, J., Kayal, J. R., Mukerji, B., & Das, A. K. (Eds.). (2021). Innovative exploration methods for minerals, oil, gas, and groundwater for sustainable development. Elsevier. https://doi.org/10.1016/C2020-0-00590-6
- Geological Survey Ireland. (2024). Geophysical Methods. [Online]. Available from: https://www.gsi.ie/en-ie/programmes-and-projects/minerals/activities/mineral-exploration/Pages/Geophysical-Methods.aspx
- Tichauer, R., Martins, A. C., Silva, R. S., & De Tomi, G. (2020). The role of geophysics in enhancing mine planning decision-making in small-scale mining. Royal Society Open Science, 7(7), 200384. https://doi.org/10.1098/rsos.200384
- Terraplus. (2024). Mineral Exploration. [Online]. Available from: https://terraplus.ca/applications/mineral-exploration/
- W.J. Scott. (2014). Geophysics for Mineral Exploration - A Manual for Prospectors. [Online]. Newfoundland and Labrador Department of Natural Resources. Available from: https://www.gov.nl.ca/iet/files/mines-prospector-matty-mitchell-virtual-geophysics-mineral-exploration-march2014.pdf
Disclaimer: The views expressed here are those of the author expressed in their private capacity and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.