What Are Geophysical Surveys?

Geophysical surveys are utilizing a range of physical principles, including seismic, magnetic, gravitational, and electrical methodologies, to detect anomalies and gather data about underlying structures, natural resources, and geotechnical conditions.

Types of Geophysical Surveys

1. Seismic Surveys

Seismic surveys are among the most common geophysical methods, particularly for engineering investigations and natural resource exploration. They involve generating controlled seismic waves, which travel through the Earth’s layers and reflect or refract back to the surface. The recorded data can then be used to interpret subsurface structures.

• Seismic Reflection:This technique is primarily used in hydrocarbon exploration and near-surface environmental surveys. It works similarly to sonar, where seismic waves reflect off subsurface layers and are captured to map geological formations. The reflections provide detailed images of subsurface layers.
• Seismic Refraction: Seismic refraction evaluates the speed at which seismic waves travel through different geological layers. As the waves refract at boundaries between layers, they offer insights into the composition and density of materials like rock or soil.
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2. Magnetic Surveys

Magnetic surveying measures variations in the Earth’s magnetic field caused by subsurface magnetic properties. Magnetic data can be collected through various means, including airborne platforms (e.g., aircraft and drones) and ground-based methods. Airborne magnetic surveys are particularly useful for covering large areas efficiently. For instance, aeromagnetic surveys are often conducted for regional geological mapping or to locate mineral deposits such as iron and chromium.

3. Gravity Surveys

Gravity surveys measure minute variations in the Earth’s gravitational field, which reflect differences in subsurface density. These anomalies can indicate the presence of voids, ore bodies, or geological structures like salt domes.

Gravity gradiometry, a more sensitive technique, measures the rate of change in gravitational acceleration. This method is used for detailed mapping of geological features, such as fault systems, reefs, and salt diapirs​

4. Electromagnetic (EM) Surveys

Electromagnetic surveys use oscillating electromagnetic fields to induce secondary fields in conductive subsurface materials. These surveys are commonly used in mineral exploration, environmental studies, and for locating groundwater.

A notable example is Time-Domain Electromagnetic (TEM) surveys, which are used to explore deep geological features by recording the decay of electromagnetic fields after the primary source is turned off. This method is useful for characterizing the electrical resistivity of subsurface layers, which can help identify mineral deposits.

5. Electrical Resistivity and Induced Polarization (IP)

Electrical resistivity surveys measure the resistance of subsurface materials to electrical current. By injecting current into the ground and measuring the voltage across electrodes, resistivity data can reveal the composition and structure of underground formations. This method is often used in hydrogeological studies and for locating contaminant plumes or archaeological features.

Induced Polarization (IP) extends this concept by measuring how materials temporarily hold an electrical charge. IP surveys are particularly effective in mineral exploration, where certain ore bodies, such as sulfides, exhibit strong polarization effects.

Data Collection and Interpretation

The success of geophysical surveys depends heavily on accurate data collection and interpretation. Surveys are typically designed based on the target and environmental conditions, with quality control measures implemented in the field to avoid collecting unnecessary data. Modern surveys benefit from real-time processing and GPS-enabled instruments, which allow for precise positioning and rapid data acquisition​.

Post-processing of geophysical data can be complex and time-consuming. It involves converting raw field measurements into meaningful models of the subsurface. Integration of multiple data sources, including geological maps and prior surveys, is often necessary to produce accurate interpretations​.

References

• Parnell, D. W. (2018). Introduction to Geophysical Surveying. GES101 Courseware. Online-PDH.