Geophysical surveying techniques (gravity, magnetic, seismic, and resistivity)

Geophysical Surveying Techniques

Gravity surveying methods work by measuring variations in the Earth’s gravitational field, usually caused by differing densities in underlying rocks.
Denser rocks such as basalt will create a greater gravitational pull than less dense sedimentary rocks.
The instrument used is a gravimeter. It measures the difference between the gravity at ground level and at sea level.
Gravity surveys can be used to locate oil and gas deposits, map fault lines, find buried archaeological features, and study crustal structures.

Magnetic surveying involves detecting the intensity and direction of the Earth’s magnetic field.
Certain rocks, like basalt or granite, have a strong magnetic influence due to their iron content. Changes in rock properties can cause local abnormalities or anomalies in the Earth’s magnetic field.
The magnetic field is measured using an instrument called a magnetometer. The data collected can determine the type and distribution of magnetic minerals in the rock, the depth of the rock, and infer the past behaviour of the Earth’s magnetic field.
Magnetic surveying is useful for detecting buried iron and steel objects, and for geological mapping and mineral exploration.

Seismic surveying is based on measuring the time taken for induced shock waves to travel through the ground and reflect back to the surface.
Two types of induced shock waves are commonly used: P-waves (Primary or pressure waves) and S-waves (Secondary or shear waves).
The speed of the waves varies depending on the properties of the rock - higher density and stiffer rocks will transmit seismic waves faster than lower density, less stiff rocks.
Measurements are taken with geophones (sound receivers) and the results are used to create a subsurface map.
Seismic surveys can be used to find oil and gas deposits, for structure mapping, and to study the Earth’s interior.

Resistivity surveying is a technique that involves the measurement of the electrical resistivity of subsurface geological structure.
An electrical current is passed into the ground and the response (the potential difference) is measured. Rocks and mineral deposits have different resistivity based on their composition, porosity, and fluid saturation.
The data gathered is used to help build a 3D image of the subsurface structure and distribution of rock types.
Resistivity surveys are often used for groundwater exploration, mineral and ore detection, determining overburden thickness, and for archaeology.