There are a number of types of geophysical surveys which are undertaken on the ground. These include:
Seismic surveys – Seismic surveys measure vibration as it passes through the Earth. This is done using a series of geophones (sensors connected to wires) placed using handheld tools and arranged in an array or specific pattern. This gives information about the properties of the rocks, often down to depths of several kilometres. The vibrations may be induced using truck-mounted vibrating weights or small explosives.
Magnetic surveys – Magnetic surveys measure the variations of the Earth’s magnetic field due to the presence of magnetic minerals. Subtle variations in the abundance of magnetic minerals are used to interpret rock types and can assist in identifying resources.
Radiometric surveys – Radiometric surveys measure gamma rays which are continuously being emitted from the earth by natural decomposition of some common radiogenic minerals. Most gamma rays emanate from the top 30 centimetres of rock or soil which can be detected by airborne surveys or on surface rocks using a hand-held spectrometer. These surveys are most often used in metallic and industrial mineral exploration.
Gravity surveys – A gravimeter measures the gravity field to determine variations in rock density in the Earth’s crust. Ground gravity surveys require a geophysical technician to take gravity measurements at set intervals of distance and record the precise height at each location.
Induced Polarisation (IP) surveys – IP surveys induce an electric field in the ground and measure the chargeability and resistivity of the subsurface. The technique can identify changes in the electric currents caused by different rocks and minerals. Readings are taken by a small crew who shift a ground array or pattern of transmission and receiver cables. These surveys are most often used in metallic mineral exploration.
Electromagnetic (EM) surveys – EM surveys induce an electromagnetic field and measure the three dimensional variations in conductivity within the near-surface soil and rock. Conductive units can be studied to locate metallic minerals, and to understand groundwater and salinity.