Geophysical Methods

Geophysical Methods

​Geophysical techniques work through applying one of several types of force to the ground, to measure the resulting energy with use of geophysical equipment and infer the geology from this. Geophysics is generally much quicker than the aforementioned methods, however, requires more data processing (office-based work) to develop the geological picture. A great advantage of these methods is that certain instruments can be attached to small aircraft for covering large areas during regional airborne surveys. This provides sparser geological information, but can highlight potential metal anomalies on a county-country scale, which can be followed up by more detailed, ground-based geophysical surveys. However, as the material is being tested indirectly, there is no 100% guarantee of its conclusions; in addition to being susceptible to contamination by many man-made metallic structures e.g. power-lines. Therefore, should geophysical surveys prove sufficiently interesting, drilling will be required afterwards to confirm the accuracy of the results.

Low-flying plane conducting airborne geophysics survey.

  • Magnetic

Certain rock-forming minerals have a naturally occurring magnetic property, which when surveying for magnetism of the rocks, allows them to be identified beneath the surface. In particular these are the iron-containing minerals magnetite and pyrrhotite. These are common constituents in volcanic metal sulphide deposits, and so may highlight an area of high copper, lead or zinc concentrations. A magnetic survey will generally occur in 2 stages. Firstly, several lines of magnetic intensity will be measured at set intervals to develop a magnetic profile over the measured lines, which may run a length of several kilometres. Secondly, if any sufficiently high readings were found from the first stage, another magnetic survey would proceed over the areas of interest, but this time over a grid pattern as opposed to lines. This latter stage allows a more detailed understanding of the magnetic intensity of the area, and from this work a map can be produced showing the magnetic intensity of the underlying rocks.

Magnetic surveys are conducted using equipment called a magnetometer, which can be operated manually as a handheld version for geologists in the field, or attached to planes for larger regional surveys. The instrument works in the form of a proton-rich fluid surrounded by an electric coil. A current is applied through the coil to polarise the fluid. When the current is switched off, the protons realign along the Earth's magnetic field at that position. Magnetic surveys have several important uses outside of mineral exploration including archaeology and locating steel and iron man-made structures beneath the surface, including looking for underground mineshafts from historic mines. This method allows deep penetration and is unaffected by high electrical ground conductivities, making them useful at sites with saline groundwater, clay or high levels of contamination where other geophysical methods struggle.

  • Electromagnetic (EM)

EM surveys differ from magnetic surveys in that as opposed to relying on the signature of naturally high intensity magnetic minerals, it induces an electromagnetic current into the ground and measures the resulting field. Because it is not limited to the presence of magnetite and pyrrhotite, it is useful for exploring for a wide range of metallic minerals, including important metals such as cobalt which is used in batteries for renewable energy sources. .

The EM system consists of a transmitter and receiver coil which is suspended above the ground. A primary electromagnetic field is output by the transmission coil which induces a secondary field in the ground – the magnitude of which is measured by the receiving coil which also measures the ratio between the primary and secondary fields. The magnitude of the secondary field is proportional to the conductivity of the material and can be used to detect signs of changes in rock type, potential mineralisation, groundwater, ground contamination and buried metal man-made objects. Similar to magnetic surveys, EM is conducted on a grid of sampling points at set intervals.

  • Electrical Resistivity

Electrical resistivity, also known as resistivity, is a bulk electric property of all material that shows how strongly it opposes the flow of electric current. A low resistivity indicates a material that readily allows the flow of electric current – i.e. it is conductive. All materials have a certain resistivity and fall within a broad range. The SI unit for resistivity is the ohmmeter (Ωm). Resistivity surveys can allow for imaging the subsurface of the Earth on both land and in water, with scans in either 2D sections or 3D volumes of rock. It is used by applying a small and highly controlled electric current across an array of electrodes, and so resistivity surveys are often coupled with induced polarisation surveys as the latter also make use of such electrodes.

Because of the conductive properties of various metal-bearing minerals, resistivity can be used in mineral exploration for metallic ore bodies. However resistivity is an important method for many other studies including geotechnical and groundwater exploration. For example, geologists can use resistivity imaging to create a time-lapse of biological or hydrological events over time, to capture % change of dynamic conditions e.g. examining how water discharges underground, or see how dangerous chemical spillages are moving underground.

Geologist conducting a resistivity-IP survey.

  • Induced Polarisation (IP)

IP is the Earth's capacity to hold an electric charge over time. It is measured through inducing a current into the ground, then switching it off and measuring how the voltage decays over time, usually a matter of seconds, eventually disappearing. Where resistivity answers "What is the resistance to the flow of electric current?" IP answers "How long do the rocks hold an electric charge?" IP often uses the same cable setup and same instrument as resistivity surveys, but instead turns off the current and measures the time component. It uses electrodes.

This method is important because it tests for the capacity of rocks to hold charge and hence properties in the subsurface that can be used to locate clay and other chargeable minerals. Usually these surveys are very informative for determining rocks because you end up with both an IP and resistivity map at the same time, allowing both measurement types to be interpreted. It is therefore important for mineral exploration, as sulphide minerals – including those hosting gold, silver and copper, generate a very large IP signature. This is especially useful when the metal-bearing minerals are more loosely distributed within the rock – as opposed to forming a concentrated ore body. IP also is a useful tool in environmental work e.g. mapping a zone of contamination to help clean-up.

  • Seismic

Seismic surveys are an extremely useful geophysical method for studying the ground conditions to a significant depth and over a large area. Seismic is utilised in many applications for subsurface investigations, mineral exploration being one of them. The method operates by emitting a percussive force to the ground, producing seismic waves which travel through the subsurface, reflecting back or refracting each time a structure of geological boundary is encountered. Thus from this, the depth and shape of underground structures can be interpreted from seismic profiles. The initial force can be generated by means of explosives, or in environmentally sensitive areas (i.e. anywhere on land) dropping a significant weight onto the ground surface using a hammer-plate or buffalo gun. Refracted waves are detected by an array of geophones which are spaced at regular intervals. The geophones record the time taken by direct waves, reflected waves and refracted waves and outputs this data to a seismograph for processing so it can be interpreted for underground structures. Analysing the time that the soundwaves take to return provides valuable information about rock types and possible fluids in the rocks, and is similar to the use of ultrasound in medicine.

Diagram illustrating the principles of seismic exploration.

Onshore seismic surveys have been used in sensitive urban locations without damaging buildings or the environment, e.g. its use in central Paris. More than four decades of offshore seismics and numerous research projects have shown the majority of evidence pointing towards such surveys having negligible effects on marine environments. A case study example of this is the rise of whale populations in Australia on both coasts, where both are near their biological maximum (10% per year), despite a significantly greater amount of offshore seismics on the West coast than the East.

  • Radiometric

Radiometric surveys (also known as Gamma ray spectrometry) are an airborne method which measures the amounts of natural radiation produced at the Earth's surface. Radioactive uranium, thorium and potassium minerals occur naturally in rocks, producing relatively high amounts of radiation which can be measured in contrast to the surrounding rocks. Due to gamma radiation only being able to penetrate several centimetres of earth/ground, the radioactive materials must be present in outcrops to be detected. However, through atmosphere, these rays can penetrate up to a couple of hundred metres, and so low-flying aircraft are useful in this method for covering large regional areas. Because the energy of gamma radiation received is distinctive of the specific element, it can be used to map uranium-thorium-potassium concentrations over wide areas. This in turn is used to infer rock types such as granite and zones within rocks that may have been altered by metal-rich fluids passing through. It is extremely important to note that these surveys measure levels of naturally occurring radiation in the rocks, and the instruments themselves do not emit any radiation, making them harmless to both people and livestock.