contains magnetite, can also be detected though the ore minerals themselves would be non-magnetic. Similarly, magnetite, hematite, and often pyrrhotite are common minerals associated with hydrothermal alteration, which can be detected to provide an inference that some mineralizing hydrothermal event has affected the rocks. Gravity surveying can be used to detect dense bodies of rocks within host formations of less dense wall rocks. This can be used to directly detect Mississippi Valley Type ore deposits, IOCG ore deposits, iron ore deposits, skarn deposits, and salt diapirs which can form oil and gas traps. Electromagnetic (EM) surveys can be used to help detect a wide variety of mineral deposits, especially base metal sulphides via detection of conductivity anomalies which can be generated around sulphide bodies in the subsurface. EM surveys are also used in diamond exploration (where the kimberlite pipes tend to have lower resistance than enclosing rocks), graphite exploration, palaeochannel-hosted uranium deposits (which are associated with shallow aquifers, which often respond to EM surveys in a conductive overburden). These are indirect inferential methods of detecting mineralization, as the commodity being sought is not directly conductive, or not sufficiently conductive to be measurable. EM surveys are also used in unexploded ordnance, archaeological, and geotechnical investigations. Regional EM surveys are conducted via airborne methods, using either fixed-wing aircraft or helicopter-borne EM rigs. Surface EM methods are based mostly on Transient EM methods using surface loops with a surface receiver, or a downhole tool lowered into a borehole which transects a body of mineralization. These methods can map out sulphide bodies within the earth in three dimensions, and provide information to geologists to direct further exploratory drilling on known mineralization. Surface loop surveys are rarely used for regional exploration, however in some cases such surveys can be used with success (e.g.; SQUID surveys for nickel ore bodies). Electric-resistance methods such as induced polarization methods can be useful for directly detecting sulfide bodies, coal, and resistive rocks such as salt and carbonates. Seismic methods can also be used for mineral exploration, since they can provide high-resolution images of geologic structures hosting mineral deposits. It is not just surface seismic surveys which are used, but also borehole seismic methods. All in all, the usage of seismic methods for mineral exploration is steadily increasing. Hydrocarbon exploration Seismic reflection and refraction techniques are the most widely used geophysical technique in hydrocarbon exploration. They are used to map