Exploration geophysics is an applied branch of geophysics and economic geology, which uses physical methods at the surface of the Earth, such as seismic, gravitational, magnetic, electrical and electromagnetic, to measure the physical properties of the subsurface, along with the anomalies in those properties. It is most often used to detect or infer the presence and position of economically useful geological deposits, such as ore minerals; fossil fuels and other hydrocarbons; geothermal reservoirs; and groundwater reservoirs. Exploration geophysics can be used to directly detect the target style of mineralization by measuring its physical properties directly. For example, one may measure the density contrasts between the dense iron ore and the lighter silicate host rock, or one may measure the electrical conductivity contrast between conductive sulfide minerals and the resistive silicate host rock. Geophysical methods The main techniques used are: Seismic tomography to locate earthquakes and assist in Seismology . Reflection seismology and seismic refraction to map the surface structure of a region. Geodesy and gravity techniques, including gravity gradiometry. Magnetic techniques, including aeromagnetic surveys to map magnetic anomalies. Electrical techniques, including electrical resistivity tomography and induced polarization. Electromagnetic methods, such as magnetotellurics, ground penetrating radar, transient/time-domain electromagnetics, and SNMR. Borehole geophysics, also called well logging. Remote sensing techniques, including hyperspectral imaging. Many other techniques, or methods of integration of the above techniques, have been developed and are currently used. However these are not as common due to cost-effectiveness, wide applicability, and/or uncertainty in the results produced. Uses Exploration geophysics is also used to map the subsurface structure of a region, to elucidate the underlying structures, to recognize spatial distribution of rock units, and to detect structures such as faults, folds and intrusive rocks. This is an indirect method for assessing the likelihood of ore deposits or hydrocarbon accumulations. Methods devised for finding mineral or hydrocarbon deposits can also be used in other areas such as monitoring environmental impact, imaging subsurface archaeological sites, ground water investigations, subsurface salinity mapping, civil engineering site investigations, and interplanetary imaging. Mineral exploration Magnetometric surveys can be useful in defining magnetic anomalies which represent ore (direct detection), or in some cases gangue minerals associated with ore deposits (indirect or inferential detection). The most direct method of detection of ore via magnetism involves detecting iron ore mineralization via mapping magnetic anomalies associated with banded iron formations which usually contain magnetite in some proportion. Skarn mineralization, which often