Exploration geophysics

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Exploration geophysics is the applied branch of geophysics which uses surface methods to measure the physical properties of the subsurface Earth, along with the anomalies in these properties, in order to detect or infer the presence and position of ore minerals, hydrocarbons, geothermal reservoirs, groundwater reservoirs, and other geological structures.

Exploration geophysics is the practical application of physical methods (such as seismic, gravitational, magnetic, electrical and electromagnetic) to measure the physical properties of rocks, and in particular, to detect the measurable physical differences between rocks that contain ore deposits or hydrocarbons and those without.

Exploration geophysics can be used to directly detect the target style of mineralisation, via measuring its physical properties directly. For example one may measure the density contrasts between iron ore and silicate wall rocks, or may measure the electrical conductivity contrast between conductive sulfide minerals and barren silicate minerals.

Geophysical methods[edit]

The main techniques used are:

  1. Seismic methods, such as reflection seismology, seismic refraction, and seismic tomography.
  2. Geodesy and gravity techniques, including gravity gradiometry.
  3. Magnetic techniques, including aeromagnetic surveys.
  4. Electrical techniques, including electrical resistivity tomography and induced polarization.
  5. Electromagnetic methods, such as magnetotellurics, ground penetrating radar and transient/time-domain electromagnetics.
  6. Borehole geophysics, also called well logging.
  7. 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[edit]

Exploration geophysics is also used to map the subsurface structure of a region, to elucidate the underlying structures, 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[edit]

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 mineralisation via mapping magnetic anomalies associated with banded iron formations which usually contain magnetite in some proportion. Skarn mineralisation, which often 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, and this alteration can be detected to provide an inference that some mineralising 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 detect a wide variety of base metal sulphide deposits via detection of conductivity anomalies which can be generated around sulphide bodies in the subsurface. EM surveys can also be used to detect palaeochannel-hosted uranium deposits which are associated with shallow aquifers, which often respond to EM surveys in conductive overburden. This is an indirect inferential method of detecting mineralisation.

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 mineralisation. These methods can map out sulphide bodies within the earth in 3 dimensions, and provide information to geologists to direct further exploratory drilling on known mineralisation. 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.

Oil and gas[edit]

Seismic reflection techniques are the most widely used geophysical technique in hydrocarbon exploration. They are used to map the subsurface distribution of stratigraphy and its structure which can be used to delineate potential hydrocarbon accumulations. Well logging is another widely used technique as it provides necessary high resolution information about rock and fluid properties in a vertical section, although they are limited in areal extent. This limitation in areal extent is the reason why seismic reflection techniques are so popular; they provide a method for interpolating and extrapolating well log information over a much larger area.

Gravity and magnetics are also used, with considerable frequency, in oil and gas exploration. These can be used to determine the geometry and depth of covered geological structures including uplifts, subsiding basins, faults, folds, igneous intrusions and salt diapirs due to their unique density and magnetic susceptibility signatures compared to the surrounding rocks.

Remote sensing techniques, specifically hyperspectral imaging, have been used to detect hydrocarbon microseepages using the spectral signature of geochemically altered soils and vegetation.[1][2]

Magnetotellurics and Controlled source electro-magnetics can provide pseudo-direct detection of hydrocarbons by detecting resistivity changes. It can also complement seismic data when imaging below salt.

Civil engineering[edit]

Ground penetrating radar is used within civil construction and engineering for a variety of uses, including detection of utilities (buried water, gas, sewerage, electrical and telecommunication cables), mapping of soft soils and overburden for geotechnical characterization, and other similar uses.

Civil engineering can also use remote sensing information for topographical mapping, planning and environmental impact assessment. Airborne electromagnetic surveys are also used to characterize soft sediments in planning and engineering roads, dams and other structures.

Magnetotellurics has proven useful for delineating groundwater reservoirs, mapping faults around areas where hazardous substances are stored (e.g. nuclear power stations and nuclear waste storage facilities), and earthquake precursor monitoring in areas with major structures such as hydro-electric dams subject to high levels of seismic activity.

BS 5930 is the standard used in the UK as a code of practice for site investigations.

Archaeology[edit]

See also: Geophysical survey (archaeology)

Ground penetrating radar can be used to map buried artifacts, such as graves, mortuaries, wreck sites, and other shallowly buried archaeological sites.

Ground magnetometric surveys can be used for detecting buried ferrous metals, useful in surveying shipwrecks, modern battlefields strewn with metal debris, and even subtle disturbances such as large-scale ancient ruins.

Sonar systems can be used to detect shipwrecks.

Forensics[edit]

Ground penetrating radar can be used to detect grave sites.

Unexploded ordnance detection[edit]

Magnetic and electromagnetic surveys can be used to locate unexploded ordnance.

References[edit]

  1. ^ Khan, S.D.; Jacobson, S. (2008). "Remote Sensing and Geochemistry for Detecting Hydrocarbon Microseepages". Geological Society of America Bulletin 120: 96–105. 
  2. ^ Petrovic, A.; Khan, S.D.; Chafetz, H. (2008). "Remote detection and geochemical studies for finding hydrocarbon-induced alterations in Lisbon Valley, Utah". Marine and Petroleum Geology 25: 696–705. 

See also[edit]

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