Introduction Of Exploration Of Geophysical In Oil & Gas Indutry - Wikipedia

Exploration geophysics - Wikipedia, the free encyclopedia

Page 1 of 4

Exploration geophysics From Wikipedia, the free encyclopedia

Exploration geophysics is the applied branch of geophysics which uses surface methods to measure the physical properties of the subsurface Earth, in order to detect or infer the presence and position of concentrations of ore minerals and hydrocarbons. 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 conductivity contrast between conductive sulfide minerals and barren silicate minerals.

Contents ■ 1 Geophysical Methods ■ 2 Uses ■ 2.1 Mineral exploration ■ 2.2 Oil and gas ■ 2.3 Civil engineering ■ 2.4 Archaeology ■ 2.5 Forensics ■ 3 Exploration geophysics in society ■ 4 See also

Geophysical Methods The following techniques are used: 1. Seismic methods, such as reflection seismology, seismic refraction, refraction microtremor (ReMi) (http://www.seismo.unr.edu/ftp/pub/louie/papers/disper/refr.html) and seismic tomography. 2. Magnetotellurics 3. Scientific drilling 4. Transient electromagnetic (EM) (see, e.g. Geonics instruments (http://www.geonics.com) ) 5. Radio frequency electromagnetic propagation (e.g., ground penetrating radar) 6. Electrical techniques, including Electrical resistivity tomography and induced polarization 7. Magnetic techniques, including aeromagnetic surveys and magnetometers. 8. Gravity and Gravity Gradiometry 9. Geodesy

http://en.wikipedia.org/w/index.php?title=Exploration_geophysics&printable=yes

8/3/2009

Exploration geophysics - Wikipedia, the free encyclopedia

Page 2 of 4

10. Remote sensing 11. Seismoelectrical Method

Uses 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 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. Electro-magnetic (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 inferrential method of detecting mineralisation. Regional EM surveys are conducted via airborne methods, utilising either fixed-wing aircraft or helicopter-borne EM rigs. Surface EM methods are based mostly on Transient EM methods utilising 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 (eg; SQUID surveys for nickel orebodies). Electric-resistance methods such as induced polarisation methods can be useful for directly detecting sulphide bodies, coal and resistive rocks such as salt and carbonates.

http://en.wikipedia.org/w/index.php?title=Exploration_geophysics&printable=yes

8/3/2009

Exploration geophysics - Wikipedia, the free encyclopedia

Page 3 of 4

Oil and gas Seismic tomography, seismic reflection and 3D seismic surveying is the geophysical exploration technique most commonly used in the onshore and offshore petrochemical exploration industries. Seismic reflection techniques are used to map the subsurface distribution of stratigraphy and its structure, which can be used to map out hydrocarbon plays; seismic refraction parameters are used to calculate the density of the subsurface layers to determine such parameters as density, porosity and rock type. Downhole geophysics tools are used within oil and gas exploration for several purposes; ■ ■ ■ ■

density tools measure rock density and porosity caliper tools measure hole diameter gamma-logging tools measure the radioactivity of the wall rocks in the bore hole EM tools measure wall-rock conductivity - to inform on porosity, sulphide content, lithology, etc.

Civil engineering Ground penetrating radar is utilised 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 characterisation, and other similar uses. Civil engineering can also utilise remote sensing information for topographical mapping, planning and environmental impact assessment. Airborne electromagnetic surveys are also used to characterise soft sediments in planning and engineering roads, dams and other structures.

Archaeology See also: Geophysical survey (archaeology) Ground penetrating radar can be used to map buried artefacts, such as graves, mortuaries, wreck sites, and other shallowly buried archaological 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 largescale ancient ruins. Sonar systems can be used to detect shipwrecks.

Forensics Ground penetrating radar can be used to detect grave sites.

Exploration geophysics in society In addition to universities and technical institutes such as the Colorado School of Mines, University of British Columbia (http://www.eos.ubc.ca/research/themes/index.html) , Curtin University (http://www.geophysics.curtin.edu.au/website) , professional societies such as the Society of Exploration Geophysicists (SEG) (seg.org (http://www.seg.org) ) and the CSEG (http://www.cseg.ca) and ASEG (http://www.aseg.org.au/) have the most recent updates of the sciences and technologies of exploration geophysics.

http://en.wikipedia.org/w/index.php?title=Exploration_geophysics&printable=yes

8/3/2009

Exploration geophysics - Wikipedia, the free encyclopedia

Page 4 of 4

Exploration Geophysics careers are in such widely varying industries as oil & gas exploration, mining, environmental studies, planetary physics and even archaeology.

See also ■ ■ ■ ■ ■ ■

Archaeological geophysics Petroleum geology Hydrocarbon exploration Kola Superdeep Borehole Mineral exploration Ore genesis

Retrieved from "http://en.wikipedia.org/wiki/Exploration_geophysics" Categories: Geophysics | Economic geology ■ This page was last modified on 2 June 2009 at 14:15. ■ Text is available under the Creative Commons Attribution/Share-Alike License; additional terms may apply. See Terms of Use for details. Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc., a non-profit organization.

http://en.wikipedia.org/w/index.php?title=Exploration_geophysics&printable=yes

8/3/2009