Exploration geophysics

Source: Wikipedia, the free encyclopedia.

Exploration geophysics is an applied branch of

hydrocarbons; geothermal reservoirs; and groundwater reservoirs. It can also be used to detect the presence of unexploded ordnance
.

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

electrical conductivity contrast between conductive sulfide minerals
and the resistive silicate host rock.

Geophysical methods

The main techniques used are:

  1. Seismic tomography to locate earthquakes and assist in Seismology.
  2. Reflection seismology and seismic refraction to map the surface structure of a region.
  3. Geodesy and gravity techniques, including gravity gradiometry.
  4. Magnetic techniques, including aeromagnetic surveys to map magnetic anomalies.
  5. Electrical techniques, including electrical resistivity tomography and induced polarization.
  6. SNMR
    .
  7. Borehole geophysics, also called well logging.
  8. 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,

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

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.

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.[1]

Hydrocarbon exploration

Seismic reflection and refraction 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, both stratigraphic and structural deposits or "traps". 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.

igneous intrusions, and salt diapirs due to their unique density and magnetic susceptibility
signatures compared to the surrounding rocks; the latter is particularly useful for metallic ores.

Remote sensing techniques, specifically hyperspectral imaging, have been used to detect hydrocarbon microseepages using the spectral signature of geochemically altered soils and vegetation.[2][3]

Specifically at sea, two methods are used: marine seismic reflection and electromagnetic seabed logging (SBL). Marine

geological traps (signalled by seismic surveys).[4]

Civil engineering

Ground penetrating radar

geotechnical characterization, and other similar uses.[5]

Spectral-Analysis-of-Surface-Waves

The Spectral-Analysis-of-Surface-Waves (SASW) method is another non-invasive technique, which is widely used in practice to detect the shear wave velocity profile of the soil. The SASW method relies on the dispersive nature of Raleigh waves in layered media, i.e., the wave-velocity depends on the load's frequency. A material profile, based on the SASW method, is thus obtained according to: a) constructing an experimental dispersion curve, by performing field experiments, each time using a different loading frequency, and measuring the surface wave-speed for each frequency; b) constructing a theoretical dispersion curve, by assuming a trial distribution for the material properties of a layered profile; c) varying the material properties of the layered profile, and repeating the previous step, until a match between the experimental dispersion curve, and the theoretical dispersion curve is attained. The SASW method renders a layered (one-dimensional) shear wave velocity profile for the soil.

Full waveform inversion

Full-waveform-inversion (FWI) methods are among the most recent techniques for geotechnical site characterization, and are still under continuous development. The method is fairly general, and is capable of imaging the arbitrarily heterogeneous compressional and shear wave velocity profiles of the soil.[6][7]

Elastic waves are used to probe the site under investigation, by placing seismic vibrators on the ground surface. These waves propagate through the soil, and due to the heterogeneous geological structure of the site under investigation, multiple reflections and refractions occur. The response of the site to the seismic vibrator is measured by sensors (geophones), also placed on the ground surface. Two key-components are required for the profiling based on full-waveform inversion. These components are: a) a computer model for the simulation of elastic waves in semi-infinite domains;[8] and b) an optimization framework, through which the computed response is matched to the measured response by iteratively updating an initially assumed material distribution for the soil.[9]

Other techniques

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.[10]

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

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.[11]

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.

Passive sonar systems are used to detect noises from marine objects or animals.[12] This system does not emit sound pulses itself but instead focuses on sound detection from marine sources.[12] This system simply 'listens' to the ocean, rather than measuring the range or orientation of an object.[12]

Geophysical survey using magnetometer

Forensics

Ground penetrating radar can be used to detect grave sites.[13] This detection is of both legal and cultural importance, providing an opportunity for affected families to pursue justice through legal punishment of those responsible and to experience closure over the loss of a loved one.[13]

Unexploded ordnance detection

Warning sign from The National Trust indicating the presence of unexploded ordnance

Unexploded ordnance (or UXO) refers to the dysfunction or non-explosion of military explosives.[14] Examples of these include, but are not limited to: bombs, flares, and grenades.[14] It is important to be able to locate and contain unexploded ordnance to avoid injuries, and even possible death, to those who may come in contact with them.[14]

The issue of unexploded ordnance originated as a result of the Crimean War (1853-1856).[15] Before this, most unexploded ordnance was locally contained in smaller volumes, and was thus not a huge public issue.[15] However, with the introduction of more widespread warfare, these quantities increased and were thus easy to lose track of and contain.[15] According to Hooper & Hambric in their piece Unexploded Ordnance (UXO): The Problem, if we are unable to move away from war in the context of conflict resolution, this problem will only continue to get worse and will likely take more than a century to resolve.[15]

Since our global method of conflict resolution banks on warfare, we must be able to rely on specific practices to detect this unexploded ordnance, such as magnetic and electromagnetic surveys.[16] By looking at differences in magnetic susceptibility and/or electrical conductivity in relation to the unexploded ordnance and the surrounding geology (soil, rock, etc.), we are able to detect and contain unexploded ordnance.[16]

See also

References

  1. ISSN 0016-8033
    .
  2. doi:10.1130/b26182.1
    .
  3. doi:10.1016/j.marpetgeo.2008.03.008
    .
  4. ^ Stéphane Sainson, Electromagnetic seabed logging, A new tool for geoscientists. Ed. Springer, 2017
  5. ^ Benedetto, Andrea., and Lara. Pajewski. Civil Engineering Applications of Ground Penetrating Radar. Ed. Andrea. Benedetto and Lara. Pajewski. 1st ed. 2015. Cham: Springer International Publishing, 2015.
  6. doi:10.1016/j.soildyn.2012.12.012
    .
  7. doi:10.1016/j.soildyn.2016.04.010
    .
  8. S2CID 122812832
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  9. S2CID 119148953
    .
  10. ^ Okazaki, Kenji et al. “Airborne Electromagnetic and Magnetic Surveys for Long Tunnel Construction Design.” Physics and chemistry of the earth. Parts A/B/C 36.16 (2011): 1237–1246.
  11. ^ Fassbinder, Jörg W. E. “Magnetometry in Archaeology – From Theory to Practice.” Rossiiskaia arkheologiia 2019.3 (2019): 75–91.
  12. ^ a b c d e f US Department of Commerce, National Oceanic and Atmospheric Administration. "What is sonar?". oceanservice.noaa.gov. Retrieved 2023-03-27.
  13. ^
    ISSN 2072-4292
    .
  14. ^ a b c Defence, National (2017-11-10). "What is Unexploded Explosive Ordnance (UXO)?". www.canada.ca. Retrieved 2023-03-10.
  15. ^
    S2CID 212963579
    , retrieved 2023-03-10
  16. ^
    ISSN 0926-9851
    .
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