Near-surface geophysics

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Automatic ground penetrating Radar (upGPR) near Swiss Camp (Greenland) Automatic ground penetrating Radar (upGPR) near Swiss Camp (Greenland) 1.jpg
Automatic ground penetrating Radar (upGPR) near Swiss Camp (Greenland)

Near-surface geophysics is the use of geophysical methods to investigate small-scale features in the shallow (tens of meters) subsurface. [1] It is closely related to applied geophysics or exploration geophysics. Methods used include seismic refraction and reflection, gravity, magnetic, electric, and electromagnetic methods. Many of these methods were developed for oil and mineral exploration but are now used for a great variety of applications, including archaeology, environmental science, forensic science, [2] military intelligence, geotechnical investigation, treasure hunting, and hydrogeology. In addition to the practical applications, near-surface geophysics includes the study of biogeochemical cycles. [3] [4]

Contents

Overview

In studies of the solid Earth, the main feature that distinguishes geophysics from geology is that it involves remote sensing. Various physical phenomena are used to probe below the surface where scientists cannot directly access the rock. Applied geophysics projects typically have the following elements: data acquisition, data reduction, data processing, modeling, and geological interpretation. [5]

This all requires various types of geophysical surveys. These may include surveys of gravity, magnetism, seismicity, or magnetotellurics.

Data acquisition

A geophysical survey is a set of measurements made with a geophysical instrument. Often a set of measurements are along a line, or traverse. Many surveys have a set of parallel traverses and another set perpendicular to it to get good spatial coverage. [5] Technologies used for geophysical surveys include:

Data reduction

The raw data from a geophysical survey must often be converted to a more useful form. This may involve correcting the data for unwanted variations; for example, a gravity survey would be corrected for surface topography. Seismic travel times would be converted to depths. Often a target of the survey will be revealed as an anomaly, a region that has data values above or below the surrounding region. [5]

Data processing

The reduced data may not provide a good enough image because of background noise. The signal-to-noise ratio may be improved by repeated measurements of the same quantity followed by some sort of averaging such as stacking or signal processing. [5]

Modeling

Once a good profile is obtained of the physical property that is directly measured, it must be converted to a model of the property that is being investigated. For example, gravity measurements are used to obtain a model of the density profile under the surface. This is called an inverse problem. Given a model of the density, the gravity measurements at the surface can be predicted; but in an inverse problem the gravity measurements are known and the density must be inferred. This problem has uncertainties due to the noise and limited coverage of the surface, but even with perfect coverage many possible models of the interior could fit the data. Thus, additional assumptions must be made to constrain the model.

Depending on the data coverage, the model may only be a 2D model of a profile. Or a set of parallel transects may be interpreted using a 2½D model, which assumes that relevant features are elongated. For more complex features, a 3D model may be obtained using tomography. [5] [6]

Geological interpretation

The final step in a project is the geological interpretation. A positive gravity anomaly may be an igneous intrusion, a negative anomaly a salt dome or void. A region of higher electrical conductivity may have water or galena. For a good interpretation the geophysics model must be combined with geological knowledge of the area. [5]

Seismology

Upper figure: a seismic profile showing intensity vs round-trip travel time. Lower figure: an interpretation of the results. RV Rafael cruise 08034 line 4.gif
Upper figure: a seismic profile showing intensity vs round-trip travel time. Lower figure: an interpretation of the results.

Seismology makes use of the ability of vibrations to travel through rock as seismic waves. These waves come in two types: pressure waves (P-waves) and shear waves (S-waves). P-waves travel faster than S-waves, and both have trajectories that bend as the wave speeds change with depth. Refraction seismology makes use of these curved trajectories. In addition, if there are discontinuities between layers in the rock or sediment, seismic waves are reflected. Reflection seismology identifies these layer boundaries by the reflections. [7]

Reflection seismology

Seismic reflection is used for imaging of nearly horizontal layers in the Earth. The method is much like echo sounding. It can be used to identify folding and faulting, and to search for oil and gas fields. On a regional scale, profiles can be combined to get sequence stratigraphy, making it possible to date sedimentary layers and identify eustatic sea level rise. [7]

Refraction seismology

Seismic refraction can be used not only to identify layers in rocks by the trajectories of the seismic waves, but also to infer the wave speeds in each layer, thereby providing some information on the material in each layer. [7]

Magnetic surveying

Magnetic surveying can be done on a planetary scale (for example, the survey of Mars by the Mars Global Surveyor) or on a scale of meters. In the near-surface, it is used to map geological boundaries and faults, find certain ores, buried igneous dykes, [8] locating buried pipes and old mine workings, and detecting some kinds of land mines. It is also used to look for human artifacts. Magnetometers are used to search for anomalies produced by targets with a lot of magnetically hard material such as ferrites. [9]

Microgravity surveying

High precision gravity measurements can be used to detect near surface density anomalies, such as those associated with sinkholes and old mine workings, [10] with repeat monitoring allowing near-surface changes over these to be quantified. [11]

Ground-penetrating radar

Ground-penetrating radar is one of the most popularly used near-surface geophysics in forensic archaeology, forensic geophysics, geotechnical investigation, treasure hunting, and hydrogeology, with typical penetration depths down to 10 m (33 ft) below ground level, depending upon local soil and rock conditions, although this depends upon the central frequency transmitter/receiver antennae utilised. [1]

Bulk ground conductivity

Bulk ground conductivity typically uses transmitter/receiver pairs to obtain primary/secondary EM signals from the surrounding environment (note potential difficulty in urban areas with above-ground EM sources of interference), with collection areas depending upon the antennae spacing and equipment used. There are airborne, land- and water-based systems currently available. They are particularly useful for initial ground reconnaissance work in geotechnical, archaeology and forensic geophysics investigations. [1]

Electrical resistivity

Electrical resistivity tomography profile Electrical resistivity tomography profile.jpg
Electrical resistivity tomography profile

The reciprocal of conductivity, electrical resistivity surveys measure the resistance of material (usually soil) between electrical probes, with typical penetration depths one to two times the electrode separations. There are various electrode configurations of equipment, the most typical using two current and two potential electrodes in a dipole-dipole array. They are used for geotechnical, archaeology and forensic geophysics investigations and have better resolution than most conductivity surveys. They do experience significant changes with soil moisture content, a difficulty in most site investigations with heterogeneous ground and differing vegetation distributions. [1]

Applications

Milsom & Eriksen (2011) [12] provide a useful field book for field geophysics.

Archaeology

Geophysical methods can be used to find or map an archaeological site remotely, avoiding unnecessary digging. They can also be used to date artifacts.

In surveys of a potential archaeological site, features cut into the ground (such as ditches, pits and postholes) may be detected, even after filled in, by electrical resistivity and magnetic methods. The infill may also be detectable using ground-penetrating radar. Foundations and walls may also have a magnetic or electrical signature. Furnaces, fireplaces and kilns may have a strong magnetic anomaly because a thermoremanent magnetization has been baked into magnetic minerals. [13]

Geophysical methods were extensively used in recent work on the submerged remains of ancient Alexandria as well as three nearby submerged cities (Herakleion, Canopus and Menouthis). [14] Methods that included side-scan sonar, magnetic surveys and seismic profiles uncovered a story of bad site location and a failure to protect buildings against geohazards. [15] In addition, they helped to locate structures that may be the lost Great Lighthouse and palace of Cleopatra, although these claims are contested. [14]

Forensics

Forensic geophysics is increasingly being used to detect near-surface objects/materials related to either a criminal or civil investigation. [16] The most high-profile objects in criminal investigations are clandestine burials of murder victims, but forensic geophysics can also include locating unmarked burials in graveyards and cemeteries, a weapon used in a crime, or buried drugs or money stashes. Civil investigations are more often trying to determine the location, amount and (more tricky) the timing of illegally dumped waste, which include physical (e.g. fly-tipping) and liquid contaminants (e.g. hydrocarbons). There are many geophysical methods that could be employed, depending upon the target and background host materials. Most commonly ground-penetrating radar is used but this may not always be an optimal search detection technique.

Geotechnical investigations

Geotechnical investigations use near-surface geophysics as a standard tool, both for initial site characterisation and to gauge where to subsequently undertake intrusive site investigation (S.I.) which involves boreholes and trial pits. [1] In rural areas conventional SI methods may be employed but in urban areas or in difficult sites, targeted geophysical techniques can rapidly characterise a site for follow-up, intensive surface or near-surface investigative methods. Most common is searching for buried utilities and still-active cables, cleared building foundations, determining soil type(s) and bedrock depth below ground level, solid/liquid waste contamination, mineshafts [17] and relict mines below ground locations and even differing ground conditions. [18] Indoor geophysical investigations have even been undertaken. [19] Techniques vary depending upon the target and host materials as mentioned.

Related Research Articles

<span class="mw-page-title-main">Seismology</span> Scientific study of earthquakes and propagation of elastic waves through a planet

Seismology is the scientific study of earthquakes and the propagation of elastic waves through the Earth or other planetary bodies. It also includes studies of earthquake environmental effects such as tsunamis as well as diverse seismic sources such as volcanic, tectonic, glacial, fluvial, oceanic, atmospheric, and artificial processes such as explosions. A related field that uses geology to infer information regarding past earthquakes is paleoseismology. A recording of Earth motion as a function of time is called a seismogram. A seismologist is a scientist who does research in seismology.

<span class="mw-page-title-main">Geophysics</span> Physics of the Earth and its vicinity

Geophysics is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis. Geophysicists, who usually study geophysics, physics, or one of the earth sciences at the graduate level, complete investigations across a wide range of scientific disciplines. The term geophysics classically refers to solid earth applications: Earth's shape; its gravitational, magnetic fields, and electromagnetic fields ; its internal structure and composition; its dynamics and their surface expression in plate tectonics, the generation of magmas, volcanism and rock formation. However, modern geophysics organizations and pure scientists use a broader definition that includes the water cycle including snow and ice; fluid dynamics of the oceans and the atmosphere; electricity and magnetism in the ionosphere and magnetosphere and solar-terrestrial physics; and analogous problems associated with the Moon and other planets.

<span class="mw-page-title-main">Geophysical survey (archaeology)</span> Non-invasive physical sensing techniques used for archaeological imaging or mapping

In archaeology, geophysical survey is ground-based physical sensing techniques used for archaeological imaging or mapping. Remote sensing and marine surveys are also used in archaeology, but are generally considered separate disciplines. Other terms, such as "geophysical prospection" and "archaeological geophysics" are generally synonymous.

<span class="mw-page-title-main">Seismic refraction</span> Deviation of seismic waves by rock or soil layers to characterize subsurface geologic structures

Seismic refraction is a geophysical principle governed by Snell's Law of refraction. The seismic refraction method utilizes the refraction of seismic waves by rock or soil layers to characterize the subsurface geologic conditions and geologic structure.

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. It can also be used to detect the presence of unexploded ordnance.

<span class="mw-page-title-main">Magnetotellurics</span> Electromagnetic geophysical technique

Magnetotellurics (MT) is an electromagnetic geophysical method for inferring the earth's subsurface electrical conductivity from measurements of natural geomagnetic and geoelectric field variation at the Earth's surface.

Geophysical survey is the systematic collection of geophysical data for spatial studies. Detection and analysis of the geophysical signals forms the core of Geophysical signal processing. The magnetic and gravitational fields emanating from the Earth's interior hold essential information concerning seismic activities and the internal structure. Hence, detection and analysis of the electric and Magnetic fields is very crucial. As the Electromagnetic and gravitational waves are multi-dimensional signals, all the 1-D transformation techniques can be extended for the analysis of these signals as well. Hence this article also discusses multi-dimensional signal processing techniques.

<span class="mw-page-title-main">Aeromagnetic survey</span> Surveying method, analyzing the magnetic properties of large regions from high altitudes

An aeromagnetic survey is a common type of geophysical survey carried out using a magnetometer aboard or towed behind an aircraft. The principle is similar to a magnetic survey carried out with a hand-held magnetometer, but allows much larger areas of the Earth's surface to be covered quickly for regional reconnaissance. The aircraft typically flies in a grid-like pattern with height and line spacing determining the resolution of the data.

<span class="mw-page-title-main">Geophysical imaging</span>

Geophysical imaging is a minimally destructive geophysical technique that investigates the subsurface of a terrestrial planet. Geophysical imaging is a noninvasive imaging technique with a high parametrical and spatio-temporal resolution. Geophysical imaging has evolved over the last 30 years due to advances in computing power and speed. It can be used to model a surface or object understudy in 2D or 3D as well as monitor changes.

Induced polarization (IP) is a geophysical imaging technique used to identify the electrical chargeability of subsurface materials, such as ore.

<span class="mw-page-title-main">Magnetic anomaly</span> Local variation in the Earths magnetic field

In geophysics, a magnetic anomaly is a local variation in the Earth's magnetic field resulting from variations in the chemistry or magnetism of the rocks. Mapping of variation over an area is valuable in detecting structures obscured by overlying material. The magnetic variation in successive bands of ocean floor parallel with mid-ocean ridges was important evidence for seafloor spreading, a concept central to the theory of plate tectonics.

The seismoelectrical method is based on the generation of electromagnetic fields in soils and rocks by seismic waves. This technique is still under development and in the future it may have applications like detecting and characterizing fluids in the underground by their electrical properties, among others, usually related to fluids.

Geomathematics is the application of mathematical methods to solve problems in geosciences, including geology and geophysics, and particularly geodynamics and seismology.

<span class="mw-page-title-main">Geothermal exploration</span>

Geothermal exploration is the exploration of the subsurface in search of viable active geothermal regions with the goal of building a geothermal power plant, where hot fluids drive turbines to create electricity. Exploration methods include a broad range of disciplines including geology, geophysics, geochemistry and engineering.

<span class="mw-page-title-main">Outline of geophysics</span> Topics in the physics of the Earth and its vicinity

The following outline is provided as an overview of and topical guide to geophysics:

<span class="mw-page-title-main">Forensic geophysics</span> Use of geophysics tools in forensic science

Forensic geophysics is a branch of forensic science and is the study, the search, the localization and the mapping of buried objects or elements beneath the soil or the water, using geophysics tools for legal purposes. There are various geophysical techniques for forensic investigations in which the targets are buried and have different dimensions. Geophysical methods have the potential to aid the search and the recovery of these targets because they can non-destructively and rapidly investigate large areas where a suspect, illegal burial or, in general, a forensic target is hidden in the subsoil. When in the subsurface there is a contrast of physical properties between a target and the material in which it is buried, it is possible to individuate and define precisely the concealing place of the searched target. It is also possible to recognize evidences of human soil occupation or excavation, both recent and older. Forensic geophysics is an evolving technique that is gaining popularity and prestige in law enforcement.

<span class="mw-page-title-main">John Call Cook</span> American geophysicist (1918–2012)

John Call Cook was an American geophysicist who played a crucial role in establishing the field of ground-penetrating radar and is generally regarded as contributing the fundamental research to develop the field. Cook is also known for demonstrating that aerial surveys can map surface radioactivity to enable much more efficient prospecting for uranium ore, for inventing electrostatic detection of hazardous ice crevasses, and for developing other novel techniques in remote sensing.

Geophysical signal analysis is concerned with the detection and a subsequent processing of signals. Any signal which is varying conveys valuable information. Hence to understand the information embedded in such signals, we need to 'detect' and 'extract data' from such quantities. Geophysical signals are of extreme importance to us as they are information bearing signals which carry data related to petroleum deposits beneath the surface and seismic data. Analysis of geophysical signals also offers us a qualitative insight into the possibility of occurrence of a natural calamity such as earthquakes or volcanic eruptions.

<span class="mw-page-title-main">Marine geophysics</span>

Marine geophysics is the scientific discipline that employs methods of geophysics to study the world's ocean basins and continental margins, particularly the solid earth beneath the ocean. It shares objectives with marine geology, which uses sedimentological, paleontological, and geochemical methods. Marine geophysical data analyses led to the theories of seafloor spreading and plate tectonics.

References

  1. 1 2 3 4 5 Reynolds, John (2011). Introduction to Applied & Environmental Geophysics. Wiley-Blackwell. ISBN   978-0-471-48535-3.
  2. Hansen, JD; Pringle, JK; Goodwin, J (2014). "GPR and bulk ground resistivity surveys in graveyards: Locating unmarked burials in contrasting soil types" (PDF). Forensic Science International. 237: e14–e29. doi:10.1016/j.forsciint.2014.01.009. PMID   24559798.
  3. Parasnis 1997 , Preface
  4. Slater et al. 2006
  5. 1 2 3 4 5 6 Mussett & Khan 2000 , Part 1
  6. Parker 1994
  7. 1 2 3 Mussett & Khan 2000 , Chapter 6
  8. Moseley, D; Pringle, JK; Haslam, RB; Egan, SS; Rogers, SL; Gertisser, G; Cassidy, NC; Stimpson, IG (2015). "Geophysical surveys to help map buried igneous intrusions, Snowdonia, North Wales, UK" (PDF). Geology Today. 31 (3): 149–182. doi:10.1111/gto.12096. S2CID   128766240.
  9. Mussett & Khan 2000 , Chapter 11
  10. Parasnis 1997 , Chapter 3
  11. Pringle, JK; Styles, P; Howell, CP; Branston, MW; Furner, R; Toon, S (2012). "Long-term time-lapse microgravity and geotechnical monitoring of relict salt mines, Marston, Cheshire, U. K." (PDF). Geophysics. 77 (6): B287–B294. doi:10.1190/GEO2011-0491.1.
  12. Milsom, J; Eriksen, A (2011). Field Geophysics, 4th Edition. Wiley-Blackwell. ISBN   978-0-470-74984-5.
  13. Mussett & Khan 2000 , Chapter 28
  14. 1 2 Lawler 2005
  15. Stanley et al. 2004
  16. Pringle, JK; Ruffell, A; Jervis, JR; Donnelly, L; McKinley, J; Hansen, J; Morgan, R; Pirrie, D; Harrison, M (2012). "The use of geoscience methods for terrestrial forensic searches". Earth-Science Reviews. 114 (1–2): 108–123. Bibcode:2012ESRv..114..108P. doi:10.1016/j.earscirev.2012.05.006.
  17. Banham, SG; Pringle, JK (2011). "GPR investigations to characterize Medieval and Roman foundations under existing shop premises: a case study from Chester, Cheshire, UK". Near Surface Geophysics. 9 (5): 483–496. doi:10.3997/1873-0604.2011028.
  18. Tuckwell, G; Grossey, T; Owen, S; Stearns, P (2012). "The use of microgravity to detect small distributed voids and low-density ground". Quarterly Journal of Engineering Geology & Hydrogeology . 41 (3): 371–380. doi:10.1144/1470-9236/07-224. S2CID   130802827.
  19. Pringle, JK; Lenham, JW; Reynolds, JR (2009). "GPR investigations to characterize Medieval and Roman foundations under existing shop premises: a case study from Chester, Cheshire, UK". Near Surface Geophysics. 7 (2): 371–380. doi:10.3997/1873-0604.2008042.

Bibliography