Gravity and Magnetic Methods

2000 ◽  
Author(s):  
Richard M. Carruthers ◽  
John D. Cornwell
Keyword(s):  
Large Scale ◽  
Gravity Data ◽  
Integrated Approach ◽  
Rock Properties ◽  
Magnetic Data ◽  
Satellite Gravity ◽  
Fully Integrated ◽  
Continental Shelves ◽  
Gravity And Magnetic ◽  
Gravity And Magnetic Data

Lateral variations in the density and magnetization of the rocks within the crust give rise to "anomalies" in the Earth's gravity and magnetic fields. These anomalies can be measured and interpreted in terms of the geology both in a qualitative sense, by mapping out trends and changes in anomaly style, and quantitatively, by creating models of the subsurface which reproduce the observed fields. Such interpretations are generally less definitive in themselves than the results from seismic surveys (see chapter 12), but the data are widely available and can provide information in areas where other methods are ineffective or have not been applied. As the different geophysical techniques respond to specific rock properties such as density, magnetization, and acoustic velocity, the results are complementary, and a fully integrated approach to data collection and interpretation is generally more effective than the sum of its parts assessed on an individual basis. Gravity and magnetic data have been acquired, at least to a reconnaissance scale, over most of the world. In particular, the release into the public domain of satellite altimetry information (combined with improved methods of data processing) means that there is gravity coverage to a similar standard for most of the offshore region to within about 50 km of the coast. Magnetic anomalies recorded from satellites provide global coverage, but the high altitude of the observations means that only large-scale features extending over many 10s of kilometers are delineated. Reconnaissance aeromagnetic surveys with flight lines 10-20 km apart provide a lateral anomaly resolution similar to that of the satellite gravity data. Oceanographic surveys undertaken by a variety of academic and research institutions are another valuable source of data in remote regions offshore which supplement and extend the more detailed coverage obtained over the continental shelves, for example, by oil companies in areas of hydrocarbon interest. Surveys over land vary widely in terms of acquisition parameters and quality, but some form of national compilation is available from many countries. A number of possible applications of the potential field (i.e., gravity and magnetic) data follow from the terms set out by UNCLOS. Paragraph 4(b) of article 76 states, "In the absence of evidence to the contrary, the foot of the continental slope is to be determined as the point of maximum change in the gradient at its base" (italics added).

Gravity, magnetics and geothermal heat flow of the Antarctic lithospheric crust and mantle

2021 ◽  
pp. M56-2020-5
Author(s):  
Folker Pappa ◽  
Jörg Ebbing
Keyword(s):  
Heat Flow ◽  
Gravity Data ◽  
Magnetic Data ◽  
Geothermal Heat ◽  
Satellite Gravity ◽  
Gravity And Magnetic ◽  
Magnetic Analysis ◽  
Gravity And Magnetic Data ◽  
The Antarctic ◽  
Broad Scale

AbstractThis chapter describes the application and coverage of gravity and magnetic data for Antarctica with emphasis on airborne and satellite models. Low resolution satellite data help to fill gaps between high-resolution airborne data. Satellite gravity data are best used to study broad-scale lithospheric architecture while airborne data, especially magnetic data, provide finer detail. We review examples of gravity and magnetic analysis and describe the possibilities and pitfalls for estimating the properties of the lithosphere as it relates to the mantle. This is followed by a discussion on geothermal heat flow and possible ways to combine different geophysical and petrological models for a better understanding of the Antarctic mantle.

Application of gravity and magnetic methods to assess geological hazards and natural resource potential in the Mosida Hills, Utah County, Utah

Geophysics ◽  
2000 ◽  
Vol 65 (5) ◽  
pp. 1514-1526 ◽  
Author(s):  
Alvin K. Benson ◽  
Andrew R. Floyd
Keyword(s):  
Gravity Data ◽  
Magnetic Data ◽  
Geological Hazards ◽  
Mafic Lava ◽  
Subsurface Geology ◽  
Gravity And Magnetic ◽  
Geophysical Surveys ◽  
Utah County ◽  
Gravity And Magnetic Data ◽  
Hills Area

Gravity and magnetic data were collected in the Mosida Hills, Utah County, Utah, at over 1100 stations covering an area of approximately 58 km2 (150 mi2) in order to help define the subsurface geology and assess potential geological hazards for urban planning in an area where the population is rapidly increasing. In addition, potential hydrocarbon traps and mineral ore bodies may be associated with some of the interpreted subsurface structures. Standard processing techniques were applied to the data to remove known variations unrelated to the geology of the area. The residual data were used to generate gravity and magnetic contour maps, isometric projections, profiles, and subsurface models. Ambiguities in the geological models were reduced by (1) incorporating data from previous geophysical surveys, surface mapping, and aeromagnetic data, (2) integrating the gravity and magnetic data from our survey, and (3) correlating the modeled cross sections. Gravity highs and coincident magnetic highs delineate mafic lava flows, gravity lows and magnetic highs reflect tuffs, and gravity highs and magnetic lows spatially correlate with carbonates. These correlations help identify the subsurface geology and lead to new insights about the formation of the associated valleys. At least eight new faults (or fault segments) were identified from the gravity data, whereas the magnetic data indicate the existence of at least three concealed and/or poorly exposed igneous bodies, as well as a large ash‐flow tuff. The presence of low‐angle faults suggests that folding or downwarping, in addition to faulting, played a role in the formation of the valleys in the Mosida Hills area. The interpreted location and nature of concealed faults and volcanic flows in the Mosida Hills area are being used by policy makers to help develop mitigation procedures to protect life and property.

A geological interpretation of gravity and magnetic data, northwest Saskatchewan

1970 ◽  
Vol 7 (3) ◽  
pp. 858-868 ◽  
Author(s):  
R. H. Wallis
Keyword(s):  
Gravity Data ◽  
Magnetic Data ◽  
Early Proterozoic ◽  
Geological Interpretation ◽  
Metasedimentary Rocks ◽  
Lower Proterozoic ◽  
Gravity And Magnetic ◽  
Northern Saskatchewan ◽  
Gravity And Magnetic Data ◽  
Sedimentary Unit

The striking 'fit' of aeromagnetic and gravity data from the Precambrian of northwest Saskatchewan, combined with known and nearby analogous, geological relationships, suggests the presence of a northeast-trending belt, 250 × 20 miles (400 × 30 km), of early Proterozoic (?) metasedimentary rocks, probably magnetite-bearing meta-arkoses. This structural–sedimentary unit might have economic possibilities analogous to other northeast-striking, Precambrian, lower Proterozoic (?), metasedimentary belts of northern Saskatchewan, the Virgin River Belt, and the Wollaston Trend.

THE ELECTRONIC COMPUTER AND GEOPHYSICS

Geophysics ◽  
1961 ◽  
Vol 26 (1) ◽  
pp. 40-44 ◽  
Author(s):  
L. S. Morrison ◽  
Robert Watson
Keyword(s):  
Digital Computer ◽  
Gravity Data ◽  
Number System ◽  
Magnetic Data ◽  
Memory Unit ◽  
Digit Number ◽  
Gravity And Magnetic ◽  
System Data ◽  
Electronic Computers ◽  
Gravity And Magnetic Data

With the advent of electronic computers a revolution in data handling has been brought about. As yet few people outside of scientific research or accounting fields are familiar enough with these computers to know how they can be applied in their particular type of work. Electronic computers consist of three basic elements: an operations register, a digital computer and a memory unit. The memory unit retains information in numeric form in locations that are identified by a coordinate system. Data stored in memory may be retrieved and processed in the digital computer and the results of the computation may be stored for later use. Computations are carried out in the binary or two‐digit number system. The operations register controls the operation of the digital computer and the memory unit and acts as a link between the intent of the programmer and the internal operation of the system. An object, given the proper code numbers describing its shape, size, color, etc., can be identified from other objects by comparison of code numbers. The speed and ability of electronic computers to compare and identify makesn possible the solution of very exacting problems that otherwise would be humanly next to impossible from time considerations. Electronic computers have been used in geophysical exploration to compute and contour derivative maps of gravity and magnetic data. They have been used to reduce gravity data to datum, compute interval and average velocities from velocity profile data and have been used to solve many non‐recurring problems.

ACLAS — A method to define geologically significant lineaments from potential-field data

Geophysics ◽  
2017 ◽  
Vol 82 (4) ◽  
pp. G87-G100 ◽  
Author(s):  
Lorenzo Cascone ◽  
Chris Green ◽  
Simon Campbell ◽  
Ahmed Salem ◽  
Derek Fairhead
Keyword(s):  
Large Scale ◽  
Magnetic Data ◽  
Data Set ◽  
New Approach ◽  
Potential Field Data ◽  
North West ◽  
Gravity And Magnetic ◽  
Lineament Analysis ◽  
The Individual ◽  
Gravity And Magnetic Data

Geologic features, such as faults, dikes, and contacts appear as lineaments in gravity and magnetic data. The automated coherent lineament analysis and selection (ACLAS) method is a new approach to automatically compare and combine sets of lineaments or edges derived from two or more existing enhancement techniques applied to the same gravity or magnetic data set. ACLAS can be applied to the results of any edge-detection algorithms and overcomes discrepancies between techniques to generate a coherent set of detected lineaments, which can be more reliably incorporated into geologic interpretation. We have determined that the method increases spatial accuracy, removes artifacts not related to real edges, increases stability, and is quick to implement and execute. The direction of lower density or susceptibility can also be automatically determined, representing, for example, the downthrown side of a fault. We have evaluated ACLAS on magnetic anomalies calculated from a simple slab model and from a synthetic continental margin model with noise added to the result. The approach helps us to identify and discount artifacts of the different techniques, although the success of the combination is limited by the appropriateness of the individual techniques and their inherent assumptions. ACLAS has been applied separately to gravity and magnetic data from the Australian North West Shelf; displaying results from the two data sets together helps in the appreciation of similarities and differences between gravity and magnetic results and indicates the application of the new approach to large-scale structural mapping. Future developments could include refinement of depth estimates for ACLAS lineaments.

Joint and constrained inversion of magnetic and gravity data: A case history from McArthur River area, Canada

Geophysics ◽  
2020 ◽  
pp. 1-76
Author(s):  
Mehrdad Darijani ◽  
Colin G. Farquharson ◽  
Peter G. Lelièvre
Keyword(s):  
Joint Inversion ◽  
Gravity Data ◽  
Real Life ◽  
Magnetic Data ◽  
Airborne Gravity ◽  
Uranium Deposits ◽  
Athabasca Basin ◽  
Compositional Approach ◽  
Gravity And Magnetic ◽  
Gravity And Magnetic Data

Magnetic and gravity data are used in the early stages of exploration for uranium deposits in the Athabasca Basin of Canada, just as for many other mineral exploration scenarios. Uranium mineralization in the Athabasca Basin is located where faults in the basement intersect the unconformity between the basement and the overlying sandstones. Both the gravity and magnetic data are dominated by signatures from the basement and an overburden of glacial sediments. The gravity and magnetic data are effective at mapping the basement geology. Any subtle gravity signal from the mineralization related to the formation of the uranium deposits is masked by the signal from the variable thickness overburden. 3D joint inversion of gravity and magnetic data, first without and then with constraints, is evaluated as a means of better determining the structure of the three main lithologies (overburden, sandstones, basement) in the Athabasca Basin. A significant amount of physical property information is available for the main rock units (and overburden), which makes the use of the compositional approach to joint inversion appropriate. For the joint inversion, the fuzzy c-mean clustering method is used. Results from representative synthetic examples show that the joint inversions can construct the overburden and basement structures better than the independent inversions of gravity and magnetic data. Furthermore, constrained joint inversion allows delineation of all three major layers in the area. The same inversion strategies were then applied to the real airborne gravity and magnetic data from the McArthur River area in the eastern Athabasca Basin. The results obtained demonstrate the capabilities of joint inversion for real-life situations.

Kowanyama deep refraction seismic survey

1989 ◽  
Vol 20 (2) ◽  
pp. 303
Author(s):  
B.M. Haines ◽  
B.A. McConachie
Keyword(s):  
Gravity Data ◽  
Seismic Refraction ◽  
Cost Effective ◽  
Seismic Survey ◽  
Magnetic Data ◽  
Gravity And Magnetic ◽  
Refraction Seismic ◽  
Intermediate Section ◽  
Cape York ◽  
Gravity And Magnetic Data

The Carpentaria Basin in the west/central portion of Cape York Peninsula is largely unexplored for petroleum, and there is an apparent ambiguity in the basement depths interpreted from gravity and aeromagnetic data. It was decided that deep seismic refraction surveys at a variety of sites should prove cost-effective in defining the geologic model for the basin. Of particular interest is the possible existence of a north-south trending elongate infrabasin inferred qualitatively from a strong gravity low shown Figure 1. Results of the refraction work indicate that the magnetic and gravity data suggestive of the presence of an infrabasin are probably related to lithological variations within basement. Furthermore, it is improbable that the thickness of the sedimentary pile anywhere within the area of investigation exceeds 1100 metres. Basement velocities are high, from 5500m/sec to 6200m/sec, typical of fresh igneous and/or metamorphic lithologies. Carbonates could not be totally excluded on the basis of these velocities alone, but are improbable in view of the gravity and magnetic data. At some locations there is evidence for the presence of an intermediate section of higher velocity within the sedimentary sequence. This is thought to be quite thin, and possibly representative of the Toolebuc Formation.

Preliminary Study on Fusion Scheme of Multi-dimensional and Multi-scale Potential Field Data

2020 ◽  
Author(s):  
Xiaolin Ji ◽  
Wanyin Wang ◽  
Fuxiang Liu ◽  
Min Yang ◽  
Shengqing Xiong ◽  
...  
Keyword(s):  
Potential Field ◽  
Field Data ◽  
Gravity Data ◽  
Magnetic Data ◽  
Potential Field Data ◽  
Multi Scale ◽  
Gravity And Magnetic ◽  
Different Dimensions ◽  
Fusion Scheme ◽  
Gravity And Magnetic Data

<p>Gravity and magnetic surveys are widely used in geology exploration because of its advantages, such as efficient and economy, green and environment-friendly, widely coverage and strong horizontal resolution. In order to well study in the geology exploration, it is required to comprehensively combine the different scales (different scales data) and different dimensions (satellite data, aeronautical data, ground data, ocean data, well data, etc.) of gravity and magnetic data that were observed in different periods, however, the comprehensive application of the multi-dimensional and multi-scale gravity and magnetic data still stays in the initial stage. In this paper, we do research on the key point of the fusion of potential field data (gravity and magnetic data): the way to fuse the different scales and different dimensions of potential field data into a benchmark and the same surface. Based on this research, we propose a scheme to fuse the multi-dimensional and multi-scale gravity and magnetic data. The synthetic models show that this fusion scheme is able to fuse the multi-dimensional and multi-scale gravity and magnetic data with great fusion results and small errors, in addition, the most important is that the fusion data conform to the characteristics of the potential field data and can meet the needs of data processing in the following steps. One of case studies in China has been accomplished to fuse aeronautical and ground gravity data that are different scales by using this fusion scheme. The fusion scheme we proposed in this study can be used in the fusion of the multi-dimensional (aeronautical, ground and ocean) and multi-scale gravity and magnetic data, which is good for interpretation and popularization.</p>

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