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We describe an example of a common geological scenario which results in magnetic bodies of limited depth extent. We then present examples of the errors in depth estimates, as high as 40% or more, incumbent in the use of many classic magnetic depth techniques when considering bodies of limited depth extent. Then using interpretation by characteristics applied to generalized dyke models we develop correction nomographs for a wide variety of classic magnetic depth techniques. We show the improvements in depth estimation that are obtained using our nomographs when applied to the ideal dyke model as well as a conceptualized passive margin cross-section model. We use the principle of isostatic equilibrium to develop one simple methodology for estimating magnetic crustal thickness in the absence of a priori information. We then demonstrate the effectiveness of these techniques by application to the 3D Bishop magnetic model. We show that by using reasonable geological assumptions one can develop acceptable first-pass estimates of the maximum thickness of magnetic bodies, which are key to accurate depth estimates. Using estimates of crustal thickness and applying appropriate thickness corrections based on the developed nomographs, errors of up to 40% in depth estimation can be reduced to a few per cent under ideal assumptions of isolated bodies and absence of noise.

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... Many methods of estimating the depth to basement from magnetic data rely on the simple assumption that the geometry of the causative source has an infinite depth extent (Flanagan and Bain, 2013). This assumption ignores the fact that the thickness of the magnetized layer will be depth limited by either the crust-mantle interface (the Moho) or the Curie temperature of the magnetic minerals, whichever depth is shallower (Wasilewski et al., 1979;Wasilewski and Mayhew, 1992;Lee et al., 2010;Salem et al., 2010;Flanagan and Bain, 2013). ...

... Many methods of estimating the depth to basement from magnetic data rely on the simple assumption that the geometry of the causative source has an infinite depth extent (Flanagan and Bain, 2013). This assumption ignores the fact that the thickness of the magnetized layer will be depth limited by either the crust-mantle interface (the Moho) or the Curie temperature of the magnetic minerals, whichever depth is shallower (Wasilewski et al., 1979;Wasilewski and Mayhew, 1992;Lee et al., 2010;Salem et al., 2010;Flanagan and Bain, 2013). Within continental areas, the Moho and Curie isotherm are likely to be at great depth and hence the effect on depth-to-source estimates is small. ...

... The problems of ignoring the depth to bottom are highlighted by Lee et al. (2010) and Salem et al. (2010). This is further addressed by Flanagan and Bain (2013) using sets of nomographs to correct a range of classic and more recent single anomaly source methods to incorporate the finite depth to bottom. ...

The local-wavenumber method estimates the depth to a magnetic source based on the spectral content of a single anomaly assuming that the base of the magnetic body is at infinite depth. However, the "infinite-depth" assumption can lead to significant underestimation of the depth to the top of magnetic bodies, especially in areas where the depth to the bottom of the magnetic layer is not large compared to the depth to the top, as would occur in high heat-flow regions and thinned continental margins. Such underestimation of depths has been demonstrated in model studies and using real data with seismic and well control. We evaluated a modification to the local-wavenumber approach to estimate the depth to the top of magnetic sources assuming that the depth to the bottom of the magnetic sources is controlled by the Curie temperature or crustal thickness. We applied this new method to a simple model of a continental margin and to magnetic survey data over the central Red Sea where the Curie isotherm is shallow. The effective structural index of this finite depth extent model is found to increase continuously from the continent to the ocean as the depth to the magnetic basement increases and the depth to the bottom of the magnetic layer decreases. We have also discovered in this study that the local-wavenumber maxima correlate well with major seafloor spreading magnetic reversal epochs in the central Red Sea segment.

... Most reliably, the depth of a magnetized body's lower edge may be determined by the application of 3-D magnetic field modeling integrated with other geophysical and geological methods (e.g., Blakely 1995; Eppelbaum and Khesin 2012; Eppelbaum and Katz 2015). In the regional magnetic data analysis, a maximum possible magnetized body lower edge occurrence may be calculated by estimating the Curie point depth (e.g., Pilchin and Eppelbaum 1997; Eppelbaum et al. 2014). Considering the estimation of the lower edge of the depth-unlimited bodies (H 2 /h 1 C 10; explanation of these parameters is given in Fig. 3) (when the lower edge of magnetized body does not reach the depth of Curie point) is a complex geophysical–geological problem. ...

... The characteristics of this anomalous body prove the intermediate-acid composition of this target (intrusion). The interpretation of the significant vertical thickness of this body agrees with the geothermic data on the depth of the Curie discontinuity in this area (about 30 km) (Eppelbaum et al. 2014). The outcroppings at the Earth's surface formed sub-volcanic and sub-intrusive bodies of various consistencies that are apparently fragments of this large magmatic massif penetrating the upper part of the section along the extended faults of the common Caucasian direction. ...

The inverse problem solution is often called "inversion procedure" or simply "inversion". The interpretation problem in determining the anomalous bodies' parameters consists of a detailed description of the sources of anomalies by the measured field taken into account with preliminary geological and geophysical information [258]. First of all, the determination of the buried anomalous bodies' parameters is shown in the example of magnetic prospecting.

... Most methodologies developed for the quantitative analysis of magnetic anomalies (e.g., [28][29][30][31][32][33][34][35]) are insufficient for complex physical-geological conditions: oblique magnetization, rugged terrain relief, and an unknown normal field level. Under oblique magnetization conditions, magnetic data are often "polarized" by calculating "pseudogravimetric" anomalies [14]. ...

With the rapid development of aeromagnetic (primarily uncrewed) methods for measuring the magnetic field, the possibility of detailed magnetic research in hard-to-reach mountainous, forested, swamp, desert, and water areas has emerged. The conditions for interpreting the magnetic field are most difficult due to the vector nature of the magnetic properties of rocks, the wide range of their properties, and the presence of residual magnetization. The physical-geological conditions of the territory of Azerbaijan are characterized by rugged terrain relief, inclined magnetization (~58^o), and complex geological environments. Along with using a probabilistic approach, deterministic methods for solving inverse and direct problems of geophysics become of great importance since it is possible to identify relatively extended reference boundaries and analyze magnetic anomalies from separate bodies of relatively simple shape. The article briefly outlines the main stages of processing and interpreting magnetic data under complex environments. The theoretical examples discussed include a block diagram of various disturbances, interpretive models of thin and thick beds, an intermediate model, a thin horizontal plate, and a horizontal circular cylinder on the flat and inclined surfaces under inclined magnetization conditions. The process of assessing magnetization on sloping terrain relief is shown. The presented field examples for the Caucasus Mts. show the quantitative interpretation of aeromagnetic data at the Big Somalit and Guton areas (southern Greater Caucasus, Azerbaijan), deep regional profile through the Lesser and Greater Caucasus, magnetic field studies in the area around the Saatly Superdeep borehole (Middle Kur Depression between the Greater and Lesser Caucasus), and 3D magnetic field modeling at the Gyzylbulag gold deposit (Azerbaijani part of the Lesser Caucasus). For the Caspian Sea was demonstrated the use of an information parameter to identify faults in the Bulla hydrocarbon field (Gulf of Baku) and for the first obtained relationship between the generalized aeromagnetic data (2.5 km over the mean sea level) and the central area of the mud volcanoes distribution in Azerbaijan.

... Gravity and magnetic data inversion help define the geometries of top basement, sedimentary basins, and crustal scale lineaments and faults (e.g., Cascone et al. 2017, Cheyney et al. 2015, Flanagan and Bain 2013, Salem et al. 2014b). This is important as conductive heat models are complicated by fluid flow circulation within sedimentary basins and fault zones (see Figure 3). ...

Favourability mapping to empower scalability of the geothermal proposition - mapping geothermal resource potential across a region in conjunction with mapping of infrastructure, market, competing resources, ESG factors and decarbonisation potential.

... Gravity and magnetic data inversion help define the geometries of top basement, sedimentary basins, and crustal scale lineaments and faults (e.g., Cascone et al. 2017, Cheyney et al. 2015, Flanagan and Bain 2013, Salem et al. 2014b). This is important as conductive heat models are complicated by fluid flow circulation within sedimentary basins and fault zones (see Figure 3). ...

Keywords
geothermal energy; prospectivity; workflow; favourability mapping; geospatial; commercial; market; gravity; magnetic; scalability; complexity; investment decision; repeatability; machine learning; heat; power; faults; permeability; volume; ranking.

... All depth estimation methods are based on different procedures of data processing and assumptions on the sources. This led Flanagan and Bain (2013) to point out that the estimation of the depth to the top of finite-extent magnetic sources could be affected by considerable error (up to 40%) if appropriate corrections are not applied to methods based on relatively simple body geometries, such as the tilt-depth method (infinitely extended dikes). ...

Spectral analysis has been used for studying a variety of geological structures and processes, such as estimation of the depth to the crystalline basement or of the Curie temperature isotherm from magnetic anomalies. However, the analysis is not standard, as it refers to different theoretical frameworks, such as statistical ensembles of homogeneous sources and uncorrelated or fractal random distributed sources. In this review, we aim to unify the approaches by reformulating all the common spectral expressions in the form of a product between a depth-dependent exponential factor and a factor, which we call the spectral correction factor, that incorporates all of the a priori assumptions for each method. This kind of organization might be useful for practitioners to quickly select the most appropriate method for a given study area. We also establish a new formula for extending the Spector and Grant method to the centroid depth estimation. Practical constraints on the depth estimation and intrinsic assumptions/limitations of the different approaches are examined by generating synthetic data of homogenous ensemble sources, random and fractal models. We address the statistical uncertainty of depth estimates using ordinary error propagation on the spectral slope. Critical parameters, such as the window size, are also analyzed in terms of the type of method used and of the geological complexity. We find that the window size is smaller for the centroid/modified centroid methods and larger for the spectral peak, de-fractal, and nonlinear parameter depth estimation methods. In any case, the window size can be large in tectonically stable regions and relatively small over volcanically, tectonically, and geothermally active areas. We finally estimate and discuss the depth to magnetic top and bottom in the Adriatic Sea region (eastern Italy) in the context of heat flow, Moho depth, and gravity data of the region.

... Khesin et al. (1996) developed methods for magnetic anomaly quantitative interpretation (improved modifications of the tangent and characteristic point methods) for complex physicalegeological conditions. Unlike some conventional procedures (Grant and West, 1965;Mikov et al., 1966;Naudy, 1970;Rao and Babu, 1984;Flanagan and Bain, 2013), these methods are applicable in conditions of rugged terrain relief and arbitrary magnetization of the objects where the normal field level is an unknown (Khesin et al., 1996). ...

Inverse problem solution plays a significant role in the potential field analysis. Visual analysis of magnetic anomalies is one of the essential steps in an investigation. All further stages may depend on the results of this initial decision-making. The horizontal component effect complicates the shape of the anomalies. The magnetic anomalies become asymmetric in the northern hemisphere when the body's magnetization is parallel to the Earth's field. Thus, anomaly maxima of sublatitudinal-oriented bodies are shifted southward.
Suppose the magnetization inclination concerning the horizon is not significant (in southern latitudes). In that case, the maximum is shifted more to the south and the intensity of the minimum increases. In contrast, the minimum is located in the northern periphery of the anomaly. Inflection points (maximum gradients) in the anomaly plots are displaced southward from the projections of lateral sides of the anomalous body on the plane. In the case of a depth-limited body, one of the minima (the southern one) may disappear along the maximum periphery. In the case of three-dimensional bodies of an approximately isometric section, magnetic anomaly distortions occur due to the inclination of the magnetization vector to the horizon and the difference in orientation of the horizontal magnetization projection for the body's axes. To analyze magnetic anomalies under complex physical-geological conditions, have been developed improved modifications of tangent, characteristic point, and areal methods. According to the proven similarity, the advanced interpreting methods developed in magnetic prospecting can be applied (with some practical assumptions) to gravity, thermal, self-potential, and electric resistivity anomalies. Numerous examples of potential field anomalies interpretation show their effectiveness.

... The former assumption may have limited applicability in relation to deeper basement sources as may be judged from the example cross- sectional model of Fig. 2b. Flanagan and Bain (2013) also note depth errors as high as 40% when the assumption of infinite depth extent is made. When considering assessments of mid-to deep- crustal sources, the practical depth extent of potential sources is also restricted by the (assumed) limit of 30 km. ...

The deep crustal magnetic structure of Britain has not previously been described in a uniform manner. We provide a new assessment of the deep crustal magnetic bodies responsible for the long wavelength magnetic features. The study area contains deep crustal relics of the destruction of early Palaeozoic oceanic lithosphere along the Thor-Tornquist Suture and primarily the Iapetus Suture separating Baltica and Avalonia from the Laurentian terranes. Spectral decomposition is applied to a merged onshore and offshore magnetic anomaly data set. Thirty idealised basement bodies are compared with a representation of the subsurface obtained by a coarse 3D inversion of the data. The central area separating Laurentia and Avalonia, is largely characterised by an absence of high susceptibilities throughout the whole crustal volume. We find that the idealised basement bodies are largely consistent with relatively high susceptibility zones at depths in excess of 10 km. The zones of higher relative susceptibility are referenced to the tectonic-terrane framework of the area and possible geological explanations for the contrasts are reviewed. In the north, the Laurentian terranes are diverse, comprising crust first created in the Archaean (Hebridean Terrane), Palaeoproterozoic (Rhinns Terrane), Mesoproterozoic? (Midland Valley Terrane), Neoproterozoic (sub-Southern Upland rocks) and Ordovician. Magnetic anomalies further record the assembly of the Gondwanan (Eastern Avalonian) part of the country through Neoproterozoic and Ordovician (Tornquist) arc magmatism and accretion. The convergence zones between Laurentia, Avalonia and Baltica have all left a magnetic imprint, as has Variscan convergence to the south.

... Obviously, the first methodology of the quantitative interpretation of magnetic anomalies produced by thick bodies was published by Peters (1949). Among other studies carried out in this direction, those by Pyatnitsky (1961), Reford and Sumner (1964), Dukhovsky et al. (1970), Am (1972), Logachev and Zakharov (1973), Tafeyev and Sokolov (1981), Rao and Babu (1984), Ravat and Taylor (1998), and Flanagan and Bain (2013) may be noted. A quantitative analysis of magnetic anomalies from models of thin beds and horizontal circular cylinders (spheres) under complex environments was given in detail in Khesin et al. (1996) and . ...

Magnetic prospecting is one of the most widely used methods for archaeological and environmental investigations worldwide. Magnetic anomaly interpretation is complicated by the inclined magnetization, complex geological (archaeological) structures of investigated sites, uneven terrain relief, and artificial noise. A non-conventional interpreting system has been developed to analyze magnetic anomalies quantitatively under the conditions mentioned above (Khesin et al., 1996; Eppelbaum et al., 2001, 2010). This system includes eight components, the most important of which is the quantitative analysis of magnetic anomalies in conditions of oblique magnetization, rugged relief, and unknown levels of the total magnetic field by using improved versions of characteristic points tangent methods. These methods were developed for the analysis of magnetic anomalies produced by three interpreting models: (1) thin bed, (2) horizontal circular cylinder (sphere), and (3) thick bed. However, many archaeological and geological bodies occupy an intermediate geometrical form between the thick bed and thin horizontal plate. This paper shows that quantitative analysis of magnetic anomalies due to such intermediate beds could be successfully carried out using the methodology developed for the thick bed model. Finally, the reliability of the interpretation methods was tested on several models and real archaeological targets.

... Obviously, the first methodology of the quantitative interpretation of magnetic anomalies produced by thick bodies was published by Peters (1949). Among other studies carried out in this direction, those by Pyatnitsky (1961), Reford and Sumner (1964), Dukhovsky et al. (1970), Am (1972), Logachev and Zakharov (1973), Tafeyev and Sokolov (1981), Rao and Babu (1984), Ravat and Taylor (1998), and Flanagan and Bain (2013) may be noted. A quantitative analysis of magnetic anomalies from models of thin beds and horizontal circular cylinders (spheres) under complex environments was given in detail in Khesin et al. (1996) and . ...

Magnetic prospecting is a rapid and inexpensive geophysical tool and one of the most widely used methods for geophysical prospection worldwide. However, the noise factors – such as inclined magnetization, complex geological structure of investigated areas, and uneven terrain relief – strongly obscure the interpretation of observed magnetic anomalies. The developed methodology of magnetic anomalies’ interpretation from models of thin beds and horizontal circular cylinders (spheres) in complex environments (oblique magnetization, rugged relief, and unknown levels of the normal magnetic field) using improved versions of characteristic points and tangents was presented in detail in the author’s previous publications. However, many geological targets have a geometrical form of thick beds, thin horizontal plates, and intermediate that crop up between these two models. This paper explicitly describes a methodology for interpreting magnetic anomalies produced by thick bed models in complex environments. It shows that quantitative analysis of magnetic anomalies due to intermediate targets could be successfully carried out using the methodology developed for the thick bed model. In the case of a thin horizontal plate with a large horizontal size, two arisen anomalies (from the left and right ends) may be interpreted as anomalies from thin beds. The interpretation methodology was successfully tested on typical models and real geological targets. It was concluded that these methods could be effectively applied for quantitative analysis of magnetic surveys for geological-geophysical mapping, archaeological target delineation, ore body searching, revealing oil and gas traps, and solving other geological and environmental problems.

The Southern Desert of Iraq is located on the stable part of the Arabian Platform. The area has a considerable thickness of Phanerozoic sediments overlying a basement. The basement has neither been seismically imaged nor drilled. Accordingly, magnetic method can help identify and map deep structure of the basement. The aeromagnetic data are used to determine the structure and approximate depth of the basement. We have used the combined results of the Tilt derivative and Phase Preserving Dynamic Range Compression (PPDRC) methods to qualitatively delineate the main basement structures and then used the Source Parameter Imaging (SPI), Tilt-depth and finite SPI (FSPI) methods to determine basement depths. The qualitative interpretation of the Tilt derivative and PPDRC methods identifies three N-S to NE-SW trending linear negative anomalies that could represent extensional grabens in the basement surface. These grabens divide the basement into three blocks, the NW block, central block, and SE block. The magnetic anomalies over the basement blocks suggest the NW and Central blocks are cut by a set of N-S to NNW-SSE and NE-SW faults. Depth estimation methods over the uplifted blocks have minimum depths of between 4 km to 5 km, while over the graben the depths range from 7 km to in excess of 12 km. The FSPI method, unlike the SPI and TD methods that use an infinite depth source body, gives depths generally deeper by up to 1.1 km if the assumed Curie point depth is at 21 km. A more realistic Curie point depth of 32 km is used in the final interpretation model. These inferred basement blocks, grabens and sub-basin structures agree in a general way with the regional structures associated with the Arabian Peninsula and could provide an important framework for developing future hydrocarbon exploration strategies of the Southern Desert.

This contribution reviews the advances of gravity (including gradiometer) and magnetic methods of exploration during the last decade. The review is restricted to airborne methods of data acquisition since they are the most common method of acquisition. During this period gradiometer (FTG and AGG) methods have 'come of age' and both systems are providing gravity tensor data that image shallow targets as never before. This in part has been due to a significant reduction in instrument and processing noise levels. For gravity acquisition systems, their improvement in design and performance has led to better acquisition in turbulent air conditions. This now makes it possible to jointly conduct gravity and magnetic drape surveys. Improvements in processing and interpretation have gone hand in hand with improvements in acquisition. The greater use of the phase signal in the form of the tilt and local wavenumber derivatives in structural mapping, the benefits of finite depth estimation and a more stable downward continuation method are discussed.

Geological 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 geological interpretation. We demonstrate that the method increases spatial accuracy, removes artefacts not related to real edges, increases stability and that it is fast 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. Here, ACLAS is demonstrated 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 to identify and discount artefacts 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 NW Australia shelf displaying results from the two data sets together helps 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.

We compute the depth to the top of magnetic basement using the Tilt-Depth method from the best available magnetic anomaly grids covering the continental USA and Australia. For the USA, the Tilt-Depth estimates were compared with sediment thicknesses based on drilling data and show a correlation of 0.86 between the datasets. If random data were used then the correlation value goes to virtually zero. There is little to no lateral offset of the depth of basinal features although there is a tendency for the Tilt-Depth results to be slightly shallower than the drill depths. We also applied the Tilt-Depth method to a local-scale, relatively high-resolution aeromagnetic survey over the Olympic Peninsula of Washington State. The Tilt-Depth method successfully identified a variety of important tectonic elements known from geological mapping. Of particular interest, the Tilt-Depth method illuminated deep (3 km) contacts within the non-magnetic sedimentary core of the Olympic Mountains, where magnetic anomalies are subdued and low in amplitude. For Australia, the Tilt-Depth estimates also give a good correlation with known areas of shallow basement and sedimentary basins. Our estimates of basement depth are not restricted to regional analysis but work equally well at the micro scale (basin scale) with depth estimates agreeing well with drill hole and seismic data. We focus on the eastern Officer Basin as an example of basin scale studies and find a good level of agreement between previously-derived basin models. However, our study potentially reveals depocentres not previously mapped due to the sparse distribution of well data. This example thus shows the potential additional advantage of the method in geological interpretation. The success of this study suggests that the Tilt-Depth method is useful in estimating the depth to crystalline basement when appropriate quality aeromagnetic anomaly data are used (i.e. line spacing on the order of or less than the expected depth to basement). The method is especially valuable as a reconnaissance tool in regions where drillhole or seismic information are either scarce, lacking, or ambiguous.

The airborne magnetometer developed during World War II for tracking submerged submarines, combined with radio-location or other means for accurately locating the airplane, constitutes a new tool of modern geophysics by means of which magnetic surveys can be made with amazing speed over land or water. This memoir presents a method of interpreting magnetic surveys which, if extensively applied, might yield new knowledge regarding the structure of the earth’s crust and shed light on specific problems of regional geology, such as the maximum depth of sedimentation in geologic basins. It can be applied also to the delineation of buried contacts and the location of probable areas of rock differentiation and of mineralization in regions where the igneous rocks crop out.¹
Airborne magnetic surveys are not essentially different from land magnetic surveys of the vertical magnetic intensity carried out in the past with the Schmidt-type magnetic balance. There is no reason to expect that airborne magnetic surveys can yield more geological information, or that what information they might give is different in kind. There is some benefit derived from removing the magnetometer a few hundred feet above glacial till and other near-surface magnetic disturbances which occasionally affect the readings of the land magnetometer. Except when prospecting in regions where the igneous rocks crop out, this factor is of minor importance. Surveying from more than one altitude is of little value because, if the magnetic intensity has been surveyed at a given height above the surface of the magnetic rocks, it is

The straight‐slope method is still popular for depth to magnetic source estimation due to its simplicity and general reliability in manual interpretation (e.g., Nettleton, 1976). Other commonly used manual slope methods are Peters rule (Peters, 1949) and Sokolov rule (Åm, 1972). The straight‐slope method uses the horizontal projection of the straightline part of the magnetic anomaly curve at the inflection point as the depth estimator (see Figure 1). Because no straight line exists mathematically, the rule is purely empirical, even though visually a certain part of a curve will appear to be straight.

This paper presents a procedure to resoive magnetic anomalies due to two-dimensional structures. The method assumes that all causative bodies have uniform magnetization and a crosssection which can be represented by a polygon of either finite or infinite depth extent. The horizontal derivative of the field profile transforms the magnetization effect of these bodies of polygonal cross-section into the equivalent of thin magnetized sheets situated along the perimeter of the causative bodies A simple transformation in the frequency domain yields an analytic function whose real part is the horizontal derivative of the field profile and whose imaginary part is the vertical derivative of the field profile. The latter can also be recognized as the Hilbert transform of the former. The procedure yields a fast and accurate way of computing the vertical derivative from a given profile. For the case of a single sheet, the amplitude of the analytic function can be represented by a symmetrical function maximizing exactly over the top of the sheet. For the case of bodies with poiygonal cross-section, such symmetrical amplitude functions can be recognized over each corner of each polygon. Reduction to the pole, if desired, can be accomplished by a simple integration of the analytic function, without any cumbersome transformations. Narrow dikes and thin ilat sheets, of thickness less than depth, where the equivalent magnetic sheets are close together, are treated in the same fashion using the field intensity as input data, rather than the horizontal derivative. The method can be adapted straightforwardly for computer treatment. It is also shown that the analytic signal can be interpreted to represent a complex “field intensity,” derivable by differentiation from a complex “potential.” This function has simple poles at each polygon corner. Finally, the Fourier spectrum due to finite or infinite thin sheets and steps is given in the Appendix.

The magnetic anomalies in vertical, horizontal, and total intensity for the thin infinite dike are shown to belong to a single mathematical family of curves for all values of dip and strike of the dike and all values of inclination of the magnetizing field. The complete family of standard curves has been constructed and is incorporated into an interpretational scheme based on superposition with observed magnetic profiles. This technique should give more reliable interpretations than methods based on only a few isolated points of a profile curve.By integration of the general thin-dike response a general expression of similar form has been derived for thick dikes, and ten sheets of curves for dikes of varying width indices have been constructed. By employing the method of subtraction of curves these serve for constructing anomaly profiles over bodies of finite depth extent. Additionally, for thin dikes the much-neglected demagnetization corrections have been incorporated in the interpretational method following verification by model studies. One important disclosure of this work is that the depth and location of the apex of an infinite tabular body may be determined without knowing the intensity or direction of magnetization within the body, assuming only that these quantities are constant throughout.

Summary The analysis of potential field data is an important tool for studying basement structure in hydrocarbon exploration settings. Numerous techniques have been developed to aid the interpretation of such data. Traditionally the effectiveness of each method has been demonstrated on highly idealized model data and/or real data where the correct interpretation is uncertain. In this paper we propose a new approach for evaluating the effectiveness of ‘depth to magnetic basement’ estimators. We present inversion results for a model magnetic dataset generated from a modified topography dataset. This strikes a balance between geological realism and ground-truthing.

A graphical method of determining the depth and other parameters of two‐dimensional tabular bodies by analysis of aeromagnetic anomalies is outlined. The method uses the inflection and half maximum slope points of anomalies having either two flanks or a single high gradient. Ratios of distances between these points are used to obtain a solution. The problem is simplified by combining angles of dip, magnetization direction and the inclination of the geomagnetic field in the plane of the profile into an apparent inclination angle. By use of the graphs, the depth, width, and apparent inclination angle can be determined rapidly from only a few simple measurements, so the method is especially suited for rapid interpretation of large aeromagnetic surveys by use of the observed profiles. Graphs are also given for locating the center or edge of the block, and the product of the intensity of magnetization and the dip of the body can be obtained by utilizing the maximum slope of the anomaly. By use of alternate values of the apparent inclination angle, the method can be used for any direction of magnetization at any magnetic latitude.

This paper discusses the solution of the inverse potential problem and its practical application in the interpretation of field data which have a scalar potential distribution. The discussion will be in terms of the interpretation of magnetic data. Among the topics discussed are: the direct calculation of basement relief, the derivation of the potential and the horizontal components of the field from the vertical intensity, the continuation of the field upward, the continuation of the field downward towards its source, the calculation of derivatives of the vertical intensity with special attention to the second and fourth, and the estimation of depths to igneous basement rocks. The uses of these tools and the information of practical value which can be obtained by their use are discussed and illustrated. Methods of rapidly making calculations using magnetic field data are given.

The paper is mainly concerned with depth estimates from magnetic data. Interpretation by characteristics is critically reviewed. Several new charts of characteristic curves for manual interpretation of magnetic profiles are presented. The charts serve different purposes, thus allowing selection of the most suitable procedure in specific situations, depending upon type and quality of anomalies and accuracy desired in the results. The parameters used are mainly based upon features located inside the inflection points.A theoretical analysis of maximum errors in depth to three-dimensional structures interpreted by some two-dimensional methods is given.

This paper attempts to present the state of aeromagnetic data processing and analysis from the perspective of its evolution since the introduction of the Gulf Airborne Fluxgate Magnetometer in 1948. The paper is concerned exclusively with aeromagnetic data applied to the search of hydrocarbons in localized structures, but the content pertains to ground magnetic data, mineral exploration, to crustal or regional studies and even to the “new global tectonics”.Although high-sensitivity magnetic instrumentation has been available for at least five years; data processing, including compilation and interpretation, has not kept abreast of this development and has hindered the efficient application of the method.

The fourth edition of SEG’s best seller is a valuable, comprehensive reference that is a must for every geophysicist, geologist, explorationist, engineer, energy adviser, economist, editor, and student involved in the field. Hundreds of terms have been added since publication of the third edition in 1991, reflecting rapid evolution of the science, especially in the areas of engineering and production problems, 3D (including multicomponent) acquisition and processing, visualization, S- and converted waves, interpretation, anisotropy, AVO, geostatistics, geohazards, neural networks, tomography, downhole measurements, horizontal drilling, and deepwater work. Definitions of hundreds of other terms have been updated. The dictionary’s title has been modified slightly to reflect growth in application of geophysical methods, with the word Applied replacing the word Exploration. The dictionary includes a guide to pronunciation and a list of reference figures and tables.

A thesis submitted to the faculty and the Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Master of Science in Geophysics. Photocopy.

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