Total Magnetic Field Anomalies Research Articles

Gridding aeromagnetic data using inverse interpolation

SUMMARY We regard the gridding of aeromagnetic data as a geophysical estimation problem and propose to use the inverse interpolation method for gridding aeromagnetic data. The gridding method we present is based on Tikhonov's regularization theory by setting up the data fitting goal and the model smoothing goal. The interpolation operator in the data fitting goal is chosen to be a Gaussian weighted operator. The smoothing filter in the model smoothing goal is chosen to be a Laplacian operator. The gridding equation is iteratively solved by using the preconditioning conjugate gradient algorithm, the preconditioning operator, which is the inverse of the Laplacian operator. The helix transform technique and Wilson's spectral factorization algorithm are used to make the filtering and inverse filtering of the Laplacian operator fast and stable. Gridding experiments were conducted on synthetic total field magnetic anomaly and real aeromagnetic data from a survey in South China and show that the method produced smooth and accurate results without obvious aliasing artefacts.

Interpretation of magnetic data in the Sinop area of Mid Black Sea, Turkey, using tilt derivative, Euler deconvolution, and discrete wavelet transform

article i nfo Article history: Received 8 March 2011 Accepted 25 May 2011 Available online xxxx The identification of lineaments in magnetic anomaly data is an important step in interpretation. In this paper, three methods for locating magnetic lineaments are applied to total field magnetic anomaly data of the Sinop area of mid Black Sea, northern Turkey. Based upon the variation in magnetic lineaments, structural map of the study area was obtained. The tilt derivative (TDR), Euler deconvolution (ED) and discrete wavelet transform (DWT) have been of great utility in the interpretation of magnetic anomaly data. These three methods were applied to enhance magnetic anomalies due to anomalous sources in part of the basement complex of study area. The TDR obtained from the magnetic anomaly reduced to pole (RTP) was designed to map geologic contacts. The source boundaries and depths are determined from the zero contours, and the half distance between ± π/4 contours or the distance between zero and + π/4 or − π/4 contour of TDR, respectively. The ED technique is used to estimate the source depth at the semi-infinite contact location. The DWT technique was carried out to infer the substructure orientation. The direction of maxima and minima coefficients of the horizontal, vertical and diagonal decompositions of the DWT works in imaging the horizontal locations of the anomalous sources. The results allow us to image and characterize subsurface lineaments for the survey area.

Depths to the magnetic layer bottom in the South China Sea area and their tectonic implications

The depths to the magnetic layer bottom (Zb) in the South China Sea (SCS) area are estimated by computing radially averaged amplitude spectra of total field magnetic anomalies. We test different sizes of moving windows in which the spectra are calculated to better understand how window sizes affect the depth estimations. Apart from lowering the resolutions of estimated Zb, larger windows do not necessarily incur presumable increases in Zb in the SCS area. Although the centroid method is taken as our primary technique for estimating Zb, for cross check, the spectral peak and the non-linear inversion methods are also applied to those windows where spectral peaks do appear. In a single window we may find discrepancy in Zb estimated from different techniques, but for all windows showing spectral peaks, the estimated Zb from one technique are grossly correlated with those from another. Our results show that most parts of the central SCS ocean basin and the northern continent–ocean transition (COT) zone have significantly smaller Zb than the surrounding continental blocks. In the surrounding continental regions Zb are averaged at about 34 km, a depth close to the Moho depth. The average Zb is about 22 km in the central basin, but this value is much larger than the Moho depth, signifying that the uppermost 10 km or so of the mantle beneath the central basin is also magnetized. The strong faulting and recent magmatism within the COT zone can account for the small Zb near the northern continental margin. The estimated Zb are also found very correlative to surface heat flow. This observation verifies dominant contributions to surface heat flow from incoming mantle heat flow due to thermal conduction. The positive correlations observed among Zb from different techniques as well as the good correlation between surface heat flow and Zb support the reasoning that our estimated Zb are within an acceptable range of accuracy.

Magnetic anomaly patterns over crustal blocks of the King Edward VII Peninsula, Marie Byrd Land, West Antarctica

Within the framework of the GITARA II project an aeromagnetic survey was performed during the GANOVEXVII expedition (1992/1993) over the King Edward VII Peninsula in northwestern Marie Byrd Land (West Antarctica). This region which may represent the eastern flank of the Ross Sea rift system had previously been explored only at reconnaissance level. New total field and upward continued (10 km) magnetic anomaly maps are produced and interpreted here to map and discuss the crustal structure of the Edward VII Peninsula. Tworound-shaped, high-amplitude magnetic anomalies are recognised over the Alexandra Mountains block. The anomalies are difficult to interpret since susceptibility data indicate the prevalence of non-magnetic rocks at the surface. A possible interpretation is that the anomalies are due to Cretaceous mafic intrusives distinct from weakly magnetic Byrd Coast Granite of the adjacent Rockefeller Mountains block. Alternatively the anomalies could be related to buried pluton-sized Devonian Ford Granodiorite intruded by dikes. If Cretaceous in age, the former intrusives revealed from the magnetics could also be responsible for contact metamorphism of the adjacent Alexandra Mountains migmatites. Lower amplitude circular anomalies over the Central Plateau and Prestrud Inlet are likely to be caused by unexposed Devonian Ford Granodiorite which crops out in the Ford Ranges. Elongated high-frequency anomalies of the Sulzberger Bay are similar to those recognised over seismically constrained Cenozoic rift-related volcanics of the Ross Sea. A broad magnetic low over the Sulzberger Ice Shelf may be indicative of a fault bounded graben-like basin with sedimentary infill. Overall recognition of magnetic anomaly patterns and trends reveals segmentation of the Edward VII Peninsula and of the adjacent marine areas in distinct crustal blocks. Faults may separate these blocks and they are interpreted to reflect multiple Cretaceous and maybe Cenozoic crustal extension\uplift phases within this part of the West Antarctic rift system.

A high-resolution aeromagnetic field test in Friuli: towards developing remote location of buried ferro-metallic bodies

High Resolution AeroMagnetic surveys (HRAM) are a novel tool experimented in several countries for volcano and earthquake hazard re-assessment, ground water exploration and mitigation, hazardous waste site characterization and accurate location of buried ferrous objects (drums, UXO, pipelines). The improvements achieved by HRAM stem from lower terrain clearance coupled with accurately positioned, real-time differential navigation on closely spaced flight grids. In field cultural noise filtering, advanced data processing, imaging and improved interpretation techniques enhance data information content. Development of HRAM approaches might also contribute to mitigate environmental hazards present throughout the Italian territory. Hence an HRAM field test was performed in July 2000 in Friuli, North-Eastern Italy to assess the capabilities and limitations of HRAM over a buried pipeline and a domestic waste site. A Cesium magnetometer in towed bird configuration was used on two separate grids. Profile line spacing was 50-100 m and bird nominal ground clearance was set to 50 m. Microlevelled total field magnetic anomaly data forms the basis for subsequent advanced processing products including 3D analytic signal, maximum horizontal gradient of pseudo-gravity and 3D Euler Deconvolution. The magnetic signatures we detected and enhanced over the environmental test site area in Friuli are also compared with similar but more extensive HRAM signatures recently observed in other countries.

Deep crustal structures of eastern China and adjacent seas revealed by magnetic data

Through reduction to the North Pole and upward continuation of the total field magnetic anomalies, we analyze magnetic patterns and spatial distributions of different tectonic blocks and crustal faults in eastern China and adjacent seas. Depths to the Curie isotherms are further estimated from radially averaged amplitude spectra of magnetic data reduced to the pole. Data reductions effectively enhance boundaries of regional tectonic belts, such as the Dabie ultra-high metamorphic belt, the Tanlu Fault, and the Diaoyudao Uplift. Curie depths are estimated at between 19.6 and 48.9 km, with a mean of 31.7 km. The Subei Basin and the south Yellow Sea Basin in the lower Yangtze block show relatively deep Curie isotherms, up to about 35 km in depth, whereas in the surrounding areas Curie depths are averaged at about 25 km. This implies that the lower Yangtze Block has experienced a unique tectonic evolution and/or has unique basement lithology and structures. From a regional perspective, sedimentary basins, such as the Subei Basin, the south Yellow Sea Basin, and the East China Sea Basin, normally show deeper Curie isotherms than surrounding uplifts such as the Diaoyudao Uplift and the Zhemin Uplifts. Curie isotherms also upwell significantly in volcanically active areas such as the Ryukyu Arc and the Cheju Island, confirming strong magmatic and geothermal activities at depth.

FORTRAN codes to implement enhanced local wave number technique to determine the depth and location and shape of the causative source using magnetic anomaly

The total field magnetic anomaly is analyzed to compute the depth and location and geometry of the causative source using two FORTRAN source codes, viz., FRCON1D and ELW. No assumption on the nature of source geometry, susceptibility contrast, etc. has been made. The source geometry is estimated by computing the structural index from previously determined depth and location. A detailed procedure is outlined for using these codes through a theoretical anomaly. The suppression of high-frequency noise in the observed data is tackled by designing a box-car window with cosine termination. The termination criterion is based on the peak position of the derivative operator computed for a pre-assumed depth of a shallow source below which the target is situated. The applicability of these codes has been demonstrated by analyzing a total field aeromagnetic anomaly of the Matheson area of northern Ontario, Canada.

3D forward and inverse modeling of total-field magnetic anomalies caused by a uniformly magnetized layer defined by a linear combination of 2D Gaussian functions

I develop a method for 3D forward modeling and nonlinear inversion of the total-field magnetic anomaly caused by a uniformly magnetized layer with its top and bottom surfaces represented by a linear combination of 2D Gaussian functions. The solution of the forward problem is found through both analytic and numerical methods of integration to calculate the theoretical magnetic anomaly. The magnetic anomalies computed by the present numerical method compare well with the ones calculated by using an analytic solution. To test the robustness of the algorithm, the inversion is performed with noisy synthetic data. The estimated parameters in the case of a synthetic model were found to deviate only modestly from the true parameters in the presence of noise. The algorithm is used to interpret a dipolar magnetic anomaly of high amplitude attributable to a laccolith of intermediate composition in northern Mexico.

Integrated geophysical survey in defining subsidence features on a golf course

Subsidence was observed at several places on the Salina Municipal Golf Course in areas known to be built over a landfill in Salina, Kansas. High-resolution magnetic survey (~5400 m2), multi-channel electrical resistivity profiling (three 154 m lines) and microgravity profiling (23 gravity-station values) were performed on a subsidence site (Green 16) to aid in determining boundaries and density deficiency of the landfill in the vicinity of the subsidence. Horizontal boundaries of the landfill were confidently defined by both magnetic anomalies and the pseudo-vertical gradient of total field magnetic anomalies. Furthermore, the pseudo-vertical gradient of magnetic anomalies presented a unique anomaly at Green 16, which provided a criterion for predicting other spots with subsidence potential using the same gradient property. Results of multi-channel electrical resistivity profiling (ERP) suggested the bottom limit of the landfill at Green 16 was around 21 m below the ground surface based on the vertical gradient of electric resistivity and a priori information on the depth of the landfill. ERP results also outlined several possible landfill bodies based on their low resistivity values. Microgravity results suggested a –0.14 g cm−3 density deficiency at Green 16 that could equate to future surface subsidence of as much as 1.5 m due to gradual compaction.

Modelling magnetic fields due to steel drum accumulations

ABSTRACTModelling the magnetic fields produced by accumulations of steel drums is a problem that is relevant to the detection and evaluation of disposal sites containing materials that are potentially hazardous to the environment. Accurate modelling is possible with existing integral equation techniques but these are numerically intensive due to the need to solve very large systems of linear equations. Use of an approximate iterative technique for the solution of the equations (system iteration) allows the integral equation technique to be extended to modelling the magnetic effect of substantially large accumulations, comprising up to several hundred drums, on very moderate computing facilities. However, even this process remains time‐consuming and suggests the use of more rapid, if less accurate, modifications. Several are available.Surprisingly, quite reasonable results can also be achieved with a very basic approximation that represents each drum by a discrete dipole located at its centroid. The dipole moments are found from the magnetic behaviour of single drums exposed to a uniform inducing field, which can be conveniently defined by a dyadic drum apparent susceptibility. The basic discrete dipole model for drum accumulations can be substantially improved by using a first‐order accommodation of the depolarizing effect produced by the shape of the accumulation.All of the above modelling techniques require details of individual drum locations and orientation. This information is generally unavailable to geophysical practitioners involved in environmental surveys and so prompts the idea of models that represent drum accumulations as a continuous distribution of magnetization. The convenience of neglecting details of drum location and orientation comes at the cost of some loss in accuracy of the modelled responses. However, for accumulations buried sufficiently deep and in which the drums are uniformly distributed, the total field magnetic anomaly is found to be reasonably approximated by the effect of a continuous magnetization, expressible in terms of an effective isotropic susceptibility. Again, the basic model can be improved by the accommodation of demagnetization effects due to the shape of the accumulation.

Use of Walsh transforms in estimation of depths of idealized sources from total-field magnetic anomalies

We developed a scheme to compute the depths of four idealized magnetized sources, viz., a monopole, a line of monopoles, a dipole and a line of dipoles from the Walsh spectra of the total-field magnetic anomalies. The sequency numbers, l max corresponding to the peaks of the differential energy density spectra over these sources are practically independent of the shape of the observed anomaly and are linearly dependent on the source depths. For each model, we derived a quantitative relation between the depth and the sequency number l max. Analyses of simulated data over idealized isolated sources reveal that (i) a profile length of about 8 times the source depth provides accurate value in computed depth; (ii) data spacing of less than one-fourth the source depth has no significant error in depth computation and (iii) the technique is capable of tolerating random error to the tune of 10% of the peak amplitude of the simulated anomaly. We compared the results of depth estimations from Walsh and Fourier spectra. Analysis of total-field magnetic anomalies over a buried water supply pipe has demonstrated the applicability of the proposed method.

Magnetic-anomaly Fourier spectrum of a 3D Gaussian source

I obtain a closed-form expression for the total-field magnetic anomaly generated by a 3D Gaussian magnetization distribution. The solution is evaluated by a novel contour integration in the complex-wavenumber domain. The spatial-domain solution is then computed by a numerical inverse Fourier transform. This calculated field can be used to interpret magnetic data because a 3D Gaussian function can represent a compact and smooth source. The method, moreover, allows for calculation of the Fourier components as a function of the distance between the depth of the source and the height of the observation level. The solution is thus useful for the inverse problem, spectral analysis, or data continuation.

Curie Point Depths Beneath Precordillera Cuyana and Sierras Pampeanas Obtained from Spectral Analysis of Magnetic Anomalies

A gravity and magnetic survey has been carried out with the purpose of investigating geophysical features of the crusts beneath three geological provinces in western Argentina: Cuyo Precordillera, the Sierras Pampeanas of San Juan and La Rioja, and Famatina System, the results of which are displayed in three maps: Bouguer anomaly, total field magnetic anomaly and total field reduced to the pole. The top and bottom boundaries of the magnetized crust were calculated from power-density spectra of the total-field anomalies from our terrestrial database in 90 2D windows. The depths obtained for the bottom of magnetized crust are assumed to correspond to Curie point depths. The values thus obtained for the Precordillera range between 29 and 40 km, whereas for the Sierras Pampeanas, in the Sierra de Pie de Palo, and other mountain chains along the Bermejo-Desaguadero lineament or Valle Fértil lineament, such depth ranges between 20 and 35 km. These results are consistent with Curie point depths determined on different continental regions in the world. A map of regional heat flow has been prepared in the present work based on the depth of the Curie point isotherm, which shows that heat flow patterns in Precordillera are different from those found in Sierras Pampeanas. A significant heat flow extending in Northeast-Southwest direction seems to be directly related to Juan Fernández Ridge trace. The results of the present investigations also point out a possible relationship between the base of the magnetized crust and the boundary separating the brittle from the ductile crustal regime.

Paleomagnetic investigation of seamounts in the vicinity of Ogasawara Fracture Zone northwest of the Marshall Islands, western Pacific

Nine seamounts located northwest of the Marshall Islands near the Ogasawara Fracture Zone were inverted for their uniform magnetization using total field magnetic anomaly and detailed bathymetric data. The paleomagnetic poles of most of the seamounts in our study area generally cluster around the Pacific Apparent Polar Wander Path (APWP). However, those that deviate significantly from the APWP are located south of the fracture zone. The seamounts in our study area can also be divided into two groups on the basis of complexity of the observed magnetic field anomaly. In general, simple conical seamounts exhibit a dipole-like field anomaly pattern with a paired anomaly low and high, and can be explained to a large extent by a uniformly magnetized source. On the other hand, those with complex morphology are larger in size, show multiple magnetic lows and highs and lie very close to or within the fracture zones, suggesting that they were formed by multi-stage volcanism.

The Magnetization Vector became important in Surface Geology and Chromite Prospecting within Ultramaphic Massifs

Ground-based magnetic survey was performed on two ultramaphic massifs, Albania, mapping magnetic anomalies possibly connected with tectonic and cumulate sections, grade of horst rocks serpentinization and/or shallow chromite deposits of variable size. The results were contradictory. We have implemented a scheme and computer code for contouring rock surface with abnormal magnetization utilizing the simplicity of magnetic bell shaped field anomaly with so-called vertical and the analytic signal for the 2-d thin sheet model. Rapid estimations for the edge of the sources, depth, intensity and angle of total magnetization for the observed shallow anomalies surveyed-line data are obtained. As an alternative of filtering technique we have used the magnetic computed field intensity map of the area with constrained source parameters. The computed magnetic field anomaly map with magnetization vector collinear within the range revealed from chromite laboratory measurements show characteristic, which hardly notices in the survey magnetic anomaly map. The predicted magnetic data on an area 4 km2 are well correlated with the shallow deposits or surface occurrences or controlled associated structures in the harzburgite host rocks. Image of total magnetization inclination data derived from ground magnetic survey acquired over a 30 km2 resembles the geology than the total field magnetic anomaly map. These data show that the total magnetization of thick residual dunites, rich in basaltic impregnation, reveals an opposite direction data against the underlain harzburgite rocks

Analytical Modeling of Magnetic and Gravity Signatures of Unexploded Ordnance

The dominant cost and time driver in unexploded ordnance (UXO) cleanup is the necessity of digging (excavating) “false alarm” gèophysical anomalies. As many as 75% of all the anomalies investigated are false alarms, e.g., scrap metal, ordnance debris, cans, wire, etc. The effort to develop capability to discriminate UXO anomalies from false alarm anomalies is a driver for the development of geophysical model-based signature modeling. The signature modeling is important for forward signature prediction and ultimately for geophysical inversion of buried object parameters from measured data. Researchers are actively developing forward and inverse modeling capability for time-domain and frequency-domain electromagnetic induction, ground penetrating radar, magnetic, and gravity anomaly signatures for realistic UXO shapes. This paper documents progress in developing forward gravity and total magnetic field analytical solutions. Gravity and magnetic methods are passive and share similarity in mathematical formalism. If both gravity and magnetic signatures of UXO can be detected in practice, the potential exists for an integrated bulk density determination as a discriminant. Fundamental concepts of the gravity and magnetic methods as applied to detection and potential discrimination of buried UXO are discussed. Analytical solutions for the gravity and total magnetic field signatures of prolate spheroid models of UXO are developed. The total magnetic field solution is a multipole expansion approximation that includes the dipole and octupole contributions, with the quadrupole contribution zero due to symmetry of the spheroid. The gravity solution is a full analytical solution for the field exterior to a spheroid. Examples and results of application of the solutions to spheroid UXO modeling are presented, along with implications for detection and discrimination. The examples include the effects of spheroid orientation on the signature characteristics. Total magnetic field anomalies of UXO are generally detectable to at least the maximum predicted penetration depths for typical soil types and low background magnetic noise. Gravity anomalies for [Formula: see text] projectiles and larger UXO are detectable at very shallow depths (i.e., just below the surface), but only the very largest UXO (e.g., [Formula: see text] projectiles) will produce detectable anomalies for burial depths approaching [Formula: see text].

Segmentation in gravity and magnetic anomalies along the U.S. East Coast passive margin: Implications for incipient structure of the oceanic lithosphere

Segmentation is a characteristic feature of seafloor spreading along the global mid‐ocean ridge system. While segmentation of active spreading centers has been the focus of much recent research, the process by which a rifted continental margin develops into a segmented mid‐ocean ridge is still poorly understood. In this study we investigate the segmentation character of the U.S. East Coast margin through a modeling study of the margin basement structure, magnetics, and gravity anomalies. The East Coast margin is of particular interest because it is one of several rifted continental margins that display thick sequences of high seismic velocity igneous crust, presumably formed during rifting. The East Coast Magnetic Anomaly (ECMA), a distinct total field magnetic high running offshore along the margin, is commonly located seaward of the thickest sections of the high‐velocity crust and displays segmentation on length scales (100–120 km) similar to the segmentation observed at the Mid‐Atlantic Ridge (MAR). Isostatic gravity anomalies were calculated by removing from free‐air gravity the predicted effects of seafloor, sediments, and a crust‐mantle model assuming local isostatic compensation. The resultant residuals show a corridor of high anomaly running along the margin, situated close to the maximum thickness of the high seismic velocity crust as determined from the two available seismic refraction lines. Reduction to the pole (R‐T‐P) of the total field magnetic anomaly shows that after the removal of skewness from the ECMA, the location of the isostatic gravity high is closely correlated to the ECMA. The isostatic gravity high is also segmented but in two distinct wave bands: 100–150 km and 300–500 km. The short‐wavelength (100–150 km) segmentation in the R‐T‐P magnetic and isostatic gravity anomalies is similar in wavelength to segmentation in magnetization and mantle Bouguer anomaly observed along the present‐day MAR. The 300–500 km segmentation in the along‐margin isostatic gravity anomaly is similar in length scale to both intermediate‐wavelength tectonic segmentation observed in the South Atlantic and variations in lithospheric strength observed along the African margin. Furthermore, two of the intermediate‐wavelength (300–500 km) isostatic gravity lows correspond to the early traces of the Kane and Atlantis fracture zones, suggesting that these two fracture zones may define boundaries of a single tectonic corridor in the North Atlantic. We hypothesize that the direct cause of the intermediate‐wavelength segmentation may be along‐margin variations in both the amount of underplated igneous crust and the strength of the lithosphere, although the relative importance of these two effects remains unresolved. Our results imply that segmentation is an important feature of margin development and that segmentation at mature oceanic spreading centers may be directly linked to segmentation during continental rifting.

Spectral prediction of magnetic source depths from simple numerical models

The depths to the top of buried magnetic sources may be inferred from the power spectrum of their field anomalies. Two examples using numerical models are presented to assess the reliability of the spectral depth estimation technique. The aeromagnetic field was derived from a computer simulation of magnetic anomalies generated by simple two-dimensional geological models. Mean depths to the geological models have been determined from two-dimensional spectral analysis of their aeromagnetic anomalies in map form. The results of the model studies show that the depths to the disturbing magnetic horizons can indeed be estimated from the spectral analysis of total magnetic field anomalies within a resolution limit of ±5%.

Microlevelling procedures applied to regional aeromagnetic data: an example from the Transantarctic Mountains (Antarctica)

The extensive application of digital enhancement and filtering as a powerful tool for aeromagnetic interpretation, not only of high resolution but also of regional data, requires an improved levelling. Two microlevelling techniques were thus compared in order to find an effective but relatively simple procedure to remove, or at least to reduce, residual magnetic errors remaining after standard levelling processes. This study was carried out on regional aeromagnetic data recently acquired at high magnetic latitudes along the Transantarctic Mountains in Antarctica, where it is particularly critical to remove time‐dependent magnetic variations. Two‐dimensional FFT filters applied to the gridded data, namely the Butterworth and a directional cosine filter, proved to be more effective than previously proposed one‐dimensional space‐domain filters in the reduction of the ‘residual corrugation’ not removed by statistical levelling. Tectonic interpretation of trends detected in the total field magnetic anomaly map and in the 3D analytic signal improved after application of frequency‐domain microlevelling. However, we also show that when interpreting microlevelled data, two factors must be considered: (i) the possible presence of real geological trends aligned along the flight lines; (ii) modifications in the results yielded by depth estimates of magnetic sources due to the FFT filters applied during the microlevelling procedure. Such changes were seen both in the well‐established 2D FFT method, based on the slope of the energy spectrum, and in the more recent 3D Euler deconvolution technique. Overall our results indicate that microlevelling could profitably be applied to older gridded aeromagnetic data sets in Antarctica, thus improving the accuracy and geological significance of future regional magnetic compilations, as already seen in other continents.

Comment on : ‘An analytic signal approach to the interpretation of total field magnetic anomalies’ by Shuang Qin

Comment on : ‘An analytic signal approach to the interpretation of total field magnetic anomalies’ by Shuang Qin