Total Field Magnetic Data 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.

3D stochastic joint inversion of gravity and magnetic data

Abstract A novel stochastic joint inversion method based on cokriging is applied to estimate density and magnetic susceptibility distributions from gravity and total magnetic field data. The method fully integrates the physical relations between density–gravity, on one hand, and magnetic susceptibility–total magnetic field, on the other hand. As a consequence, when the data are considered noise-free, the responses from the inverted density and susceptibility data exactly reproduce the observed data. The required density and magnetic susceptibility auto- and cross covariance are assumed to follow a linear model of coregionalization (LCM). The parameters of the LCM are estimated from v–v plot fitting of the gravity and total magnetic experimental covariances. The model is tested on two synthetic cases and one real data set, the Perseverance mine (Quebec, Canada). Joint inversions are compared to separate inversions. The joint inversions better recover the known models in the synthetic cases. With the real data set, better definition and location of the mineralized lenses are achieved by joint inversion.

Joint processing of total-field and gradient magnetic data

The processing of aeromagnetic data to account for levelling has been improved using gradient data. Utilising multiple magnetometers allows measurements of magnetic gradients and minimises the diurnal variation and other common-mode noise. We develop an equivalent source technique for jointly processing total-field and gradient data that makes use of a well known but rarely used relationship between the derivatives of the magnetic field and the derivative of its source to relate both datasets to a common equivalent source distribution. This approach treats the observed gradients as an additional and independent dataset instead of being just supplemental information. The direct result of joint processing is a set of enhanced data that incorporates information from both types of observed data as well as a higher signal-to-noise ratio. The methodology of the joint equivalent source processing technique is presented and demonstrated with a field example. Our method diminishes higher frequency noise, accentuates mid-frequency signals, and has higher resolution than that of total-field data alone.

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.

Interpretation of aeromagnetic data using eigenvector analysis of pseudogravity gradient tensor

This study has shown that the same properties of the gravity gradient tensor are valid for the pseudogravity gradient tensor derived from magnetic field data, assuming that the magnetization direction is known. Eigenvectors of the pseudogravity gradient tensor are used to estimate depth to the center of mass of geologic bodies. The strike directions of 2D geological structures are estimated from the eigenvectors corresponding to the smallest eigenvalues. For a set of data points enclosed by a square window, a robust least-squares procedure is used to estimate the source point which has the smallest sum of squared distances to the lines passing through the measurement points and parallel to the eigenvectors corresponding to the maximum eigenvalues. The dimensionality of the pseudogravity field is defined from the dimensionality indicator I, derived from the tensor components. In the case of quasi-2D sources, a rectangular window is used in the robust least-squares procedure to reduce the uncertainty of estimations.Based on synthetic data sets, the method was tested on synthetic models and found to be robust to random noise in magnetic field data. The application of the method was also tested on a pseudogravity gradient tensor derived from total magnetic field data over the Särna area in west-central Sweden. Combined with Euler deconvolution, the method provides useful complementary information for interpretation of aeromagnetic data.

Estimation of the normal geomagnetic field and geological interpretation of the normal field anomalies over middle-northern Croatia

Based on the ground survey of total-field magnetic data at 53 sites, which recorded a time-series over the interval 2003–2005, the core field + long wavelength lithospheric field over the middle-northern Croatia region was calculated. The area of the survey was 18900 km2, an average distance between the neighbouring sites being 12 km. The results were reduced to 2004.5 epoch. This “normal” total-field was estimated using the firstorder Taylor polynomial as a function of geographic coordinates, and the polynomial coefficients were calculated with three methods of adjustment: simple and weighted least squares fits and adjustment according to the most frequent value. The stability of the normal field was tested using the Monte Carlo-type test, by decreasing the input data set in each adjustment (up to 90%). All obtained field residuals (measured-“normal”) were mutually compared, as well as fit coefficients of the Taylor polynomials. The residual values indicate the presence of shorter-wavelength anomalies, specifically three major residual anomalies were found (−106 nT, 74 nT and 57 nT). The geostatistical analysis of the ground survey data and the normal field residuals (respectively), using the median absolute deviation method, was further conducted in order to evaluate the calculated anomalies. The geological situation around the anomalies derived by the median absolute deviation method, and around the normal field residual anomalies, is given. The correlation was found between higher value anomalies of the normal field residuals, and shallow volcanic rocks and oil field, respectively.

Structural and tectonic interpretation of geophysical data along the Eastern Continental Margin of India with special reference to the deep water petroliferous basins

The study area encompasses the Eastern Continental Margin of India (ECMI) and the adjoining deep water areas of Bay of Bengal. The region has evolved through multiple phases of tectonic activity and fed by abundant supply of sediments brought by prominent river systems of the Indian shield. Detailed analysis of total field magnetic and satellite-derived gravity data along with multi channel seismic reflection sections is carried out to decipher major tectonic features, basement structure, and the results have been interpreted in terms of basin configuration and play types for different deep water basins along the ECMI. Interpretation of various image enhanced gravity and magnetic anomaly maps suggest that in general, the ENE–WSW trending faults dominate the structural configuration at the margin. These maps also exhibit a clear density transition from the region of attenuated continental crust/proto oceanic crust to oceanic crust based on which the Continent Ocean Boundary (COB) has been demarcated along the margin. Basement depths estimated from magnetic data indicate that the values range from 1 to 12 km below sea level and deepen towards the Bengal Fan in the north and reveal horst–graben features related to rifting. A comparison of basement depths derived from seismic data indicates that in general, the basement trends and depths are comparable in Cauvery and Krishna–Godavari basins, whereas, in the Mahanadi basin, basement structure over the 85°E ridge is clearly revealed in seismic data. Further, eight multichannel seismic sections across different basins of the margin presented here reveal fault pattern, rift geometries and depositional trends related to canyon fills and channel–levee systems and provide a basic framework for future petroleum in this under explored frontier.

Rapid construction of equivalent sources using wavelets

We have developed a fast algorithm for generating an equivalent source by using fast wavelet transforms based on orthonormal, compactly supported wavelets. We apply a 2D wavelet transform to each row and column of the coefficient matrix and subsequently threshold the transformed matrix to generate a sparse representation in the wavelet domain. The algorithm then uses this sparse matrix to construct the the equivalent source directly in the wavelet domain. Performing an inverse wavelet transform then yields the equivalent source in the space domain. Using upward continuation of total-field magnetic data between uneven surfaces as examples, we have compared this approach with the direct solution using the dense matrix in the space domain. We have shown that the wavelet approach can reduce the CPU time by as many as two orders of magnitude.

English

  Ground magnetic survey of eastern part of Ilesa town in Osun State, Southwest Nigeria was performed. Total field magnetic data was recorded using high resolution proton precision geometric magnetometer which implores total components of the ground magnetic anomaly data running through fifteen traverses. This research focused on delineation of faults in this part of the highly mineralized Ilesa schist belt. The field data was qualitatively and quantitatively interpreted and the results gave values for the total component measurements of ground magnetic anomaly that varied between a minimum negative peak value of about -330 gammas and a maximum positive peak value of about 80 gammas. Depth to the basement rock was estimated using Peters half slope method which gave a maximum depth to basement of about 160 m. Information on wide range of magnetic intensities over the different rock types in this area were obtained from the interpretations. The results generated were used to delineate rock boundaries, major and minor faults in this area.   Key words: Magnetic intensities, residual ground magnetic, geomagnetic sections, Ilesa east.

Decorrugation, edge detection, and modelling of total field magnetic observations from a historic town site, Yellowstone National Park, USA

AbstractCinnabar, Montana is a historic town site and railroad depot near the northern edge of Yellowstone National Park and was inhabited between 1883 and 1903. Remains of foundations and old photographs help determine the area of the town, but the south and east limits are unknown. We acquired total field magnetic intensity data to help determine the full extent of the town. Randomly distributed ferrous magnetic sources on the surface and typical noise associated with acquisition complicate the signal. To separate signal and noise we applied filtering and edge detection techniques common in the aeromagnetic industry to our data. Regional removal, decorrugation, upward continuation, and edge detection successfully separated signal and noise. Following filtering, we extracted two larger anomalies from the data set. For those two anomalies, we estimated the edges of their causative sources by calculating the maxima in the horizontal gradient of their anomalies and by inverse modelling those sources; both methods yield similar results. An archaeological test unit excavation within one of the anomalies clearly indicates the remains of buried domestic features, the foundation to a house or other building associated with the late nineteenth to early twentieth century use of Cinnabar. Thus the southeast extent of Cinnabar is greater than previously thought. The lack of surface indicators or adequate historic photography precluded the identification of this buried feature without the aid of the magnetic study. Copyright © 2009 John Wiley & Sons, Ltd.

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.

The structure of the ophiolitic beltin Albania inferred from geomagnetic anomalies

The ground magnetic measurements in Albania were used for the compilation of the Total Magnetic Field (TMF) anomaly map of Albania. The magnetic data were processed and interpreted in order to study the structure of the ophiolitic belts of Albania. The ophiolites of Albania are placed at the Mirdita zone and are divided into two parallel alignments which are called the eastern and western ophiolitic belts. They are associated with strong potential field anomalies and their characteristics are considered crucial for a better understanding of the tectonic settings of Albania. The ground TMF data used in this study were acquired over various campaigns (1990-1994) and cover most of Albania's territory. The data were compiled to a map after reduction to the epoch 1990.4. The strongest magnetic anomalies in Albania appear along the known ophiolitic belts which trend NE-SW to the north and NW-SE to the south. Several processing steps were applied to the unified and gridded data in order to obtain information on the distribution of the magnetic sources. The magnetic sources were subsequently modeled using a 2.5D inversion technique. The magnetic properties of the ophiolites determined from laboratory measurements on rock samples, while their lateral extent was calculated from the processing of the magnetic data and used as constraints to the inversion procedure. The bottom of the ophiolitic belts is considered to be predominated by harzburgites. They exhibit lower magnetization than other rocks of the complex, i.e. gabbros, basalt. In fact gabbros are associated with the observed high frequency magnetic anomalies. The modeling results indicate that the thickness of the ophiolites is reduced from east to west. The eastern ophiolitic belt has a maximum thickness of approximately 12 km at its northern section (Kukesi and Lura massifs). The magnetic sources appear with relatively small thickness at the western ophiolitic belt. Boundaries of the eastern ophiolites are characterized by vertical contacts.

Matched filter separation of magnetic anomalies caused by scattered surface debris at archaeological sites

ABSTRACTRandomly scattered debris, some of it ferromagnetic, on or near the ground surface regularly degrades magnetic data acquired for archaeological purposes. These sources create short‐wavelength high‐amplitude dipolar anomalies in total field magnetic intensity maps and they can dominate maps of the vertical magnetic gradient. Thus, we generally wish to separate longer wavelength anomalies created by features of interest, such as foundations or building perimeters, from the shorter wavelength anomalies created by debris on or near the ground surface. Matched bandpass filtering, employed extensively in the aeromagnetic industry, is an effective way to separate magnetic anomalies arising from different depths. It entails fitting the radially averaged power spectrum of the total field magnetic data with a series of power spectra corresponding to simple equivalent layers at the archaeological site. We show that applying matched bandpass filtering to a set of total field magnetic intensity data yields two equivalent layers with excellent separation of near‐surface sources from deeper sources of interest. The benefit of this approach over enhancing surface features using magnetic gradiometry, analysis by upward continuation, or the analytic signal is that we isolate the source layers without losing information.

Portable Magnetic/Frequency Domain Electromagnetic Induction Sensor System Development

An unexploded ordnance (UXO) survey instrument that simultaneously collects total field magnetic data and frequency domain electromagnetic (FDEM) induction data was developed and tested for the detection and characterization of buried UXO. The system is comprised of an FDEM sensor operating at 9.8 kHz and a cesium vapor magnetometer. The system was initially tested in dynamic survey (detection) and static cued survey (characterization) modes at the Naval Research Laboratory Blossom Point UXO test facility in Maryland. During these tests, electromagnetic (EM) induced bias in the magnetic data was mitigated by physically offsetting the magnetometer from the EM transmitter coils. In the dynamic survey, the aggregate performance exceeded the detection rates for the individual component sensor technologies. The cued analysis tests showed that target features can be determined by using model-based analyses, and the location estimate errors provided by these analyses were consistently better than the dynamic survey results.

A New Algorithm for Depth Determination from Total Magnetic Anomalies due to Spheres

We have developed an automatic method to determine the depth of a buried sphere from numerical second horizontal derivative anomalies obtained from total field magnetic data. The method is based on using a relationship between the depth and a combination of observations at symmetric points with respect to the coordinate of the projection of the center of the source in the plane of the measurement points with a free parameter (graticule spacing). The problem of depth determination has been transformed into the problem of finding a solution of a nonlinear equation of f(z) = 0. Procedures are also formulated to determine the magnetic moment and the effective angle of magnetization. The method is applied to synthetic examples with and without random errors and tested on a field example from Senegal. In all cases, the depth solutions are in good agreement with the actual ones.

Completing the Spectrum of the Australian Digital Magnetic Anomaly Map

Continental-scale merges of Australian airborne magnetic data are only accurate for wavelengths less than about 100 km, due to limitations of survey size and data processing. Wavelengths greater than 400 km are available from satellite data; thus, there is a ``gap' in the spectrum for intermediate wavelengths between 100 km and 400 km. Geoscience Australia is filling this gap by acquiring new airborne magnetic data as part of the Australian Government?s new energy initiative. Intermediate wavelengths are important, for example, for better definition of sedimentary basins for petroleum prospectivity evaluation, for interpreted depths to bottom of magnetic sources in attempts to define the Curie point isotherm, and for regional removal in modelling. The AWAGS2 project (Australia-wide Airborne Geophysical Survey) is collecting both gamma-ray spectrometric and total-field magnetic data across Australia with long north-south lines spaced 75 km apart and 80 m terrain clearance. Accurate spatial crustal magnetic data depend upon accurate removal of time variations of Earth?s magnetic field. These are recorded at pairs of ground sites, concurrently with the airborne acquisition, and supplemented by geomagnetic observatory and other data. Routine airborne magnetic surveys rarely use more than one base magnetometer and the long lines of AWAGS2 present a challenge. It cannot be assumed that time variations are spatially uniform (eg., induction in the oceans and crust creates spatial non-uniformities). Removal of time variations depends upon interpolation of data recorded at ground sites and knowledge of induction effects. Removal of the latter is aided by using high-resolution vector data recorded at the base sites. Using the AWAGS2 corrected traverses will improve the accuracy of intermediate wavelengths in the Australian Digital Magnetic Anomaly Map.

Large-Area Remote Sensing Coverage of Archaeological Sites with a Dual-Track Cart-Mounted Cesium Magnetometer

This paper describes the benefits of collecting total field magnetic data at archaeological sites with a mobile cart holding two cesium magnetometer sensors and a separate stationary base station magnetometer to correct for global-scale magnetic fluctuations. To date, the Texas Historical Commission has surveyed 40 sites from various time periods and geographical settings in Texas. The cart has allowed rapid surveys over large areas while minimizing signal-degrading noise. Four examples from Texas show that large-area and high-density magnetic surveys can be completed in relatively short periods of time producing maps that aid significantly in archaeological site interpretation. In some instances, no field excavation was required to understand quickly what the magnetic patterns represented in terms of buried archaeological features. In all cases, the magnetometer maps provided important information to help guide future field excavations.

Magnetic models of unexploded ordnance

Magnetometry is widely used for the characterization of areas contaminated by unexploded ordnance (UXO). To successfully discriminate hazardous UXO from nonhazardous clutter requires accurate models of the ordnance response. This paper develops an ordnance library with 15 different items using total-field magnetic data collected over a test stand. The induced and remanent magnetizations were obtained by varying the three-dimensional orientation of each item and measuring the magnetic field on a horizontal plane in the dipolar regime. Replicate measurements using multiple specimens of the same ordnance returned very similar induced magnetizations. The fitted moments were used to estimate the detection depths for different sensor noise floors. A prolate spheroid with a 3.5 aspect ratio was used to provide a good approximation to the detection depths for many of the ordnance items. Assuming a 1-nT noise floor, these orientation-dependent detection depths ranged from 10 to 17 times of the object's diameter

Optimizing the use of aeromagnetic data for predictive geological interpretation: an example from the Grenville Province, Quebec

Predictive geological mapping relies largely on the empirical and statistical analysis of aeromagnetic data. However, in most applications the analysis remains essentially visual and unconstrained. The lithological and structural diversity of rock units underlying the Mingan Region make it an ideal test area to apply more rigorous approaches to magnetic data processing and interpretation, and to assess their usefulness and limitations. In the application discussed here, various derivatives and transformations of the total field magnetic data are evaluated empirically by photo-interpretation using a Geographic Information System. We show that rock types are best represented using the total field and vertical derivative of the magnetic data, whereas contacts between rock types are best delineated using the horizontal derivative of the total field and the analytic signal. In addition, the maxima of the analytic signal are used to estimate the direction of dip of large-scale geological units. Statistical analyses show that the correlation between geology and magnetic data is not directly proportional. Finally, the source of discrepancies between mapped geological units and magnetic response are evaluated through theoretical data modeling of representative geological bodies.

Curie Point Depths of Albania Inferred from Ground Total Field Magnetic Data

Ground total magnetic field data of Albania were used to produce estimates of the Curie point isotherm. The strategy followed was to estimate the depth to the bottom of the deepest magnetic sources. Firstly, the average depth to the top of the deepest crustal block, zt, was computed by linear fitting to the second lowest-frequency segment of the azimuthally averaged power spectrum of the total magnetic field data. Then, the depth to the centroid of the deepest crustal block, z0, was computed by linear fitting to the lowest-frequency segment of the azimuthally averaged power spectrum of a distribution of magnetic dipoles. Finally, the depth to the bottom, the inferred Curie point depth, zb, was calculated from zb=2z0−zt. Curie depth estimates for Albania vary from about 17 to 25.5 km (below sea level). These results are consistent with the depths inferred by extrapolating geothermal gradient and heat-flow values, suggesting that the Curie point depth analysis is useful to estimate the regional thermal structure. It also suggests that the approach was valid and that ground total magnetic field data can be used for this purpose.