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Estimating the nature and the horizontal and vertical positions of 3D magnetic sources using Euler deconvolution
"... We have developed a new method that drastically reduces the number of the source location estimates in Euler deconvolution to only one per anomaly. Our method employs the analytical estimators of the base level and of the horizontal and vertical source positions in Euler deconvolution as a function ..."
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We have developed a new method that drastically reduces the number of the source location estimates in Euler deconvolution to only one per anomaly. Our method employs the analytical estimators of the base level and of the horizontal and vertical source positions in Euler deconvolution as a function of the x and ycoordinates of the observations. By assuming any tentative structural index (defining the geometry of the sources), our method automatically locates plateaus, on the maps of the horizontal coordinate estimates, indicating consistent estimates that are very close to the true corresponding coordinates. These plateaus are located in the neighborhood of the highest values of the anomaly and show a contrasting behavior with those estimates that form inclined planes at the anomaly borders. The plateaus are automatically located on the maps of the horizontal coordinate estimates by fitting a firstdegree polynomial to these estimates in a movingwindow scheme spanning all estimates. The positions where the angular coefficient estimates are closest to zero identify the plateaus of the horizontal coordinate estimates. The sample means of these horizontal coordinate estimates are the best horizontal location estimates. After mapping each plateau, our method takes as the best structural index the one that yields the minimum correlation between the totalfield anomaly and the estimated base level over each plateau. By using the estimated structural index for each plateau, our approach extracts the vertical coordinate estimates over the corresponding plateau. The sample means of these estimates are the best depth location estimates in our method. When applied to synthetic data, our method yielded good results if the bodies produce weak and midinterfering anomalies. A test on real data over intrusions in the Goiás Alkaline Province, Brazil, retrieved spherelike sources suggesting 3D bodies.
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"... Towards automated mapping of depth to magnetic/gravity basement — examples using new extensions to an old method ..."
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Towards automated mapping of depth to magnetic/gravity basement — examples using new extensions to an old method
75th Anniversary The historical development of the magnetic method in exploration
"... The magnetic method, perhaps the oldest of geophysical exploration techniques, blossomed after the advent of airborne surveys in World War II. With improvements in instrumentation, navigation, and platform compensation, it is now possible to map the entire crustal section at a variety of scales, fr ..."
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The magnetic method, perhaps the oldest of geophysical exploration techniques, blossomed after the advent of airborne surveys in World War II. With improvements in instrumentation, navigation, and platform compensation, it is now possible to map the entire crustal section at a variety of scales, from strongly magnetic basement at regional scale to weakly magnetic sedimentary contacts at local scale. Methods of data filtering, display, and interpretation have also advanced, especially with the availability of lowcost, highperformance personal computers and color raster graphics. The magnetic method is the primary exploration tool in the search for minerals. In other arenas, the magnetic method has evolved from its sole use for mapping basement structure to include a wide range of new applications, such as locating intrasedimentary faults, defining subtle lithologic contacts, mapping salt domes in weakly magnetic sediments, and better defining targets through 3D inversion. These new applications have increased the method’s utility in all realms of exploration — in the search for minerals, oil and gas, geothermal resources, and groundwater, and for a variety of other purposes such as natural hazards assessment, mapping impact structures, and engineering and environmental studies.
Magnetic Basement Depth Inversion in the Space Domain
"... Abstract—We present a totalfield anomaly inversion method to determine both the basement relief and the magnetization direction (inclination and declination) of a 2D sedimentary basin presuming negligible sediment magnetization. Our method assumes that the magnetic intensity contrast is constant an ..."
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Abstract—We present a totalfield anomaly inversion method to determine both the basement relief and the magnetization direction (inclination and declination) of a 2D sedimentary basin presuming negligible sediment magnetization. Our method assumes that the magnetic intensity contrast is constant and known. We use a nonspectral approach based on approximating the vertical cross section of the sedimentary basin by a polygon, whose uppermost vertices are forced to coincide with the basin outcrop, which are presumably known. For fixed values of the x coordinates our method estimates the z coordinates of the unknown polygon vertices. To obtain the magnetization direction we assume that besides the totalfield anomaly, information about the basement’s outcrops at the basin borders and the basement depths at a few points is available. To obtain stable depthtobasement estimates we impose overall smoothness and positivity constraints on the parameter estimates. Tests on synthetic data showed that the simultaneous estimation of the irregular basement relief and the magnetization direction yields good estimates for the relief despite the mild instability in the magnetization direction. The inversion of aeromagnetic data from the onshore Almada Basin, Brazil, revealed a shallow, eastwarddipping basement basin. Key words: Depthtobasement estimate, magnetic inversion, nonspectral information. 1.
Summary
"... The Euler method of automated depth estimation from potential field data has been extended to permit solution for the structural index as part of the inversion process. Examples across several areas of Australia are discussed. A more concentrated 3D model of Broken Hill using 3DWEG also makes use of ..."
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The Euler method of automated depth estimation from potential field data has been extended to permit solution for the structural index as part of the inversion process. Examples across several areas of Australia are discussed. A more concentrated 3D model of Broken Hill using 3DWEG also makes use of Euler. The Euler method generates many solutions, which require culling or clustering. In some cases, depth estimates within 15 % of actual source depths are achieved. It also benefits from full 3D visualization.
Automatic interpretation of magnetic data using Euler deconvolution with nonlinear background
"... The voluminous gravity and magnetic datasets demand automatic interpretation techniques like Naudy, Euler and Werner deconvolution. Of these techniques, the Euler deconvolution has become a popular choice because the method assumes no particular geological model. However, the conventional approach t ..."
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The voluminous gravity and magnetic datasets demand automatic interpretation techniques like Naudy, Euler and Werner deconvolution. Of these techniques, the Euler deconvolution has become a popular choice because the method assumes no particular geological model. However, the conventional approach to solving Euler equation requires tentative values of the structural index preventing it from being fully automatic and assumes a constant background that can be easily violated if the singular points are close to each other. We propose a possible solution to these problems by simultaneously estimating the source location, depth and structural index assuming nonlinear background. The Euler equation is solved in a nonlinear fashion using optimization technique like conjugate gradient. This technique is applied to a published synthetic dataset where the magnetic anomalies were modeled for a complex assemblage of simple magnetic bodies. The results for close by singular points are superior to those obtained by assuming linear background. We also applied the technique to a magnetic dataset collected along the western continental margin of India. The results are in agreement with the regional magnetic interpretation and the bathymetric expressions. 1