Estimating source location using normalized magnetic source strength calculated from magnetic gradient tensor data

Abstract

For a number of widely used models, normalized source strength (NSS) can be derived from eigenvalues of the magnetic gradient tensor. The NSS is proportional to a constant q normalized by the nth power of the distance between observation and integration points where q is a shape factor depending upon geometry of the model and n is the structural index. The NSS is independent of magnetization direction, and its amplitude is only affected by the magnitude of magnetization. The NSS is also a homogenous function and satisfies Euler's homogeneity equation. Therefore, Euler deconvolution of the NSS can be used to estimate source location. In our algorithm, we use data points enclosed by a square window centered at maxima of the NSS for simultaneously estimating the source location and structural index. The window size is increased until it exceeds a predefined limit. Then the most reliable solution is chosen based on some statistical analysis (minimum uncertainty). One of the advantages of the presented method is that it allows automatic identification of the structural index as the constant background field is eliminated. Another advantage is reduction of interference effects from neighboring sources by differentiation of the NSS. We have compared our method with the analytic signal amplitude and when the magnetic source contains remanent magnetization with a different direction to the inducing field, the NSS provides more reliable information about source geometry. Application of the method has been demonstrated on an aeromagnetic data set from the Tuckers Igneous Complex, Queensland, Australia. The NSS has improved interpretation of magnetic anomalies for this igneous complex, for which available geologic information shows relatively strong remanent magnetization.

Corrigendum to:New methods for interpretation of magnetic vector and gradient tensor data I: eigenvector analysis and the normalised source strength

“…For all 2D sources, for spheres, for compact 3D bodies that can be represented by a dipole, and for narrow, axially magnetised pipe-like bodies (poletype sources) m is completely independent of the magnetisation direction. Beiki et al (2012) show that for more complex 3D sources m is only weakly dependent on magnetisation direction, substantially less so than the 3D total gradient (which is the usual generalisation of the ASA to 3D).…”

“…Other contributors to this effort include Schmidt et al (2004) and Schmidt (2006), who extended the Euler deconvolution method to magnetic gradient tensor data and demonstrated its utility for mapping depth to sources and structural index variations over the Tallawang magnetite deposit ; Fitzgerald and Holstein (2006) and Fitzgerald et al (2009Fitzgerald et al ( , 2010, who developed improved methods for gridding and levelling of gradient tensor data; and Heath (2007) who derived expressions for magnetic gradient tensor component anomalies produced by simple geological models and multipole sources. More recently Clark et al (2009) andClark (2010) presented new methods for dipole detection, localisation and characterisation (DLC); Holstein et al (2011) developed efficient interpretation methods for gradient tensor data over dykes; Beiki et al (2011) gave methods for estimation of source depths and strike directions from eigenvector analysis of the pseudo-gravity gradient tensor, calculated from TMI data; Clark (2012a, b) presented methods for interpreting vector and tensor data, using components calculated from high quality TMI data over the Mount Leyshon goldmineralised system and the Tallawang magnetite skarn deposit as illustrations; and Beiki et al (2012) developed an algorithm for depth estimation using Euler deconvolution of the NSS and applied it to analysis of gradient tensor data calculated from a TMI survey.…”

“…the point pole, point dipole, line of dipoles (horizontal cylinder), thin dipping sheet, and contact models) the NSS is a homogeneous function with a simple 1/r n fall-off, suitable for Euler deconvolution, and the width of the m anomaly is simply related to the source depth. Beiki et al (2012) and Clark (2012b) expressed the NSS of some simple sources as…”

“…Using this formulation, Beiki et al (2012) demonstrated the efficacy of Euler deconvolution of the NSS for simultaneous estimation of structural indices, locations of geological boundaries, and source depths in an area that exhibits strong remanent magnetisation. Clark (2012b) showed the utility of the NSS for interpreting the gradient tensor anomaly over a tabular magnetite skarn.…”

2D sequential inversion of total magnitude and total magnetic anomaly data affected by remanent magnetization

“…The amplitude data were calculated by use of equivalent source processing in the space domain avoiding the invalidation of wavenumber-domain method due to large variations in the data elevation. In addition, the normalized magnetic source strength (NSS) introduced by Beiki et al (2012) is also minimally affected by the direction of remanent magnetization present. Thus, Pilkington and Beiki (2013) use a standard 3D inversion algorithm (e.g., Pilkington, 2009) to invert the NSS data from an area, in which varying remanence direction is apparent.…”