Dhananjay Ravat

An altitude-normalized magnetic map of Mars and its interpretation

Techniques developed for the reduction and analysisofterrestrialsatellitemagneticelddataareusedto better understand the magnetic eld observations made by Mars Global Surveyor. A global distribution of radial (Br) magnetic eld observations and associated uncertainties is invertedfor an equivalentsource magnetization distribution and then used to generate an altitude- normalized map of Br at 200 km. The observations are well-represented by a potential function of crustal origin, consistent with a rema- nent origin for the Martian magnetic features. The correla- tion between the 40546 Br observations andBr calculated from the magnetization solution at observation locations is 0.978. For a magnetization distribution connedto a 50 km layer,calculatedmagnetizationsrangefrom-22to+17A/m. We see correlations with tectonics that were only hinted at in earlier maps. Magnetic features appear to be truncated against Valles Marineris and Ganges Chasma, suggestive of a major change in crustal properties associated with fault- ing.

Tilt-depth method: A simple depth estimation method using first-order magnetic derivatives

Aeromagnetic data are routinely presented as contour or color-shaded maps of the total magnetic intensity (TMI). An interpreters task is to identify features (anomalies) contained within the map and qualitatively and/or quantitatively interpret them into geologic structures at depth. If the map contains anomalies that have large magnetic intensities, the bodies might be considered to have large magnetizations, or to be at shallow depths. Small amplitude anomalies superimposed on these anomalies could be masked or even missed by an interpreter. Thus the task of the interpreter is to use the spectral content of the anomalies to try and resolve these ambiguities. Part of this process is also to obtain estimates of the depth and shape of the body causing the anomalies.

Interpretation of magnetic data using tilt-angle derivatives

We have developed a new method for interpretation of gridded magnetic data which, based on derivatives of the tilt angle, provides a simple linear equation, similar to the 3D Euler equation. Our method estimates both the horizontal locationandthedepthofmagneticbodies,butwithoutspecifying prior information about the nature of the sources structural index. Using source-position estimates, the nature of the source can then be inferred.Theoretical simulations over simple and complex magnetic sources that give rise to noisecorrupted and noise-free data, illustrate the ability of the method to provide source locations and index values characterizingthenatureofthesourcebodies.Ourmethodusessecondderivativesofthemagneticanomaly,whicharesensitive to noise high-wavenumber spectral content in the data. Thus, an upward continuation of the anomaly may lead to reduce the noise effect. We demonstrate the practical utility of the method using a field example from Namibia, where the results of the proposed method show broad correlation with previous results using interactive forward modeling.

National Geophysical Data Center candidate for the World Digital Magnetic Anomaly Map

Marine and airborne magnetic anomaly data have been collected for more than half a century, providing global coverage of the Earth. Furthermore, the German CHAMP satellite is providing increasingly accurate information on large-scale magnetic anomalies. The World Digital Magnetic Anomaly Map project is an international effort to integrate all available near-surface and satellite magnetic anomaly data into a global map database. Teams of researchers were invited to produce candidate maps using a common pool of data sets. Here we present the National Geophysical Data Center (NGDC) candidate. To produce a homogeneous map, the near-surface data were first line-leveled and then merged by Least Squares Collocation. Long wavelengths were found to agree surprisingly well with independent satellite information. This validates our final processing step of merging the short-wavelength part of the near-surface data with long-wavelength satellite magnetic anomalies.

A combined analytic signal and Euler method (AN-EUL) for automatic interpretation of magnetic data

We present a new automatic method of interpretation of magnetic data, called AN-EUL (pronounced “an oil”). The derivation is based on a combination of the analytic signal and the Euler deconvolution methods. With AN-EUL, both the location and the approximate geometry of a magnetic source can be deduced. The method is tested using theoretical simulations with different magnetic models placed at different depths with respect to the observation height. In all cases, the method estimated the locations and the approximate geometries of the sources. The method is tested further using ground magnetic data acquired above a shallow geological dike whose source parameters are known from drill logs, and also from airborne magnetic data measured over a known ferrometallic object. In both these cases, the method correctly estimated the locations and the nature of these sources.

Analysis of the Euler Method and Its Applicability in Environmental Magnetic Investigations

The analysis of the Euler method from the perspective of three‐dimensional semi‐compact magnetic sources shows that for arbitrarily shaped sources, the anomaly attenuation rate varies with increasing source‐to‐observation distance. For such sources, the value of n (commonly known as the structural index) is a function of the size of the window and the distance to the window. The variation of the n in the case of arbitrarily shaped sources is an inherent source of scatter in the derived solutions because no single n is correct at all source‐to‐observation distances. Because the Euler method is able to place the horizontal positions of the sources more precisely, much of the error due to an incorrect choice of n is mapped into the depth error. The percent depth error resulting from an incorrect choice of n is scale‐independent, resulting in smaller physical errors in an application involving small distances and larger physical errors in an application involving large distances. It is shown with heuristic an...

Global vector and scalar Magsat magnetic anomaly maps

Empirical and analytical techniques for modeling ionospheric fields in Magsat data have been developed that facilitate ionospheric field removal from uncorrected anomalies to obtain better estimates of regional lithospheric anomalies. This task has been accomplished for equatorial ΔX, ΔZ, and ΔB component and polar ΔZ and ΔB component measurements. The techniques for modeling ionospheric fields have been integrated into a processing sequence that incorporates some of the important data-processing techniques developed during the last decade. Data-processing techniques include retention of common signal in a correlation analysis of adjacent passes ; analysis of field differences between dawn and dusk data at points where their orbits cross ; and retention of common signal in a covariant spherical harmonic analysis procedure. Results suggest that implementation of the above processing scheme leads to the mapping of the most robust magnetic anomalies of the lithosphere (vector components as well as scalar).

Analytic signal approach and its applicability in environmental magnetic investigations

Abstract We investigate the analytic signal method and its applicability in obtaining source locations of compact environmental magnetic objects. Previous investigations have shown that, for two-dimensional magnetic sources, the shape and location of the maxima of the amplitude of the analytic signal (AAS) are independent of the magnetization direction. In this study, we show that the shape of the AAS over magnetic dipole or sphere source is dependent on the direction of magnetization and, consequently, the maxima of the AAS are not always located directly over the dipolar sources. Maximum shift in the horizontal location is obtained for magnetic inclination of 30°. The shifts of the maxima are a function of the source-to-observation distance and they can be up to 30% of the distance. We also present a method of estimating the depths of compact magnetic objects based on the ratio of the AAS of the magnetic anomaly to the AAS of the vertical gradient of the magnetic anomaly. The estimated depths are independent of the magnetization direction. With the help of magnetic anomalies over environmental targets of buried steel drums, we show that the depths can be reliably estimated in most cases. Therefore, the analytic signal approach can be useful in estimating source locations of compact magnetic objects. However, horizontal locations of the targets derived from the maximum values of the AAS must be verified using other techniques.

Interpretation of magnetic data using an enhanced local wavenumber (ELW) method

This paper presents an enhancement of the local-wavenumber method (named ELW for “enhanced local wavenumber”) for interpretation of profile magnetic data. This method uses the traditional and phase-rotated local wavenumbers to produce a linear equation as a function of the model parameters. The equation can be solved to determine the horizontal location and depth of a 2D magnetic body without specifying a priori information about the nature of the sources. Using the obtained source-location parameters, the nature of the source can then be inferred. The method was tested using theoretical simulations with random noise over a dike body. It was able to provide both the location and an index characterizing the nature of the source body. The practical utility of the method is demonstrated using field examples over dikelike bodies from Canada and Egypt.

Linearized least‐squares method for interpretation of potential‐field data from sources of simple geometry

We present a new method for interpreting isolated potential-field (gravity and magnetic) anomaly data. A linear equation, involving a symmetric anomalous field and its horizontal gradient, is derived to provide both the depth and nature of the buried sources. In many currently available methods, either higher order derivatives or postprocessing is necessary to extract both pieces of information; therefore, data must be of very high quality. In contrast, for gravity work with our method, only a first-order horizontal derivative is needed and the traditional data quality is sufficient. Our proposed method is similar to the Euler technique; it uses a shape factor instead of a structural index to characterize the buried sources. The method is tested using theoretical anomaly data with and without random noise. In all cases, the method adequately estimates the location and the approximate shape of the source. The practical utility of the method is demonstrated using gravity and magnetic field examples from the United States and Zimbabwe.