New Magnetic and Gravity Interpretation Methodologies and Their Innovative Application for Environmental, Exploration, and Planetary-Scale Potential-Field Data I Posters
Presiding: D Ravat, Southern Illinois University at Carbondale; D K Butler, US Army Engineer Research and Development Center
GP13A-01 1330h
Model-based Separation Filtering of Magnetic Data
Separating the fields produced by sources at different depths is a common requirement in the interpretation of potential field data. Approaches to this problem are generally data- or model-based. Data-based methods rely on breaks in the slope of the logarithmic power spectrum of the observed field to design filters that can effect the separation. When the power spectrum shows no identifiable spectral slope breaks, other approaches are necessary. We outline a model-based method that does not depend on power spectral information but requires estimates of the average depths of the source distributions. An ensemble of models is computed based on these known values and a set of filter parameters are determined that produce the closest fit (in a least-squares sense) to the theoretical fields generated by each source distribution. This approach is used to separate basement effects from intrasedimentary sources in magnetic data collected over the Colville Hills, Northwest Territories, Canada.
GP13A-02 1330h
Spherical Cap Harmonic Modeling of the Antarctic Magnetic Anomaly Map
During the last decade the Antarctic Digital Magnetic Anomaly Project (ADMAP) produced a representation of the Antarctic crustal magnetic anomalies. All the ground, marine and aeromagnetic data collected south of 60°S since the IGY 1957-58 were compiled and reprocessed to produce a regional crustal magnetic anomaly map with a 5-km grid interval. Satellite-altitude crustal anomalies from the CHAMP (400 km) and Ørsted (700 km altitude) missions were also processed and used to fill in regional gaps in the near-surface survey coverage. In this paper, we report on our efforts to develop a Spherical Cap Harmonic (SCH) model of the multi-altitude crustal magnetic observations. The purpose of our work is to produce a regional model that will depict the crustal magnetic anomalies anywhere between the surface and satellite altitude with an accuracy not achieved by global-Earth models. The new SCH model synthesizes almost 50 years of magnetic survey observations to facilitate our future studies of the Antarctic magnetic field.
GP13A-03 1330h
Spectral Correlation of Antarctic Satellite Magnetic and Gravity Anomalies
Large areas of the Antarctic continent lack terrestrial or airborne magnetic and gravity survey coverage due to the harsh climate and extended distances involved. Satellite missions therefore play an important role, often being the only source of information for the study of remote regions. NASA's GRACE satellite mission provides global-scale gravity measurements with a much higher spatial resolution than previous missions. The free-air gravity anomalies in Antarctica from GRACE offer new insights on the poorly understood Antarctic crust. New interpretations and candidates for further investigation are presented from spectral correlation analysis between the new GRACE free-air gravity anomalies and magnetic anomalies measured by the CHAMP and Ørsted satellites. We quantify the anomaly correlations using correlation filters based on Poisson's relation. Favorability indices are derived that highlight the positive and negative correlations between the satellite observed geopotential anomalies. Several positively and negatively correlated regional anomalies yield new insights on the enigmatic crustal tectonics of Queen Maud Land, Enderby Land and other regions of East Antarctica, and the West Antarctic Peninsula.
GP13A-04 1330h
Comparison of Methods of Mapping the Depth to the Top and the Bottom of Magnetic Sources Using Layered and Random Synthetic Magnetic Models
Several methods of mapping depths to the top and the bottom of the magnetic sources have been suggested since the Spector and Grant's (1970) landmark paper showing how an assumption of random sources could be exploited in generating radially-averaged power spectrum, the slopes of which give an estimate to the depth to the top of the sources of magnetic anomalies when all the qualifying assumptions of the method are met. An application to the magnetic bottom was investigated by Shuey et al. (1977, the spectral peak approach) and Bhattacharyya and Leu (1975, the slope of the frequency-scaled power spectrum approach). Despite the difficulty in determining the bottom of the magnetic layer due to the requirement of large window sizes over which precise regional magnetic anomalies must be known of sources that meet the assumptions of the techniques, the appeal of these methods is significant because of the important exploration and geodynamical problems it can address: for example, the issues of direct interest range from finding the range of tops and bottoms of regional magnetic sources, regions of elevated temperatures in the crust due to temperature sensitivity of magnetic minerals, the nature of magnetic minerals in the deep crust and upper mantle, and investigation of whether the Moho is a magnetic boundary or not, etc. In this study, we take a closer look at these techniques using windows of various dimensions (from 100 to greater than 300 km) over simulated magnetic anomalies from synthetic models of upwarped layered and random magnetic crustal sources distributed over a large model space of 800 km x 800 km in areal extent and situated at realistic crustal depths. Our initial results suggest that even in the ideal situations, many times the shape of the power spectrum is unsuitable for obtaining the estimates of the depth to the bottom because no unambiguous spectral peak was observed and even when it was felt that the spectral peak was observed, the derived bottom estimate was not always accurate. We also observed that, in most instances, the method of Bhattacharyya and Leu (1975) yielded much shallower depths than the true depths, indicating that the assumptions of the method were not being met by the variety of sources we generated. We found that all results had to be cross-checked by modeling the spectral slopes and the locations of the peaks - the approach recently taken independently by Ross et al. (2004, Fall AGU, &35; T31A-1287) and Finn and Ravat (2004, Fall AGU, &35; T11A-1236).
GP13A-05 1330h
On the Application of Euler Deconvolution to the Analytic Signal
In the last years papers on Euler deconvolution (ED) used formulations that accounted for the unknown background field, allowing to consider the structural index (N) an unknown to be solved for, together with the source coordinates. Among them, Hsu (2002) and Fedi and Florio (2002) independently pointed out that the use of an adequate m-order derivative of the field, instead than the field itself, allowed solving for both N and source position. For the same reason, Keating and Pilkington (2004) proposed the ED of the analytic signal. A function being analyzed by ED must be homogeneous but also harmonic, because it must be possible to compute its vertical derivative, as well known from potential field theory. Huang et al. (1995), demonstrated that analytic signal is a homogeneous function, but, for instance, it is rather obvious that the magnetic field modulus (corresponding to the analytic signal of a gravity field) is not a harmonic function (e.g.: Grant & West, 1965). Thus, it appears that a straightforward application of ED to the analytic signal is not possible because a vertical derivation of this function is not correct by using standard potential fields analysis tools. In this note we want to theoretically and empirically check what kind of error are caused in the ED by such wrong assumption about analytic signal harmonicity. We will discuss results on profile and map synthetic data, and use a simple method to compute the vertical derivative of non-harmonic functions measured on a horizontal plane. Our main conclusions are: 1. To approximate a correct evaluation of the vertical derivative of a non-harmonic function it is useful to compute it with finite-difference, by using upward continuation. 2. We found that the errors on the vertical derivative computed as if the analytic signal was harmonic reflects mainly on the structural index estimate; these errors can mislead an interpretation even though the depth estimates are almost correct. 3. Consistent estimates of depth and S.I. are instead obtained by using a finite-difference vertical derivative of the analytic signal. 4. Analysis of a case history confirms the strong error in the estimation of structural index if the analytic signal is treated as an harmonic function.
GP13A-06 1330h
Geomagnetic Data Assimilation with Satellite Measurements Using an Optimal Interpolation Approach
Geomagnetic data assimilation involves combining surface geomagnetic observations with model outputs. Optimal interpolation is a simplification of the Kalman filter in which the relative weights of the model and observations are specified through a covariance model. Because geomagnetic data assimilation is in its early stages, there is not yet an accurate set of initial conditions available. We therefore use initial weighting of the model output which is substantially lower than the observation weighting. Weighting of the model is gradually increased during the course of the assimilation as information from previous observations is incorporated into the simulation. For this study, we use satellite geomagnetic measurements and the MoSST core dynamics model. The results are presented in terms of the evolution of the observed minus forecast value of the surface magnetic field over the past several decades
GP13A-07 1330h
Band Iron Formations and Satellite Magnetic Anomalies
Band Iron Formations (BIF) are mainly Precambrian (2.5-1.8 Ga) sedimentary deposits and are composed of alternating layers of iron rich material and silica (chert). Precambrian BIF mark growth in the level of free oxygen in the atmosphere and the ocean which happened about 2.2 Ga. Distribution of main BIF includes Hamersley Range, Australia; Transvaal-Griquatown, South Africa; Minas Gerais, Brazil; Labrador Trough, Canada, and Kursk-Krivoi Rog (Russia). Together these five very large BIF deposits constitute about 90 percent of Earth's total estimated BIF (5.76*10 14 ). On each continent these ancient rocks usually metamorphosed and crystallized include what are variously described as hematite-quartzites, banded iron formations, banded jaspers or calico-rocks. West African, Hudson Bay and Western Australian Satellite Magnetic Anomalies coincide with distribution BIF deposits. The Kursk Satellite Magnetic Anomaly (KMA) (about 22 nT at the altitude=400km, centered at 51o N, 37o E) also was identified by ground and aeromagnetic observations and is recognized as one of the largest magnetic anomaly on the Earth. Magnetic modeling shows that immense Precambrian iron ore deposits (iron bands) of Voronezh uplift are the main source of KMA. Magnetic properties of 10000 BIF samples outcropped in the KMA area have been measured and analyzed (Krutikhovskaya et al., 1964) Rockmag BIF dataset is presented at: http://core2.gsfc.nasa.gov/MPDB/datasets.html. Mean NRM value is about 42 A/M, Qn about 1.4. Demagnetization tests suggest that hard and stable NRM component is caused by hematite occurring in BIF in different forms and grain sizes. Hematite deposits discovered on Mars in western equatorial area with layered topography of Aram Chaos and Sinus Meridiani could be of hydrothermal origin and may be formed similar to hematite precipitated in BIF on Earth.