GP31A-01
Improving the Magnetic Anomaly Map of the United States
We have improved magnetic anomaly map of the United States using National Uranium Reconnaissance & Evaluations (NURE) aeromagnetic surveys collected during the 1970s. Previous versions of these data processed using IGRF/DGRF do not mesh well at the survey boundaries because of leveling artifacts. Similarly, the U.S. component of the North American magnetic anomaly map has long wavelength errors caused by warping of hundreds of state and local aeromagnetic surveys during the merging process. The main difference in our processing that has allowed us to retain proper base levels is the use of the temporally continuous main field Comprehensive Model (CM4) by Sabaka et al. (2004, GJI, 159, 521-547). The advantage of using the NURE surveys is that most of these surveys have time information and diurnal variation observed with basestation magnetometers is removed from them. Furthermore, we have cleaned the NURE data by removing many spurious values through visual inspection. Some NURE surveys did not have total field values or time information. For these surveys, we reintroduced the IGRF for their approximate date and removed the core field determined by CM4. We compare the results of our processing and improvements with the U.S. aeromagnetic anomaly data prepared by different merging techniques. The improved map is more suitable for regional geologic and geodynamic interpretations.
GP31A-02
Gravity and Magnetic 3-D inversion of Morro do Engenho Alkaline Intrusion, Central Brazil
The Goias Alkaline Province, in Central Brazil, is a suite of alkaline rocks whose composition varies from mafic- ultramafic alkaline complexes in the northern area, to subvolcanic alkaline rocks in the central part and volcanics (kamafugitic systems) in the south. In this Late Cretaceous Province some intrusives outcrop, including the Morro do Engenho Complex (ME), and some can be defined only using the aeromagnetic signal, like A2 anomaly, completely covered by Quaternary sediments. K-Ar radiometric ages are in the interval of 90-80 Ma for the intrusions. Geologic mapping of these two bodies is hampered by extensive alluvial sedimentation and strong weathering of the mafic/ultramafic rocks. This strong weathering is responsible for the Nickel enrichment of the region, increasing economic interest in the alkalines. The bodies are all marked by strong gravity and magnetic anomalies that are very effective in mapping them, so a 3-D inversion of gravity and magnetic data was carried out in order to estimate their subsurface geometry. In the 3D inversion the models are representative of the structures covering the place where ME outcrops. In this inversion, the maximum depth indicated by the density distribution was 10 km for ME and 6 km for A2. The recovered density model indicates a NE-SW preferential direction for the density contrast distribution in subsurface. Since the remanent magnetization is very important in the area, the 3-D inversion was accomplished considering three different possibilities: only induced field, only remanent field, and total field. Magnetic inversion resulted in narrower bodies that are almost 2 km deeper. These results are compatible with 2.5D direct modeling, where the magnetic susceptibility was more centralized along the body axis. It is clear that the adopted procedure of using a-priori information on magnetic remanesce before final inversion was very useful.
GP31A-03
Source location estimation from noisy magnetic data using Euler's homogeneity equation
The development of automated techniques for the determination of the equivalent source location and characteristics from total field magnetic anomalies is an important task in potential field geophysics. This is especially convenient when large data sets are available, such as those measured in aeromagnetic surveys. In the context of hydrocarbon exploration, magnetic depth estimation is useful to know the thickness of the sedimentary section, since most sedimentary rocks have very low magnetizations. For mineral exploration these tools are applicable to the location of ore bodies containing magnetic minerals which motivated the interest of many authors in this field. In this work we focus our attention in the well known Euler's deconvolution procedure, in which the homogeneity properties of the magnetic fields associated to simple concentrated sources are used to estimate their horizontal position and depth. Although this approach has the advantage of not assuming a priori any particular geologic model, it requires the numerical computation of derivatives of the magnetic anomalies. These operations can be efficiently performed in the spatial wavenumber domain taking into account some general properties of the Fourier transforms of the scalar magnetic potentials. We present numerical examples to analyze the applicability of the method for the estimation of the source parameters associated to synthetic two-dimensional anomalies (i.e. profile data) for different models and geomagnetic inclinations, analyzing the convenience of using pole-reduced anomaly data. A parametric analysis of the solutions in the presence of random noise is also performed. The validity of the depth of the equivalent source as an indicator of the true depth to the top of the magnetic structures is also discussed, aiming at practical applications with real aeromagnetic data measured over the Argentine Atlantic ocean.