New Magnetic and Gravity Interpretation Methodologies and Their Innovative Application for Environmental, Exploration, and Planetary-Scale Potential-Field Data III
Presiding: D Ravat, Southern Illinois University at Carbondale; D K Butler, US Army Engineer Research and Development Center
GP24A-01 INVITED 15:30h
Recent Advances in Quantitative Interpretation of Magnetic Data in the Presence of Strong Remanent Magnetization
Three-dimensional (3D) inversion of magnetic data to recover a distribution of magnetic susceptibility has been successfully used for mineral exploration for the last decade. However, such inversion algorithms depend critically upon a known magnetization direction, which is commonly assumed to be the same as the direction of the ambient field. This central assumption has severely limited the application of these methods to allied disciplines where the presence of remanent magnetization is a rule rather than exception. The difficulty arises because the total magnetization, which is a vector sum of the induced and remanent magnetization, can be rotated away from the inducing field direction if the remanent magnetization is strong and not aligned with the inducing field. To overcome this difficulty, we develop two parallel attacks. In the first, we estimate the direction of total magnetization and supply it to the inversion algorithm, assuming that the magnetization direction does not vary greatly within the target region. We have developed two methods for estimating magnetization direction. Alternatively, we accept the fact that a single direction may not be estimated for various reasons and proceed to directly invert quantities that are derived from magnetic data but are weakly dependent upon the magnetization direction. We have developed an algorithm that constructs 3D distributions of the magnitude of magnetization by inverting either the amplitude of anomalous magnetic field or the total gradient of a component. This algorithm does not require explicit knowledge of magnetization direction. In this paper, we present the details of both approaches and examine their effectiveness using synthetic and field examples. It is now possible to invert virtually any magnetic data arising from geophysical problems by using these two complementary approaches. They open the door to 3D quantitative interpretation of magnetic data in a variety of applications including archaeology, environmental problems, crustal study, and planetary exploration.
GP24A-02 INVITED 15:50h
An overview of new methods for deriving and interpreting satellite magnetic anomaly maps
Since July 2000, the low orbiting CHAMP satellite is measuring the magnetic field in unprecedented resolution and accuracy. After 2009, the Swarm constellation of satellites will provide even more accurate measurements, including the east-west vector gradients of the field. Even with the exceptional data quality, compiling high resolution magnetic anomaly maps remains a formidable task. Due to attenuation with altitude, crustal features of scale lengths smaller than the satellite altitude are very weak and easily masked by the signatures of magnetospheric, ionospheric and ocean flow induced electric currents. Particularly challenging is the derivation of global anomaly maps which are not only valid at satellite altitude, but can be downward continued to the surface of the Earth and combined with marine and aeromagnetic compilations. Global and regional interpretation of satellite magnetic anomalies is also making considerable progress. Geographical Information System (GIS) methods are used to effectively combine geological, tectonic, petrological and seismic information of the crust. The merit of including the rather sparse heat flow data is currently being investigated. GIS models can be used to predict magnetic anomalies and compare them with the observations in order to test different hypotheses on the structure and composition of the crust. Apart from this forward modeling approach, new inverse methods have been developed to map the vertically integrated magnetization of the crust and to determine the thickness of the magnetized layer.
http://www.ngdc.noaa.gov/seg/geomag/geomag.shtml
GP24A-03 16:15h
An alternative 3D source inversion method for magnetic anomalies with depth resolution
In this paper we present a new method to invert magnetic anomaly data aimed at characterizing magnetic sources in a variety of geological contexts. Our algorithm makes use of a dipolar approximation to compute the magnetic field of discrete blocks. We show how this approximation does not affect significantly the success of the inversion method. Furthermore, it presents numerous advantages in terms of computing speed and portability, so that magnetic sources can be resolved directly in the field using a laptop computer. In addition, the introduction of analytic, rather than objective or empirical, weighting functions aimed at contrasting the natural decay of the signal amplitude with depth, makes the method robust and general enough to allow depth resolution in a large variety of case studies. Geophysical exploration and crustal modelling would be benefited by the use of such a method. The algorithm is tested on synthetic as well as on real field data sets.
GP24A-04 16:30h
New Model Alternatives for Improving the Representation of the Core Magnetic Field of Antarctica
The use of the International Geomagnetic Reference Field Model (IGRF) in constructing magnetic anomaly maps can lead to problems in the determination of magnetic anomalies. The major problems occur at the edges of local or regional magnetic surveys carried out in different epochs, which appear due to inaccuracies in the continuation of long wavelength anomalies. The interpretation of the resulting anomaly maps can therefore be erroneous. The situation is much worse in polar regions like Antarctica where magnetic activity of external origin is intense and only a few ground magnetic observatories exist; thus, it is even more difficult to separate ionospheric variations properly from the secular variation of the core magnetic field. In this paper, we examine two alternatives to the piecewise-continuous IGRF core magnetic field in Antarctica for the last 45 years: the present global Comprehensive Model (CM4) and the new version of the Antarctic Reference Model (ARM). It will be shown through different examples that both these continuous models represent the secular variation in Antarctica more adequately than IGRF: for instance, the improvement of ARM relative to IGRF-9 model is higher than 30% for the secular change of the total intensity field since 1960 for the overall Antarctic observatory compilation. Other examples concerning ground, shipborne, and aeromagnetic measurements will be presented. We therefore recommend the use of either the CM4 or the regional ARM model as possible candidates for defining the crustal magnetic field of Antarctica (e.g., the next generation of the Antarctic Digital Magnetic Anomaly Map).
GP24A-05 16:45h
Signature and Thickness of Magnetic Crust Determined From Satellite Data
Intermediate wavelength magnetic anomalies determined from satellite data exhibit patterns interpretable in terms of crustal thickness. Previous work has demonstrated that seismic, compositional and thermal models of crustal thickness, converted into magnetic rock susceptibilities and magnetizations, predict satellite anomalies similar to those observed, with the greatest discrepancies where crustal thickness is least well known. Inversions of satellite anomalies for crustal magnetization assuming a constant thickness magnetized layer also suggest a correlation between vertically integrated magnetization and crustal thickness. Here, we present quantitative comparisons between magnetization models and crustal thickness estimates, using both forward and inverse magnetization models (some including both induced and remanent magnetization), and several determinations of crustal thickness including Mooney et al.'s (2004) database.