HR: 17:45h
AN: NS34A-08    [Abstracts]
TI: Terrane Definition From Textural Measures of Aeromagnetic Data
AU: * Gettings, M
EM: mgetting@usgs.gov
AF: U.S. Geological Survey, 520 N. Park Ave. Rm 355, Tucson, AZ 85719, United States
AB: The vertical and horizontal magnetization in the Earth's crust is an anisotropic multifractal distribution, and this results in a horizontal multifractal distribution of magnetic anomalies at and above the Earth's surface. Discreet lithologic terranes are frequently observed to exhibit a characteristic pattern or "texture" of anomalies in aeromagnetic maps. Multifractal measures provide some tools useful in quantifying different textures and the scaling properties of aeromagnetic anomalies in map view can be used to define boundaries between terranes of different magnetic textures. If the source depths are not too large, the magnetic textural measures of lithologic units can be used to map their extent beneath cover. Although magnetic anomaly textures are generally visible to the eye on aeromagnetic anomaly images, the actual boundary between two textures is frequently difficult to determine with certainty. The use of quantitative textural measures provides a more objective framework for the boundary definition problem. A high resolution aeromagnetic survey over an area of highly variable geology has been used as a test area for these studies. Two robust measures that have proven useful for textural analysis are: the number of extrema per unit area; and the surface area per unit area in a window moving over the gridded aeromagnetic data. The former measures the "noisiness" of the data, and the latter depends on anomaly amplitudes and discriminates between large and small magnetizations. Window sizes for texture analysis are typically a few km square because geologic terranes of interest are generally of the order of tens of km in characteristic dimension. Other measures investigated are based on the scaling properties of the field within the window computed from the structure function for various exponents. The minima of the structure function define the characteristic sizes of anomalies, analogous to the power spectrum for a periodic function, and the maxima define characteristic sizes of large amplitude changes within the window. Ratios of the two-dimensional extrema to the total extrema also provides a measure of elongation within the window. Together, maps of these measures provide tools that improve the definition of terrane boundaries in both exposed and covered terranes and constitute a powerful tool for extending bedrock geological maps beneath cover.
DE: 1517 Magnetic anomalies: modeling and interpretation
DE: 3252 Spatial analysis (0500)
DE: 4440 Fractals and multifractals
DE: 4460 Pattern formation
SC: Near-Surface Geophysics [NS]
MN: 2007 Fall Meeting