Spectral analysis and ridge-regression of magnetic anomalies from the northern continental margin of the Yucatán Peninsula, Mexico Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/p2676x621

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  • Marine magnetic data from the northern Yucatan continental margin were used to obtain the radially-averaged power-density spectra (RAPDS) of 34 grids, each with dimensions of 128 by 128 km, overlapping 50 percent. Depths to the tops of three magnetic horizons were estimated from the slopes of linear segments in the spectra. The depth to the base of the magnetic crust was estimated using an implicit relationship between the deepest depth estimation and the spectral peak position. The depth determinations agree with the limited drillhole data available and show some consistency with seismic refraction interpretations for the study area. Results of similar studies, based on the spectral analysis of magnetic anomalies in different tectonic settings, seem to agree with other geophysical and geologic data. In this area however, the limited depth of resolution achieved by seismic methods, and the absence of deep drillhole data and heat-flow measurements, makes it difficult to assess the validity of this interpretation. I therefore examined the simplifications implicit in the data processing techniques commonly applied in the spectral analysis of magnetic anomalies, and the assumptions upon which the method is based. Finally, I tested the method itself by interpreting synthetic anomalies generated from model source bodies. I found that depth interpretations, based upon assuming that the slope of linear segments in the RAPDS are proportional to the average depth to the top of magnetic horizons, and thickness determinations using the position of the spectral peak, are unreliable. The problem has been oversimplified. I also show that the RAPDS is independent of the direction of both the geomagnetic field and the magnetization vector. It can be represented by a Functional in terms of depth to the top of the source, its thickness and its horizontal dimensions. The problem of interpreting the RAPDS was then formulated as an inverse problem. A solution was obtained through minimizing, iteratively, the sum of squares of residuals between a real-data spectrum and a synthetic spectrum. The minimization was based on a linearized model, using the ridge-regression algorithm. This technique provides acceptable solutions for synthetic anomalies produced by model source bodies. Depth and thickness determinations, obtained using this technique on the Yucatan spectra, were used to make contour maps of the average depths to the top and the base of the magnetic crust. The top of the magnetic crust is at an average depth of 1.8 km. The base of the magnetic crust is at an average depth of 25 km. Beneath the central Campeche Bank and Campeche Terrace the depth to the base is close to the depth to the crust-mantle boundary determined by gravity modeling and mass column analysis. This suggests that in those regions the mantle is the lower magnetic boundary.
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