HR: 1330h
AN: GP13A-04    [Abstracts]
TI: Comparison of Methods of Mapping the Depth to the Top and the Bottom of Magnetic Sources Using Layered and Random Synthetic Magnetic Models
AU: * Ravat, D
EM: ravat@geo.siu.edu
AF: Southern Illinois University Carbondale, Geology 4324 SIUC, Carbondale, IL 62901-4324 United States
AU: Pignatelli, A
EM: pignatelli@ingv.it
AF: INGV, Via di Vigna Murata 605, Roma, 00143 Italy
AU: Nicolosi, I
EM: nicolosi@ingv.it
AF: INGV, Via di Vigna Murata 605, Roma, 00143 Italy
AU: Chiappini, M
EM: chiappini@ingv.it
AF: INGV, Via di Vigna Murata 605, Roma, 00143 Italy
AB: 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).
DE: 0925 Magnetic and electrical methods
DE: 1517 Magnetic anomaly modeling
DE: 1545 Spatial variations (all harmonics and anomalies)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2005 Joint Assembly