HR: 0800h
AN: NG31B-0871    [Abstracts]
TI: 3D Inversion of magnetic total gradient in the presence of remanent magnetization
AU: * Shearer, S E
EM: sshearer@mines.edu
AF: Center for Gravity, Electrical and Magnetic Studies, Department of Geophysics, Colorado School of Mines, 1500 Illinois St., Golden, C0 80401 United States
AU: Li, Y
EM: ygli@mines.edu
AF: Center for Gravity, Electrical and Magnetic Studies, Department of Geophysics, Colorado School of Mines, 1500 Illinois St., Golden, C0 80401 United States
AB: We present a general approach for inverting magnetic data in the presence of strong remanent magnetization. Inversion of magnetic data in the presence of remanence has been hampered by the need to specify the direction of magnetization. In such cases, the remanent magnetization can have a direction significantly different from that of the current inducing field and a magnitude large enough to alter the direction of total magnetization. As a result, the magnetization direction becomes an unknown quantity. Current interpretation techniques such as 3D inversion for susceptibility rely upon knowledge of this direction and their application becomes problematical. However, the total gradient (defined as the magnitude of the gradient vector of magnetic anomaly data) is independent of the magnetization direction when the causative bodies are two-dimensional and is weakly dependent upon the magnetization direction in three dimensions. Therefore, we can invert total gradient directly to recover the magnitude of magnetization without a precise knowledge of its direction. This is a nonlinear inverse problem because the total gradient depends nonlinearly upon the magnetization. It is also necessary to impose a bound constraint to ensure that the inverted magnitude of magnetization is nonnegative. This introduces further nonlinearity into the problem. We formulate the inversion by using Tikhonov regularization with the added bound constraints, and solve the resulting optimization using a primal logarithmic barrier method. We will present the algorithm and illustrate it with both synthetic and field examples. The ability to invert magnetic data with little information about the nature of remanent magnetization increases the areas in which we can apply 3D inversion of magnetic data. It opens the door to more quantitative interpretation of data in a variety of practical problems ranging from kimberlite imaging in diamond exploration, structural imaging in strongly magnetic terrain in mineral exploration, archeological investigations, and crustal studies.
DE: 3200 MATHEMATICAL GEOPHYSICS (New field)
DE: 3210 Modeling
DE: 3260 Inverse theory
DE: 1517 Magnetic anomaly modeling
SC: Nonlinear Geophysics [NG]
MN: 2004 AGU Fall Meeting