1. Formerly Gravity and Magnetics Research Consortium, Colorado School of Mines, Department of Geophysics, Golden, Colorado 80401; presently Petrobras, Av. Chile 65 s.1301, Rio de Janeiro 20031‐912, Brazil.
2. Colorado School of Mines, Department of Mathematics and Computer Sciences, Golden, Colorado 80401.
3. Colorado School of Mines, Department of Geophysics, Gravity and Magnetics Research Consortium, Golden, Colorado 80401.