1. Los Alamos National Laboratory, Geophysics Group, Los Alamos, New Mexico, U.S.A.
2. Oklahoma Geological Survey, University of Oklahoma, Norman, Oklahoma, U.S.A.
3. Sandia National Laboratories, Geophysical Detection Programs, Albuquerque, New Mexico, U.S.A.
4. Roy M. Huffington Department of Earth Sciences, Southern Methodist University, Dallas, Texas, U.S.A.