1. Department of Geophysics Stanford University, 397 Panama Mall Stanford CA 94305 USA
2. U.S. Geological Survey, 11 Sherman Place, Unit 5015 Storrs CT 06269 USA
3. Kansas Geological Survey University of Kansas, 1930 Constant Ave. Lawrence KS 66047 USA
4. U.S. Geological Survey Upper Midwest Water Science Center, 8505 Research Way Middleton WI 53562 USA