Submitted on: 17 July 2003.
Revised on: 18 October 2003.
Accepted on: 05 December 2003.
___________________JOURNAL OF APPLIED GEOPHYSICS---ELSEVIER
Submitted on: 24 June 1999.
Revised on: 20 September 1999.
Accepted on: 14 November 1999.
___________________JOURNAL OF APPLIED GEOPHYSICS---ELSEVIER
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