s.surf.idw - interpolation technique

Hello,

Please can someone tell me the references/papers where I can get more
details and descriptions on the numerical approximation (weighted
averaging) interpolation techniques that is used in the command s.surf.idw
- surface generation from sites data program. This command was written by
Michael Shapiro of the USACERL.

Your help is very much appreciated.
Thanks.

Norkhair Ibrahim
Nottingham University
UK