Making a VLF metal detector?

I've been thinking it might be interesting to try to make a metal detector using an Arduino, and it seems like a VLF sensor would be the best way to go; outputting signal at ~10KHz should be feasible. My question is, what would be the best way to go about making the actual sensor?

I know that the general design is two concentric loops of wire, with signal being 'written' to the outer one and 'read' from the inner one. But how big do the loops need to be? How circular? Should they be single lengths of wire, or coiled like a homemade inductor?

Any ideas?

You will find some topics about metal detectors on this forum.

The Arduino output is a square wave, not a sine wave. A sine wave would be a single frequency.

High-end metal detectors use lower frequencies and higher frequencies to be able to distinquish different metals.

http://www.hobby-hour.com/electronics/metal_detectors.php

Suggestion: identify and start with a known design. Once working on your workbench, adapt it for the Arduino...

You will find some forum posts on the subject, too, like: http://forum.arduino.cc/index.php/topic,143402.0.html

See https://github.com/dc42/arduino/tree/master/MetalDetector for an Arduino-based induction balance metal detector that I am developing. The detection hardware and software is complete, but I still need to add the user interface. There is some background on this project in the thread at http://forum.arduino.cc/index.php?topic=65712.