CREG members use loop-to-loop induction systems for determining underground locations (particularly for establishing when the surface station is vertically above the underground one).  The transmitter emits a sequence of beeps, usually on 874 Hz or 3496 Hz and is run off a small 9 V alkaline battery.

Brian Pease's site at http://radiolocation.tripod.com/ has a lot about building and using equipment for this application.  He says "Horizontal range is on the order of several thousand feet and a depth of nearly 700 feet has been done".

Complete issues of CREGJ (ISSN 1361-4800) have not been placed on line because of copyright problems.  If you go to  http://bcra.org.uk/creg/jnl/index.html you can get contents lists for all issues.  Back numbers can be ordered.

73 John F5VLF/G3PAI
Formerly Commissioning Editor, Cave Radio & Electronics Group Journal

*********** REPLY SEPARATOR  ***********

On 11/03/2010 at 13:45 M0FMT wrote:

>Try
>http://www.marcspages.co.uk/tech/index.htm
>73 es GL petefmt
>
>--- On Thu, 11/3/10, Roger Lapthorn <[email protected]> wrote:
>
>
>From: Roger Lapthorn <[email protected]>
>Subject: LF: Induction DX?
>To: [email protected]
>Date: Thursday, 11 March, 2010, 20:54
>
>
>If I've asked this before please forgive me....
>
>With all the discussion recently on how to radiate a signal on sub-9kHz
>I'm left wondrering how far, practically, one could expect to get simply
>with sub-9kHz magnetic induction using multi-turn loops at each end?  The
>Cave Radio Group (CREG) may have some answers but I'm not a subscriber so
>can't read any of their documents.
>
>It would be quite easy for anyone to generate 50-100W or so and feed this
>into a largish diameter multi-turn loop. Correctly aligned and with a
>similar loop at the far end and using decent weak signal processing just
>what range might be possible using modes like QRSS or WSPR? Has anyone
>tried this to see just what is practical, simply using induction fields?
>
>Also, can anyone point me to some webpages with some practical audio
>frequency induction communication systems? There are lots of references to
>near field comms for on-body and inter-device comms but I've not found
>much otherwise.
>
>73s
>Roger G3XBM
>
>
>--
>http://g3xbm-qrp.blogspot.com/
>http://www.g3xbm.co.uk
>http://www.youtube.com/user/G3XBM
>G3XBM    GQRP 1678      ISWL G11088