Hi Andy and Richard,


> I can imagine that modelling ground strata of a rocky hill top
will be almost impossible. 

Our cave radio experience certainly supports that. The geological maps will 
give you the underlying rock type, but overlaying it you often have an unknown 
depth of soil. 

The next problem is that you need the conductivities of these. For the rock, 
the map will give the specific type of (e.g.) sandstone, but the reference 
works on conductivity will probably just list "sandstone" and give 
conductivities ranging over several orders of magnitude.*

The final cause to reach for the bottle is when you realise that the 
conductivities of the soil, and of the rock unless it's impermiable to water, 
will depend on how saturated they are. This will depend on the recent rainfall 
history. If you want to try modelling that I'll come back in a few years and 
read your doctoral thesis (or put flowers on your grave) ...

Some of us in the cave radio community have in fact speculated that it might be 
more profitable to turn this round and make the sort of electrical measurements 
that Alan alluded to to find out the electrical properties of the rock.


* If anyone does know of reference works that give the conductivities for many 
specific rock types (e.g. Great Scar Limestone as against just limestone), I'd 
be very grateful to be told.

73,
Chris G4OKW