Re: [Phys-l] Need advice to build earth resistivity meter

[Brian Whatcott <betwys1@sbcglobal.net>]

At 04:33 PM 9/18/2007, Roberto, you wrote:
> > Hello Friends:
> >
> > One studient has the idea of building an earth resistivity meter capable of
> > sounding very deeply, for example,  to detect water at 1500 feet. The
> > problem involves metering of very low current values with good  accuracy,
> > and good knowledge of earth current distribution density laws.
> > It is an interesting problem but we need help finding circuits that can be
> > used in this project,  especially because the idea is to use standard
> > commercial equipment (low  cost digital meters and common batteries).
> > Perhaps somebody has seen such experimental equipment. Any help would be
> > appreciated.
> >
> > My best regards.
> > Roberto

Without any great insight into Earth resistance testing, I am going to
suppose that this method is not practical as proposed.
 There is a practical method into which the student's interest might
be diverted.  It's called hammer and plate seismics.

Briefly: a sheet of (say) 1/4 inch thick steel plate  measuring perhaps
3 ft on a side, is placed on a terrain of interest.
Several microphones are stuck into the ground on two lines extending
from the plate in different directions. One microphone near the plate
 is to act as a trigger for a multi pen recorder, or some computer-based
data acquisition equivalent.

A sledge-hammer is swung so as to impact near the center of the plate.
 This is noisy!   The echoes seen at various pens give some sense of
subterranean surfaces when the various path lengths are accounted for,
 in favorable circumstances. Even so, I seem to recall it needs explosive
stimuli to visualize targets as deep as 1500 ft.

As a more speculative alternative, I seem to recall that sub-surface
radars are now depicting layers at respectable depths.
 Such results for a particular location may even be available
from some agency like NASA for example.



Brian Whatcott    Altus OK    Eureka!