1D VERSUS 2D VERSUS 3D MODELLING

We would all like to have inversion and modelling codes that are fully 3D, but
they are expensive and often tricky to use. If the earth varies significantly
in the direction perpendicular to a line of survey electrodes then any 2D
code, smooth, blocky, whatever, will be unable to recover a "true" model
because 2D codes know nothing about the earth off to the sides of the survey.
This issue is even more important for 1D codes - they can produce models that
emmulate the "true" earth only if the earth can be modelled in terms of layers
that are uniform in both horizontal directions.

On the other hand, 1D codes often think in terms of a few discrete layers. If
this is really what the earth is like under the survey then these
interpretation methods may be the best choice. My feeling is that a 2D survey
should be interpreted first using 2D methods. Then, if the result looks like
layers, a 1D approach could be used in the vicinity of the most uniform looking
region to constrain the layer properties and interface depths. But I would be
unhappy using nothing but 1D intepretation methods because there is no way of
telling from such surveys whether the earth is uniform in all directions under
the interpretation location.

Are there any inexpensive interpretation codes? The web-based
geophysics course at the Colorado School of Mines
   (http://www.mines.edu/fs_home/tboyd/GP311/)
has applets to forward model Schlumberger or Wenner soundings over
a 3-layer earth, and the notes and exercise instructions are very complete.

There are a couple of soundings programs advertised on the rockware web site
http://www.rockware.com/ under the software/geophysics/resistivity heading. For
teaching, we (UBC) still use a very inexpensive (<$100 for academics) DOS program
that was made available from Gradient Geophyiscs, 921 Spruce, Missoula, Montana,
United States, 59802,  Phone: 406-542-0340, Fax: 406-542-0308, Contact: Garry J.
Carlson. It is not complicated, nor sophisticated, but it serves our
educational purposes. I doupt if it is still "for sale" although you might like
to contact Garry Carlson.

Francis Jones, UBC-GIF Outreach coordinator