RES2DINV is a computer program that will automatically determine a twodimensional
(2-D) resistivity model for the subsurface for the data obtained
from electrical imaging surveys (Griffiths and Barker 1993). Since it is a
Windows based program, all Windows compatible graphics cards and printers
are automatically supported. It has been tested with video screen modes of up
to 1600 by 1200 pixels and 16 million colours.
Figure 1 shows an example of the electrodes arrangement and measurement
sequence that can be used for a 2-D electrical imaging survey. This program is
designed to invert large data sets (with about 200 to 21000 data points)
collected with a system with a large number of electrodes (about 25 to 16000
electrodes).
The 2-D model used by the inversion program, which consists of a number of
rectangular blocks, is shown in Figure 2. The arrangement of the blocks is
loosely tied to the distribution of the data points in the pseudosection. The
distribution and size of the blocks is automatically generated by the program
using the distribution of the data points as a rough guide. The depth of the
bottom row of blocks is set to be approximately equal to the equivalent depth
of investigation (Edwards 1977) of the data points with the largest electrode
spacing. The survey is usually carried out with a system where the electrodes
are arranged along a line with a constant spacing between adjacent electrodes.
However, the program can also handle data sets with a non-uniform electrode
spacing.
A forward modelling subroutine is used to calculate the apparent resistivity
values, and a non-linear least-squares optimisation technique is used for the
inversion routine (deGroot-Hedlin and Constable 1990, Loke and Barker
1996a). The program supports both the finite-difference and finite-element
forward modelling techniques. This program can be used for surveys using the
Wenner, pole-pole, dipole-dipole, pole-dipole, Wenner-Schlumberger and
equatorial dipole-dipole (rectangular) arrays. In addition to these common
arrays, the program even supports non-conventional arrays with an almost
unlimited number of possible electrode configurations! You can process
pseudosections with up to 16000 electrodes and 21000 data points at a single
time on a computer with 1 GB RAM. The largest electrode spacing can be up
to 36 times the smallest spacing used in a single data set. The program data
limits will be extended in the future as larger field data sets are encountered.
Besides normal surveys carried out with the electrodes on the ground surface,
the program also supports underwater and cross-borehole surveys!
