2D RESISTIVITY & IP INVERSION SOFTWARE
For Windows 3.1 and 95 - Supports
land and underwater surveys
Figure 1 Rathcroghan
mound survey (a) apparent resistivity pseudosection, and (b) computer
Figure 2 Magusi
River ore body IP survey. (a) Apparent resistivity pseudosection,
(b) resistivity model section, (c) apparent metal factor pseudosection
and (d) metal factor model section.
Two-dimensional (2D) electrical imaging surveys are widely used
to map areas of moderately complex geology where conventional resistivity
sounding and profiling techniques are inadequate. The results from
such surveys are usually plotted in the form of a pseudosection
(Figure 1a) which gives an approximate but distorted picture of
the subsurface geology.
The RES2DINV program uses the smoothness-constrained least-squares
method inversion technique to produce a 2D model of the subsurface
from the apparent resistivity data alone. It is completely automatic
and the user does not even have to supply a starting model. This
program has been optimised for the inversion of large data sets.
The use of available memory is optimised so as to reduce the computer
time by minimising disk swapping. On a Pentium based microcomputer,
the inversion of a single pseudosection is usually completed within
minutes. The Wenner, pole-pole, dipole-dipole, pole-dipole, Wenner-Schlumberger
and rectangular arrays are supported. Topographic corrections can
also be carried out by this program. Together with the free 2D forward
modeling program RES2DMOD, it forms a complete 2D resistivity forward
modeling and inversion package.
The program will automatically choose the optimum inversion parameters
for a particular data set. However, the parameters which affects
the inversion process can be modified by the user. Three different
variations of the least-squares method are provided; a very fast
quasi-Newton method, a slower but more accurate Gauss-Newton method,
and a moderately fast hybrid technique which incorporates the advantages
of the quasi-Newton and Gauss-Newton methods. The smoothing filter
can be adjusted to emphasize resistivity variations in the vertical
or horizontal directions. Two different variations of the smoothness
constrained least-squares method are provided; one optimised to
reduce the difference between the calculated and measured apparent
resistivity values, the other which guaranties models with smooth
resistivity variations even with noisy data sets. Resistivity information
from borehole and other sources can also be included to constrain
the inversion process.
An example of the results obtained from an electrical imaging survey
in an area with fairly complex subsurface geology and significant
surface topography is shown in Figure 1. The survey was carried
out across a circular mound which is thought to contain some important
Irish archaeological burial chambers (Waddell and Barton 1995).
This data set was provided by Dr. Kevin Barton of the Applied Geophysics
Unit , University College Galway, Ireland.
Figure 2 shows the results from an IP survey over the Magusi River
ore body where the survey was conducted with 30.5 meters (100 feet),
61.0 meters (200 feet) and 91.4 meters (300 feet) dipoles (Edwards
1977). The resulting pseudosection has a very complex distribution
of the datum points due to the overlapping data levels. The ore
body shows up as a low resistivity body with high IP values near
the middle of the survey line in the model sections.
As RES2DINV is a Windows based program, any graphics card or color
printer with a Windows driver is automatically supported. It has
been tested for graphics modes with a maximum resolution of 1600
by 1200 pixels and 256 colors. The current version of RES2DINV also
supports underwater surveys.
The main limitation in the accuracy of the models produced by 2D
inversion programs is probably effects due to 3D structures. A free
complimentary 3D resistivity and IP forward modeling program, RES3DMOD,
is provided so that the user can assess the effects of 3D structures
on the apparent resistivity measurements.
Waddell, J. and Barton, K, 1995, Seeing
beneath Rathcroghan. Archaeology Ireland, Vol. 9, No. 1, 38-41.
Edwards, L.S., 1977, A modified pseudosection for resistivity and
induced-polarization. Geophysics, 42, 1020-1036.