• For Windows 95/98/2000/NT/XP
• Supports on land, underwater and cross-borehole surveys
• Supports the Wenner (alpha,beta,gamma), Wenner-Schlumberger, pole-pole, pole-dipole, inline dipole-dipole, equatorial dipole-dipole and non-conventional arrays.
• Supports exact and approximate least-squares optimisation methods
• Supports smooth and sharp constrasts inversions
• Supports up to 4000 electrodes and 16000 data points on computers with 512MB RAM

Over the last 10 years, there has been a revolutionary change in the resistivity (and IP) surveying method. Until the early 1990's, the resistivity method has been used as a one-dimensional (1D) tool where quantitative modelling was confined to simple horizontal layers which are not sufficiently accurate for complex geological environments. However due to recent developments in field equipment design, interpretation software and microcomputer technology, two-dimensional (2D) and even three-dimensional (3D) surveys are now practical geophysical exploration tools for environmental and engineering studies.

Two-dimensional (2D) electrical imaging surveys are now widely used to map areas of moderately complex geology where conventional 1D 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. Four different techniques for topographic modelling are available in 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 for areas where the subsurface resistivity varies in a smooth manner (such as chemical plumes), and another optimised for areas with sharp boundaries (such as massive ore bodies). A robust data inversion option is also available to reduce the effect of noisy data points. Resistivity information from borehole and other sources can also be included to constrain the inversion process.

Three examples of the of data sets inverted with this software are shown to illustrate the speed, robustness and versatility of the RES2DINV. The first example is the well known Sting cave survey data set (www.agiusa.com). A known cave shows up as a high resistivity region near the center of the line in the inversion model, while a new cave was discovered at the left end of the line (Figure 1b). On a 90 Mhz Pentium computer, the inversion of this data set took about 1.6 (one point six) minutes with RES2DINV. In comparison, another commercial 2D inversion program quotes an inversion time of 18 (eighteen) minutes on a 75Mhz Pentium computer for the same data set!

Figure 1 The (a) apparent resistivity pseudosection for the Sting Cave survey together with (b) an inversion model.

The second example is from a combined resistivity and IP survey over the Magusi River massive sulphide ore (Edwards L.S., 1977. A modified pseudosection for resistivity and induced-polarization. Geophysics, 42, 1020-1036.). This survey was conducted with 30.5 meters (100 feet), 61.0 meters (200 feet) and 91.4 meters (300 feet) dipoles. The resulting pseudosection has a very complex distribution of the data points with overlapping data levels measured with different dipole spacings. The measured apparent resistivity and IP pseudosections, together with the model sections obtained are shown in Figure 2. The ore body shows up as a distinct low resistivity body with high IP values near the middle of the survey line in the model sections. Note the sharp boundaries between ore body and the surrounding rocks.

Figure 2. Magusi River ore body survey. (a) Apparent resistivity pseudosection, (b) resistivity model section, (c) apparent metal factor pseudosection and (d) metal factor model section.

RES2DINV is probably the first, and so far the only, commercial 2D inversion software that also supports underwater surveys! The following example is the most unusual data set that I have come across. It is not only the longest in physical length and number of electrode positions, but it also uses a highly asymmetrical non-conventional electrode arrangement collected by an underwater mobile surveying system. This survey was carried out by Sage Engineering of Belgium along a river to to map the near surface lithology of the riverbed where there were plans to lay a cable. The total length of the of the survey line was 8 kilometres. Due to limitations in the screen size and resolution with a web browser, only the first 670 meters of the survey line is shown below. This particular data subset has 600 electrode positions and 516 data points, whereas the inversion model has 1576 blocks. On a 550 Mhz Pentium III computer, it took about 44 minutes to process this data set. The inversion model together with the river bottom topography is shown in Figure 3b. Most of the riverbed materials have resistivities of less than 120 ohm.m. There are several areas where the near-surface materials have significantly higher resistivities of over 150 ohm.m. Unfortunately, geological information in this area is rather limited. In the high resistivity areas, the divers faced problems in obtaining sediment samples. The lower resistivity materials are possibly more coherent sediments.

(information above is taken from the Geotomo website (www.geoelectric.com) and is copyright Y.Gan 2000)

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