RES2DINV ver. 3.2
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 inversion model.



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.

RES3DINV ver. 2.0
3D RESISTIVITY & IP INVERSION SOFTWARE
For Windows 3.1, 95 and NT - Now available as a combined package together with RES2DINV, the 2D Resistivity & IP inversion program.


The 3D model obtained from the inversion of the Lernacken Sludge deposit survey data set. The model is shown in the form of horizontal slices through the earth in seven different depth intervals.

In areas where the geological structures are approximately two-dimensional (2D), conventional 2D electrical imaging surveys have been successfully used. The main limitation of such surveys s probably the assumption of a 2D structure. In areas with complex structures, there is no substitute for a fully 3D survey. This program is designed to invert data collected from E-SCAN (Li and Oldenburg 1992) type of surveys where the electrodes are arranged in a rectangular grid. The arrays which are most commonly used for 3D surveys are the pole-pole, pole-dipole and dipole-dipole. Other arrays probably do not provide sufficient horizontal coverage to fully map the area of interest.

The RES3DINV program uses the smoothness-constrained least-squares inversion technique to produce a 3D 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. A Pentium Pro or Pentium II based microcomputer with at least 64 megabytes RAM and a 3.2 gigabyte hard-disk is recommended. Depending on the size of the survey grid and the number of field measurements, as well as the speed of the computer system used, the inversion of a single 3D data set can take anywhere from a few minutes to more than 12 hours! Topographic corrections can also be carried out with this program by using a distorted finite-element grid such that the surface of the gird matches the topography.

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. 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.

An example of the results obtained from an electrical imaging survey in an area with a complex subsurface geology is shown in Figure 1. This survey was carried out at Lernacken in Southern Sweden over a closed sludge deposit (Dahlin and Bernstone 1997). A resistivity survey using a grid of 21 by 17 electrodes with a 5 metres spacing between adjacent electrodes was carried out. The pole-pole array was used. The former sludge ponds containing highly contaminated ground water show up as low resistivity zones in the top two layers. This was confirmed by chemical analysis of samples. The low resistivity areas in the bottom two layers are due to saline water from a nearby sea.
As RES3DINV is a Windows based program, all graphics cards and printers with a Windows driver are automatically supported. It has been tested for graphics modes with a maximum resolution of 1600 by 1200 pixels and 256 colours.

RES3DINV is only available as a combined package together with RES2DINV, the 2D resistivity and IP inversion program. The combined RES3DINV and RES2DINV package costs only about 25% more than the RES2DINV program alone. Those who had earlier bought the RES2DINV program will get a 40% discount for the new combined RES3DINV and RES2DINV software package.