Description of RES2DINV software

RES2DINV ver. 3.55 - 2D RESISTIVITY & IP INVERSION SOFTWARE

For Windows 98/Me/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, gradient and non-conventional arrays.

Supports exact and approximate least-squares optimisation methods

Supports smooth and sharp constrasts inversions

Supports up to 16000 electrodes and 21000 data points on computers with 1GB RAM

Seamless inversion of very long survey lines using sparse inversion techniques

(RES2DINV only license includes limited used of RES3DINV 3D inversion program)

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.

The figure below shows an example from an electrical imaging survey in an area with fairly complex subsurface geology and significant surface topography. This survey was carried out across a circular mound which is thought to contain some important Irish archaeological burial chambers (Waddell, J. and Barton, K, 1995, Seeing beneath Rathcroghan. Archaeology Ireland, Vol. 9, No. 1, 38-41.). The inversion of this data set, which has 67 electrode positions and 339 data points, took about 81seconds on a 266 MHz Pentium II computer.

Figure 1 Rathcroghan mound survey (a) apparent resistivity pseudosection, and (b) computer 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 data set has a total of 7479 electrode positions and 6636 data points, whereas the inversion model used has 19936 cells. For such a long survey line, sparse inversion techniques were used so that the entire data set can be inverted at a single time to produce a continuous seamless model. The use of sparse inversion techniques enable the inversion of very long survey lines to be carried out without greatly increasing the computer time and memory required. On a 3.2 GHz Pentium 4 computer with 1 GB RAM, it took about 101 minutes to process this data set. The inversion model together with the river bottom topography is shown in Figure 3. Most of the r