Tecnologie e materiali per Geotecnica - Geofisica - Monitoraggi Ambientali

 

Geofisica

Casella di testo: 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 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.

 

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

Casella di testo: 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 . 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!

 

 

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