Automated 2D Inversion Basics

The ideas posed by Loke and Barker В основе представленного алгоритма лежат идеи, опубликованные Локе и Баркером ("Geophysical prospection", 1995) form the basis of the presented algorithm. Loke's and Barker's "deconvolution" idea is to represent the cross-section under the VES observation line by a set of 2D blocks with fixed boundaries and certain initial values of blocks' resisitivity. The theoretical apparent resistivity field is calculated for the model thus obtained. Then the corrections for the blocks' resisitivities are estimated by means of the least square method to minimize the difference between the measured apparent resistivity field and the calculated one. Loke and Barker apply the finite elements method to estimate the resistivities' partial derivatives in the least square method and use the expressions for DC point sources.
The presented algorithm, being ideologically close to Loke's and Barker's "deconvolution", differs from the latter significantly. The integral equation method is applied to estimate the partial derivatives in the least square method. The expressions for the linear current sources are used, i. e. merely 2D model is considered. The possibility i simplemented to crate the initial model as thge horizontally layered medium. The class of problems to solve is thus broadened and additional ways to consider the a priory information are provided.

## TRF_2D for Windows Principal Specifications

The TRF_2D for Windows program processes the  traverse of VES with current line along the observation line.
Three arrays date can be processed:
• AMNB Schlumberger array (MN<<AB/2) .
• Amn + mnB half-Schlumberger combined array (MN<<AО).
• Amn + mnB pole - dipole array  (MN<<AО) with fixed current electrode and moving potential electrodes (dipole); the registration point is attched to the current electorde position.
Up to 100 sounding points on the observation line.
Up to 20 arbitrary current electrode spasings per sounding point.
The modelled medium is divided into rectangular blocks (up to 1000) making columns (up to 15 rows of blocks) under each sounding position, separate column for each sounding point.
The horizontally layered medium can be set as the embedding cross-section.
The inversion results can be presented in the format fitting the SURFER ((c) Golden Software) XYZ data format.

## TRF_2D for Windows - Limitations for Usage

Some simplifications adopted to increase the speed of calculation result naturally in some limitations for usage:
the resistivity contrast cannot exceed 1:10 in the cross-section as a whole and should not exceed 1:5 for the neighbouring layers of the initial horizontally-layered medium (if any).

1Alexei A. Bobachev
Dept. of Geophysics, Geological Faculty, MSU
Vorobievy Gory 119899, Moscow, Russia
Phone /fax: (+7-095) 939 49 63
E-mail: boba@geophys.geol.msu.ru