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The difficulty to use master curves as well as classical techniques for the determination of layer distribution (ei, ρi) from a resistivity sounding arises when the presumed number of layers exceeds five or six. The principle of the method proposed here is based on the identification of the resistivity transform. This principle was recently underlined by many authors. The resistivity transform can be easily derived from the experimental data by the application of Ghosh's linear filter, and another method for deriving the filter coefficientes is suggested. For a given theoretical resistivity transform corresponding to a given distribution of layers (thicknesses and resistivities) various criteria that measure the difference between this theoretical resistivity transform and an experimental one derived by the application of Ghosh's filter are given. A discussion of these criteria from a physical as well as a mathematical point of view follows. The proposed method is then exposed; it is based on a gradient method. The type of gradient method used is defined and justified physically as well as with numerical examples of identified master curves. The practical use for the method and experimental confrontation of identified field curves with drill holes are given. The cost as well as memory occupation and time of execution of the program on CDC 7600 computer is estimated.

Cheops pyramid. The terrain correction is a way to correct the initial slab model which for each station is at the station elevation. In underground surveys of small horizontal but large vertical extent, I have shown (Cheops pyramid for example) that the model used (or the standard terrain corrections used) should extend greatly as compared to the 1000 m radius initally used in this case (one should bear in mind that we were looking for cavities of a few cubic meters only). Figure 8 of my paper shows that the slab + terrain correction model is quite wrong as long as the terrain correction radius is small. Note that apart from the pyramid itself, the ground is practically flat and located at the low level of the pyramid.

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