Abstract: Methods for evaluating the properties of formations surrounding a borehole, include the steps of measuring electrical properties of the formation from within the borehole; deriving a model of parameters of the formation surrounding the borehole; and estimating the properties of the formation using the model to interpret the measured electrical properties. In one aspect of the invention in which the formation includes a series of distributed beds, each bed having specific properties. Another aspect includes grouping similar beds, assigning one or more properties to all of the beds in a group, and using the assigned properties to estimate the properties of the formation for measurements relating to the beds of that group. Another aspect includes making several different measurements of formation electrical properties and simultaneously estimating the formation properties from the different measurements to provide a joint inversion.

Abstract: A method of measuring the resistivity of drilling mud in a borehole passing through a terrestrial formation includes the steps of a) inserting a sonde into the borehole, the sonde having an elongate body provided with at least one annular current electrode (A0, A′0) and at least two annular guard electrodes (A, A′, A1, A′1, A2, A′2) situated on either side of the annular current electrode; b) performing computed focusing to simulate an operating mode in which at least one current I0 is emitted into the surrounding formation from the annular current electrode, the current I0 being focused in the formation by emitting two currents I1 and I′1 from the two annular guard electrodes situated on either side of the annular current electrode; and c) producing a signal representative of the resistivity Rm of the drilling mud from the simulated operating mode.

Abstract: The invention relates to a method of measuring the resistivity of a drilling mud by means of a sonde provided with an annular current electrode (A.sub.0) and with annular guard electrodes (A, A'), current I.sub.0 being emitted into the surrounding formation together with focusing currents I and I'. A signal representative of R.sub.m is produced in response to I.sub.0. Various apparatuses for implementing the method are described. Another method implements a technique of computed focusing. Corresponding apparatuses are also described.