New approximation for a magnetic field of a dipole on a borehole axis

New, more exact approximated solution to the direct problem of identifying a signal of a vertically oriented magnetic dipole on a borehole axis is reported. This solution makes possible to process the data of the high-frequency induction logging isoparametric sounding, including negative values of the measured phase difference. The new approximation enables taking into account the variable borehole radius that is of special importance when a highly-conductive drilling mud is used. As the obtained expression is explicit, it can be used for the quick data processing and for solving inverse problems of the borehole geophysics. __________ Translated from Fiziko-Tekhnicheskie Problemy Razrabotki Poleznykh Iskopaemykh, No. 4, pp. 3–9, July–August, 2006.