Method for determining the axial position of formation layer boundaries

Abstract

A method for estimating axial positions of formation layer boundaries from transverse electromagnetic induction signals. A first derivative is calculated with respect to depth of the induction signals. A second derivative of the signals is calculated. The second derivative is muted. Layer boundaries are selected at axial positions where the muted second derivative is non zero, and the first derivative changes sign. The selected boundaries are thickness filtered to eliminate boundaries which have the same axial spacing as the spacing between an induction transmitter and receiver used to measure the induction signals, and to eliminate boundaries having a spacing less than an axial resolution of the induction signals. In a preferred embodiment, the process is repeated using transverse induction measurements made at another alternating current frequency. Layer boundaries selected in both frequencies are determined to be the layer boundaries. An alternative embodiment includes Fourier transforming the induction signals into the spatial frequency domain, low pass filtering the Fourier transformed signals at a band limit about equal to the axial resolution of the induction signals, calculating a central first derivative of the filtered, Fourier transformed signals, calculating an inverse Fourier transform of the central first derivative, determining roots of the inverse Fourier transform, and testing localized properties of the inverse Fourier transform within a selected number of data sample points of the selected roots, thereby providing indications of formation layer boundaries at axial positions most likely to be true ones of the formation layer boundaries.