Methods for determining values for earth formation properties

Abstract

Methods are provided for processing spectral data detected by a gamma ray detector of a borehole tool in order to obtain quantitative determinations of formation and/or borehole properties. According to one invention aspect, spectral data are compressed using a principal components technique in order to find principal component coefficients. Those coefficients are supplied to an inverse filter which processes the data by inverting, deconvoluting, and filtering the data to provide determinations of formation properties such as formation density, Pe, and borehole parameters such as the mudcake thickness times the difference of the densities between the formation and mudcake. The principal components for which coefficients are found, as well as the inverse filter are generated from a calibration database. Prior information on noise and formation and/or borehole parameters are also utilized in forming the inverse filter. According to another invention aspect, spatial deconvolution is obtained by obtaining sets of principal components for each gamma ray detector and providing different pluralities of sets (spatial intervals) to the inverse filter. Also, where a backscatter detector is used in conjunction with far detector, spectral data from the far detector are processed by taking the logarithm thereof, while the spectral data from the backscatter detector is processed without taking a logarithm.