Abstract: In the illustrative embodiments of the invention disclosed, the composition of an earth formation is investigated by repetitively irradiating the formation with bursts of fast neutrons and generating an energy spectrum of the gamma rays resulting from the inelastic scattering of such neutrons by nuclei of the formation. The inelastic scattering gamma ray spectrum is analyzed by comparing it with a composite spectrum, made up of standard spectra of constituents postulated to comprise the formation, to determine the proportions in the formation of the postulated constituents. A background energy spectrum is generated from gamma rays detected during periods between neutron bursts and is utilized to provide one or more standard background spectra for use in the analysis of the inelastic scattering gamma ray spectra. The standard background spectrum or spectra are preferably updated on a repetitive basis to reflect the current background component in the detected inelastic scattering gamma ray spectrum.