Pulse-height spectra recorded by a passive spectral gamma-ray logging system were simulated by radiation-transport calculations for several combinations of gamma ray sources and tool environments. Derived from the calculated log responses were (1) source-and energydependent environmental corrections and (2) calibration factors ?nd an environmental correction methcd for thre.-window potmsiupduraniumhhorium analysis. IntrocluclionA passive spectral gamma ray (SGR) logging tool records gamma my count data from rock formations that contain naturally occur-.tig mdioelements. Counts am colhted in a W-channel muMcbannel analyzer, with gain set to span an energy range of Oto 3 MeV. Bemuse the tool is capable of recording sp&ml data over a broad energy range, environmental cmrectiom should be expressed as .energy-depemdemt fqnctions.This paper describes the derivation of enviroruqental corrections through computer mcdeling calculations. Brief descriptions of the computer codes and calculations are given, and the v~idity of the modeling t.e+ique is demonstrated through comptisons of computer-generated pulse-height spectra, with pulse-height spectra recordsd in weU-charactedzed test formations. Over most of me pulse-height energy range, the theoretical spsc~a agree with the measured spectra to within a few percent. .' For a tool centered in a borehole surrounded by a homogeneous, radioactive reck medium of infinite extent, pulse-height sF?c&&? were calculated for (1) calibration conditions, (2) all combination of seven borehole diameters and three mud types, and (3) four ca3edhole modds.. Separate calculations were.performed for.each of the thIee "mmauy occurring gamma ray sources (potassium, uranium, and thorium) S Spectral correction factors were derived fmm the calculated Pulse-hei~ht spectra. These factors quantify, according to g--a ray source, tie hole size, mud weight, and casing effects as timction.s of energy. Examples are presented for the thorium source.For each pulse-height spectrum, window count rates were cdcukited over three energy windows. The energy range of each window was set to envelop a specific photopeak. The photopeaks of interest were the photopeakmresulting from the 1,461-keV gamma ray of potassium, from the 1,764-keV gamma ray of uranium, and fmm the 2,615 -k.&V gamma ray of !.borium. The calculated wiqdgw count rata were used to determine tool-calibration constants and an environmental correction scbeznefor tie tkee+vindow potmsiunuuraniudhorium analysis. Computer Modeling Method Fig. 1.ilhstrates atypical configuration for computer calculations. Transport materials in such a modeI are represented by tables of photon cross sections. These tables are compild by a computer cede lamed DINT.1 DINT uses Setiempirica.1 am?lydcat fonmdas to calculate the energy-dependent cross sections for photoelectric &sorptim and pair production. The energy-and dhecdondependent cross sections for incoherent and coherent scattering (examples of which are Comptcm and Raleigh scattering)"are calculated by Df rom the iile...

No abstract

No abstract

No abstract

No abstract