The principal modes of interaction between gamma rays and atoms are reviewed. Special attention is paid to the need for using only Compton scattering in order to determine the electron density of a sample or earth formation and for eliminating chemical composition effects arising from low‐energy photoelectric absorption. The nature of the so‐called Z/A effect is discussed in relation to the conversion of electron density, the immediately measured quantity, into bulk density. Since gamma‐ray diffusion results from repeated scattering in the formation, the Boltzmann transport equation is used as the framework for understanding empirical observations. The method‐of‐moments solution provides both an understanding of the spatial variation of the gamma‐ray energy spectrum and also a basis for reducing the Boltzmann equation to the simple diffusion equation. A discussion is presented comparing experimental results with the diffusion solution for the point‐source‐at‐plane‐interface problem. The comparison confirms the validity of the mathematical and physical approximations made.

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To overcome the problems of mudcake and hole irregularities, the new compensated formation density logging device employs two detectors spaced at different intervals from the source. The detector at the shorter spacing is particularly sensitive to the density of material immediately adjacent to the face of the pad. The contribution of this material, which includes mudcake and minor wall irregularities, affects the response of each detector to a different degree. The signals from both detectors are combined to give automatically a density correction which is added to the uncompensated density information from the detector at the longer spacing. Both the compensated density measurement and the amount of compensation are then recorded on the log. With the unwanted borehole effects removed, the measurement is recorded directly in terms of bulk density on a linear scale. Introduction Formation density is a very useful and diagnostic parameter for formation evaluation. When the matrix lithology is known, porosity is accurately and readily computed from density data. When the density measurements are used with other porosity-responsive measurements, such as Sonic and Neutron logs, both lithology and porosity may be defined with good accuracy. In the past, however, application of density logs was complicated by the need for corrections to obtain true formation densities from the log values. Mudcakes on permeable formations and roughness of the borehole walls have been particularly troublesome since, in each case, the device was prevented from direct contact with the formation. Attempts have been made to correct for such conditions by making a simultaneous caliper log, estimating pad standoff, and assuming knowledge of the composition of the intervening materials. While such corrections are frequently accurate, they are not always reliable because of uncertainties in the pad standoff and material composition. A further complication was the necessity for manual conversion of log counting rates to density. The newly introduced compensated formation density, logging device (FDC) automatically corrects for Mudcake and minor wall irregularities, and provides a log that is scaled directly in bulk density. The purpose of this paper is to discuss briefly the equipment, to explain the theoretical considerations involved in the compensation, and to present field examples of this new log. COMPENSATED FORMATION DENSITY EQUIPMENT The FDC tool incorporates two major advances over previous density logging devices: a second detector, spaced closer to the gamma ray source to provide increased sensitivity to material close to the pad; and the automatic derivation of a corrected formation density from the two counting rates. The arrangement of the source and detectors is shown in Fig. 1. The longer spacing, detector is at the same spacing from the source and gives the same response as the single detector in the previous uncompensated device. By appropriate combination of the outputs of the two detectors, the FDC tool is able to compensate in large measure for effects of mudcake and hole rugosity. This is achieved, as will be described later, without specific knowledge of either the thickness or the nature of material between the pad and formation. To obtain corrected formation density, the two count rates are automatically processed by an analog computer in the surface panel. JPT P. 1411ˆ

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A sidewall epithermal neutron tool has been developed to substantially reduce environmental effects that have previously complicated neutron log interpretation. Designed for operation in uncased wells, the device provides increased accuracy in both liquid-filled and empty holes. A brief discussion of neutron moderation, diffusion and capture shows that logs using epithermal neutron detection depend on a smaller number of formation-characterizing parameters than those using thermal neutron or capture gamma ray detection. Thus, they come closer to providing an unambiguous determination of hydrogen content. In this new device a directionally sensitive epithermal neutron detection system has been incorporated in a sidewall source-detector skid to minimize borehole effects. The effects of variations in borehole size and shape, mud type, temperature and salinity are greatly reduced. Small residual borehole effects are then computationally accounted for in the surface control panel to provide a borehole-corrected neutron log. The log presents a direct recording of neutron-derived porosity on a linear scale. With corrections for bore hole effects already applied, this direct recording of porosity simplifies log interpretation. Furthermore, comparison with a linear porosity presentation of a formation density log permits sandstones, limestones and dolomites to be readily identified. Thus, in complex or variable lithology, porosity is determined with greater accuracy and reliability that heretofore. Laboratory data and field results demonstrate the improvements in neutron logging afforded by this new sidewall epithermal neutron logging device. Introduction The new Sidewall Neutron Porosity (SNP) logging system, designed for use in uncased wells, provides reliability and accuracy never before achieved with neutron logs. The effects of variations in borehole diameter and shape, fluid salinity, mud weight and temperature parameters that have long complicated neutron log interpretationare suppressed or corrected for by this sidewall epithermal neutron detection system. Furthermore, to simplify interpretation the SNP log presents a direct recording of Computed porosity on a linear scale. The performance improvements achieved by this new system arise primarily from the combination of two important design features. First, though not unique to this new tool, an epithermal neutron detection system is used, Epithermal neutron detection substantially reduces the perturbing influences of the thermal neutron absorption properties of rock matrices and water salinity. Second, the neutron detection system is mounted in a directionally sensitive sidewall skid to greatly minimize borehole effects. The purpose of this paper is to explain briefly the advantages offered by the detection of epithermal neutrons, to describe the SNP equipment and the log, to present calibration and correction data and to give examples of interpretation methods made especially convenient by this new system. ADVANTAGES OF EPITHERMAL NEUTRON DETECTION Epithermal neutron detection, of all the neutron methods in current commercial use, provides the simplest determination of formation hydrogen content. Advantages of epithermal neutron detectors, as compared with thermal neutron and gamma ray detectors, are probably best explained by a brief review of the life of a source-emitted neutron. A fast neutron from the source will eventually be captured by the nucleus of an atom. However, before capture is likely to occur, the fast neutron (energy of 100,000 electron volts (ev) or more) will be slowed down until it is in thermal equilibrium with its surroundings. The neutron is then considered to be a "thermal" neutron (average energy of 0.025 ev at 25C). The overwhelming majority of the slowing down thus occurs in reaching epithermal energies (energies just above thermal 0.5 to several ev). After reaching thermal energy, the neutron will "diffuse" through the formation with, on the average, no further energy change until capture. Upon the capture of the thermal neutron, relatively high energy gamma rays are usually emitted. Hydrogen plays a very important role in the process of slowing down fast neutrons. On the other hand, the absorption effects of water salinity and low concentrations of other strong thermal neutron absorbers relatively unimportant in the moderation process are very important in thermal diffusion, capture and the production of gamma rays. Thus, by detecting only epithermal neutrons with the SNP, spurious effects due to these thermal neutron absorbers are greatly minimized as compared to the effects on tools using thermal neutron or gamma ray detectors. The epithermal neutron detection system affords another important advantage. JPT P. 1351ˆ