Abstract: A borehole logging tool employing a pulsed neutron source and a pair of spaced-apart epithermal neutron detectors is lowered into a borehole traversing a subsurface formation. The formation is irradiated with bursts of fast neutrons and the epithermal neutrons returning to the borehole as a result of such irradiation are detected by the pair of epithermal neutron detectors. These detected epithermal neutrons are both time resolved and time integrated during their epithermal die-away spectrum to provide indications of lithology independent porosity and lithology dependent porosity respectively.

Abstract: A borehole logging tool for neutron lifetime logging of subterranean formations surrounding a borehole includes a pulsed source of fast neutrons and a radiation detector for measuring neutrons returning to the borehole from the irradiated formations surrounding the borehole. The radiation detector system comprises a neutron absorbing and gamma ray radiating material which is moved by an articulating arm into contact with the borehole wall and is azimuthally oriented so that such neutron absorbing and gamma ray radiating material is firmly pressed against the formation. A shielding material minimizes absorption of neutrons from the borehole fluid. A gamma ray detector located in the main body of the logging tool detects gamma rays from the radiating material as a measure of the epithermal or thermal lifetime characteristic of the formation adjacent the borehole.

Abstract: A borehole logging tool includes a pulsed source of fast neutrons and a neutron sensitive counter. A neutron filter encases the counter and has a thickness to permit the longest possible delay time following each burst of fast neutrons before epithermal neutrons from the irradiated formation are detected by the neutron counter while at the same time minimizing thermal neutron effects.

Abstract: A directional epithermal neutron detector for use in well logging employs a plurality of neutron counters and a neutron moderating material. A first thermal neutron shield provides a cylindrical housing for the counters and moderating material. The counters are clustered to one side of the housing and the moderating material fills the remaining portion of the housing. A second thermal neutron shield separates the counters from the moderating material.

Abstract: Method and apparatus for pulsed neutron logging employing detectors of different energy-dependent sensitivities to neutrons at energy levels less than the chemical binding energy of hydrogen in hydrogenous fluids found in subterranean formations. Fast neutrons from the pulsed neutron source enter the subterranean formation under investigation and are moderated therein to form populations of slow neutrons during a thermalization period occurring subsequent to the fast neutron burst and a thermal equilibrium diffusion period occurring subsequent to the thermalization period. The detectors are employed to obtain count rate measurements during at least one and preferably two time windows falling within the thermalization period and during a later time window occurring within the thermal equilibrium diffusion period.

Abstract: Method and apparatus for epithermal neutron decay logging wherein the formation under investigation is irradiated with bursts of fast neutrons which are moderated therein to form a population of epithermal neutrons. The decay rate of epithermal neutrons within the formation is measured within an energy range having lower limit which is less than the chemical binding energy of bound hydrogen in the formation. In addition, the decay rate of epithermal neutrons within the formation is measured within a second energy range having a lower limit which is greater than the lower limit of the first energy range.

Abstract: A borehole logging tool includes a source of fast neutrons and a fast neutron detector. A discriminator connected to the detector rejects pulses from the detector attributable to nonsource neutrons from the formation and passes to a count rate meter those pulses from the detector attributable to source neutrons. The output of the count rate meter is recorded as a measure of neutron source output.

Abstract: Methods and apparatus for radioactive well logging in which the formation is irradiated with bursts of primary radiation and the resulting secondary radiation is measured during a plurality of time windows occurring subsequent to the primary radiation burst. A plurality of ratio functions are established which are representative of the radiation count rates measured during adjacent pairs of the time windows. The ratio functions are compared with each other and at least one ratio function is selected. The selected ratio function is employed to generate a signal representative of the decay rate of the secondary radiation. The invention may be employed in epithermal neutron decay logging and at least a portion of the time windows may be of successively greater lengths.

Abstract: A radioactivity well logging tool employs an epithermal neutron detector having a thermal neutron counter surrounded by a thermal neutron shield. Located between the counter and the shield is a neutron moderating material for slowing down epithermal neutrons penetrating the thermal neutron shield to enable their counting by the thermal neutron counter.

Abstract: A technique is provided for assaying the formations surrounding a borehole for uranium-bearing ore. A borehole logging tool employs a pulsed neutron source to cyclically irradiate the formations with neutrons. The tool measures neutron fluxes produced during the period of time that prompt neutrons are being produced by the neutron fission of uranium in the formations. The tool also measures the total neutron fluxes produced in response to each burst of the neutron source, such measurements being utilized to correct the neutron flux response of the borehole logging tool during the prompt fission neutron period for the effects of epithermal/thermal neutron moderation, scattering, and absorption within the borehole itself.

Abstract: A borehole logging tool includes a source of fast neutrons, an epithermal neutron flux detector, and a thermal neutron flux detector. A count rate meter is connected to each detector. A ratio detector provides a signal indicative of the ratio of the count rates of the two detectors obtained during the time that prompt neutrons are emitted from neutron fission of uranium in the formation.

Abstract: Porosity measurements are carried out by employing a pulsed source of high energy neutrons actuated to irradiate a medium; some of the neutrons returning from the medium are detected by a specially filtered epithermal neutron detector. Control states are generated to define each of an ordered series of successively longer time intervals, the first of which is a few microseconds in length. Counters connected to the detector count representative numbers of the epithermal neutrons during each of the control states. Ratio functions are generated representative of the ratios of the epithermal neutrons counted during successively later equal length combinations of the time intervals. The ratio functions are compared with a predetermined threshold level and a porosity function is established in dependence upon the ratio function which earliest in time with reference to one of the irradiating pulses bears a predetermined relation to the threshold level.

Abstract: The specification discloses a technique for assaying for delayed fission neutrons from uranium to obtain a quantitative measure of uranium ore grade. In one embodiment, a pulsed neutron source of 14-Mev neutrons and a neutron detector are located in a borehole at the level of a formation of interest. The source is operated cyclically to irradiate the formations with bursts of fast neutrons, and the resulting neutrons from the formations are detected. Measurements are made of those neutrons detected between neutron bursts and indicative of delayed neutrons emitted as a result of neutron fission of uranium. Measurements also are obtained in a nonore-bearing formation to record the count of delayed neutrons emitted from oxygen when irradiated. These measurements are compared with those obtained in the ore-bearing formations of interest to correct for the effect of the oxygen background.