Abstract: A magnet assembly for nuclear magnetic well logging apparatus includes two diametrically opposed magnets which produce a static and substantially homogeneous magnetic field. Each magnet has a magnet axis and is partitioned into a first and a second magnet segment by a plane substantially parallel to the magnet axis.

Abstract: This invention makes an NMR measurement at extended depths into the formation using the earth's magnet field to generate an NMR measurement and using a magnet field from a permanent magnetic to suppress the NMR signal in the borehole and shallow formation. The magnetic field from a permanent magnet increases the Larmor frequency in the borehole and the shallow earth formation. An oscillator applies an RF magnetic field to the formation for a period of time in order to align nuclei spins approximately perpendicular to the earth's magnetic field. At the end of this period, the field is turned off rapidly. Since the spins are unable to follow this sudden change, they are left aligned perpendicular to the earth's field. These nuclei precess about the earth's field at the approximate Larmor frequency of the earth's field. The RF receiver detects the RF signals in the approximate frequency range of the Larmor frequency.

Abstract: The present invention is an improved method and tool for determining formation density by using an array of gamma-ray detectors. This invention can correct for large standoffs encountered in abnormally shaped boreholes and in particular for the increased standoffs typically encountered by mandrel tools. In this invention, the collimated detectors have varying depths of investigation into the formation. At small standoffs a short spaced (SS) detector investigates mainly the mud and mudcake and a shallow layer of the formation. Unlike the SS, a mid spaced (MS) detector has a deeper depth of investigation and is sensitive to borehole and formation even at increased standoffs. A long spaced (LS) detector is mainly sensitive to the formation density and its density reading is corrected by using the standoff information from the MS and SS detectors. In addition to measuring density, this invention can measure the photo-electric factor (PEF) of the formation.

Abstract: A method and apparatus for detecting natural gamma-ray signals of an earth formation and correcting these gamma-ray signals for effects from borehole gamma-rays. Gamma-ray signals are detected at two detector locations in the borehole. The borehole effects are derived from the differences between the detected signals. After the borehole effects are determined, the detected gamma-ray signal is corrected for these effects and measurements of the formation elements are derived. A differential signal between the two detectors is created by excluding a small volume of borehole fluid from one of the detectors from the borehole fluid, before detecting gamma-rays at both detectors. The excluder displaces a known volume of borehole fluid equal to the volume of the excluder. Both detectors record gamma-ray generated from the formation and borehole. The additional volume of borehole fluid at one detector causes the detected signals to be different.

Abstract: A nuclear spectroscopy method and apparatus for obtaining qualitative and quantitative information related to water flow, comprising the steps of:(1) irradiating the water flow with a source of neutrons of sufficient energy to interact with oxygen atoms in the water according to the activation reaction O.sup.16 (n,p)N.sup.16 ;(2) detecting and counting, with at least at a detector, the gamma rays emitted during disintegration of N.sup.16 ;(3) making a plot of the counts versus time; and(4) deriving from said plot information related to said water flow.The irradiating is advantageously interrupted after a given period of time, and is preferably immediately followed by the detection. The water flow velocity "V" is calculated from the formula V=d/t, where "d" is the distance between the source and the detector(s), and "t" is the time period between the irradiation and the time corresponding to a characteristic on said plot, representative of the water flow and departing from the N.sup.16 exponential decay curve.

Abstract: Method and apparatus for identifying and determining the concentrations of subsurface formation elements next to a borehole, by detecting and counting the gamma rays resulting from the activation of Al atoms by neutrons emitted by a high energy source. The total count rates from Al are corrected for Si interference by (1) calculating the ratio of the count rates of gamma rays from Si activation to the count rates of the gamma rays resulting from the inelastic collisions of neutrons with Si atoms, thus providing a reference ratio independent of environmental parameters; (2) detecting and counting the gamma rays resulting from the Si inelastic collisions; (3) determining from the count rates of silicon inelastic gamma rays and from said ratio, the count rates of Si activation gamma rays; and (4) subtracting, from the total count rates of activation gamma rays, the count rates of Si activation gamma rays.

Abstract: A nuclear spectroscopic method and apparatus for determining hydrocarbon saturation or water saturation of a formation adjacent a well bore is disclosed. Near and far inelastic gamma ray spectra are detected in response to pulses of fast neutrons irradiating the formation and materials in the borehole. Near and far carbon/oxygen (C/O) ratios are extracted from each spectra using predetermined spectra of postulated elements. The near and far C/O ratios are combined to determine a representation of formation hydrocarbon saturation substantially corrected for borehole gamma rays produced by inelastic reaction of fast neutrons and hydrocarbons in the well bore. Shielding of near and far detectors is provided to make the near detector more sensitive to gamma rays emanating from borehole gamma rays and to make the far detector less sensitive to borehole gamma rays and more sensitive to formation gamma rays.

Abstract: A nuclear spectroscopic method and apparatus for determining hydrocarbon saturation or water saturation of a formation adjacent a well bore is disclosed. Near and far inelastic gamma ray spectra are detected in response to pulses of fast neutrons irradiating the formation and materials in the borehole. Near and far carbon/oxygen (C/O) ratios are extracted from each spectra using predetermined spectra of postulated elements. The near and far C/O ratios are combined to determine a representation of formation hydrocarbon saturation substantially corrected for borehole gamma rays produced by inelastic reaction of fast neutrons and hydrocarbons in the well bore. Shielding of near and far detectors is provided to make the near detector more sensitive to gamma rays emanating from borehole gamma rays and to make the far detector less sensitive to borehole gamma rays and more sensitive to formation gamma rays.