Abstract: Measurement-while-drilling apparatus includes a 14 MeV neutron accelerator, a near-spaced neutron detector which primarily senses source neutrons and whose output is proportional to source strength, one or more intermediately-spaced epithermal neutron detectors eccentered against the drill collar wall and primarily responsive to formation hydrogen concentration, and a third far-spaced radiation detector, either gamma ray or neutron, primarily responsive to formation density. The intermediately-spaced and far-spaced detector outputs, normalized by the near-spaced detector output, are combined to provide measurements of porosity, density and lithology and to detect gas. A thermal neutron detector and/or a gamma ray detector may also be provided at intermediate spacings to provide additional information of interest, such as standoff measurements and spectral analysis of formation composition. Tool outputs are related to the angular or azimuthal orientation of the measurement apparatus in the borehole.

Abstract: Inelastic scattering gamma ray spectra taken in a borehole are analyzed by a least squares spectral fitting process to determine the relative elemental contributions thereto of chemical elements postulated to be present in unknown earth formations and contributing to the measured spectra from the formations. The relative inelastic yields for silicon, calcium and magnesium are calibrated to provide straightforward estimates of the respective elemental concentrations for those elements and of the volumetric fractions of the elements or associated rock types, such as sandstone, limestone and dolomite, in the formations. The ratio of the relative inelastic yields for magnesium and calcium provides an indication of the degree of dolomitization of a formation. A technique for determining formation iron content from the relative inelastic iron yield in open or cased holes is also disclosed.

Abstract: The first and second moments of an epithermal neutron slowing down time curve are determined and are combined in accordance with a predetermined empirical relationship to derive a raw measurement of the porosity of an earth formation surrounding a borehole. The raw porosity value is then combined with the second moment in accordance with further predetermined empirical relationships to derive a standoff measurement and a standoff-correction factor to be applied to the raw porosity measurement to produce a standoff-corrected porosity measurement. Where the raw porosity value changes appreciably from depth level to depth level, a differential spatial filter is applied to the raw porosity measurement and to the standoff-correction factor to provide a more precise standoff-corrected porosity measurement.

Abstract: A gamma-gamma sonde for use in determining the bulk density of an earth formation adjacent a borehole comprises a source of gamma radiation and three attenuation-type gamma ray detectors DN, DI and DF spaced apart from the source, respectively, at successively greater distances. The count rate of each detector is converted into an apparent formation density .rho. based on data derived from calibration of the sonde in calibration media having known uniform density, thereby to obtain apparent formation densities .rho..sub.F, .rho..sub.I and .rho..sub.N. The ratio(.rho..sub.F -.rho..sub.I) to (.rho..sub.F -.rho..sub.N)is substantially independent of mudcake density and provides an estimate of the mudcake thickness. The calibration data involving mudcakes of known density and thickness enable the use of the mudcake thickness estimate to determine the value of k in the algorithm.rho..sub.apparent =.rho..sub.F +k(.rho..sub.F -.rho..sub.N),from which formation density is determined.

Abstract: A compact circular magnetic induction accelerator (betatron) for use as a borehole gamma ray source includes a field magnet and generally circular pole pieces composed of a class of ferrite having the general formula M.sup.2+ Fe.sub.2.sup.3+ O.sub.4, where M represents two or more divalent metal ions from the group consisting of Mn, Zn and Ni. The core magnet is in the form of two symmetrical closed loops, with one leg of each loop passing axially through the circular pole pieces. The field coil and the core coil may be arranged in series or in parallel, and switching circuits are provided for effecting electron beam capture and ejection. In an illustrative borehole application, the betatron is used as a gamma ray source in a bulk density logging tool.

Abstract: A modulator circuit for a betatron includes an independent low voltage D.C. powder supply, an intermediate low voltage capacitor connected to one side of the betatron winding, and a high voltage capacitor connected to the other side of the betatron winding. Unidirectional current devices normally permit current flow from the voltage capacitor, through the betatron winding to the high voltage capacitor. Energy is thereby transferred from the power supply and low voltage capacitor through the betatron winding to the high voltage capacitor. Switches are provided selectively to reverse the direction of current flow and thereby discharge the energy stored in both capacitors into the betatron winding to excit the betatron magentic circuit. Upon discharge of the high voltage capacitor, the unidirectional current devices once again restore normal current flow, so that the energy stored in the betatron electromagnet is returned to the high voltage capacitor.

Abstract: The porosities of subsurface earth formations surrounding a borehole are investigated using a logging tool run in the wellbore by repeatedly irradiating the formations with discrete bursts of high energy neutrons, measuring the populations of epithermal neutrons at near and far locations from the neutron source, and also measuring the rate of decay of the epithermal neutron population at a third location following each neutron burst and deriving therefrom a measurement of the epithermal neutron slowing down time. Formation porosity values derived from the near-and-far location population measurements are corrected for detector standoff effects by use, in accordance with a predetermined empirical relationship, of the epithermal slow down time measurement.