Abstract: A nuclear magnetic resonance well logging apparatus, comprising an antenna for inducing a radio frequency magnetic field in earth formations surrounding the apparatus and for detecting nuclear magnetic resonance signals from the earth formations; and a magnet for inducing a static magnetic field within the earth formations. The magnet is formed from a powdered, electrically conductive permanent magnet material. The grain size of the magnet material small enough with respect to the frequency of the radio frequency magnetic field to substantially prevent intragranular power loss of the radio frequency magnetic field. The magnet is formed from electrically isolated blocks of the magnet material each having a thickness less than a skin depth of the radio frequency magnetic field within the magnet material. The boundaries between the blocks are oriented to interrupt flow of eddy currents in the magnet resulting from the radio frequency magnetic field.

Abstract: A method for determining the thermal neutron capture cross-section of an earth formation penetrated by a wellbore from counts of capture gamma rays. The capture gamma rays are detected at spaced apart locations after bombarding the formations with high energy neutrons. The method includes determining an apparent capture cross-section at each one of the spaced apart locations from the counts of capture gamma rays, determining a difference in the apparent capture cross-sections determined between pairs of the spaced apart locations, and calculating the capture cross-section by combining the apparent capture cross-sections and the differences into an empirical relationship of known values of the capture cross-section with respect to the differences and the apparent capture cross-sections.

Abstract: A well logging cable including an optical fiber and a pressure sealed enclosure surrounding the fiber. The enclosure is adapted to be able to be elongated to the maximum expected axial strain on the logging cable without permanent deformation. In one embodiment, the enclosure is a corrugated-wall tube surrounding the optical fiber. The tube contains toroidally shaped rings within the maximum diameter portions of the corrugations in the tube wall. The cable includes armor wires wound around the corrugated-wall tube. In the preferred embodiment, the tube is surrounded by an elastomeric jacket, and is filled with hydraulic oil or the like to prevent entry of wellbore fluids into the tube under hydrostatic pressure.