Permanent magnet material composition and structure for eddy current suppression in a nuclear magnetic resonance sensing apparatus

摘要

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. In a preferred embodiment, the magnet material comprises powdered, epoxy resin-bonded Samarium-Cobalt.