CROSSED-COIL NUCLEAR MAGNETISM WELL LOGGING APPARATUS AND METHOD UTILIZING A POLARIZING FIELD AND AN ADDITIONAL POLARIZING FIELD TO SHIFT THE DIRECTION OF POLARIZATION

摘要

A crossed-coil nuclear magnetism well logging apparatus and method is designed to function efficiently at magnetically equatorial regions of the world, as for example in Venezuela, and in slanted boreholes where the earth's magnetic field is substantially perpendicular to the axis of the borehole. When a polarization field Hp is established in the earth formation, the protons of the formation fluids can acquire nuclear magnetism polarization M. After the occurrence of a predetermined polarizing period, Tpc, the polarizing field Hp is removed in the presence of a rather weak polarizing field, Hl, (generated by passing a polarizing current through the receiving coil) at a select decay rate, i.e., adiabatically, not rapidly, so that the directions of the nuclear magnetic polarization M can be angularly and adiabatically shifted to new directions prior to precession about the earth's field. In order to provide adiabatic shifting of the polarization M, the resultant magnetic field Hr formed between the collapsing polarizing Hp and Hl fields must rotate at a rate that is small compared to the Larmor frequency of the protons in the Hrfield.