Nuclear magnetic resonance well logging

Claim 1

A NUCLEAR MAGNETIC RESONANCE LOGGING SYSTEM WHICH COMPRISES: (A) AN EXPLORING UNIT ADAPTED TO BE POSITIONED AT SELECTED DEPTHS IN A WELLBORE, (B) A FIELD COIL IN SAID UNIT, (C) MEANS FOR APPLYING A RELATIVELY HIGH CURRENT IN SAID FIELD COIL TO PRODUCE A POLARIZING MAGNETIC FIELD SUCCESSIVELY AT EACH OF SAID DEPTHS IN A SUBSTANTIALLY HORIZONTAL DIRECTION TO AND BEYOND THE WALLS OF SAID WELLBORE, (D) MEANS FOR ABRUPTLY CHANGING SAID HIGH CURRENT TO A REDUCED LEVEL FOR MAINTAINING FOR A PREDETERMINED PERIOD THE MAGNITUDE OF SAID POLARIZING MAGNETIC FIELD AT A LEVEL OF THE ORDER OF ABOUT TEN TIMES THE EARTH''S MAGNETIC FIELD, (E) MEANS FOR EXCITING SAID FIELD COIL WITH ALTERNATING CURRENT TO PRODUCE AN ALTERNATING MAGNETIC FIELD BEGINNING AT THE EXPIRATION OF AN INTERVAL FOLLOWING THE ABRUPT DECREASE OF SAID HIGH CURRENT ABOUT EQUAL TO THE TRANSVERSE RELAXATION TIME OF WATER, AND (F) MEANS FOR MEASURING THE AMPLITUDE OF THE LARMOR PRECESSION SIGNAL FROM PRECESSING NUCLEI IN THE EARTH''S MAGNETIC FIELD BEGINNING A LIKE INTERVAL APPLICATION OF SAID ALTERNATING MAGNETIC FIELD.