Computer-linked nml tool and method having a series of sham polarizing

Abstract

The present invention decreases the time needed between collection cycles of an NML tool located in a wellbore penetrating an earth formation by zeroing the effect of prior-in-time residual polarization via a surprising change in the operations, while guarding against the effects of tuning errors, viz., using a series of sham polarizing cycles wherein the coil circuit is permitted to ring at a Q' value higher than normal during the initial stages of each sham cycle followed by the establishment of a low Q thereafter. Result: the effect is as if sham ringing is at the higher Q' value, but without the consequences of slow cutoff of the decaying magnetic field introduced during ringing. Hence, the effects of error fields are minimized and previously non-precessing components of the residual polarization undergo precession and can be subsequently scattered by the next-in-time field. Since the sham fields as well as the next-in-time polarizing field are all provided with slow-rising amplitude v. time turn-on segments, any residual polarization precessing at the start of the buildup of the fields can be easily scattered. Ultimately, all aspects of residual polarization are destroyed whereby cyclic NML logging speed can be greatly improved.