HIGH VERTICAL RESOLUTION ANTENNAS FOR NMR LOGGING

摘要

A high vertical resolution antenna design is provided for use in an NMR measurement apparatus. Multiple coils are situated along the length of a magnet. A primary coil is energized to cause an oscillating magnetic field in a portio n of earth formation surrounding a borehole. A secondary coil having smaller dimensions than the primary coil is operated to receive spin echoes from a depth of investigatio n associated with the secondary coil. A distance sufficient to minimize electrical coupli ng separates the coils. The separation distance can be reduced by selecting a secondary coil with orthogonal polarization to the primary coil. Alternatively, a cross coil configuration can be implemented where the orthogonal secondary coil at least partially overla ps the primary coil, thereby reducing the overall length necessary for the polarizi ng magnet.