The present invention relates generally to an apparatus and method for
obtaining an azimuthally resolved nuclear magnetic resonance measurement
of an earth formation traversed by a borehole. The measurement can be made
while drilling or using a wireline tool. A receiving rf antenna having a
non-axisymmetric response pattern is used to obtain the azimuthally
resolved nuclear magnetic resonance measurement. The antenna employs axial
currents which excite an azimuthally oriented rf magnetic field. For this
situation, the static magnetic field is either radial or axial in its
orientation. The antenna generates a relatively long, in axial extent,
region of generally uniform static field magnitude and polarization in the
formation. The present invention facilitates low profile antenna
configurations that can permit dispensing with the reduction of the inner
diameter of the drill collar at the rf antenna location.