The method and apparatus relies on a modified induction approach and preferably is incorporated into a metal drill collar for MWD measurements. Directional nulls are created in a primary changing magnetic field formed in a formation by a transmitting antenna which is in a groove on the surface of the...http://www.google.co.uk/patents/US5089779?utm_source=gb-gplus-sharePatent US5089779 - Method and apparatus for measuring strata resistivity adjacent a borehole
