| 摘要 |
<p>A method and apparatus is described for measuring the resistivity of a formation adjacent a borehole. It 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 drilling collar. Detecting antennas are placed at the directional nulls to enable the primary magnetic field to be decoupled from the same and allow it to be distinguished from the secondary field caused by the current induced in the formation by such primary field. The drill collar also carries a plurality of current detectors which detect the value of current which is induced in the formation to provide a resistivity profile immediately adjacent the borehole. The preferred embodiment described uses a hybrid modified induction and electrical approach.</p> |
