Methods and apparatus for induction logging in cased boreholes

摘要

Methods and apparatus are disclosed which allow induction logging of geological formations through borehole casing. Longitudinal and azimuthal excitation A.C. magnetic fields applied on the interior of the borehole casing penetrate the walls of the borehole casing under certain circumstances. These A.C. magnetic fields produce eddy currents in the adjacent geological formation, thereby producing secondary A.C. magnetic fields characteristic of the formation. Under certain circumstances, the longitudinal and azimuthal components of the secondary A.C. magnetic fields consequently penetrate to the interior of the casing and are measured to provide the resistivity and changes in the resistivity of the formation.