Induction logging apparatus including non-linear means for translating a detected signal

Claim 1

IN INDUCTION LOGGING APPARATUS FOR INVESTIGATING EARTH FORMATIONS TRAVERSED BY A BOREHOLE, THE COMBINATION COMPRISING: A COIL SYSTEM ADAPTED FOR MOVEMENT THROUGH THE BOREHOLE; MEANS FOR ENERGIZING THE COIL SYSTEM WITH ALTERNATING CURRENT TO DEVELOP AN ELECTRICAL SIGNAL WHICH IS DEPENDENT ON THE ELECTRICAL CHARACTERISTICS OF THE ADJACENT FORMATION MATERIAL, SUCH SIGNAL INCLUDING A NONLIEAR COMPONENT CAUSED BY ELECTRICAL SKIN EFFECT PHENOMENA; MEANS COUPLED TO THE COIL SYSTEM FOR DETECTING THE MAGNITUDE OF A PARTICULAR PHASE COMPONENET OF THE COIL SYSTEM SIGNAL HAVING A PREDETERMINED PHASE RELATIVE TO THE ENERGIZING CURRENT AND DEVELOPING AN ELECTRICAL OUTPUT SIGNAL PROPORTIONAL TO SUCH MAGNITUDE; MEANS RESPONSIVE TO THE OUTPUT SIGNAL FOR PRODUCING A MODIFIED SIGNAL HAVING A MAGNITUDE WHICH IS A NONLINEAR FUNCTION OF THE MAGNITUDE OF SUCH OUTPUT SIGNAL, SUCH NONLINEAR FUNCTION BEING PROPORTIONED TO MINIMIZE THE NONLINEAR SKIN EFFECT COMPONENT; AND MEANS FOR PROVIDING AN INDICATION OF THE MODIFIED SIGNAL.