Telemetering system for use in borehole logging to control downhole tool from surface

Claim 1

IN A BOREHOLE LOGGING SYSTEM WHEREIN A PLURALITY OF RELATIVELY LOW SIGNAL LEVEL FUNCTIONS ARE TO BE TRANSMITTED BETWEEN ABOVE GROUND EQUIPMENT AND A BOREHOLE INSTRUMENT ASSEMBLY, AND ONE OR MORE LOW IMPEDANCE DEVICES ARE TO BE OPERATED DOWNHOLE AT OCCASIONAL INTERVALS IN RESPONSE TO SIGNALS FROM ABOVE GROUND EQUIPMENT; THE COMBINATION COMPRISING, A CABLE CONDUCTOR AND A GROUND RETURN, A FIRST TERMINAL AT THE ABOVE GROUND END PORTION OF SAID CONDUCTOR, A SECOND TERMINAL AT THE DOWNHOLE END PORTION OF SAID CONDUCTOR, MEANS FOR APPLYING A PLURALITY OF RELATIVELY LOW LEVEL SIGNALS BETWEEN SAID SECOND TERMINAL AND THE GROUND RETURN, A FIRST "ZENER" DIODE, A SECOND SEMICONDUCTOR DIODE, MEANS CONNECTING THE CATHODE OF SAID FIRST DIODE TO SAID SECOND TERMINAL AND FOR CONNECTING THE ANODE OF SAID FIRST DIODE TO THE ANODE OF SAID SECOND DIODE, A LOW IMPEDANCE DEVICE CONNECTED BETWEEN THE CATHODE OF SAID SECOND DIODE AND SAID GROUND RETURN, SAID "ZENER" DIODE HAVING A PREDETERMINED REVERSE BREAKDOWN VOLTAGE, AND SAID SECOND DIODE HAVING A PREDETERMINED REVERSE BREAKDOWN VOLTAGE, ABOVE GROUND MEANS FOR GENERATING A DIRECT CURRENT PULSE OF POSITIVE POLARITY AND OF VOLTAGE MAGNITUDE GREATER THAN THE REVERSE BREAK-