Well survey system modulation technique

Abstract

A single pair of conductors in a drill cable in a borehole is used not only to transmit power into the hole, but to transmit control commands into the hole, and to communicate sensor data back up to the surface. A frequency for power and communication of 1 kHz is employed; but this is frequency modulated by deviation of about 5 percent to transmit commands down-hole. A different non-interfering phase modulation is used to send data back to the surface by causing a susceptance (preferably capacitive) to be connected across the supply cable to signify a binary pulse. The alteration in phase between current into the cable and potential across it is observed at the surface and the pulses so detected are processed by conventional data processing techniques. A novel coding method is used for sending commands into the hole: Each of a series of possible commands is assigned a binary value so that, for example, if commands #2 and #4 are to be sent, a total of 2 + 8 pulses (all of equal weight) forming a command word are sent down-hole where they are counted by a binary counter. When the total number has been counted, outputs #2 and #4 of the counter (2.sup.1 and 2.sup.3) will have signals on them which constitute the transmitted orders. This method is wasteful of time-bandwidth product (which is in abundant supply in this invention's applications) but avoids problems of timing inherent in serial transmission of conventional binary signals.