Multiple transmit frequency induction logging system with closed loop conversion circuitry for phase and gain variation elimination

摘要

An induction logging tool is disclosed for utilization in conjunction with surface electronic equipment for measuring a characteristic of subsurface formations. A transmit signal which is comprised of a repetitive sequential series of at least three transmit frequencies is generated by coupling an oscillator to a transmitter coil and a plurality of selectable tuning capacitors. A signal characteristic of the response of the formation to this transmit signal is detected at a receiver coil array and two phase sensitive detectors are then utilized to generate output signals indicative of the magnitude of the X and R components of the formation signal which is in phase with a reference phase signal. The output signal of each phase sensitive detector is then utilized to accurately determine the X and R component of the receiver coil signal for each frequency by nulling the output of the phase sensitive detector utilizing a closed loop conversion circuit which eliminates errors due to phase and gain variations. The output signal is then compensated by convolving the output of the phase sensitive detector with one of multiple filter coefficient sets which are each associated with a particular transmit frequency, for each portion of the output signal which is attributable to a particular transmit frequency. Each filter coefficient set preferably comprises a Finite Impulse Response (FIR) filter, which is derived by altering each filter coefficient to induce a specified response of the induction tool to a simulated step function.