Abstract: An MWD measuring apparatus and technique using a single transducer mounted on an MWD drill collar near a tubular member, such as an upset or stabilizer blade. The minimum stand-off is the radial difference between the MWD drill collar and the tubular member when the drillstring is side-walled against the borehole wall. Control apparatus connected to the transducer determines elapsed time for each pulse transmitted towards and reflected from the borehole wall. A plurality of round-trip transit time (RTT) values are measured and the results statistically analyzed to identify and verify the RTT.sub.min value corresponding to the minimum stand-off distance. The RTT values may be analyzed directly or using histogram techniques. Once determined, the sound speed of the drilling fluid is calculated as twice the radial difference between the MWD collar and stabilizer blade or upset divided by the RTT.sub.min value.

Abstract: An ultrasonic stand-off gauge for use in measuring the instantaneous stand-off distance between the drill stem and the borehole wall while drilling. A primary transducer mounted on the drill collar sends and receives ultrasonic pulses between the drill collar and the borehole wall. A second, reference transducer mounted on the pressure barrel within the drill collar measures the sound speed of the drilling fluid. The drilling fluid flows through a conduit within the drill collar, where the reference transducer transmits a pulse through the drilling fluid in the conduit. The pulse from the reference transducer is reflected off an internal reflector mounted on the internal surface of the drill collar. Since the distance between the reference transducer and the reflector is known and constant, the sound speed of the drilling fluid is determined accurately.