E - Fixed Constructions – 21 – B
Patent
E - Fixed Constructions
21
B
166/3
E21B 47/06 (2006.01) E21B 47/18 (2006.01)
Patent
CA 1206089
METHOD AND APPARATUS FOR SIGNAL RECOVERY IN A LOGGING WHILE DRILLING SYSTEM Abstract of the Disclosure A method and an apparatus are provided for recovery of data in a well logging while drilling system wherein a data signal originating from a downhole apparatus is transmitted to the earth's surface through drilling mud. The method includes measuring pressure pulsations in a mud flow line at the earth's surface in two spaced apart locations then combining the data signals to cancel downhole propagating pressure waves such as surface generated interference noise. The combined data signal is then processed by convolution filters, and then passed through a detector to determine the presence or absence of a valid data signal. In the apparatus these data signals are passed through the drilling mud by pressure fluctuations originating at a downhole apparatus. Two closely spaced pressure sensors measure mud pressure pulsations in a mud line at a surface location. One of the measured pressure pulsation signals is time shifted an amount equal to the travel time of the group propagation acoustic velocity in the mud between the two pressure sensors. One of the two signals has its polarity reversed and the two signals are combined to cancel any downward propagating energy from the combined pressure data signal. The combined pressure data signal is then processed by each of n matched filters in a baseband PCM system to determine the presence or absence of one of n symbols in the residual data signal. Each output of the filtered data signals is then input to a "Largest of" Bayesian detector to identify either the presence of a symbol or absence of the data signal.
427966
Dresser Industries Inc.
Macrae & Co.
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