Downhole Timing Recovery and Signal Detection

Abstract

The present invention relates to telemetry apparatus and methods, and more particularly to acoustic telemetry apparatus and methods used in the oil and gas industry. More specifically, the invention relates to a method for enhancing a received signal transmitted by acoustic telemetry through a drill string by modifying the received signal by a multiplication of the received signal with a second waveform.

1 . A method for enhancing a received signal transmitted by acoustic telemetry through a drill string comprising the step of modifying the received signal by a multiplication of the received signal with a second waveform. 
   
   
       2 . The method of  claim 1  where the received acoustic signal is a linear frequency chirp. 
   
   
       3 . The method of  claim 2 , where the received acoustic signal is a linear frequency upchirp and the second waveform is a linear frequency downchirp. 
   
   
       4 . The method of  claim 2 , where the received acoustic signal is a linear frequency downchirp and the second waveform is a linear frequency upchirp. 
   
   
       5 . The method of  claim 2  wherein an autocorrelation function of the received signal is calculated and the autocorrelation function is optimized to compensate for limited chirp bandwidth. 
   
   
       6 . The method of  claim 5  wherein the autocorrelation function is optimized to remove dispersion effects. 
   
   
       7 . A method of enhancing a received chirp signal within a receiver wherein the received chirp signal has been transmitted by acoustic telemetry through a drill string, comprising the step of:
 applying a non-constant frequency local oscillator signal to the received chirp signal to selectively shift component frequencies of the received chirp signal by spreading the received chirp in the frequency domain while maintaining baud rate in order to create a processed signal having an increased time-bandwidth.   
   
   
       8 . The method as in  claim 7  wherein the non-constant frequency local oscillator is a linear chirp waveform. 
   
   
       9 . The method as in  claim 7  wherein step a) includes adjusting an autocorrelation waveform to a desired form during down-conversion in the receiver. 
   
   
       10 . The method as in  claim 7  wherein the local oscillator signal is a down chirp opposite in frequency span to the received chirp signal. 
   
   
       11 . The method as in  claim 8  wherein the local oscillator signal is a down chirp opposite in frequency span to the received chirp signal. 
   
   
       12 . The method as in  claim 9  wherein the local oscillator signal is a down chirp opposite in frequency span to the received chirp signal. 
   
   
       13 . The method as in  claim 10  wherein the frequency span of the down chirp is chosen to obtain a desired correlation waveform. 
   
   
       14 . The method as in  claim 7  wherein the processed signal is subjected to multiple frequency sweeps and the time-bandwidth is increased with each frequency sweep and wherein the frequency sweeps are limited to twice the received chirp's frequency sweep. 
   
   
       15 . The method as in  claim 8  wherein the processed signal is subjected to multiple frequency sweeps and the time-bandwidth is increased with each frequency sweep and wherein the frequency sweeps are limited to twice the received chirp's frequency sweep. 
   
   
       16 . The method as in  claim 9  wherein the processed signal is subjected to multiple frequency sweeps and the time-bandwidth is increased with each frequency sweep and wherein the frequency sweeps are limited to twice the received chirp's frequency sweep. 
   
   
       17 . The method as in  claim 13  wherein the processed signal is subjected to multiple frequency sweeps and the time-bandwidth is increased with each frequency sweep and wherein the frequency sweeps are limited to twice the received chirp's frequency sweep.