Burst QAM downhole telemetry system

Abstract

A downhole telemetry system that transmits a burst-QAM uplink signal to the surface of the well is disclosed. In a preferred embodiment, a downhole instrument coupled to a pair of conductors in a wireline or composite tubing string transmits a burst-QAM uplink signal to a surface system. The burst-QAM signal preferably comprises a series of data frames carrying telemetry data. Each data frame is preferably preceded by a quiet interval (when no signal is present), a timing synchronization sequence, and a training sequence. The timing synchronization sequence is designed for easy timing recovery at the surface, and the training sequence is designed to aid the adaptation of the equalizer. The data frame itself preferably includes a synchronization field, a data count, and a checksum in addition to the data. Direct digital synthesis is preferably employed to modulate the uplink signal.

1 . A method of communicating telemetry information comprising: 
 grouping data words to form data frames;    transmitting a preamble before each data frame; and    transmitting the data frames immediately after the respective preamble.    
     
     
         2 . The method of  claim 1 , wherein the preamble includes a quiet interval.  
     
     
         3 . The method of  claim 1 , wherein the preamble includes a timing synchronization sequence.  
     
     
         4 . The method of  claim 1 , wherein the preamble includes a training sequence.  
     
     
         5 . The method of  claim 1 , wherein the act of transmitting the data frames includes using quadrature amplitude modulation to impress the data onto a carrier signal.  
     
     
         6 . The method of  claim 1 , further comprising: 
 prepending a synchronization word and a data word count to each group of data words; and    appending a checksum to each group of data words.    
     
     
         7 . The method of  claim 6 , further comprising: 
 scrambling each group of data words by combining the group of data words with a mask.    
     
     
         8 . The method of  claim 1 , further comprising: 
 receiving the preambles;    generating a local clock signal that is synchronized with a timing synchronization sequence in the preambles; and    updating coefficients of an adaptive filter using a training sequence in the preambles.    
     
     
         9 . A telemetry transmitter comprising: 
 a scrambler configured to exclusive-or data frames with a mask;    a preamble generator configured to provide a preamble before each data frame;    a modulator coupled to the preamble generator to receive the preambles and coupled to the scrambler to receive scrambled data frames, wherein the modulator is configured to modulate the preambles and scrambled data frames to form a quadrature amplitude modulated (QAM) signal,    wherein the preamble is identical from frame to frame.    
     
     
         10 . The telemetry transmitter of  claim 9 , wherein the preamble includes: 
 a quiet interval;    a timing synchronization sequence; and    a training sequence.    
     
     
         11 . The telemetry transmitter of  claim 9 , further comprising: 
 a high pass filter configured to block low-frequency components of the QAM signal; and    an isolation transformer configured to couple the filtered QAM signal to telemetry conductors contained within composite tubing.    
     
     
         12 - 30 . (canceled)