Apparatus and method for communication between downhole components

Abstract

A method of synchronization between downhole components includes: generating a dual tone synchronization signal by a signal generator in a first downhole component disposed in a borehole in an earth formation, the dual tone signal including a first constituent periodic signal having a first frequency f 1 and a second constituent periodic signal having a second frequency f 2 that is different from the first frequency; transmitting the synchronization signal to a second downhole component disposed in the borehole; receiving the synchronization signal by a signal processor in the second downhole component, calculating a phase difference between the first constituent signal and the second constituent signal, and calculating a transmission delay based on the phase difference; and synchronizing operation of the first and second downhole components based on the delay.

What is claimed is: 
     
         1 . A method of synchronization between downhole components, the method comprising:
 generating a dual tone synchronization signal by a signal generator in a first downhole component disposed in a borehole in an earth formation, the dual tone signal including a first constituent periodic signal having a first frequency f 1  and a second constituent periodic signal having a second frequency f 2  that is different from the first frequency;   transmitting the synchronization signal to a second downhole component disposed in the borehole;   receiving the synchronization signal by a signal processor in the second downhole component, calculating a phase difference between the first constituent signal and the second constituent signal, and calculating a transmission delay based on the phase difference; and   synchronizing operation of the first and second downhole components based on the delay.   
     
     
         2 . The method of  claim 1 , wherein the synchronization signal is transmitted directly to the second downhole component over a transmission line connected between the downhole components and a surface unit. 
     
     
         3 . The method of  claim 2 , wherein the transmission line is a single conductor power and telemetry bus. 
     
     
         4 . The method of  claim 2 , wherein the synchronization signal is transmitted in a first frequency band that is different than a second frequency band used to transmit communications between the surface unit and the downhole components. 
     
     
         5 . The method of  claim 1 , further comprising transmitting a trigger signal from the first downhole component to the second downhole component, the trigger signal indicating a time value measured by the first downhole component. 
     
     
         6 . The method of  claim 5 , wherein the time value is a zero crossing point at a selected cycle of the synchronization signal. 
     
     
         7 . The method of  claim 5 , wherein the first downhole component includes a transient electromagnetic (TEM) transmitter, the second downhole component includes a TEM receiver, and the trigger signal indicates a time at which the TEM transmitter commences transmitting TEM signals into the earth formation. 
     
     
         8 . The method of  claim 1 , wherein the first downhole component and the second downhole component are connected via a current loop communication system. 
     
     
         9 . The method of  claim 8 , wherein the first downhole component includes a current loop transmitter configured to convert voltage signals from the first downhole component to current signals and transmit the current signals over a current loop formed by the transmission line. 
     
     
         10 . An apparatus for communicating between downhole components, comprising:
 an interface coupled to a first downhole component, the interface configured to communicatively couple the first downhole component to a transmission line and transmit signals to a second downhole component over the transmission line, the interface including a current loop transmitter configured to convert voltage signals from the first downhole component to current signals and transmit the current signals on a current loop formed by the transmission line.   
     
     
         11 . The apparatus of  claim 10 , further comprising a second interface coupled to the second downhole component, the second interface including a current loop receiver configured to convert current signals received from the first downhole component over the transmission line to voltage signals. 
     
     
         12 . The apparatus of  claim 10 , wherein the current loop is formed by the transmission line coupled to a surface power source and a return path formed by a borehole string that includes the downhole components. 
     
     
         13 . The apparatus of  claim 12 , wherein the transmission line is a single conductor power and telemetry bus. 
     
     
         14 . The apparatus of  claim 12 , wherein the current loop transmitter is configured to transmit the current signals in a first frequency band that is different than a second frequency band used to transmit communications between the surface unit and the downhole components. 
     
     
         15 . The apparatus of  claim 14 , wherein the current loop transmitter includes a resonant decoupling device configured to prevent coupling of signals in the second frequency band with the current loop transmitter. 
     
     
         16 . The apparatus of  claim 14 , wherein the current loop transmitter includes a band pass filter tuned to frequencies in the first frequency band. 
     
     
         17 . The apparatus of  claim 11 , wherein the first interface and the second interface are configured to both transmit and receive current loop signals. 
     
     
         18 . The apparatus of  claim 17 , wherein the first interface and the second interface are configured as a half-duplex system to allow for two-way communication. 
     
     
         19 . The apparatus of  claim 17 , wherein the first interface and the second interface are configured as a full-duplex system to allow for simultaneous two-way communication. 
     
     
         20 . The apparatus of  claim 10 , wherein the first downhole component includes a signal generator configured to generate a dual tone synchronization signal, the dual tone signal including a first constituent periodic signal having a first frequency f 1  and a second constituent periodic signal having a second frequency f 2  that is different from the first frequency, and the second downhole component includes a processor configured to receive the synchronization signal, calculating a phase difference between the first constituent signal and the second constituent signal, and calculate a transmission delay based on the phase difference.