Cross talk noise reduction technique for downhole instrumentation

Abstract

In order to reduce crosstalk effects, cables that carry contaminating signals can be routed using physically adjacent cables in pairs that carry replica signals of opposite polarity. Keeping the cables in close proximity will reduce loop area and will cause the magnetic fields induced by the wires carrying the paired signals to cancel. Accordingly, the crosstalk induced both by the differential and common-mode components of these two signals on any other medium or device cancel each other.

1 . A downhole tool, comprising:
 a plurality of electrical conductors extending through a channel connecting a first portion of the downhole tool with a second portion of the downhole tool, at least some of the plurality of electrical conductors electrically connecting one or more electronics modules in the first portion of the downhole tool with one or more electronics modules in the second portion of the downhole tool, the plurality of electrical conductors comprising:   a first electrical conductor;   a second electrical conductor and a third electrical conductor, the second and third electrical conductors being part of an electrical circuit,   wherein, during operation of the downhole tool, the first electrical conductor carries a first electrical signal, and one of the electronics modules provides a second electrical signal to the second electrical conductor and a third electrical signal to the third electrical conductor such that cross-talk induced by the second electrical signal on the first electrical signal is reduced by cross-talk induced by the third electrical signal on the first electrical signal.   
     
     
         2 . The downhole tool of  claim 1 , wherein the one electronics module provides the third electrical signal, wherein the third electrical signal is a replica of the second electrical signal with opposite polarity. 
     
     
         3 . The downhole tool of  claim 1 , wherein the one electronics module comprises a signal generator for generating an input electrical signal and the one electronics module is configured to generate the second and third electrical signals based on the input signal. 
     
     
         4 . The downhole tool of  claim 3 , wherein the one electronics module comprises a signal divider which divides the input electrical signal to provide the second and third electrical signals. 
     
     
         5 . The downhole tool of  claim 3 , wherein the one electronics module comprises an induction module which induces the second and third electrical signals in the second and third electrical conductors, respectively, based on the input electrical signal. 
     
     
         6 . The downhole tool of  claim 1 , comprising a sensor assembly, wherein the first electrical conductor is a component of the sensor assembly. 
     
     
         7 . The downhole tool of  claim 6 , wherein the first electrical conductor is an antenna. 
     
     
         8 . The downhole tool of  claim 6 , wherein the sensor assembly comprises a sensor selected from the group consisting of a force sensor, a nuclear magnetic resonance sensor, a magnetometer, a magnetic field sensor, an acoustic sensor, a temperature sensor, an electric field sensor, a pressure sensor, an optical sensor, a proximity sensor, and an accelerometer. 
     
     
         9 . The downhole tool of  claim 1 , wherein the second electrical conductor and the third electrical conductor extend parallel to each other in the channel. 
     
     
         10 . The downhole tool of  claim 1 , wherein the second electrical conductor and the third electrical conductor are twisted together to form a twisted pair. 
     
     
         11 . The downhole tool of  claim 1 , wherein the channel has a length between approximately 6 inches and 10 feet. 
     
     
         12 . The downhole tool of  claim 1 , wherein the channel has an inner diameter of about 3 inches or less. 
     
     
         13 . The downhole tool of  claim 1 , wherein one or more of the signals are AC signals. 
     
     
         14 . The downhole tool of  claim 1 , wherein one or more of the signals are DC signals. 
     
     
         15 . The downhole tool of  claim 1 , wherein the first electrical signal has a signal to noise ratio (SNR) of approximately 10 or greater. 
     
     
         16 . The downhole tool of  claim 1 , wherein the first electrical signal has an SNR of between approximately 2 and 5. 
     
     
         17 . The downhole tool of  claim 1 , further comprising one or more additional cables extending through the channel. 
     
     
         18 . The downhole tool of  claim 1 , wherein at least ten cables extend through the channel. 
     
     
         19 . The downhole tool of  claim 1 , wherein the downhole tool is a well-logging tool. 
     
     
         20 . The downhole tool of  claim 1 , wherein the downhole tool is a sonde. 
     
     
         21 . A method for reducing cross-talk in a first electrical signal, comprising:
 transmitting a first electrical signal along a first electrical conductor running through a channel;   simultaneously to transmitting the first electrical signal, generating a second electrical signal along a second electrical conductor running through the channel and generating a third electrical signal along a third electrical conductor running through the channel, the second and the third electrical conductors being in sufficient proximity to the first electrical conductor to induce cross-talk in the first electrical conductor,   wherein the second and third electrical signals are generated so that cross-talk induced by the second electrical signal on the first electrical signal is reduced by cross-talk induced by the third electrical signal on the first electrical signal.   
     
     
         22 . The method of  claim 21 , wherein one or more of the signals are AC signals. 
     
     
         23 . The method of  claim 21 , wherein one or more of the signals are DC signals. 
     
     
         24 . The method of  claim 21 , wherein the first electrical signal has a signal to noise ratio (SNR) of approximately 10 or greater. 
     
     
         25 . The method of  claim 21 , wherein the first electrical signal has an signal to noise ratio (SNR) of between approximately 2 and 5. 
     
     
         26 . A method of using a downhole tool, comprising directing one or more electrical signals between different portions of the downhole tool,
 wherein directing one or more electrical signals between different portions of the downhole tool comprises:
 transmitting a first electrical signal along a first electrical conductor running through a channel of the downhole tool, the channel extending between the different notions of the downhole tool; 
 simultaneously to transmitting the first electrical signal, generating a second electrical signal along a second electrical conductor running through the channel and generating a third electrical signal along a third electrical conductor running through the channel, the second and the third electrical conductors being in sufficient proximitv to the first electrical conductor to induce cross-talk in the first electrical conductor, 
 wherein the second and third electrical signals are generated so that cross-talk induced by the second electrical signal on the first electrical signal is reduced by cross-talk induced by the third electrical signal on the first electrical signal. 
   
     
     
         27 . The method of  claim 26 , further comprising:
 disposing the downhole tool in a borehole; and   performing a logging operation with the downhole tool.