Receiver And Transmitter Synchronization For Crosswell Electromagnetic Imaging Logging Tool Based On GPS

Crosswell Electromagnetic imaging logging Tool (CEMIT) has a large magnetic fieldtransmitter in one well and also has a sensitive low-noise receiver (receiver string) inanother well. Firstly, the transmitter generates an electromagnetic field with thefrequency range of5Hz to1000Hz. Then, the receiver senses the primary field of thetransmitter combined with the secondary field produced by the formation between twowells. And the phase difference Ф between receiving and transmitting electromagneticwave signal is closely related to formation conductivity. The higher synchronizationprecision between receiver and transmitter, the measurement error of Ф we get will besmaller, and then we can more accurately calculating formation conductivity.Firstly, some formulas are presented to explain the relationship between the Ф withformation conductivity. And some figures are presented to explain that transceiversynchronization is critical to measure the Ф. Secondly, according to current GPSsynchronization applications, some synchronization methods based on GPS for CEMITare presented, then the best method is chosen. The point is that four real-time clockmodules are implemented in two Surface Model Systems (SMS) and two Downhole ToolModel Systems (DHTMS), and clock modules between SMS and DHTMS should besynchronization. Thirdly, a new synchronization method between SMS and DHTMS ispresented. RS-485bus or CAN bus is used for communication between SMS andDHTMS. First of all, based on the reference time from GPS receiver, the clocks in SMSsare calibrated. And then after transmission delay between SMS and DHTMS has beenmeasured and two clock in DHTMSs are calibrated by corresponding SMS’s clock withthe transmission delay.Lastly, an experiment is implemented for the method using RS-485bus and CAN bus separately. The experiment results show that synchronization between receiver andtransmitter is achieved. And a better result has been obtained when using RS-485busthan using CAN bus. The synchronization clock error is less than400ns between receiverclock and transmitter clock when using RS-485bus, and which corresponds to a phaseaccuracy of0.144degree at1KHz. The method can meet the needs of phase differencemeasurement in CEMIT.