Precision timing synchronization in Crosswell Electromagnetic logging Tool

In Crosswell Electromagnetic Imaging logging Tool (CEMIT), the phase difference Φ between receiving and transmitting electromagnetic wave signal is closely related to formation conductivity. The higher synchronization precision of receiver and transmitter, the measurement error of Φ will be smaller, and then we can more accurately calculating formation conductivity. Based on GPS a synchronization method is presented for CEMIT in the paper. First of all, four real-time clock modules are implemented in two Surface Model System (SMS) and two Downhole Tool Model System (DHTMS). Based on the reference time from GPS receiver, the clocks in SMSs are calibrated. And then after transmission delay between SMS and DHTMS has been measured and two clock in DHTMSs are calibrated by corresponding SMS's clock with the transmission delay. Lastly, the synchronization between receiver and transmitter is achieved. Experiment results show that synchronization clock error is less than 400ns between receiver clock and transmitter clock when communication protocol between SMS and DHTMS uses RS-485, and which corresponds to a phase accuracy of 0.144 degree at 1 KHz. The method can meet the needs of phase difference measurement in CEMIT.