井间电磁成像测井仪收发同步技术研究

Receiver and Transmitter Synchronization for Crosswell Electromagnetic Imaging Logging Tool

井间电磁成像测井仪中接收信号与发射信号的相位差与地层电导率密切相关,研究接收与发射信号的精确同步技术能为准确测量其相位差、准确解算地层电导率以及精确分析地层剩余油储量提供支持。提出一种基于GPS的收发同步方法。在地面系统和井下系统实现4个实时时钟,以GPS接收机提供的基准时间对地面系统进行精确授时;精确测量地面系统和井下仪器的传输时延,地面控制系统根据传输时延对井下系统精确授时,实现井下系统时钟与GPS提供的基准时间同步,达到了接收机与发射机的时间同步。初步实验结果表明,使用RS-485总线进行通讯其同步时间误差小于400ns(信号频率为1kHz时,相位误差为0.144°)。

From crosswell electromagnetic imaging logging tool （CEMIT）, the phase difference between receiving and transmitting electromagnetic wave signal is closely related to formation conductivity. The higher synchronization precision of receiver and transmitter, the smaller measurement error of the phase difference, therefore we can more accurately calculate formation conductivity and remaining oil reservoira. 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 clocks in DHTMSs are calibrated by corresponding SMS＇s clock with the transmission delay. Lastly, the synchronization between receiver and transmitter is achieved. Experimental results show that synchronization clock error is less than 400 ns between receiver clock and transmitter clock when communication protocol between SMS and DHTMS uses RS-485, which corresponds to a phase accuracy of 0. 144 degree at 1 kHz.