| Magnetic resonance sounding(MRS)is a geophysical method that can detect groundwater directly and non-invasively,which has been widely used in arid and semi-arid regions in China due to its advantages of detecting groundwater qualitatively and quantitatively.With the development of MRS technology from 1D to the precise 2D/3D groundwater imaging achieved within a few hundred meters generally.In addition,the MRS system with one transmitter matches and multiple receivers to work in the mode of multi-point synchronous measurement is also required.However,the wired synchronization methods of current transmitting and receiving control systems cannot meet the requirements for actual work.The signal attenuation and distortion occur when the synchronization signal transmitted over long wires(>100m),which leads to delayed triggering or false triggering.In view of the above problems,this paper proposed a high-precision wireless synchronization technology which was based on the MRS system for the first time,then a corresponding transmitting and receiving control system was developed,and finally gave the field detection results.The main research works of the paper are as follows:1.By studying the basic principle of the surface MRS control system and the limitations of wired synchronization methods,a long wire transmission signal model was established,and the effect of cable length on the synchronization signal was analyzed.Furthermore,the overall design scheme and work flow of the new control system were proposed based on the idea of wireless synchronization technology.More importantly,the synchronization pulse signal cycle was determined by analyzing the pulse law and capacity charging technology of the MRS device,which lays the foundation for the achievement of the system design.2.In consideration of the problems of delayed or false triggering in wired synchronization,a self-calibrated high-precision synchronous timing technology was proposed to ensure the stable and precise work of system,which was based on the theory of Global Positioning System(GPS)and Oven Controlled Crystal Oscillator(OCXO)Taking the advantages of GPS(without accumulated error for a long time)and OCXO(output high-precision clock in a short time)into account,the wireless synchronization compensation was designed.The high-precision second pulse of GPS was used to provide a reference for frequency and phase self-calibration of the OCXO.In this work,the synchronous timing module was developed to continuously provide high-precision synchronization pulses and accurate time information for transmitters and receivers.3.On the basis of traditional MRS transmitting and receiving control system,a new control system was designed based on GPS and OCXO synchronization technology.Using the ARM+CPLD architecture scheme,an transmission control system that possess many functions including transmission timing control,transmission current acquisition,IGBT driving,and human-computer interaction was designed.The receiving control system was designed using LabVIEW software platform,which was based on the GPS and OCXO synchronization timing module.The system achieved long-distance and high-precision synchronization between transmission and reception through presetting protocols,analyzing information,and so on.Finally,the new control system of the MRS instrument based on the GPS and OCXO synchronization technology was tested successfully.The test results show that the accuracy of the synchronization pulse signal can be kept within 250 ns under the condition that the GPS is effective.Even after the GPS suddenly loses of lock,the instrument can still maintain high-precision synchronization work for 60 minutes.If GPS repositioning is effective during this period,the system can quickly achieve self-calibration,and provide high-precision synchronization pulse and time information for the instrument again.To sum up,the research achievement in this paper meets the requirement of the synchronization accuracy and realizes the leap from the wired synchronization to the wireless synchronization of the MRS device,which provides a powerful technical support for the large-area 2D/3D detection in China. |
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