| The Transient Electromagnetic Method(TEM) has played an important role ingeological survey, groundwater exploration and disaster water detection in recent years,and now it’s an important way to detect the unknown underground target. Although thestudy of Transient Electromagnetic Method in our country is relatively late comparedwith other countries, the ATEM instrument developed by Jilin University and WTEMdeveloped by Chongqing Pentium Numerical Control Technology Research Institutehave achieved good detection results in ground applications. However, due to the limitof coil parameters in tunnel environment, the turn-off time of emission current is long,so the operating range of Transient Electromagnetic Method is limited to some extent.In view of the shortcomings of existing instruments, this paper proposed tunnelelectromagnetic instrument in order to meet the detection needs, and gave specificdesign solutions and test results.By researching the design scheme of transient electromagnetic transmitter at homeand abroad it is known that:1. Most of the transmitter control units are designed basedon single-chip microcomputer or PC104. They both have the disadvantages of slowprocessing speed and heavy power consumption, which make them unsuitable for longtime work.2. The turn-off time of emission current is an important parameter for thetransmitter, and it’s the key factor affecting the reliability of subsequent dataprocessing result.3. The feature of transient electromagnetic signal is that theamplitude of early signal is big but the middle-late signal is small. In order to solvethese problems, this paper proposed following solutions:1. The transmitter is designedbased on Complex Programmable Logic Device(CPLD), which has the advantages offast processing speed, low power consumption and simple peripheral circuit.2. Thispaper proposed the power transformation part based on MOSFET which has quick switch speed. Through absorbing energy by TVS diode, short emission current turn-offtime is realized. The transmitter has a variety of transmission frequency and duty ratioto meet the needs of different detection situations.3. In view of low sampling noiseand high signal-to-noise ratio of MPS-140801, this paper proposed the hardware partof the receiver which is designed on the basis of this collection card. This paperproposed the segmented amplification technology. Five channels of differentmagnifications collect signals at the same time, and then the transient electromagneticsignal can be created from the five signals after the signals are selected from the fivechannels at different time separately and magnifications are unified. The purpose ofsmall magnification collection for early signal and big magnification collection formiddle-late signal can be realized in this way and this method improves the collectionprecision of the middle-late signal.The receiver control software is programmed based on LabVIEW, and the user cancontrol the receiver through this software. Control software can mainly realize thefunctions of parameter setting, current collecting and signal collecting. Parameterssuch as data storage location, sampling rate, superposition times and sampling lengthcan be set before the receiver is operated. Current collecting part can display emissioncurrent waveform, save current data, calculate emission current turn-off time and saveturn-off time. Signal collecting part can display superposition times and signalwaveform in real time by linear coordinate or logarithmic coordinate.To verify the instrument design parameters, the transmitter and receiver are testedindoor and outdoor. The indoor test includes emission current turn-off time,transmitting frequency and duty ratio, receiver background noise, data saving functionand so on. Indoor test result proved that when using5m×5m,8-turn transmitting coilapplicable to tunnel environment and emission current is20A, the transmitting currentturn-off time is50μs and the receiver background noise is15μV. Finally, a fieldexperiment was conducted in Maijishan Tunnel of Gansu Province. The validity of thetransient electromagnetic instrument was verified by analyzing the experiment result. |
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