Transient Electromagnetic Transmitter Circuit Design And Calibration Study Of Response

Transient electromagnetic method is an important mean of geological technologyand its application is very extensive, such as the oil and gas exploration, mineexploration, hydraulic engineering, construction engineering and other fields. Westerncountries have had a lot of achievements in theoretical and experimental study abouttransient electromagnetic system in the1980s, and already produced commercialinstruments, then, China just has researched transient electromagnetic system. Thereare various issues such as theory cutting off from experiment in research, lack ofevaluation mechanism of instruments, so we must do more basic theory andexperiment research about transient electromagnetic system.This paper analyzes development and actual state of transient electromagnetictransmitter and finds its trend is light, miniaturization, high integration, and strongstability. In China, the research of transmitter is mostly with high current, shortshutdown and linear off as evaluation criteria, but these ideas are lack of theoreticalbasis. We can’t simply rely on the size, length or linear and nonlinear to evaluatetransmitter performance. Transmitter performance is mostly determined by the standor fall of exploration results, so it is necessary to study the relationship of explorationresults and transmitter parameters. This paper researches transmitter circuit designand transmit current for the effect of responses.Transmitter circuit design includes1st generation based on MCU and2ndgeneration based on DSP. The problems of1st generation transmitter include signaldelay and low baseband precision, so PWM technology and nonlinear optocoupler areused. In order to in-depth study of this subject, the2nd generation transmitter isdesigned including DSP minimum system, man-machine interface, GPS, transmitsignal acquisition, power supply circuit, design of other peripheral hardware andsoftware. Its advantages include miniaturization and full function. The effect of transmitting current for responses is studied. In theory aspect, thispaper establishes a free space calibration model, analyzes the calculation formula ofthe calibration loop current and the calibration loop response, and carries on the cycleexpansion analysis. In order to ensure the effect of response, noise characteristicsof transient electromagnetic sensor system are studied. The research verifies thetheory of model comparing with measured. With the help of model, the actual sensorbackground noise is63.5pV/m2which is lower than the ambient noise, so the noisesource of transient electromagnetic system is ambient noise. In measured aspect, thispaper analyzes the impact of parasitic oscillation of transmitting current early turn-offon the calibration loop current, and results show the parasitic oscillation oftransmitting current early turn-off has little effect on the response. Derivative ofcalibration loop current is its response. The influences of different turn-off times oncalibration loop response are compared with three different time constant, and resultsshow that fast turn-off is useful for early detection, slow turn-off for deep detection;turn-off time should be chosen based on electrical conductivity of different detectiontarget according to the actual demand; the standard to evaluate the transmitter that thebetter turn-off is the faster is no theoretical basis, these parameters mainly decide onconductivity of detecting targets and buried depth.