Development Of TEM Transmitter

The transient electromagnetic method is closely related to the exploration ofmineral resources, geological structure, engineering and geological survey,environmental survey, archaeological and urban underground pipeline investigationand other various socio-economic development activities. Due to the vast territory andabundant resources, the exploration requires a lot of instruments. But compared withsimilar foreign products, the practical effect of our independently developed TEMinstruments is relatively poor, which greatly limits the development of our own TEMtechnology. It has important significance for the state and society to develop andenhance the TEM level. Based on the study of the entire time domain transientelectromagnetic system, this paper mainly introduces the transmitting circuit and theinfluence of the turn-off time of the emission current. The entire work as follows:Discussed the basic principle and process of the time domain transientelectromagnetic method; relatively large dynamic range and wide bandwidth are themain features of the transient electromagnetic signal; the primary field and secondaryfield are received simultaneously, the signal attenuation of the early primary field isquickly, but its amplitude is relatively large, and the late secondary field signalattenuates slowly but its amplitude becomes very weak. Through one of the keytheory of TEM-"smoke ring theory", we know that the early secondary field signalwhich has large amplitude and decays fast reflects the shallow geological information,and slow changed late signal reflects the deep geological information.Combined with the basic theory, we proposed the basic working device for generaltransient detecting, and different devices play very important role in the detection ofdifferent geological conditions. The basic components of the TEM system (includingthe transmitter and receiver, data acquisition, preprocessing, interpretation software)as well as some forms of the emission current are introduced. With above discussion, we know that the launch technology is a very important part in the TEM detectingtechnology, the quality of the excitation current directly affects the post-dataacquisition and inversion interpretation of the target. Because the emission currentcannot produce the desired rectangular pulse, it needs certain off-time. During theoff-time, the primary field and secondary field exist at the same time, which makesthis part of the data cannot be used. Since the signal during this time precisely reflectsthe early geological information, we must shorten the off-time as much as possible.For the transmitter using the conventional full-bridge circuit structure which has noclamp circuit, the linearity of the current is good. However, its turn-off time isrelatively long, which is unable to meet the experiment demands and needs to beimproved. We analyzed several announced TEM transmitter circuits and discussedtheir advantages and disadvantages. Although they have reduced the turn-off time ofthe emission current, there are always some drawbacks that are experimentalinconvenience or poor linearity. Combined with the characteristics of TVS tube, weproposed a new TVS clamp circuit whose feasibility is proved by experiment. Theworking principle and choice of the parameters of the circuit elements are described.Next, discuss the transition process of the receiving coil from the time domain andfrequency domain. The simulation result shows that the primary field signals in thelow-bandwidth, amplitude appears decay, at the same time its rising and the fallingedge response slowly. It is difficult to observe the early signal of the secondary field.With the change of the bandwidth of the receiver coil, the linear part of the secondaryfield signal will vary degrees of delay, the earliest observation time corresponding lag.Finally, according to the measured data, analyze the relationship of the primary field,the secondary field signal and the coil bandwidth. To get the best observations, weneed an appropriate match in the measured coil bandwidth and off-time of the current.