Study On The Key Technology Of The GREATEM Transmitter

The need for mine resources is greatly increased because of their important application in industrial production, agricultural production and daily lives. The insufficient of mine supply in our country is because of the limited exploration abilities but not the shortage of mines. It makes significant sense to improve new prospecting ways to promote the exploration abilities.GREATEM(Grounded Electrical-source Airborne TEM) was newly proposed in recent years. Using a grounded emitting source and an airborne magnetic field receiver, GREATEM can obtain a greater transmit power combined with the advantage of conventional of AEM. It releases the work of movement and increases the depth of exploration. Although some theories have been proposed, there is still no report about mature GREATEM system in China. The design of GREATEM system makes great sense for study of the theory and improvement of the exploration abilities. Transmitter design is the first step for a whole system.Relying on the public welfare program of Ministry of Land and Resources, this thesis did research on the correlated theories and the key technologies of transmitter system of GREATEM. The main achievements and conclusions are as follows:(1) Based on the basic theory of electromagnetic, the forward modeling algorithm was derived for GREATEM with dipole-dipole array under the stratified earth to guide the design of transmitter parameters, such as current, magnetic torque, last period of current, etc. The influence of resistivity was calculated under the condition of uniformization torque and receiver area. Combining with the designed indexes of the receiver, the transmitter-receiver distance and receiver height influence were deduced. The distribution of the field of magnetic was also given with xV 、yV and zV on the height of 50 m. A new transformation model named multi-point source array was prompted to achieve larger and more homogeneous response.(2) A simplified model of ground loop was prompted according to the reverse thinking of the quantification of the AEM by ground loop, through which can guide the waveform of the GREATEM transmitter. The spectral analysis and power calculation of four kinds of different wave forms were completed, and the responses to ground loop were calculated as well. The study illustrates that all these familiar wave forms achieve the lower response than the ideal negative step signal. When the width of the square wave appears 2.5 times of the ground loop parametert, it can achieve the 90% of the ideal response, which allows the approximate of the square to step. Under the model of the ground loop, trapezoidal flat time should not 22 times lower than the slope time, otherwise the error will more than 10% of the square response. Triangular response will only be 33.9% and half-sine response 45.7% of the step wave response, which means more poor than square or trapezoidal. The compared results combing with normal spectral analysis and power calculation guided the wave selection of emitting.(3) The analysis of two typical switch-off procedure is done to compare the practical turn-off current response to the ideal one. An important conclusion is that the influence of the turn-off time can Not be ignored by delaying the observe time, because it continue to add about 2~3% on the ideal response. In the condition of 50 A current, the switch off time should not be longer than 200 ms, otherwise the relative error would exceed 10%, therefore the influence of the response should be carefully controlled. The research in the exponential damping switch off model shows that, when selecting the parameter of the coil, close time constant should be selected to achieve higher response. A brief introduction was done to the correction of the switch off time and the study of the quick turn off circuit was executed. Simulations, such as the classical H bridge simulation, the RCD absorption circuit, the constant voltage clamping circuit, the linear switch-on current, the control of constant current, are carried out through Matlab and Multisim.(4) The transmitter system was designed in this thesis, especially the kernel module of the hardware and software. The hardware design includes high power supply, the driver and protection circuit, the sample of voltage and current, the micro-controller circuit, the communication and storage of the data, the synchronous circuit, and the anti-interference design. The software design includes the structure of embedded development, the PC software, the anti-interference of software, etc. The field experiment was taken to test the transmitter in Yeshan Geology Park in Nanjing. Both the electrical and magnetic sources were generated in the shape of bipolar square wave form. The ground data acquisition was done to test the transmitter and the improvement suggestion was given in the end.