| As the mineral sources are continuously being exploited, mineral products covered in 500 meters near-surface are exhausted. It is an important way to solve the shortage of mineral resources that exploring mineral products in the second layer of the underground space(500m-2000 m below). TEM, short for the transient electromagnetic method, is a geophysical method widely accepted. As is restricted by the bandwidth and the sensitivity,the TEM method cannot reach the depth of more than 800 m. SQUID, short for superconducting quantum interference device, processing the characteristics as follows:low noise, high sensitivity and wide bandwidth can replace the cores to achieve the aim of deep exploration. This paper focus on the problem that peaks of current and the slope of falling edges leading to the loss of lock among SQUIDs and invent a high power transmitter in transient electromagnetic system based on Low-Tc SQUID. With the transmitter, we can solve the problems that transmitting high power, restricting current peaks to a high quality and controllable slow turn-off are hard to achieve. The main contents are as follows.1.With the magnetic calculations based on the operating means of transient electromagnetic central-loop source, I calculated the magnetic response in stratiform low resistivity model and the relationship graph between transmitting magnetic moment, the minimum resolution ratio in magnetic field and the exploring depth. Also I discussed the influence on the loss of lock caused by the gradient of magnetic field using different transmitting magnetic moment. In addition, I made the key technological index clear and definite.2.With the 24-bit A/D of high precision and intelligent power module(IPM), I designed a high power transmitting bridge circuit making the output of bipolar 100 A come true. Then, I designed a collecting circuit focus on the slope of the current falling edge. To achieve this, I used the SPI and DMA technology to transfer data in high speed. By controlling the numbers of GPS, baud rate and the output time series, I settled the conflictbetween collecting time of current and reading the GPS information.3.To find a solution for the loss of lock among SQUID caused by current peaks, I analyzed the characteristics and the generation mechanism of the current peaks and used the advantages of traditional circuit for reference. Eventually a peak-restrained circuit with resistors and capacitors cascaded and many capacitors in parallel was designed. It makes transmitting current oscillogram of high quality realized, and restrains the current peaks effectively.4.To make the change rate of the central loop in magnetic field meet the requirements of the SQUID slew rate, I invented a controllable slow turn-off circuit based on the property of the transmitter capacitive load and ARM processor through controlling PWM waveform cutting off the bridge-arm in delayed-time, affiliated with the technology of TVS low-voltage clamping. In this way, I extended the decline time of the transmitting current, reduced the gradient of the current falling edge.5.Through the indoor experiment, I tested the waveform and the key technological index of the transmitter. Using the SQUID transient electromagnetic transmitting and receiving system, I conducted outdoor experiment on Hengsha island in Shanghai. After the experiment, I conclude that the transmitting system is able to run stably in a long time with the output current up to 100 A. During the experiment, a current waveform of high quality was obtained. In addition, the transmitting system is able to control the current declining time, which means the loss of lock is primarily settled. |
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