Development Of Transient Electro-magnetic Data Acquisition System Based On DSP

The Transient Electromagnetic data acquisition system based on DSP( Digital Signal Processor ) was designed in this thesis. Transient Electromagnetic methods (TEM) can be called as Time Domain Electromagnetic methods. The measuring equipments of TEM include Two units: transmitter and receiver. Emission current falls down from I to 0 suddenly when step current passes through the transmitter loop. According to the electromagnetic induction theory, abrupt variation of current in the transmitter loop will induce magnetic field around it ,which is called the first field. In the transmission of the first field, when it meets good conductor underground, induce current will be created, which is called erratic current of second current. Because the second current changes with time, so it will create new magnetic field, which is called the second magnetic field. Because of the thermal wastage in the good conductor underground, the second magnetic field fades down as exponential rule approximately, which creates transient magnetic field. The second magnetic field is sourcing from the induced current of good conductor, so it includes geology information relating to the geologic body. Observing the second magnetic field through receiver loop, and analyzing and processing the data, we can explain the correlative physical parameter with the geologic body. With the development of digital and intellectual Transient Electromagnetic instruments , the use of forward computation and inversion of digital model computer, the improvement of explanation level, TEM can resolve a large range of geology problems which almost includes every field of geophysical exploration: ore exploration, constitution exploration, hydrogeology and engineering geology investigation, environment survey supervising and archaeology and so on. In recent years, TEM has made better outcomes in searching water, city government engineer, salt and alkali soil, pollution survey and shallow layer petroleum constitution graph and so on. The Transient Electromagnetic data acquisition system designed in this thesis utilized DSP(Digital Signal Processing) chip as CPU. CPLD(Complex Programmable Logic Device)was designed as decoder to realize floating-point amplification by controlling the programmable amplifier, which made the data acquisition system has a large dynamic range of 144 dB. The system with equivalent 24-bit resolution has the highest sampling rate of 200Ksps. At the present time in the home area, the TEM receiver system often uses microchip ,PC104 or DSP as CPU。But they all have limitations. Although microchip is cheap, it's speed is slow, the memory capacity is limited and it's interface is bad. PC104 has perfect function and abundant interface, it also has big memory capacity, while the noise of PC104 is very serious, the signal of late stage in the noise is hard to extract, so losing part of usable information. And the power consumption of PC104 is very heavy, it is inconvenient to long-duration working in the field. The DSP chip integrates real time processing and control abilities, with low power consumption, low noise, smart serviceability and high-speed operational capability, it supplies ideal solving project in control field . This system uses the TMS320LF2407A as CPU, which is manufactured by TI. It has low power consumption, good capability of anti-interference, low cost and small capacity, and the TMS320LF2407A can control data acquisition in real-time and proceed immediate processing to the complicated algorithm of after-data. In modern measure instruments, most have data acquisition system. Usually the conversion precision of the data acquisition system lies on the word length of A/D, in other words , the measure precision of instrument depends on the quantization word length of the A/D converter. Sometimes, we use the signal dynamic range to measure the system's standard, which is the ratio of the breadth value of the maximal input signal allowed and theminimal signal identified of the allowed input signal. In many occasions, it is hard to meet the need of the dynamic range to use A/D converter with broad word length. The parameter of TEM data acquisition system mainly depends on the dynamic range and frequency range of the measured signal. The wave received by TEM receiver is in the beginning period of transmitter cut-off. It is the transient electromagnetic signal. The second field (TEM signal) with large dynamic range quickly decreases in the beginning period and has a large breadth; in middle and late period, the TEM signal slowly decreases and has a small breadth. In order to collect the TEM signal in the beginning period of transmitter cut-off exactly and identify the weak signal in middle and late period effectively, the sample time of the system must reach to several us and the resolving power of the system must reach to several uV . In the Transient Electromagnetic data acquisition system based on DSP, the dynamic range of the TEM signal is greater than 138dB. That is to require the expensive 24bits A/D, but the speed cannot be satisfied. So, we applies the floating point amplification technique in the data acquisition system, which attains higher signal dynamic range in lower cost. The floating point amplification technique is to magnify the signal with appropriate value firstly, then to convert the signal with A/D. We can get the original signal by data processing. The system includes two main sections, namely the front acquisition—fast speed 8bits A/D and the rear acquisition—16bits A/D. The front A/D samples the signal magnified by anti-saturation circuit and the CPLD decoding circuit compares the data to provide appropriate value magnifying the analog signal, then 16bits A/D convert the signal. So the result consists of exponent-code (the output of decode circuit) and tail-code (the output of 16bits A/D). DSP reverts the signal by processing the exponent-code and tail-code. In order to sample with no distortion, subsection magnification technique is used in anti-saturation circuit. It is to magnify the signal in early period with 1 time...