Research On Techniques Of Quadrature Detection Used In Controlled Source Electromagnetic Detection

The controlled source electromagnetic detection method uses the artificial excitation source to obtain the electromagnetic information of the underground medium and realize the detection of underground medium structurewith the electric field and magnetic field sensors.With the continuous development of modern science and technology, and the increase of human social activities, it has been more and more widely used in the water, geothermal resources, mineral resources exploration, the monitoring and management of engineering and underground pipeline route determination, and other fields.When working in the field, the transmitter produces square waves as the excitation source. The number of the receivers ranges from several units to a dozen working at the same time, and the distance between the receivers is tens of meters. In order to ensure the effectiveness and accuracy of field work, it needs to monitor and evaluate the large amount of time domain data collected by the controlled source electromagnetic detection system. Generally, the adopted monitoring schemes are:(1) Each receiver is equipped with a computer to conduct frequency-domain change;(2) Upload the large amount of time domain data to the upper monitor computer through the controller by cable or wireless method. Obviously, the above two methods have shortcomings of high costs, great power consumption and high ber of wireless transmission, etc..Based on a kind of quadrature detection technology widely used in signal detection field and the deficiency of the extraction of amplitude and phase of the controlled source electromagnetic detection using the traditional quadrature detection technology, an improved quadrature detection technology has been proposed, which is more suitable for the real-time monitoring and evaluation of large amount of time domain data with square-wave excitation source. Due to the implementation of the proposed approach using the FPGA and MCU as the master controller to realize the accurate operation, it can meet the demand of low cost, low power consumption, real-time, accuracy, portability and other requirements for the monitoring of real-time data. As to provide hardware platform and experimental basis for the controlled source electromagnetic detection with square-wave excitation source and its related work, the work content of this paper is as follows:1) It analyzed and introduced the basic principle of controlled source electromagnetic detection. Combined with the actual field work situation and the characteristic, it expounded the research background and significance of the topic by analyzing the characteristics and insufficiency of the current monitoring and assessment schemes.2) Based on the analysis of the traditional principle of quadrature detection and the characteristics of the controlled source electromagnetic detection using the square-wave excitation source, the implementation process and the test results of the traditional sinusoidal-wave and square-wave referenced quadrature detection have been derived and analyzed. Combined with the advantages and disadvantages of the traditional quadrature detection technology, referenced signalswith specificfrequencies have been added. An improved quadrature detection technology has been proposed in this paper, which adapts better to the square-wave excitation source. It reduces the hardware requirements, and also improves the precision of the final results at the same time.3) It designed the whole detection scheme of quadrature detection of the controlled source electromagnetic detection. According to the implementation process of the improved algorithm, the matching low-pass filter circuitis have been designed to filter the redundant high frequency harmonic components after the main harmonic components have been filtered by the reasonable algorithm. The earth medium influence with high-pass properties is discussed by adding the matching high-pass filter(HPF).4) Based on the improved quadrature detection principle, it adopted low sampling rate method and sampling integral technology. In order to lower the AD chip index requirements and avoid the spectrum leakage, it selected reasonable sampling points, meanwhile, it improved the precision of the sampling system.5) It selected the 16-bit parallelled AD676 modulus conversion chip to conduct sampling and data conversion. Use FPGA and MCU MSP430 as the master controller of the acquisition system. It set up square wave output, AD control, quadrature detection control, complementation and quadrature detection operation modules in FPGA to realize the improved quadrature detection technology. In MSP430, it achieved a good human-computer interaction by programming and adding an external LCD, keyboard and other equipments. At the same time, in order to guarantee the high precision requirements in the field experiment, the GPS clock source cooperated with the high-precision temperature crystal oscillator as the system coordination clock.6) Based on the hardware platform, it conducted the improved quadrature detection experiment to extract the acquisition signals’ amplitude and phase information of the controlled source electromagnetic detection. With comparison and analysis of the measured results, it showed that the proposed algorithm had higher detection accuracy, and the extracting information is relatively accurate. It verified the feasibility and reliability of the proposed algorithm, and it has laid a solid foundation for more real-time and more quickly to monitor and evaluate the controlled source electromagnetic detection data.The experimental results indicated that the improved quadrature detection technology is feasible. The amplitude error is 0.01% ~ 0.72% compared with the sinusoidal-wave referenced quadrature detection, and the phase error is within 4?. The detection precision improves at least one order of magnitude in comparison with the square-wave referenced quadrature detection algorithm. The improved quadrature detection algorithm is multiplication-free, in high real-time property, and of low hardware requirements. Meanwhile, it can restrain the earth medium influence with high-pass properties. It has a good application prospect in the controlled source electromagnetic detection.