Design Of Array Mrs Receiver For Groundwater Detection

Groundwater is the important water supply resource in our whole country. Particularly in the northern part, water of life and industrial is ninety percent of groundwater. The groundwater in the northern is mostly layered water, which can be detected directly and effectively by the existing one-dimensional MRS (Magnetic Resonance Sounding) instrument. However in the southwest area, due to karst water is difficult to detect, the exploitation level of groundwater is low. This paper introduces design of array MRS receivers for detecting which can do two-dimensional and three-dimensional groundwater detection by several signals collecting at the same time with the original MRS water detector transmitter.MRS method is depend on nuclear magnetic resonance of water proton itself so that we can transmit a Larmor frequency current to excitation magnetic which makes the proton go into the higher energy level. Then cut off the transmit current, the hydrogen protons return to its original state, in the process will produce a weak macro alternating magnetic field. The electromotive force generated in loops is called MRS signal. It is a sinusoidal signal with e exponential decay, if we got its initial amplitudes, relaxation time and initial phase then groundwater information would clear by the inversion calculation.This paper designed the array MRS receiver instrument in the condition of existing transmitter. The communication circuit is the typical serial communication Standard form RS-485bus structure, the computer is host and the receiving units are slaves. The synchronization signal is generated by the COMS uPA1901and the Opto-isolation device6N137, which can reduce the mutual interference between the receiving units, also can extend receiving unit conveniently.The weak signal extraction circuit is depended on the narrowband spectrum of the MRS signal. The Receiving circuit is composed of the coil and the harmonic capacitor. Then the signal goes through preamp part which can effectively amplify the signal and not introduce noise. The broadband filter will filter out the50Hz frequency interference with a fixed gain. The narrowband filter will filter the signal in the frequency range with the adjusted bandwidth and gain. The programmable gain amplifier can adjust the gain. Therefore the total gain range is from80.6dB to130.1dB and the bandwidth is from110Hz to60Hz.The signal acquisition board use FPGA+MCU architecture to control the AD7656doing signal acquisition. Because of the work frequency of FPGA is high so we can achieve the sampling frequency to568KSPS. This paper use the4N times sampling algorithm to extract the signal which is implemented in FPGA. The Computer sends the acquisition parameters to MCU, then MCU process the parameters and send them to the FPGA, FPGA do fast sample and arithmetic operation based on the parameters. The calculated data is stored in the FIFO waiting for read by the host computer. The host computer sequentially read and save the data, also calculated the envelope card and graphical display.The receiver designed in this paper had already did some indoor test, the background noise and input short-circuit noise met the design requirements. We have a good result during the indoor coil simulation experiment too. We also do test in the field where groundwater distribution is known and have confirmed the availability and stability of the instrument.