The Development Of NMR Experimental Prototype For Groundwater Investigation

Nuclear magnetic resonance (NMR) is the only geophysical method currently used to direct investigation of groundwater. NMR technology is cutting-edge technology in today's world. It is the new domains that using the Nuclear Magnetic Resonance technology to investigate groundwater. This paper's aim is to study of the overall plan and key technology of Nuclear Magnetic Resonance experimental prototype for groundwater investigation.First, this paper expounds the principle of NMR technology for investigating groundwater and how the instrument works, anatomizes the development of NMR instrument for groundwater investigation in domestic and aboard, and finally gives a design of the Nuclear Magnetic Resonance experimental prototype for groundwater investigation.The instrument includes two parts: transmitter system and receiver system. In order to make the H proton of groundwater bring the NMR phenomenon, the high power of transmitter system with Lamor frequency excitation pulses is provided for the loop. The alternating current generates alternating magnetic field. Receiver system is used for receiving NMR signals from induction coil, amplification, and filtering.This paper is based on the principle of resonant, high-power, high-precision transmit mode. Transmitter power source uses the DC_DC to carry on the charge for the 0.1F capacity, and then the capacity carries on the transmitter as the transmit circuit power supply. DDS is used to generate the local Larmor frequency, and then the whole-controlled H-bridge circuit is drove, which is formed by power IPM (PM400DSA060), to generate power Larmor frequency square-wave signal. Its output-voltage reaches up to 600V and output-current reaches to 400A, and then realize high-power transmit by the interaction of resonance coil and the resonant capacitor. A high voltage vacuum relay is employed to realize transmit fast shutting off the receive cut and the isolation. When the instrument transmits, vacuum relay disconnect receiver circuit. After the instrument stop transmitting, waiting for around 30 ms, and then connecting relay to the receiver circuit, receiver circuit will work.Because of the weakness of the NMR signal, the ratio of signal to noise is very low, this impose very high request to the NMR amplifier. In this paper Low-Noise amplifier MAX410 is used as the input circuit, and the input impedance matching and election-frequency amplifier, high-performance 8-order band-pass filter latter, reasonable PCB board layout and shielding boxes are used to further reduce the technology amplifier noise. On the other hand, the way of superposition of many times is employed to grow SNR, by diminishing noise, making receiver met our requirements.By indoor and outdoor experiments during a period over one year, accompanied by exploration discussion and improving equipment, the feasibility of this plan is proved at last, which meets our standard.