Abstract:

In order to inverse to obtain undergeound water information, determine quantity of groundwater and structure information of stratum, according to multiplicative orthogonal vector lock-in amplifier principle, SNMR(Surface Nuclear Magnetic Resonance)  signal collection system based on analog orthogonal vector lock-in amplifier has been designed. Taking advantage of CPLD(Complex Programmable Logic Device） and D/A converter, SNMR signal and reference signal multiplies to achieve signal orthogonal functions. The free induction decay FID(Free Induction Decay) signal is extracted and then the water parameters of SNMR detecting are obtained. The hardware and software of the analog orthogonal method are simple and the SNR(Signal-to-Noise Ratio) is advanced. After field tests, the SNMR FID signal collection system using the analog orthogonal is tested in the practice detecting groundwater. According to different Lamor frequency, sampling frequency can be adjusted at 250～1 000 Hz by programming.

Key words: orthogonal vector, lock-in amplifier, free induction decay(FID), surface nuclear magnetic resonance(SNMR)