| NMR(Nuclear Magnetic Resonance) Logging Instrument is one of representative instruments of low field pulse NMR Tool,which provide lots of eesential information in international oil and gas development. Instruments include the sensor and the interpretation of the system, Sensor and circuits directly decide the performance of the instrument. The core of paper is NMR echo signals and SNR (signal to noise ratio). Mainly expounds NMR principle, sensor development,the formation and action of nuclear magnetism instrument system,magnetic field and antenna, sensor noise matches, the extraction and experimental validation of singal.It introduces the formation of logging sensor and the merits and shortcomings of application about geomagnetic field, uniform Magnetic field, gradient Magnetic field of NMR. It also describes basic principles of NMR from the Classical Mechanics and Quantum Mechanics to get mathematical expressions of Free Induction Decay(FID) signal. SNR indicate the emphasis of Echo sensor and its noise matching design;It proposes the general framework of the NMR system modules structure and function.Sensor is one of the most important parts of NMR Instrument,which includes the magnetic body and the antenna. It calculates the magnetic around the saddle coil. It utilizes the simulation software Ansoft Maxwell to analyze and verify the relations between the depth of probe exploring and the strength of magnetic and coil. It raises the influence factors of electromagnetic field shield of NMR.It discusses the demands of sensor Noise Match. It analyzes, simulates and designs a simple circuit which is common-mode rejection ability and superior noise performance of the circuit for the characteristics of low echo SNR,special exponential decay signal and complex work environment, finally, it illustrates the worse degree of echo signal SNR for the circuit designed by the experiment of common-mode rejection and circuit bandwidth. |
