Study On Some Problems Of NMR Logging

Echo signal is the original signal of NMR Logging, and the main source of gettingstrata information by ground person. That the original signal is very weak is onecharacteristic of NMR Logging, and one of difficulties of measurement. From theinfluence factors of echo signal, this thesis discuss the question below: the effects of thechange of exciting pulse on echo signal; the effects of temperature on static magnetic field;researching on the matching of NMR logging tools.Based on the Bloch equation, combined with CPMG sequence, deduced the formulaof echo signal amplitude. Described the standard SVD inversion and the forward method.Through the simulation on the Matlab platform, concluding the effect of the change ofexciting pulse on the echo amplitude and T2spectrum. Proposing an approximateprocessing method.Based on the structure of NMR probe and the finite element method, using Ansoft as atool, discussing the effects of temperature on static magnetic field. Then using the relationbetween echo signal and static magnetic, concluding the approximate relationship oftemperature effect on static magnetic field.From the perspective of information theory, discussing the meaning of matchingsignificance, giving the equivalent model of NMR probe, analyzing and calculating thematching situation of NMR logging tool, getting the fact that the noise factor is low atevery frequency point, and it will get low noise factor when the testing frequency deviatesfrom the resonance frequency.According to the result, the effects of the change of exciting pulse will make the echosignal wave and make the T2spectrum translate; the approximate relationship betweentemperature and static magnetic field is proportional; under the condition of not changing parameter of NMR probe, making the testing frequency deviate from the resonancefrequency will get lower noise factor.