Improving of the Probing Signal’s Spectral Content for Devices Nuclear Magnetic Logging
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Abstract
The method of nuclear magnetic logging (NML) allows to distinguish easily enough the intervals in which there are hydrocarbons and to forecast their extraction. Moreover, the principles of measuring using this method are simple enough, despite the difficulties of processes. There were described method of improving spectral composition of probing voltage NML device in this article. The disadvantage of the method is that the resonance area covers all reservoir fluids. Therefore, search for a way to reduce the frequency spectrum and extend the range of the device of nuclear magnetic resonance is a rather urgent task. In the article is stated that within conducting researches is necessary probing signals of large capacity. In this case, it is more appropriate to use switch mode of the power converters. The system of multilevel inverter is created on the base of these converters that allows to get probing signal simply enough. A three-level inverter that generates high-frequency pulses is considered. A virtual model of the converter is constructed together with smoothing and resonant filters, which generate the probing voltage in nuclear magnetic resonance (NMR) devices, using a modified Carr-Purcell-Meiboom-Gill sequence. The result is a wide range of voltages with side harmonics, which have the significant influence on the output signal relative to the base frequency which the NMR device is tuned. The analysis of the harmonic composition of this voltage is conducted. Calculations of intermodulation distortion coefficients for the first and two side harmonics are given. To reduce the influence of side harmonics in the output signal on the quality of measurement by NMR devices, it is proposed to implement additional generators with the phase difference between the side harmonic and the initial phase will be a half of period. The inductors used are transformers that have a ratio of primary and secondary windings of 1: 1 and the secondary winding has the same value of inductance as the replaced coil that was in the model without modifications. Anti-phase side harmonic generators are connected to the primary windings. In this way additionally implemented a galvanic isolation between the generator and the main signal. As a result of simulation of such a converter is obtained an alternative spectral composition of the output signal. After calculations and comparison of the obtained coefficients, it is shown that the proposed method really improves the spectral composition of the output signal. The disadvantages of using this method of spectrum improvement are considered as well. The relative voltage improvement in the case of a 30 ohms load is calculated. The simulation results of the proposed method show an improvement in the spectral composition of the probing voltage by ~ 2.68 times. With this method, it is possible to dampen unwanted side harmonics by adjusting the appropriate frequencies and initial phases so that the generator is always running in antiphase.
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