Abstract
Low-field NMR technology has been widely used in many fields, among which the most representative method is the use of CPMG sequence for T2 measurement. Important parameters such as permeability, saturation and fluid type of porous media can be obtained by T2 spectrum inversion calculation of NMR echo sequence signal. However, T2 spectrum inversion calculation is easily affected by noise signals. Improving the signal-to-noise ratio (SNR) of echo signal is one of the key problems with the application of low-field NMR technology. Therefore, in order to obtain more accurate analysis results, it is necessary to develop corresponding noise reduction methods. The past researches on NMR signal noise reduction mainly focus on echo string. In this paper, a complete NMR signal model is constructed, and the problem of the echo and echo string de-noising is discussed. FIR filter and orthogonal modulation filter (OMF) are studied for noise reduction of echo. To solve the problem of echo string de-noising, an improved complementary ensemble empirical mode decomposition (CEEMD) threshold de-noising method is proposed and compared with the heuristic threshold de-noising method based on sym4 wavelet and the previous empirical mode decomposition (EMD) threshold de-noising method. Based on the construction model and practical verification, it is proved that the proposed method can invert the more accurate T2 spectrum while improving the signal-to-noise ratio. The research results of this paper provide a strong support for the application of low-field NMR technology in the environment of strong noise.
Highlights
Low-field NMR technology characterized by relaxation measurement has been widely used in petroleum exploration, biomedicine, quality control and other fields, among which the most representative method is the use of CPMG sequence [1] for T2 measurement
In order to evaluate the effect of noise reduction, it is analyzed from the following two aspects: first, the change of signal-to-noise ratio; the second is the comparison of inversion effects
Simulation results show that orthogonal modulation filter can effectively reduce echo noise in low signal-to-noise ratio (SNR) environment
Summary
Low-field NMR technology characterized by relaxation measurement has been widely used in petroleum exploration, biomedicine, quality control and other fields, among which the most representative method is the use of CPMG sequence [1] for T2 measurement. The existing NMR noise reduction methods generally adopt the different distribution of signal and noise in time domain, frequency domain or time-frequency domain to reduce noise. Due to their advantages in NMR signal processing, these noise reduction methods are taken seriously [2]–[5]. The wavelet noise reduction method, which can maintain the peak signal while noise reduction features, has become a current research hotspot.
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