An improved pulse sequence and inversion algorithm of spectrum

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• 作者：Xinmin Ge^a ; ^b ; ^{gexinmin2002@163.com} ; Hua Chen^c ; Yiren Fan^a ; ^b ; Juntao Liu^a ; ^b ; Jianchao Cai^d ; Jianyu Liu^a ; ^b
• 关键词：Nuclear magnetic resonance ; Multiple relaxation components ; Improved pulse sequence ; Inversion algorithm
• 刊名：Computer Physics Communications
• 出版年：2017
• 出版时间：March 2017
• 年：2017
• 卷：212
• 期：Complete
• 页码：82-89
• 全文大小：2309 K
• 卷排序：212

文摘

The nuclear magnetic resonance transversal relaxation time is widely applied in geological prospecting, both in laboratory and downhole environments. However, current methods used for data acquisition and inversion should be reformed to characterize geological samples with complicated relaxation components and pore size distributions, such as samples of tight oil, gas shale, and carbonate. We present an improved pulse sequence to collect transversal relaxation signals based on the CPMG (Carr, Purcell, Meiboom, and Gill) pulse sequence. The echo spacing is not constant but varies in different windows, depending on prior knowledge or customer requirements. We use the entropy based truncated singular value decomposition (TSVD) to compress the ill-posed matrix and discard small singular values which cause the inversion instability. A hybrid algorithm combining the iterative TSVD and a simultaneous iterative reconstruction technique is implemented to reach the global convergence and stability of the inversion. Numerical simulations indicate that the improved pulse sequence leads to the same result as CPMG, but with lower echo numbers and computational time. The proposed method is a promising technique for geophysical prospecting and other related fields in future.