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| Titel |
Optimization of CPMG sequences to measure NMR transverse relaxation time T2 in borehole applications |
| VerfasserIn |
M. Ronczka, M. Müller-Petke |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
2193-0856
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| Digitales Dokument |
URL |
| Erschienen |
In: Geoscientific Instrumentation, Methods and Data Systems ; 1, no. 2 ; Nr. 1, no. 2 (2012-11-21), S.197-208 |
| Datensatznummer |
250000171
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| Publikation (Nr.) |
 copernicus.org/gi-1-197-2012.pdf |
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| Zusammenfassung |
Nuclear Magnetic Resonance (NMR) can provide key information such as porosity
and permeability for hydrological characterization of geological material. In
particular the NMR transverse relaxation time T2 is used to estimate
permeability since it reflects a pore-size dependent relaxation process. The
measurement sequence (CPMG) usually consists of several thousands of
electromagnetic pulses to densely record the relaxation process and to avoid
relaxation processes that are due to diffusion. These pulses are
equidistantly spaced by a time constant tE.
In NMR borehole applications the use of CPMG sequences for measuring the
transverse relaxation time T2 is limited due to requirements on energy
consumption. For measuring T2, it is state-of-the-art to conduct at
least two sequences with different echo spacings (tE) for
recording fast and slow relaxing processes that correspond to different
pore-sizes. We focus on conducting only a single CPMG sequence and reducing
the amount of energy while obtaining both slow and fast decaying components
and minimizing the influence of relaxation due to diffusion. Therefore, we
tested the usage of CPMG sequences with an increasing tE and a
decreasing number of pulses.
A synthetic study as well as laboratory measurements on samples of glass
beads and granulate material of different grain size spectra were conducted
to evaluate the effects of an increasing tE. We show that T2
distributions are broadened if the number of pulses is decreasing and the
mean grain size is increasing, which is mostly an effect of a significantly
shortened acquisition time. The shift of T2 distributions to small decay
times as a function of tE and the mean grain size distribution is
observed.
We found that it is possible to conduct CPMG sequences with an increased
tE. According to the acquisition time and increasing influence of
relaxation due to diffusion, the sequence parameters need to be chosen
carefully to avoid misinterpretations. |
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