Abstract
Abstract. 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.
Highlights
The method of nuclear magnetic resonance (NMR), discovered in the forties of the last century (Bloch et al, 1946; Purcell et al, 1946), has found widespread use in scientific and daily life
Additional errors for small decay times occur if the echo spacing (τ ) is increased, which is probably caused by a bad sampling of fast decaying signals
As part of a project for developing an NMR borehole tool to characterize near surface groundwater layer in unconsolidated material, optimized CPMG sequences for measuring T2 decay time were examined
Summary
Discuss.: 19 July 2012 Revised: 9 October 2012 – Accepted: 25 October 2012 – Published: 21 November 2012
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