Realization and assessment of T1 measurements with surface nuclear magnetic resonance

Abstract

The method of Surface Nuclear Magnetic Resonance (SNMR) is based on the excitation of hydrogen protons in the mobile pore water by a magnetic field oscillating with the local Larmor frequency. In practice the excitation is realized by an alternating current flowing through a transmitter loop at the surface. The excitation intensity defines the investigation depth of the method and is characterized by the pulse moment q (the product of the current I and the pulse duration τ). Increasing the pulse moment q, the NMR excitation focuses on greater depths. After the termination of the stimulating pulse the magnetic resonance field caused by the precession of the hydrogen protons (acting as small dipoles) around the axis of the geomagnetic field, is measured using the same loop as receiver [1, 2]. The signal amplitude is directly proportional to the amount of free water in the pore space. The relaxation behavior of the NMR signal (decay time constant) is dependent on the effective pore size of the material (Fig. 1). SNMR is the only geophysical method, that allows a direct characterization of aquifers by surface measurements. In SNMR usually the free induction decay (FID) of the excited hydrogen protons is measured (Fig. 2, A). This decay is described by the relaxation time T2* and is strongly affected by diffusion processes caused by static geomagnetic field inhomogeneities (Fig. 1). In well logging NMR and laboratory NMR such field dependent diffusion processes can be eliminated by the application of suitable pulse sequences (e.g. Inversion Recovery, CPMG). Thus, the measured relaxation times are defined only by the pore sizes of the material and are not affected by diffusion processes [3]. With the upgraded NUMIS Plus system it is possible to determine the longitudinal relaxation time T1 of SNMR signals that is independent of diffusion processes. Analogous to a 90° – 90° pulse sequence in the laboratory NMR, two successive NMR pulses of the same intensity q are emitted (Fig. 2, B). However, in SNMR there is no constant excitation over the full range of the investigated volume (i.e. constant tilt angle distribution of the magnetic moments), but rather an area of maximum excitation (tilt angle) within the focused depth. Therefore, for T1 measurements with SNMR only a quasi 90° - 90° pulse sequence can be realized. From the NMR response of the ground water a corresponding time constant T1 can be derived [4]. For sake of measurement progress default T1 measurements are conducted using only a single pulse sequence, i.e. two reading points. With the recent upgrade of the NUMIS Plus system of the Federal Institute for Geosciences and Resources (BGR) this technique is now also available in Germany.