Magnetic Resonance Velocimetry for porous media: sources and reduction of measurement errors for improved accuracy

Abstract

Nuclear Magnetic Resonance (NMR) is a powerful tool for noninvasively investigating static and dynamic properties of fluids inside complex and opaque structures. Thus, Magnetic Resonance Velocimetry (MRV) allows for the in situ analysis of the local flow velocity of fluids. Such an analysis characterises mass transport properties and helps to validate or improve numerical predictions for porous media. The benefit of such validations is lowered by several problems worsening the measurement accuracy. This publication addresses systematic errors and the influence of noise, which may reduce the accuracy of MRV measurements. Techniques are described to minimise or correct displacement and phase errors. A new multi-echo MRV-sequence is proposed as a compromise between Velocity-to-Noise Ratio (VNR), total measurement time, spatial resolution and displacement errors. For improved VNR, a dual-VENC encoding scheme was used and repetition time T_{text{R}} was optimised. As an example, the proposed MRV sequence was used to measure the three-directional flow velocity of water in an Open Cell Foam (OCF) with 10 pores per inch with three-dimensional isotropic spatial resolution.