A study to apply nuclear magnetic resonance porosity measurements to seabed sediments

Abstract

The method of Nuclear-Magnetic-Resonance (NMR) Quantitative-Relaxation-Tomography (QRT) of 1H nuclei is proposed for cores of ocean bottom sediments in order to determine porosity as functions of position within a core without disturbing the material. This information, with cm or mm resolution, is relevant to the understanding of acoustic properties of ocean bottom sediments, which affect sonar and echo sounding. The feasibility of the method is shown by QRT on samples up to 8 cm in diameter of glass beads ranging from 0.3 to 16 mm in diameter and also of small 2 mm plastic cylinders. All samples were fully saturated with water. Three kinds of maps are produced for each section, with 0.73 × 0.73 mm pixel size and 5 mm slice thickness: Proton Density map, and two relaxation times ( T 1 and T 2) maps, where T 1 and T 2 are the time constants for the return to the equilibrium of the longitudinal and transverse components of the nuclear magnetization, respectively. For each voxel the relaxation data are fitted to single exponential functions, giving values of T 1, T 2 and also of extrapolated signal, giving voxel porosity. Since the tomograph has a minimum echo time of 10 ms, any part of the signal having T 2 < 10 ms is not seen. In the systems tested, the NMR porosity Φ NMR is, for this reason, approximately 3 percentage porosity units less than the porosity Φ measured destructively by wet and dry weight differences.