Open AccessThe NMR Relaxation Response of Unconsolidated Sediments during Drainage and Imbibition
Author(s) -
Falzone Samuel,
Keating Kristina
Publication year - 2016
Publication title -
vadose zone journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.036
H-Index - 81
ISSN - 1539-1663
DOI - 10.2136/vzj2015.11.0153
Subject(s) - imbibition , drainage , hysteresis , soil science , relaxation (psychology) , geology , soil water , porosity , geotechnical engineering , mineralogy , materials science , physics , germination , condensed matter physics , psychology , ecology , social psychology , botany , biology
Core Ideas NMR relaxation rates are a function of water content but do not exhibit hysteresis. NMR sum of echoes does not exhibit hysteresis with imbibition and drainage. The results are consistent across the synthetic sands and natural soils investigated. Results suggest NMR can characterize the WRC but not distinguish drainage from imbibition. In this laboratory study, nuclear magnetic resonance (NMR) relaxation data were collected on unconsolidated sediment to determine the NMR response, characterized by the mean‐log transverse relaxation rate, T 2ML −1 , and the sum of echoes, SOE, during drainage and imbibition. Measurements were made on four synthetic sands, with a range of grain sizes and Fe content, and two natural loamy sand soils. A porous ceramic plate apparatus was used to induce drainage and imbibition. Water content (θ) was plotted vs. matric potential (ψ) to give the water retention curve (WRC). The drainage and imbibition branches of the WRC were then compared with the corresponding branches of the θ– T 2 ML −1 and θ–SOE curves. We observed the expected linear trend between NMR signal magnitude and θ. The θ– T 2ML −1 or θ–SOE curves did not exhibit drainage–imbibition hysteresis, even though T 2ML −1 and SOE varied with θ and the WRC did exhibit hysteresis. Using a simple pore network model, we show that, for well‐connected networks, the surface‐area/volume ratio of the water‐occupied porosity is similar during drainage and imbibition, explaining the lack of hysteresis present in the measured θ– T 2ML −1 or θ–SOE curves. For materials with poorly connected pore networks, hysteresis may be observed. We conclude that, for the materials used in this study, it is not possible to distinguish drainage from imbibition using T 2ML −1 or SOE. However, because T 2ML −1 and SOE depend on θ, NMR data may be useful for characterizing a single branch or an average of the two branches of the WRC by relating these NMR parameters to ψ.