NON-INVASIVELY ESTIMATING POROSITY IN A GRANITE CATCHMENT USING ROCK PHYSICS, SEISMIC REFRACTION, NUCLEAR MAGNETIC RESONANCE AND SURFACE WAVE DISPERSION

摘要

Fresh water is a critical resource that supports ecosystems, shapes landscapes and sustains human civilization. Many high mountain catchments act as sources for streams and rivers and provide water that recharges shallow groundwater aquifers. It is important to understand the hydrologic properties of high mountain catchments to improve understanding of the water cycle, predict how streams, rivers and shallow groundwater aquifers might react under a changing climate, and ultimately improve water balance budgets. This study focuses on estimating porosity in a high mountain granite catchment using noninvasive geophysical methods. The data set includes a 240 channel seismic refraction line optimized to obtain s-wave velocities by the dispersion of surface waves, a surface nuclear magnetic resonance (NMR) sounding, and two boreholes with downhoie NMR logs. We estimate porosity by providing p-wave and s-wave velocities to a Bayesian inversion based on a Hertz-Mindlin rock physics model. To validate geophysically estimated porosities in saprolite we compare them to measured porosities on 25 core samples, ranging between 0.5-9 m depth. Geophysically estimated porosities and measured porosities show good correlation and are, in most cases, within 0.1 of each other. Both measured porosities and geophysically estimated porosities indicate the saprolite is homogenous with an average porosity of 0.36 +/- 0.03. NMR data indicate a lack of water in the saprolite. The saprolite is probably the most porous unit in the watershed but it does not function as the main aquifer. We interpret the lack of storage as water passing through the saprolite to either recharge the fractured rock aquifer or leave the watershed through stream flow.