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Soils are excellent reflectors of ground-penetrating radar (GPR) signals because of the ability of organic matter to hold water. In this paper, GPR profiles of an eolian sedimentary succession are combined with textural, dielectric, and moisture-retention characteristics to illustrate the influence of soil moisture on radar-wave reflection. Organic matter in this succession varies strongly, from Ͻ 0.15% for clean sand to 7% for the most prominent soil, whereas grain-size distributions are comparable. Moisture-retention curves show a complex relationship between suction potential (pF) and volumetric water content (). As a result of their uniform pore-size distribution, clean sand and weakly developed soils with Ͻ 1% organic matter experience a sudden loss of water between pF 1.5 and pF 1.8, going directly from saturated to almost dry conditions. In contrast, the most prominent soil shows a more gradual decrease in with increasing suction potential. It follows that the dielectric contrast between clean sand and this soil increases sharply above pF 1.5, reaches a maximum value at field-capacity conditions, and then decreases slowly. Synthetic GPR images for different suction potentials show that field-capacity conditions, when reflection coefficients are high, are favorable for tracing one single soil. Dry sediments are preferable when imaging widely spaced soils, whereas saturated sediments are best when imaging closely spaced soils.

531G are often faced with making quantitative predictions of the migration of multiple fl uids through the subsurface. Software packages for simulating multiphase fl uid fl ow through porous media are well developed (e.g., ECLIPSE Schlumberger reservoir simulator, HYDRUS-1D and 2D [Šimůnek et al., 1998, 1999], and STOMP [White and Oostrom, 2000]) and fi nd wide applications in both research and industry. Among the physical properties of the porous medium that are required as model inputs, the capillary pressure and unsaturated hydraulic conductivity (or relative permeability) vs. fl uid saturation functions are often considered the most critical (Christiansen, 2001). Accurate and effi cient measurements of these functions are therefore of considerable importance for ensuring accurate simulations.Th e advantages of measuring the capillary pressure and relative permeability functions on core samples using centrifuges have been recognized from as early as the beginning of the 20th century (Briggs and McLane, 1907;Gardner, 1937). Th e fact that the centrifugal acceleration is many times greater than Earth's gravitational acceleration and that it acts equally on all fl uids throughout the sample (which accelerates the adaptation to changes in boundary conditions) is seen as an attractive way to speed up multiphase fl uid fl ow experiments. Besides this advantage, the fl ow and transport processes take place under more ideal and controlled conditions than experiments conducted under normal gravity, leading to more accurate measurements that extend across a wider range of saturations (Nimmo, 1990;Nakajima and Stadler, 2006).Conventional permeameter techniques for measuring the intrinsic permeability, k, under normal gravity conditions have also been applied to a centrifugal fi eld. A clear distinction is made between experiments in which water is supplied from an internal or an external source, and if the fl ow experiment is conducted under constant-or falling-head conditions (Nimmo et al., 2002). Th e relevant analytical equations describing saturated fl ow under these diff erent combinations of experimental conditions were presented in Nimmo and Mello (1991) and Nimmo et al. (2002).Centrifuge techniques for measuring the relative permeability function can be divided into steady-and transient-state methods. In steady-state centrifuge experiments, the sample and fl uids are subject to a time-invariant centrifugal acceleration. Pressure and fl ow conditions at the inlet end face (the sample face closest to the center of rotation) are set so that steady-state fl ow conditions develop for the wetting fl uid with time. Two steady-state methods, referred to by Nimmo et al. (2002) as internal fl ow control (IFC) and unsaturated fl ow apparatus (UFA), have been used for multiphase fl ow applications. A: IFC, internal fl ow control; UFA, unsaturated fl ow apparatus. R AConduc ng drainage or imbibi on experiments in a centrifugal force fi eld has long been recognized as a valid and efficient way to determine capillary pressure-...

All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. Comparison of Average and Point Capillary Pressure-Saturation Functions Determined by Steady-State Centrifugation Soil Physics T he capillary pressure-saturation function is an important hydraulic property of variably saturated rocks and soils. Th is function is needed for simulating multiphase fl uid fl ow and chemical transport in porous media in applications such as agricultural crop production, enhanced oil recovery, subsurface C sequestration, and remediation of contaminated soils. In soil physics, the capillary pressure-saturation function is traditionally determined in the laboratory using a hanging water column (Dane and Hopmans, 2002a) or pressure plates (Dane and Hopmans, 2002b). In these methods, a series of capillary pressures, ψ, are imposed at a particular point and the corresponding volumetric water contents for the entire porous medium, 〈θ〉, are determined gravimetrically or manometrically. If the densities of the nonwetting and wetting fl uids are diff erent, ψ will vary with height within the porous medium (Dane et al., 1992; Liu and Dane, 1995a). As a result, the volumetric water content at the point where ψ is controlled, θ, can deviate signifi cantly from the measured 〈θ〉. Th us, use of the capillary pressure-average saturation function, 〈θ〉(ψ), instead of the point capillary pressure-saturation function, θ(ψ), in fl ow and transport models can produce erroneous predictions of important hydraulic properties such as the relative permeability function (Peters and Durner, 2006). Because point measurements of θ are rarely available in hanging water column and pressure plate experiments, computational procedures have been developed to extract the θ(ψ) function from the measured 〈θ〉(ψ) function. Liu and Dane