Sediment permeability plays a key role in controlling both the flux of solute and heat through marine sediments and patterns of hydrothermal circulation within the underlying oceanic crust. Interlayered silt‐rich and clay‐rich sediments almost completely bury basement on the eastern flank of the Juan de Fuca Ridge. Laboratory measurements indicate that silt‐rich sediment permeability is 1 to 2 orders of magnitude greater than clay‐rich sediment permeability at similar depths. Numerical simulations of coupled fluid flow and heat transfer using a simplified model of a sedimented ridge flank with smooth basement topography illustrate how differences in permeability between the two sediment types can influence patterns of hydrothermal circulation and the flux of heat and solute across the seafloor. The layered structure of the model domain is inferred from reflection seismic and seafloor heat flow data. Two distinct patterns of hydrothermal circulation are obtained, depending on whether silt‐rich or clay‐rich sediments compose the sediment layer above a permeable upper basement aquifer. A clay‐rich sediment column nearly isolates circulation within the basement aquifer, resulting in closed convection. Computed seafloor heat flow profiles resemble heat flow measurements made on the eastern flank of the Juan de Fuca Ridge. Computed vertical fluid fluxes across the sediment column are small, in agreement with fluid fluxes estimated from sediment geochemical profiles. When the sediments are silt‐rich, 17% of the flow within a convection cell crosses the sediment column. Open convection where basement topography is smooth is probably rare because sediment columns overlying oceanic crust are unlikely to be entirely silt. Each pattern of convection persists over a wide range of sediment thicknesses for a given sediment permeability and basal heat flow. A transition from open to closed convection occurs when the effective permeability of the sediment column is only slightly less than that of an entirely silt‐rich column. Thus the addition of only a small amount of clay to an otherwise silt‐rich sediment column may effectively isolate the crustal aquifer from the ocean.

A three‐dimensional (3‐D) 100 MHz ground‐penetrating radar (GPR) data volume is the basis of in‐situ characterization of a fluvial reservoir analog in the Ferron Sandstone of east‐central Utah. We use the GPR reflection times to image the bounding surfaces via 3‐D velocity estimation and depth migration, and we use the 3‐D amplitude distribution to generate a geostatistical model of the dimensions, orientations, and geometries of the internal structures from the surface down to ∼12 m depth. Each sedimentological element is assigned a realistic fluid permeability distribution by kriging with the 3‐D correlation structures derived from the GPR data and which are constrained by the permeabilities measured in cores and in plugs extracted from the adjacent cliff face. The 3‐D GPR image shows that GPR facies changes can be interpreted to locate sedimentological bounding surfaces, even when the surfaces do not correspond to strong GPR reflections. The site contains two main sedimentary regimes. The upper ∼5 m contain trough cross‐bedded sandstone with average permeability of ∼40 md and maximum correlation lengths [Formula: see text]. The lower ∼7 m contain scour and fill fluvial deposits with average permeability varying from ∼30 md to ∼15 md as clay content increases, and maximum correlation lengths [Formula: see text]. These representations are suitable for input to fluid flow modeling.

As part of the 3D characterization of a fluvial reservoir analog site in the Ferron Sandstone in east-central Utah, new lab measurements of porosity, permeability, water content, and complex dielectric permittivity are collected and analyzed. Petrographic analysis of thin sections extracted from the same samples produced data on bulk, macro- and microporosity, lithology, and cementation. Thus, we have an unusually comprehensive data base for analysis. Debye models of complex dielectric permittivities are fitted using three frequency-dependent Debye relaxation mechanisms. Most ambient and dry samples are dominated by low-frequency relaxation mechanisms. The average dielectric constant and electrical conductivity at the typical ground-penetrating radar (GPR) frequency of [Formula: see text] are directly related to volumetric water content and are 3.86 and [Formula: see text] for oven-dried samples, 4.50 and [Formula: see text] for ambient saturated sam-ples, and 15.42 and [Formula: see text] for fully saturated samples. Electrical conductivity is poorly estimated from the oven-dried samples (for all clay content) because ion mobility is significantly reduced; thus, the dry conductivity is less useful for estimating petrophysical variations. Multivariate regressions with the petrophysical parameters estimate the electrical properties at [Formula: see text] and [Formula: see text] with average correlation coefficients of [Formula: see text] and [Formula: see text], respectively. Empirically derived predictions of dielectric constant as a function of water content will always provide better fits to the observed values than either generic models (such as the CRIM model) or fits to other data sets (such as the Topp formula, which was derived for soils). The Topp model consistently underestimates the dielectric constant, while the CRIM model generally overestimates it, at both 75 and [Formula: see text]. The overall regression procedures can be applied to data from other sites and potentially used as the basis of inversion of petrophysical properties from measurements of electric and dielectric properties.

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Ideally, characterization of hydrocarbon reservoirs requires information about heterogeneity at a submeter scale in three dimensions. Detailed geologic information and permeability data from surface and cliff face outcrops and boreholes in the alluvial part of the Ferron Sandstone are integrated here with three-dimensional (3-D) ground-penetrating radar (GPR) data for analysis of a near-surface sandstone reservoir analog in fluvial channel deposits. The GPR survey covers a volume with a surface area of 40 ‫ן‬ 16.5 m and a depth of 12 m. Five architectural elements are identified and described in outcrop and well cores, using a sixfold hierarchy of bounding surfaces. Internally, the lower four units consist of fine-grained, parallel-laminated sandstone, and the upper unit consists of medium-grained, trough cross-bedded sandstone. The same sedimentary architectural elements and associated bounding surfaces are distinguished in the GPR data by making use of principles developed in seismic stratigraphic analysis. To facilitate comparison of geologic features in the depth domain and radar reflectors in the time domain, the radar data are depth migrated. The GPR interpretation is carried out mainly on migrated 100 MHz data with a vertical resolution of about 0.5 m. Measures of the spatial continuity and variation of the first-and second-order bounding surfaces are obtained by computing 3-D experimental variograms for each architectural element (each radar