Dawn Lavoie

Porometry and fabric of marine clay and carbonate sediments: Determinants of permeability

Abstract The porometry of a marine sediment is determined by the fabric, i.e., the shape, orientation, arrangement (spatial distribution) and associations of the solid particles. Fabric ultimately determines the permeability of the sediment by controlling the size, shape and continuity of the pore space. Electron microscopy studies have revealed complex fabrics and porometries of surficial and deeply buried sediments from various geological environments. When viewed in profile using thin sections, the pores of unconsolidated, high-porosity marine clays are often very irregular in shape and span a broad size range. In general, however, the pore profiles of these sediments can be described as having aspect ratios (length-to-width ratios) which approach 1.0. Pore profiles of consolidated, low-porosity clays are characterized by aspect ratios which approach infinity. Coefficients of permeability for marine clays typically range from 10 −5 cm/s (porosities of 70–75%) to 10 −8 cm/s (porosities of about 50%). The permeability coefficient was 10 −13 cm/s and the porosity was 27% for bentonite (a smectite) consolidated in the laboratory to a maximum load of 68.9 × 10 3 kPa ∗ . The pore space characteristics in carbonate sediments vary greatly depending on the constituent particles. Aspect ratios for pore profiles can range from 1.0 in a coccolith ooze to 2.0 in an aragonitic needle matrix consolidated uniaxially at pressures of 4.32 × 10 3 kPa. The permeability of shallow-water (3–4 m) oolitic carbonates is largely controlled by the permeability of the aragonite needle matrix. Despite the large quantity (in one case 94% by weight) of fine-sand-size ooids in these deposits, the comparatively impermeable ooids are matrix supported. This results in a sediment with relatively high porosity and medium permeability, similar to fine sands that are tightly packed and, therefore, grain supported. Permeability coefficients of these oolitic carbonate sediments range from 10 −2 to 10 −4 cm/s. Carbonate sediments recovered from intermediate water depths (970–1980 m) are composed of grains (mostly large shell fragments) embedded in either a coccolith matrix or an aragonite needle matrix. The permeability coefficients for these carbonates range from 10 −5 to 10 −6 cm/s. The sediments studied range in size from sands to clays, and the differences in porosity and permeability observed for these sediments reflect differences both in the fabrics of the fine-grained matrices and in the grain-size distributions. The data presented suggest that the sediment fabric is a function of the characteristics of the constituent particles and the physical and chemical environments of deposition and that the fabric plays a major role in determining the permeability of the deposit. Limited but revealing computerized image analyses have been carried out on scanning and transmission electron micrographs of these sediments. Initial results indicate that this approach may prove useful for quantifying fabric parameters and providing a statistical basis for fabric descriptions.

Laboratory Measurements of Acoustic Properties of Periplatform Carbonate Sediments

Geoacoustic models of the seafloor are essential components of underwater and seafloor acoustic studies. Propagation velocities for compressional and shear waves in the seafloor material are important geoacoustic parameters. Previously reported values for shear wave velocities in seafloor carbonate oozes have shown wide disparity. New laboratory measurements of compressional and shear wave geoacoustic parameters have been made as a function of effective stress. Periplatform carbonate samples from the Little Bahama Bank and Exuma Sound in the western North Atlantic Ocean were used for these measurements. Conventional pulse timing compressional wave velocities were measured. Shear wave parameter measurements were made with a new technique using duomorph sensors embedded in the seafloor sample. The resulting compressional wave velocity values are consistent with previously reported measurements for similar material (e.g. Hamilton). The shear wave dynamic property values obtained are more consistent with the lower shear wave velocity values reported, for example by Schultheiss. These lower shear wave velocities also agree with values obtained by pulse timing measurements with bender bimorph transducers in samples from the same seafloor region.

Permeability Characteristics of Continental Slope and Deep-Water Carbonates from a Microfabric Perspective

Permeability, the rate at which fluids move through a porous medium, affects the consolidation and reduction in porosity of a sediment with time and overburden pressure. Most marine clays consist of smectites and illites that are fine-grained and platey. In contrast, carbonate sediments are composed of multishaped, multisized components. In general, clay-rich sediments consolidate to lower porosities and are less permeable than carbonates at given overburden pressures. This difference is related to microfabric.

Consolidation-related fabric changes of periplatform sediments

Geotechnical properties of carbonate sediments result from several factors such as the particular constituents comprising the sediment, the mineralogy, fabric, and effective stress. Investigating the effects of increasing effective stress by mechanical consolidation using permeability, porosity, and porometry determinations reveals fabric changes that could not be determined in scanning electron micrographs. Porometry changes, determined using an image analyzer, are a function of matrix composition and grain support. Samples that were predominantly matrix-supported exhibited an increase in pore numbers per unit area and a pore size decrease resulting from consolidation. Samples composed predominantly of aragonite needle matrices displayed opposite behavior.

In situ geoacoustic systems and measurements in a variety of carbonate and siliciclastic environments

In situ geoacoustic data collected from the Dry Tortugas and Marquesas as part of the Coastal Benthic Boundary Layer Special Research Program using a new suite of in situ probes developed by the Naval Research Laboratory agree well with predicted values of shear wave velocity. When compared with data from a number of siliciclastic environments, it is evident that for a given porosity, carbonate sediments always have a higher shear wave velocity than siliciclastic sediments. This is because the effective porosity in carbonate sediments is interparticle porosity, only part of the bulk porosity but in siliciclastic sediments, the measured bulk porosity and interparticle porosity are the same.

Duomorph sensing for laboratory measurement of shear modulus

The authors developed a simple laboratory test procedure to measure the shear modulus of a sediment under controlled loading conditions. A duomorph sensor has been constructed for this purpose. Compressional wave velocity has been measured using compressional wave transducers before and after consolidation to validate the duomorph results. Initial results indicate that the concept is feasible, and continued testing is planned.<<ETX>>

The use of a towed, direct-current, electrical resistivity array for the classification of marine sediments

Model studies indicate that the DC electric resistivity technique is feasible for sediment classification and layer structuring. A prototype array was built to test the hypothesis that such a technique can be used in an underway mode in the marine environment. A 60-m, inverted array was towed both on and off the seafloor with electrode spacings appropriate for a penetration depth of 10 m below the seafloor. Three different bottom types, namely mud, gassy mud, and sand, were surveyed in the Mississippi Sound using the array. Ground truth was provided with an acoustic seafloor classification system, CTD (conductivity, temperature, and depth) casts, and numerous sediment cores. Data were analyzed using SUBVERT, an inversion routine adapted for an IBM-PC AT.<<ETX>>

In situ, laboratory, and modeled permeability of unconsolidated sands

Bulk physical sediment properties, especially permeability, are required to predict sediment geoacoustic and geotechnical behavior. Of the variables controlling permeability in surficial sediments, pore space distribution is probably the most significant. The objective is to quantify the relationship between sediment microfabric and sediment flow properties. Relatively undisturbed samples were collected by divers from the upper 25 cm of seafloor from several locations on the Florida shelf and near Bimini in the Bahamas. The undisturbed sediment microfabric was maintained by impregnating the samples using a polyester resin. Sediment microfabric was examined using petrographic and scanning electron microscopes. Representative images were scanned and analyzed using Image Tool and Image Tool Plug Ins. Using erosion‐dilation analysis, the 2D imaged pores and pore network were defined and quantified. CT images were used to determine sediment isotropy and to generate 3D structural images, which were used as inpu...

Interpretation of in situ logging p‐wave velocities in sediments over very young crust

Laboratory and downhole logging measurements on sediments in the Lau back arc basin display none of the expected trends in physical and acoustic properties as a function of depth. In situ, the sediments are underconsolidated, although in the lab the sediments can consolidate normally, as shown by measuring both compressional and shear wave velocity in a controlled laboratory triaxial setting. Both compressional and shear wave velocity, measured in the triaxial cell after consolidation to estimated in situ pressures, are significantly higher than those measured in situ. The primary reason that sediments are not consolidating in situ, and therefore, compressional and shear wave velocity are not decreasing downhole, is that water lost during the consolidation process is rapidly replaced by fluids circulating through the system convectively. The undisturbed microfabrics of these sediments were examined by scanning electron microscopy equipped with energy dispersive x‐ray spectral analysis and image analysis s...