The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the burden, to Department of Defense, Washington Headquarters Services, Directorate for Information 14. ABSTRACT The mechanics of uncemented soft sediments during bubble growth are not widely understood and no rheological model has found wide acceptance. We offer definitive evidence on the mode of bubble formation in the form of X-ray computed tomographic images and comparison with theory. Natural and injected bubbles in muddy cohesive sediments are shown to be highly eccentric oblate spheroids (disks) that grow either by fracturing the sediment or by reopening preexisting fractures. In contrast, bubbles in soft sandy sediment tend to be spherical, suggesting that sand acts fluidly or plastically in response to growth stresses. We also present bubble-rise results from gelatin, a mechanically similar but transparent medium, that suggest that initial rise is also accomplished ABSTRACT idization, e.g., gravity flows, during some natThe mechanics of uncemented soft sediments during bubble growth are not widely ural disturbances have suggested that such understood and no rheological model has found wide acceptance. We offer definitive ev-sediments can act fluidly or plastically in reidence on the mode of bubble formation in the form of X-ray computed tomographic sponse to stress. Past mechanical models of images and comparison with theory. Natural and injected bubbles in muddy cohesive bubbles in these sediments have visualized the sediments are shown to be highly eccentric oblate spheroids (disks) that grow either by bubbles as essentially spherical (e.g., Wheeler, fracturing the sediment or by reopening preexisting fractures. In contrast, bubbles in soft 1988; Sills et al., 1991), with the implication, sandy sediment tend to be spherical, suggesting that sand acts fluidly or plastically in intentional or not, that the surrounding mediresponse to growth stresses. We also present bubble-rise results from gelatin, a mechan-um reacts fluidly or plastically to their growth ically similar but transparent medium, that suggest that initial rise is also accomplished and rise. Scientists and engineers have develby fracture. Given that muddy sediments are elastic and yield by fracture, it becomes oped an impressive understanding of bubble much easier to explain physically related phenomena such as seafloor pockmark forma-growth in fluids, and a vast literature covers tion, animal burrowing, and gas buildup during methane hydrate melting.the topic (e.g., Clift et al., 1978; Lohse, 2003). However, we show here that muddy sediment Keywords: bubbles, mud, fracture, methane.does not respond mechanically either a...

[1] While soil evaporation studies have typically focused on pure or low salinity water evaporation, higher salinity soil conditions are becoming more prevalent. This work explores the combined effect of matrix heterogeneity and salt precipitation on evaporation from soils. Long-term evaporation processes were studied using sand columns, in which heterogeneity consisted of two layers with different grain sizes, and X-ray computed tomography (CT) scanning to quantify salt deposition within pores. For saline solutions, three new stages of evaporation were defined: SS1, SS2, and SS3. SS1 exhibits a low and gradual decrease in evaporation rate because of increasing osmotic potential. During SS2, evaporation rate falls progressively because of salt-crust formation. SS3 is characterized by a constant low evaporation rate. Even though phenomenologically similar to the well-defined classical evaporation stages for pure water, these saline stages correspond to different mechanisms. It is shown that SS2 and SS3 take place while matrix water content can still support first-stage evaporation. Salinity suppressed evaporation more strongly in homogeneous rather than in heterogeneous media. CT scans indicated preferential salt precipitation in the fine-textured regions. Heterogeneous spatial distribution of salt precipitates within the media enabled vapor transport via large pores, while small pores were clogged with precipitated salts. A mathematical model was formulated that simulates evaporation for saline solutions from homogeneous and heterogeneous soils. The model was used to differentiate and quantify the mechanisms controlling each stage of the evaporation process.Citation: Nachshon, U., N. Weisbrod, M. I. Dragila, and A. Grader (2011), Combined evaporation and salt precipitation in homogeneous and heterogeneous porous media, Water Resour. Res., 47, W03513,

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Flow‐through tests are completed on a natural fracture in novaculite at temperatures of 20°C, 80°C, 120°C, and 150°C. Measurements of fluid and dissolved mass fluxes, and concurrent X‐ray CT imaging, are used to constrain the progress of mineral dissolution and its effect on transport properties. Under constant effective stress, fracture permeability decreases monotonically with an increase in temperature. Increases in temperature cause closure of the fracture, although each increment in temperature causes a successively smaller effect. The initial differential fluid pressure‐drop across the fracture increases by two orders of magnitude through the 900 h duration of the test, consistent with a reduction of an equivalent hydraulic aperture by a factor of five. Both the magnitude and rate of aperture reduction is consistent with the dissolution of stressed asperities in contact, as confirmed by the hydraulic and mass efflux data. These observations are confirmed by CT imaging, resolved to 35 microns, and define the potentially substantial influence that benign changes in environmental conditions of stress, temperature, and chemistry may exert on transport properties.

[1] Results are reported for water flow-through experiments conducted on an artificial fracture in limestone at room temperature and under ambient confining stress of 3.5 MPa. Tests are concurrently monitored for mineral mass loss or gain and for changes in differential pressure between the inlet and outlet, throughout the 1500-hour duration of the experiment. Periodic imaging by X-ray computed tomography augments the fluid and mineral mass balance data and provides a third independent constraint on dissolution processes. The sample is sequentially circulated by water of two different compositions through the 1500-hour duration of the experiment, the first 935 hours by sampled groundwater (pH % 8), followed by 555 hours of distilled water (pH % 6). Large changes in the differential pressure are recorded throughout the experiment, for the constant flow rate of 2 cm 2 /m; these are used as a proxy for recorded changes in fracture permeability, under invariant effective stress conditions. Mass of Ca and Mg were net-removed throughout the experiment. During the initial circulation of groundwater, the differential pressure increased almost threefold and is interpreted as a net reduction in permeability as the contacting asperities across the fracture are removed and the fracture closes. With the circulation of distilled water, permeability initially reduced threefold and ultimately increased by 2 orders of magnitude as a ''wormhole'' developed in the sample. This spontaneous switch from net decrease in permeability to net increase occurred with no change in experimental conditions of flow rate or applied effective stress, and Ca was net dissolved throughout. This behavior is attributed to the evolving localization of mass removal, triggered as free-face dissolution outcompetes stress-mediated dissolution at the asperity contacts.

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[1] A medical-based X-ray CT scanner was used to monitor the diffusion of NaI into the matrix of a 20-cm-long, 5-cm diameter fractured chalk core. The core was retrieved from a core hole at a depth of 18.3 m and was artificially fractured along its axis using a Brazilian-like test. The NaI solution flowed continuously along the vertically oriented fracture and the transient lateral concentration distribution within the matrix at different cross sections along the core was monitored by two-dimensional 2-mm-thick slices through the sample and an in-plan pixel resolution of about 0.25 mm. The lateral concentration distribution within the matrix was characterized by a sharp decrease at a thin matrix layer adjacent to the fracture/matrix interface followed by diffusion-type concentration distribution elsewhere. This concentration variation suggests that a thin transition layer exists along the fracture/matrix interface where the diffusion coefficient is higher than that of the bulk matrix. The higher diffusion coefficient of the transition layer is possibly related to minifissures that develop when fractures are formed. After 6 days of tracer injection into the fracture inlet, distilled water was injected for 11 days, forming a reverse concentration gradient and back diffusion. A mathematical model that assumes diffusion within the matrix and a linear concentration variation through the transition layer from its value in the fracture to its time-dependent value at the transition layer/matrix interface was developed. Very good agreement was obtained between the predicted and measured concentrations during both the diffusion and back diffusion phases. Application of the model to a field site in the Negev desert, Israel, suggested that the rock matrix that had been subjected to 20 year of contaminant diffusion would require more than 200 years before it would stop releasing contaminants into the intersecting fractures (a parabolic process). According to these calculations, remediation efforts based on clean water injection into the fractures are not feasible.