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The application of nuclear magnetic resonance imaging (NMRI) to the direct three-dimensional measurement of flow in rough-walled water-saturated rock fractures is presented for the first time. The study demonstrates the abilities of NMRI to noninvasively measure rock-water interfaces and water flow velocities in these fractures and investigates the effects of wall morphology on flow patterns inside a typical rock fracture. Two- and three-dimensional flow-encoded spin-echo pulse sequences were applied. The stability and reproducibility of the water flow patterns were confirmed by analyzing two-dimensional velocity images. A variety of geometrical and hydraulic features were determined from three-dimensional velocity images, including the rock-water interfaces, the fracture aperture distribution, and the critical aperture path; velocity profiles and volumetric flow rates; flow and stagnant regions; and the critical velocity path. In particular, the effects of a sharp step discontinuity of the fracture walls and the applicability of the cubic law were examined. As a result of the complex three-dimensional geometry, velocity profiles are generally parabolic but often highly asymmetric, with respect to the fracture walls. These asymmetric velocity profiles are clustered together, with significant correlations; they are not just local random phenomena. However, theoretical considerations indicate that the effects of the measured asymmetry on volumetric flow rates and hydraulic conductivities are insignificant, in that the overall flow inside rough fractures still obeys the cubic law. The features discussed in this study emphasize the strong heterogeneity and the highly three-dimensional nature of the flow patterns in natural rock fractures and consequently the need for three-dimensional flow analysis.
Karstic carbonate aquifers are extremely heterogeneous with a distribution of permeability that spans many orders of magnitude. They often contain open conduit flow paths with hydraulic characteristics more like surface streams than ground water. Karstic carbonate aquifers have highly efficient interfaces with surface water through swallets and springs. Characterizing parameters include: area of ground-water basin, area of allogenic recharge basins, conduit carrying capacity, matrix hydraulic conductivity, fracture hydraulic conductivity, conduit system response time, and conduit/fracture coupling coefficients. The geologic setting provides boundary conditions that allow the generalized conceptual model to be applied to specific aquifers.
Two recent papers have proposed radically different modes of origin for cave folia. Audra et al. (2009) propose subaqueous origin of carbonate folia via hypogenic CO2 bubble trapping, with concurrent condensation-corrosion and evaporative precipitation within individual folia gas pockets. Queen (2009) proposes that at least some folia are analogous to suboceanic tufa-tower “flanges” and may result from subaqueous freshwater mixing into a briny environment. The purpose of this paper is to show that neither of these mechanisms can be the fundamental process responsible for folia morphology in cave deposits, and that accretion from adherent particles at fluctuating interfaces is the only mechanism that has been shown to apply to folia of all compositions and in all cave environments where they are known to occur.
Recognition of the wide occurrence, significance, and specific characteristics of hypogene speleogenesis during last twodecades signifies a major paradigm shift in karst science, previously overwhelmingly dominated by epigene concepts and models. Hypogene karst is one of the fundamental categories of karst, at least of equal importance with more familiar epigenic karst. Hypogene and epigenic karst systems are regularly associated with different types, patterns, and segments off low systems, which are characterized by distinct hydrokinetic, chemical, and thermal conditions. Hypogene speleogenesis is the formation of solution-enlarged permeability structures by water that recharges thecavernous zone from below, independent of recharge from the overlying or immediately adjacent surface. It develops mainly in leaky confined conditions, although it may continue through unconfined ones. Hydraulic communication along cross-formational flow paths, across lithological boundaries, different porosity systems, and flow regimes allows deeper ground waters in regional or intermediate flow systems to interact with shallower and more local systems, permittinga variety of dissolution mechanisms to operate. A specific hydrogeologic mechanism acting in hypogenic transverse speleogenesis (restricted input/output) suppresses the positive flow-dissolution feedback and speleogenetic competition seen in the epigenic development. Hypogenic caves occur in different soluble rocks in a wide range of geological and tectonic settings, basinal throughorogenic. Overall patterns of cave systems are strongly guided by the spatial distribution of the initial (prespeleogenetic) permeability features and hydrostratigraphic barriers and interfaces within the soluble and adjacent units, by the mode of water input to, and output from, cave-forming zones and by the overall recharge–discharge configuration in the multiple aquifer system. Because of their transverse nature, hypogene caves have a clustered distribution in plan view, althoughinitial clusters may merge laterally across considerable areas. Hypogene caves display remarkable similarity in their pattern sand mesomorphology, strongly suggesting that the type of flow system is the primary control. The rapidly evolving understanding of hypogene speleogenesis has broad implications for many applied fields such asprospecting and characterization of hydrocarbon reservoirs, groundwater management, geological engineering, and mineral resources industries