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Abstract:
Most dissolutional conduits develop along fissures (bedding planes, joints or faults) in which the initial opening varies considerably from place to place, i.e. they are anisotropic. The propagation of conduits between input and output points in an anisotropic fissure was studied systematically with three types of hardware models: (1) electrical current analogs and (2) sandbox analogs, to investigate flowfield geometry; (3) artificial fissures cast in Plaster of Paris, with random anisotropic elements on their surfaces. A first series of experiments studied the case of a single input to the fissure. The earliest proto-conduits were observed to extend in radial (Darcian) array but one principal and several secondary tubes quickly extended down the hydraulic gradient, robbing competitors of their flow. This mode of propagation was tested with a resistor network computer model using 100 by 100 nodes, Weyl's (1958) equation for dissolution and transport with parameters appropriate for plaster, and three choices of opening - isotropic, anisotropic but homogeneous, heterogeneous. The computer model results closely matched those of the hardware models. In second and third series of plaster hardware experiments, the cases of (1) multiple inputs to the fissure in one rank, and (2) multiple inputs in multiple ranks, were explored. In the competition between inputs, some principal tubes in near ranks first breakthrough to the output boundary. This re-orients the flowfields of failed nearby competitors, which then extend to join the principal via their closest secondaries. The process extends outwards and to the rear, linking up all inputs in a cascading system.