[1] The simplicity of a pan-and-bag seepage meter makes it an attractive tool for a variety of hydrologic investigations, but the performance of this device is far from simple. Laboratory experiments show that the hydraulic head required to fill collection bags can range from 0.5 mm to several millimeters at flow rates typical of seepage meters, and this additional head will divert water away from the pan. A theoretical analysis gives a semianalytical expression for the flux captured by a seepage meter using a collection bag. The analysis shows that bag conductance, radius of the pan, and hydraulic conductivity of the stream bed can be combined to give a dimensionless term that characterizes seepage meter performance. Field experiments in a sandy streambed show that conventional seepage meters can resolve spatial differences in ground water flux but only when using a highly conductive collection system that is isolated from the current.

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SUMMARYThe response of deformable fractures to changes in fluid pressure controls phenomena ranging from the flow of fluids near wells to the propagation of hydraulic fractures. We developed an analysis designed to simulate fluid flows in the vicinity of asperity-supported fractures at rest, or fully open fractures that might be propagating. Transitions between at-rest and propagating fractures can also be simulated. This is accomplished by defining contact aperture as the aperture when asperities on a closing fracture first make contact. Locations on a fracture where the aperture is less than the contact aperture are loaded by both fluid pressure and effective stress, whereas locations where the aperture exceeds the contact aperture are loaded only by fluid pressure. Fluid pressure and effective stress on the fracture are determined as functions of time by solving equations of continuity in the fracture and matrix, and by matching the global displacements of the fracture walls to the local deformation of asperities.The resulting analysis is implemented in a numerical code that can simulate well tests or hydraulic fracturing operations. Aperture changes during hydraulic well tests can be measured in the field, and the results predicted using this analysis are similar to field observations. The hydraulic fracturing process can be simulated from the inflation of a pre-existing crack, to the propagation of a fracture, and the closure of the fracture to rest on asperities or proppant. Two-dimensional, multi-phase fluid flow in the matrix is included to provide details that are obscured by simplifications of the leakoff process (Carter-type assumptions) used in many hydraulic fracture models. Execution times are relatively short, so it is practical to implement this code with parameter estimation algorithms to facilitate interpretation of field data.

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