Development of Neutrally Buoyant Sensors for Tracing Conduits in Karst Aquifers, 20-R9707

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Principal Investigators
Ronald T. Green
Ben A. Abbott
Gordon W. Wittmeyer

Inclusive Dates:  04/02/07 – Current

Background -  Effective management of groundwater resources relies heavily on realistic and accurate models of the flow of groundwater in subsurface reservoirs. Although models for porous media aquifers are sufficiently sophisticated for meaningful simulations, model capabilities to account for the complexity of simultaneous conduit and matrix flow in limestone aquifers containing dissolution cavities and conduits (karst aquifers) are currently inadequate to permit realistic management simulations. SwRI recently developed MODFLOW-DCM with the capability to simulate both conduit flow and matrix flow in a karst aquifer. Early applications have highlighted the critical need to better locate and characterize the properties of the conduits. Conventional methods currently used to directly characterize karst conduits include tracer tests using water soluble chemical tracers and dyes, and mapping by cave divers. Even when successful, these techniques provide limited information on conduit morphology. A great need exists, therefore, to develop new tools to characterize conduits to support karst aquifer flow modeling tools.

Approach - The approach of this project is to develop and demonstrate an inexpensive, neutrally buoyant, sensor node technology for measuring velocity, pathway traveled, and conduit size based on sensors that float with water conveyed via karst conduits. The sensors would be emplaced in conduits up-gradient from spring orifices. Access to the conduits can be gained either at sinkholes that connect to conduits or wells that intersect conduits. The sensors will be instrumented to record velocity and path, and with ultra-sound sensors to measure conduit dimensions. In addition, the sensors will be instrumented with transponders that would emit a signal when they exit the spring. In this way, sensors that "daylight" at a spring other than the one expected could still be detected using satellite coverage. Currently, there is no other technology that can provide this type of detailed information.

Accomplishments - Initial research has focused on developing the first prototype field units and acquiring site access to conduct field testing. The prototype contains six pairs of ultra-sound sensors for mapping conduit morphology, accelerometers for mapping speed and relative direction, and a magnetometer for providing absolute direction. A prototype has been successfully tested under laboratory conditions. Algorithms for resolving collected data into conduit paths and geometries have been developed. Additional units are being fabricated for use in outdoor field testing. Field deployment will be performed in Honey Creek Cave just north of San Antonio near Guadalupe River State Park. Honey Creek Cave is a wet cave with a known path and morphology.

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