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In Situ Permeable Flow

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AR #1133
AR #1252

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Tech ID: 99
Project Overview
The In Situ Permeable Flow Sensor uses a thin cylinder heater buried in the ground to directly measure the direction and magnitude of 3-D groundwater flow in porous aquifers. Temperature distribution on the cylinder surface varies as a function of groundwater flow magnitude and direction. Previous technologies were labor intensive and required that large volumes of contaminated water be pumped to the surface for storage and disposal.

Technology Description
 Groundwater and soil gas flow sensors are a new technology for measuring directly the full 3-dimensional fluid flow velocity vector at essentially a single point in porous media. Each probe consists of a rod approximately 30 inches long by 2 inches in diameter, fabricated of low thermal conductivity polyurethane foam. Deployed on the surface of the rod are a thin-film, flex circuit style heater and an array of 30 temperature sensors (thermistors). The probe is buried in the ground at the point where the flow is to be monitored. When the heater is activated, a temporally and spatially uniform heat flux from the probe is established. In the absence of any flow past the probe, the temperature distribution observed on the surface of the probe is independent of azimuthal position of the probe and symmetric about the vertical midpoint of the probe. If there is significant groundwater flow past the instrument, then the temperature distribution on the surface of the tool is perturbed as some of the heat emanating from the probe is advected around the tool by the moving fluid. The downstream side of the probe will be relatively warm compared to the upstream side. The direction and magnitude of the full 3-dimensional flow velocity vector can be deduced from the measured temperature distribution on the surface of the probe. In water-saturated sediments the probes are capable of accurately measuring groundwater flow velocities in the range of approximately 5 × 10-6 to 5 × 10-3 cm/s. Because the heat capacity of a given volume of air is much less than that of the same volume of water, the probes can measure air flow velocity in dry sediments in the range of 1 × 10-3 to 1 cm/s. Changes in flow about one order of magnitude smaller than this can be resolved. A critical aspect of obtaining reliable data from the flow sensors is the method of deployment. In order to avoid negative impacts on the flow velocity caused by the presence of a borehole, well screen, and gravel pack, the flow sensors must be buried directly in the ground, in intimate contact with the formation. This limits the range of applicability of the technology to sites where the sediments are unconsolidated. The probe is installed in a borehole at the desired monitoring location. The borehole can either be backfilled with appropriate media, or soil can be allowed to collapse around the probe. Although this deployment strategy means that the relatively inexpensive probes cannot be recovered once deployed, they can be monitored remotely on a continuous basis for long periods of time (months to years).

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