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Monitoring the Maximum Depth of Drainage in Response to Pumping Using Borehole Ground Penetrating Radar

L.A. Ferré, Dept. of Hydrology and Water Resources, University of Arizona, 1133 E. North Campus Drive, P.O. Box 210011, Tucson, AZ 85721-0011

View larger version (39K): [in a new window]   Fig. 1. Schematic diagram of borehole ground penetrating radar used in zero offset profiling mode. Travel paths between the transmitter (Tx) and receiver (Rx) of direct and critically refracted waves are shown.

View larger version (47K): [in a new window]   Fig. 2. The field experimental layout including three pumping wells (PW), four borehole ground penetrating radar (BGPR) access tubes (small circles), and a nearby shallow piezometer (PZ). The two BGPR access tubes used in this study are denoted by the filled circles.

View larger version (20K): [in a new window]   Fig. 3. The average travel time offset between measurements made between 4.4 and 5.4 m bgs at any given elapsed time compared with that measured before pumping began.

View larger version (24K): [in a new window]   Fig. 4. Three travel time profiles measured with 100-MHz borehole antennae in zero offset profiling mode on two different dates. Measurement depths are referenced to the middle of the antennae. The depth of the water table was 2.06 m bgs on 2 Sept. 2001 and 1.99 m on 8 Aug. 2002. The length of a 100-MHz antenna is shown for scale.

View larger version (33K): [in a new window]   Fig. 5. Water content profiles determined from travel times measured with a 100-MHz borehole antennae in zero offset profiling mode before and 0.07, 0.14, and 3.10 d after the beginning of operation of a pumping well.

View larger version (22K): [in a new window]   Fig. 6. Water table depth and maximum depth of drainage of at least 0.02 cm3 cm-3 based on borehole ground penetrating radar (BGPR) water content profiles collected during pumping and recovery. Borehole ground penetrating radar measurements are referenced to the middle of the antennae; BGPR results are shown with and without a upward shift of 0.50 m.

View larger version (38K): [in a new window]   Fig. 7. Stippled blue boxes show zones of full saturation. Dashed blue lines show the top of the capillary fringe. (A) Vertical hydraulic gradients cause differences between the water table elevation and the water level measured in a piezometer. (B) The maximum depth of water content change in a static profile lies above the water table at the top of the capillary fringe. (C) The first-arriving energy for antennae placed immediately below the water table may be associated with critical refractions that occur above the capillary fringe. (D) A typical water content change as a function of elevation is shown in response to a lowering of the water table. The maximum depth of drainage is the top of the capillary fringe. If the center of the antennae (dashed black line) is centered below this depth, a water content change may be measured because drainage occurs over part of the sampled depth interval.