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2121 Geoscience Dr., Austin, TX 78726, USA
phone: +1(512)335-3338, fax: +1(512)258-9958
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AGI STING/Swift case history 1
Amistad Dam site
Summary:
At the Amistad Dam site an investigation was made with a 56 electrode STING/Swift system to determine the location of leaks which have effectively drained the giant dam of water. As it was not immediately obvious which array type would return the best data three different arrays were recorded on the same electrode layout. The resulting three 2D profiles show both similarities and differences. The conclusion is that dipole-dipole and Schlumberger are good choices for lateral structures whereas Wenner gives high resolution near the surface but also "noisy" data towards increasing depths. Wenner seems to have a strong signal/noise ratio but averages fine information and thus smooths the picture. Schlumberger seems to be a good compromise, good detail with lateral resolution and good signal/noise ratio resulting in good depth penetration.
Description
The array of 56 steel electrodes were located at the
bottom of the dam in a drained part near the wall. They were
placed at 5m intervals along the survey line. A Swift Smart
Electrode cable consisting of four 14 switch segments were
connected together for this test. STING was prepared with two
user defined measurement programs, one for the Schlumberger
method and the other for the Wenner method. The measurements were
automatically recorded by Sting using these two loaded programs
and the built-in Dip-Dip program for a total of three different
recordings on the same site using the exact same electrodes.
The recording times for the three methods were: Dip-Dip (688 readings) 2h35m, Wenner (495 readings) 1h56m, Schlumberger (426 readings) 1h39m. All these readings could be recorded in an afternoon at the dam site.
The results obtained show that both Dip-Dip and
Schlumberger methods were able to clearly resolve the leaky area
at about 565m along the profile. Both agreed very well about the
near surface structure with some resolution benefit visible for
the Dip-Dip method. With the Wenner method near surface
structures agree well with the other two but it seems like it is
not so good at mapping lateral structures at greater depths. The
large anomaly shows up only as a minor low resistivity undulation
at the proper coordinate in the Wenner data.
The anomalies centered around coordinates 565 and 620 were drilled and grouted. The anomaly around 565 is a fracture zone with associated cave and the anomaly at 620 is a cavity. The two systems have been injection grouted using 703 cubic meters of a mixture consisting of 50% water, 14% cement and 36% sand. The water in the dam is now reported to be raising.
Images from the survey
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The survey was performed along the embankment on the bottom of the almost dry dam |
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View of the almost dry dam as seen from the top of the embankment |
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One of more than 15 concrete plugged holes, where water leaked down and then out under the dam. The hole is 5 meter diameter. The dam is seen in the background. |
Last modified: 2006-09-07 18:30:40 EDT
