Case Study - Amistad Dam: Scan for Leaks in Earthen Dam


Courtesy of Advanced Geosciences, Inc.

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.


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.

Results after the survey

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.

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