Marine geophysics has played an important role in a lo-year investigation and repair program at<br>the Ludington Pumped Storage Plant, an 1872 megawatt hydroelectric pumped storage facility<br>located on the eastern shore of Lake Michigan (Figure 1). The upper reservoir is 842 acres in size<br>and lined with clay on the bottom and lower inside slopes. Excessive seepage was noted in 1973<br>after the reservoir was filled for the first time. In 1975, trench-like features were first observed<br>extending through the clay liner. Geophysical and diver investigations later found that these<br>“trench features” ranged from four to ten feet in width, three to forty feet in depth, and hundreds<br>of feet in length (Figure 2). A geophysical testing program conducted in 1990 showed that sidescan<br>sonar and fathometer surveys combined with an accurate vessel positioning system were the<br>most effective methods to map the trench features. Annual side-scan sonar and fathometer<br>surveys have been performed since 1992 to assess the current status of the trench features and<br>provide a reliable map for diver inspections and underwater repairs. The digital trench feature<br>map is used as control for the real-time positioning of the boats and divers using differential GPS<br>and ultra-short baseline (USBL) tracking systems, respectively.


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