High Resolution Seismic Reflection Survey At Keechelus Dam

Replacement of the existing Bureau of Reclamation dam (Keechelus) located at the headwaters<br>of the Yakima River in Washington became necessary after the incidental encounter of a void during<br>trenching along the dam crest. Subsequent geophysical surveys discovered several other voids along the<br>structure, mandating the replacement of this high-risk dam. High resolution seismic reflection surveys<br>conducted along the downstream toe provided a continuous image of till, swamp bog, alluvial, outwash,<br>and lacustrine sediments which make up the Quaternary unconsolidated material that lay on Tertiary<br>rhyolite bedrock. Each of the three seismic lines possessed uniquely different signal-to-noise and resolution<br>characteristics. One of the profiles was acquired perpendicular to the long axis of the dam and<br>through a swamp area. This profile provided an excellent image of the till/lacustrine sediment contact<br>with indications of very steeply dipping contacts between the lacustrine and outwash sediments. Delineation<br>of these features, as shallow as 20 ft, is significant to the engineering of a new structure, which<br>must provide a watertight seal through the very permeable outwash and alluvial fan sediments. Resolution<br>potential of this 24-fold, 150+ Hz data set is around 10 ft (~½ wavelength). A dramatic channel type<br>feature separating a thin sequence of outwash and lacustrine sediments from a thickening wedge of<br>lacustrine type sediments is interpretable directly beneath the trace of the proposed new structure. Based<br>on the seismic reflection data set it should be possible to more precisely fit the preconstruction engineering<br>designs to the actual geology, which will be encountered during construction.