Shallow Seismic Reflection Survey at Wickiup Dam in Central Oregon

Concerns about potential liquefaction and resulting flowslide type failures at Wickiup Dam in Central Oregon prompted geophysical studies of the left wing dike, studies which included a high resolution seismic reflection survey. High resolution seismic reflection data effectively delineated structures and correlated lithologies to shallow borehole data above the surface of the dipping, basalt bedrock on the downstream side of the dam. Unconsolidated, Quaternary lacusnine, fluvial, and volcanic-derived sediments overlay the bedrock surface, which is encountered within 50 feet of the ground surface near the dam outlet works and plunges to depths of at least 1000 fi below ground surface 5000 ft west of the outlet works. Sediments that filled the seismically defined bedrock surface possess a wide range of bedding geometries likely indicative of multiple episodes of erosion and deposition, driven by the modern and ancestral Deschutes River system. Dominant frequencies in excess of 250 Hz evident on stacked sections provide vertical bed resolution on the order of 10 ft at depths of over 900 ft. Layering interpreted on stacked seismic data within the basaltic bedrock has been inferred to represent contacts between different basalt flow episodes and volcanic sediments (ash, cinder, etc.) originating from Wickiup Butte. Frequency inversions, which are common indicators of inappropriately stackedinterpreted seismic sections, evident on these stacked data are actual inversions that can be verified by correlating the stacked sections with shot gathers, a process rarely possible on high resolution stacked seismic sections. If geologic interpretations, instrumental in engineering designs for resistance to seismic loading, would have been based on existing drill data from the upper 300 ft alone, a relatively flat laying, cyclic sequence of sands, gravels, volcanic ash, silt, and clay overlying the mudflow and basalt materials would have been inferred to an estimated depth of several hundred to more than one thousand feet. Seismic data provided an extremely detailed image of the subsurface, delineating channel features within the sediments and a dipping basalt bedrock surface. Amplification potential of these kinds of geometries may impact the levels of expected ground motion for a given size seismic event.