Geophysical Seepage Detection Studies, Mill Creek Dam, Walla Walla, Washington

摘要

A geophysical seepage detection study which used the self-potential (SP) method was conducted at Mill Creek Dam and Reservoir in Walla Walla, Washington. The reservoir has experienced excessive loss of stored water due to seepage since its first test filling in 1941. The seepage pattern was not altered by remedial measures attempted at various times from 1941 to 1979. The objective of this investigation was to monitor SP values over two permanently installed electrode arrays as the reservoir filling progressed. Array 1 was installed from Station 18+00 to Station 41+50 along the elevation 1,225-foot contour on the upstream face of the dam, just downstream of the location of the cutoff wall. Array 2 ties into Array 1 at Station 32+00 and proceeds approximately perpendicular to Array 1 in the upstream direction beginning at Station 0+00 and commencing at Station 18+00. For each array, anomalous SP values, especially those which increase or decrease as a function of time, were flagged as possible indicators of seepage paths. A qualitative ranking scheme was developed to rank the relative importance of detected anomalous zones.