Shallow Marine MASW: a Case History and Lessons Learned

摘要

The concrete spillway associated with a small dam in Rapid City, SouthDakota has evidenced persistent seepage since its construction in the 1970’s. Visualinspection appears to indicate signs of worsening seepage. Geotechnical boringsfrom dam pre-construction as well as modern boring logs taken on either side of thespillway have been consulted; however, budgetary, access, and logistical constraintsrestricted a more in-depth geotechnical boring program. A geophysical investigationwas ordered to better define the subsurface stratigraphy. This data would be used toperform seepage/flow net analysis and determine some remedial solutions to mitigateseepage and extend the life of the structure.Seismic surface wave techniques were employed in a shallow marineenvironment to map depth to bedrock between available boreholes. Hydrophonestreamers with 1m and 3m receiver spacing were used as receiver arrays. MASWand microtremor data were collected using various sources located on both the marine(within the river) and land (adjacent the spillway) sides of the rolling receiver arrays.CMP gathers were constructed using data from four different source-receiver arrayconfigurations. Geotechnical borings were used to constrain the inverse model andverify interpretation of the geophysical data.A traditional means of collecting 2D MASW data utilizes a “walkaway” testto determine a single optimal source-receiver offset per survey profile. We show thatmultiple source-receiver offsets, and receiver-receiver spacings, were necessary togenerate complete surface wave dispersion curves suitable for modeling total depth ofinvestigation. We present challenges observed during the data collection, modelingof the data, and interpretation caused by using this traditionally land-based method ina marine environment.