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Shallow Marine MASW: A Case History

Published –
SAGEEP, 2009.
ASCE GeoFlorida Conference proceedings, 2010.

Authors –
JB Shawver, Zonge Geosciences, Inc., Minneapolis, Minnesota, U.S.A.
Alex Fisher, FMG Engineering, Rapid City, South Dakota, U.S.A.
Choon Park, Park Seismic LLC, Shelton, Connecticut, U.S.A.

Paper – [pdf] 2009 Sageep MASW offest method- Park Shawver
Paper – [pdf] 2010 GeoFlorida -Shawver-Fisher-Park2_shallow marine MASW

Abstract
The concrete spillway associated with a small dam in Rapid City, South Dakota has evidenced persistent seepage since its construction in the 1970’s. Visual inspection appears to indicate signs of worsening seepage. Geotechnical borings from dam pre-construction as well as modern boring logs taken on either side of the spillway have been consulted; however, budgetary, access, and logistical constraints restricted a more in-depth geotechnical boring program. A geophysical investigation was ordered to better define the subsurface stratigraphy. This data would be used to perform seepage/flow net analysis and determine some remedial solutions to mitigate seepage and extend the life of the structure.

Seismic surface wave techniques were employed in a shallow marine environment to map depth to bedrock between available boreholes. Hydro-phone streamers with 1m and 3m receiver spacing were used as receiver arrays. MASW and micro-

tremor 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 array configurations. Geotechnical borings were used to constrain the inverse model and verify interpre-tation of the geophysical data.

A traditional means of collecting 2D MASW data utilizes a “walkaway” test to determine a single optimal source-receiver offset per survey profile. We show that multiple source-receiver offsets, and receiver-receiver spacings, were necessary to generate complete surface wave dispersion curves suitable for modeling total depth of investigation. We present challenges observed during the data collection, modeling of the data, and interpretation caused by using this traditionally land-based method in a marine environment.