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Appendix C: Hydrogeologic and Depth-to-Bedrock Seismic Refraction Profiles
During the period from May 6 - 16, 1996, the Maine Geological Survey conducted 18 seismic refraction surveys in order to gather subsurface geologic and hydrogeologic information in the area of the Rockland landslides of 1996 and 1973. Total horizontal distance of all lines cumulatively is 4,305 feet (1.23 km). Site survey lines were located roughly perpendicular as well as parallel to the directions of the slides. Actual locations selected were made on the basis of access and surface suitability.
Seismic refraction techniques, following field procedures described by Haeni (1988), were used to obtain profiles showing the depth to water table, depth to bedrock, and topography of the bedrock surface. In seismic exploration, seismic waves are generated at the surface by an energy source such as a small explosion or a sledge hammer blow to a metal plate. These compressional waves travel through the ground at different velocities depending upon the material-the denser the material, the faster the wave velocity. The seismic waves are then detected by geophones positioned in a line along the ground. In order to detect different layers of underground material by refraction, the seismic velocity must increase with depth and there must be a significant velocity contrast between the layers.
In this study, seismic refraction was used to distinguish between unsaturated glacial deposits (above the water table), saturated glacial deposits (below the water table), and bedrock, which were found to have seismic velocities appropriate for the method. A 12-channel, EG&G Geometrics ES-1225 seismograph was used in this study. The instrument is sensitive and delivers a relatively high degree of resolution. Individual seismic lines varied from 230 to 280 feet (70 - 85 m) in length. Three longer profiles, up to 1000 feet (300 m) total, were obtained by overlapping several lines end to end. Shot points and geophones for each line were surveyed to determine their elevations relative to mean sea level (NGVD). A computer program (described by Scott and others, 1972) was used to determine each layer's seismic velocity and to generate a profile of the water table and bedrock surface beneath each line.
In the area studied, the average seismic velocity measured in unsaturated glacial deposits has a range from 582 to 1426 feet per second (ft/s) (177 - 435 m/s), with an average velocity for all lines of 903 ft/s (275 m/s). Saturated glacial deposits have average velocities of 4403 to 5225 ft/s (1342 - 1592 m/s) with an average velocity for all lines of 4717 ft/s (1438 m/s). Bedrock seismic velocities in the study area vary from 12,679 to 18,767 ft/s (3864 - 5720 m/s) with an average velocity for all lines of 14,895 ft/s (4540 m/s). Thickness of glacial deposits varies from 25 to 61 ft (8 - 18 m), with an average thickness of 40 ft (12 m). Bedrock surface elevation varies from -18 feet (-6 m) below sea level to 41 feet (12 m) above sea level, with an average elevation of 14 feet (4 m) above sea level.
Test drilling information collected by Robert G. Gerber, Inc. (Appendix B), after the April landslide was used to verify and calibrate the lines. Such direct measurement of the depth to bedrock was available only for the holes drilled near the head of the 1996 landslide. The seismic information collected from the other areas has not been confirmed by drilling.
Last updated on October 6, 2005
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