MIL-99-02

Using MASW to map bedrock in Olathe, Kansas

Abstract

The shear wave velocity field, calculated using the Multichannel Analysis of Surface Waves (MASW) method (Park et al., 1999; Xia et al., in press) was used to map the bedrock surface at depths of 2 to 7 m and identify potential fracture zones within bedrock at a site in Olathe, Kansas. Preliminary analysis of this site’s hydrologic characteristics, based primarily on borehole data, suggested that fractures and/or an unmapped buried stream channel was influencing fluid movement along the drill-defined bedrock surface. Since topographic variations on the surface of bedrock can influence the transport and eventual fate of contaminants released at or near the ground surface, determining the nature and location of anomalous bedrock was critical to establishing the environmental risk at this site. High velocity gradients within the shear wave velocity field were used as diagnostic of the bedrock surface, while localized lateral decreases in the shear wave velocity below the bedrock surface were considered characteristic of fracture zones or erosional channels. Calculating the shear wave velocity field from surface wave arrivals can generally be accomplished with a high degree of accuracy regardless of cultural noise. The insensitivity of MASW to cultural obstacles and noise was demonstrated at this site (e.g., a 185,000 m2 asphalt parking lot, electrical and mechanical noise from nearby industrial facilities, traffic noise from the adjacent highway, exploratory drilling on the asphalt parking lot, and aircraft noise). The depth-to-bedrock map produced using shear wave velocity data only possesses significantly higher resolution with less than 0.3 m in difference observed between the interpreted bedrock depth from surface wave data and from drill confirmed bedrock. Advantages of mapping the bedrock surface with the shear wave velocity field calculated from surface waves include the insensitivity of MASW to velocity inversions, ease of generating and propagating surface wave energy in comparison to body wave energy, and its sensitivity to lateral changes in velocity.

 

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