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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