Kansas Geological Survey, Open-file Report 1999-9
by Richard D. Miller,
and David Laflen
Kansas Geological Survey
KGS Open-file Report 1999-9
The shear wave velocity field, calculated using the Multi-channel 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 6 to 23 ft 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 fractures and/or an unmapped buried stream channel could be 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 introduced at or near the ground surface, determining the nature and location of anomalous bedrock was critical to hydrologic characterization of 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., approx 220,000 sq yd 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 only shear wave velocity data possesses significantly higher resolution than maps produced using drilling alone. There is less than 1 ft of difference in the depth-to-bedrock interpreted from surface wave data versus the depths determined through drilling. Geophones used for this study were equipped with steel baseplates. 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.
Localized anomalies were observed on lines 2 and 4 that are likely representative of anomalies in the bedrock. Anomalously low velocities observed on the western portion of the east-west lines is very abrupt and localized, suggesting a zone of either fractures, a relatively shear-sided channel, or a fault. Based on the seismic data alone it is not possible to determine which of these scenarios is correct. Speculating from the character and apparent vertical extent of this feature, it is more likely a fault or fracture than a channel. The physical dimensions of this feature would require a channel to be over 20 ft deep and 30 ft wide with vertical side walls. A channel with these dimensions is possible, of course, but would not be expected in this setting.
The general topographic trend of the bedrock surface suggests a north dip with an apparent change in the material composition of the shallow bedrock at a localized low in the bedrock surface. Bedrock lows are present at the north end of lines 1 and 3. Based on the drop in shear wave velocities, these lows are also areas with either increased weathered bedrock near the contact between unconsolidated material and bedrock or the bedrock material changes slightly (i.e., limestone to shale or possibly the shale limestone thickens). Lines 2 and 4 possess a low velocity bedrock material near their eastern ends. It appears to transition from high velocity to low velocity very subtly on line 2 while on line 4 the feature is very abrupt and similar in character to the fault/fracture/channel interpreted on the western end of both these lines. Bedrock toward the eastern ends of lines 2 and 4 can be interpreted to be either flat or maybe slightly shallower than near the building on the west.
The depth-to-bedrock contour map produced using both drilling and seismic data is significantly more detailed and represents a closer approximation to the real bedrock surface than either drilling or seismic data could have produced alone. The match between drilling and seismic is excellent. Improved resolution on the surface of the bedrock provides insight into the texture of bedrock and permits identification and appraisal of short wavelength variations in the bedrock surface. The goals and objectives of this survey were met.
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Kansas Geological Survey, Geophysics
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