Bedrock Depth and Seismic Velocity Estimates at SRBA Training Sites in Comal, Hamilton, Pecos, Taylor, and Travis Counties, Texas

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2002

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During field training exercises for Texas Department of Transportation (TxDOT) staff, seismic refraction data were acquired at seven sites in six TxDOT districts using the prototype Seismic Refraction Bedrock Analyzer (SRBA). These data were acquired in a variety of settings that included bedrock depths ranging from a few tens of centimeters to more than 6 meters and bedrock types of sandstone, limestone, and mudstone. Analysis of the data acquired using the SRBA consisted of analyzing first seismic arrivals, attributing the arrivals to a direct wave traveling in the fill below the pavement and a more rapidly propagating wave that is critically refracted along the underlying bedrock surface, and creating seismic velocity models that match the observed first arrivals and allow seismic velocities and bedrock depths to be estimated. SRBA data were acquired on U.S. 281 in the San Antonio district, I-10 in the Midland district, U.S. 84 in the Abilene district, U.S. 290 in the Austin district, U.S. 190 in the Brownwood district, and S.H. 36 in the Waco district. Compressional wave velocities measured in layer 1 and interpreted to represent compacted fill between pavement and bedrock ranged from 551 m/s to 1,117 m/s. Compressional wave velocities measured in layer 2 and interpreted to represent bedrock ranged from 1,079 m/s in sandstone to 2,964 m/s in limestone. Estimated depths to layer 2, interpreted to approximate bedrock depth, ranged from 0.33 m to 6.46 m. Depths to bedrock deeper than 6 m cannot be reliably estimated using the SRBA in its current configuration. During the training exercises, the SRBA prototype proved to be easy to deploy, acquired data sufficient for shallow refraction analysis in a few minutes, and was used to produce reasonably accurate estimates of bedrock depth and seismic velocity in bedrock and overlying layers. Analysis of SRBA data is currently cumbersome, requiring first arrivals to be picked in one software package and then exported to another package for refraction analysis. Experience is required to pick first breaks reliably and thus obtain reasonable bedrock depth estimates. Near-term development of the SRBA prototype should include integration of first arrival picking and refraction analysis. Longer-term development might include full integration of data acquisition, first break picking, and refraction analysis in a custom software environment. These improvements would shorten the time required to estimate bedrock depth from an hour or more to near real-time.

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