1887

Abstract

Shear-wave (S-wave) refraction is a powerful tool for investigation<br>of the shallow subsurface. Compressional-wave (P-wave) refraction,<br>while very successful for bedrock mapping, is not successful at<br>differentiating bedding within the alluvial section. In poorly<br>consolidated and saturated materials P-wave velocities are<br>determined by the speed of sound in water (about 5200 feet/second).<br>S-wave propagation is not greatly affected by water content, thus<br>layering within the alluvial section can often be mapped with Swave<br>refraction.<br>Travel-time curves for models illustrating these points include a<br>P-wave model with two alluvial layers over bedrock. The P-wave<br>velocities of lOOO'/s, 3OOO'/s and 7OOO'/s are shown to be masked<br>by saturation within the alluvial section. Modeled S-wave<br>velocities for the same section predict clear detection of the Swave<br>first arrivals. The acoustic (through the air) arrival can<br>complicate the S-wave results.<br>A landfill investigation case-history is an example of the use of<br>S-wave refraction. A suite of geophysical tools were proposed to<br>assist in the sitting of monitor wells. The geologic setting is<br>lo-70 feet of loess over 20-100 feet of till over limestone<br>bedrock. Water table was within the loess but the loess-till<br>interface (a potential aguiclude) was the primary target. GPR, DC<br>resistivity, EM-34, P-wave refraction, P-wave reflection, and Swave<br>refraction were tested. GPR penetration was less than 15 feet<br>and the resistivity contrasts between the loess and till were less<br>than 30% thus the electromagnetic methods were discarded. P-wave<br>refraction mapped the top of the water table, a secondary target.<br>P-wave reflection mapped the top of bedrock (also of secondary<br>interest). S-wave refraction produced excellent data and mapped<br>the horizon of interest.

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/content/papers/10.3997/2214-4609-pdb.209.1993_011
1993-04-18
2024-03-29
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http://instance.metastore.ingenta.com/content/papers/10.3997/2214-4609-pdb.209.1993_011
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