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Shear-wave Investigations in Poorly Consolidated Materials

Publisher –
Environmental and Engineering Geophysical Society (EEGS), 1993 Symposium on the Application of Geophysics to Engineering and Environmental Problems (SAGEEP) proceedings.

Authors –
Edward P. Pedersen, Jacobson Helgoth Consultants, Lakewood, Colorado, U.S.A.;
David Butler, MicroGeophysics Corporation, Wheat Ridge, Colorado, U.S.A.

Paper – [pdf] ENV_Butler_1993_Shear-Wave_Investigations_in_Poorly_Consolidated_Materials

Shear-wave (S-wave) refraction is a powerful tool for investigation of the shallow subsurface. Compressional-wave (P-wave) refraction, while very successful for bedrock mapping, is not successful at differentiating bedding within the alluvial section. In poorly consolidated and saturated materials P-wave velocities are determined by the speed of sound in water (about 5200 feet/second). S-wave propagation is not greatly affected by water content, thus layering within the alluvial section can often be mapped with S-wave refraction. Travel-time curves for models illustrating these points include a P-wave model with two alluvial layers over bedrock. The P-wave velocities of lOOO’/s, 3OOO’/s and 7OOO’/s are shown to be masked by saturation within the alluvial section. Modeled S-wave velocities for the same section predict clear detection of the S-wave first arrivals. The acoustic (through the air) arrival can complicate the S-wave results.
A landfill investigation case-history is an example of the use of S-wave refraction. A suite of geophysical tools were proposed to assist in the siting of monitor wells. The geologic setting is 1o-70 feet of loess over 20-100 feet of till over limestone bedrock. Water table was within the loess but the loess-till interface (a potential aguiclude) was the primary target. GPR, DC resistivity, EM-34, P-wave refraction, P-wave reflection, and S-wave refraction were tested. GPR penetration was less than 15 feet and the resistivity contrasts between the loess and till were less than 30% thus the electromagnetic methods were discarded. P-wave refraction mapped the top of the water table, a secondary target. P-wave reflection mapped the top of bedrock (also of secondary interest). S-wave refraction produced excellent data and mapped the horizon of interest.