Shear-wave Investigations in Poorly Consolidated Materials
Environmental and Engineering Geophysical Society (EEGS), 1993 Symposium on the Application of Geophysics to Engineering and Environmental Problems (SAGEEP) proceedings.
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.