NEAR-SURFACE SH-WAVE REFRACTION AND REFLECTION AND THEIR APPLICATIONS IN OREGON

Thick, unconsolidated soils with high ground-water levels are present in most of western Oregon. These conditions often result in a lack of P-wave impedance contrasts necessary for reflection/refraction signal generation, thereby making the conventional energy source ineffectual for defining the near-surface soil properties. Shear waves, however, propagate only in the soil matrix and are not influenced by pore fluids. This makes shear-wave refraction/reflection more effective for characterizing near-surface soil properties and subsurface geometry. The SH-wave is a shear wave with a particle motion parallel to the ground surface, and is easily generated with hammers or other sources. SH-wave refraction and reflection techniques have been used to characterize near-surface site conditions and neotectonic features in western Oregon. The case histories include shear- wave velocity measurements of the top 30 meters of soils in communities throughout western Oregon, as well as fault characterizations in the Willamette Valley. These case histories show that SH-wave refraction and reflection methods are very effective for characterizing near-surface soil conditions.