oa Results Of Shear-Wave Measurements

Because seismic shear waves (S waves) propagate in earth materials differently than do<br>compressional waves (P waves), the addition of a shear-wave capability to the engineering<br>geophysicist’s shallow-depth seismic system markedly broadens its usefulness. In general, S waves<br>respond only to lithologic changes, whereas P wave are affected by both the lithology and the<br>fluids contained. In one area studied, both the saturated overburden and the bedrock had nearly<br>the same P-wave velocity, and therefore the boundary between them was undetectable by P-wave<br>methods; however, it was detectable by S-wave methods. Since the water table could be seen with<br>P-wave methods, the volume of the unconfined aquifer could be estimated--P waves for its top; S waves for its base.<br>The amplitude, frequency content, and arrival time of shear waves all change significantly<br>across a near-vertical boundary. I have used these S-wave characteristics to map earth fissures<br>and to locate the edge of a concealed high wall of a reclaimed surface mine.<br>Reflections of horizontal shear (SH) body waves are commonly masked by SH surface<br>waves, but these surface waves do not occur where the surface layer’s shear-wave velocity is close<br>to or higher than that of the lower layers. Because of this lack of surface-wave noise, and<br>because of the inherent simplicity of SH-wave propagation (for example, no wave-type conversion<br>at interfaces), studies have obtained usable SH reflections from a tar sand underlying a<br>limestone sequence, possible SH reflections from within an ignimbrite section, and excellent<br>shear-wave reflections interpreted as having come from lenticular clay bodies at depths of 12 and<br>32 m beneath the surface of an alluvial fan.