Chapter 14: Laboratory Measurements of Velocity and Attenuation in Sediments
Seismic velocities and attenuations collected in the field with, for example, reflection, refraction, and crosshole surveys serve as valuable constraints on subsurface lithology, porosity, permeability, and fluid saturation. Until recently, the measurement resolution of seismic velocities in the shallow subsurface had limited the use of the complete power of seismic attribute analysis in shallow soils. Recent advances in field techniques for seismic measurements in shallow soils (e.g., Bachrach et al., 1998; Bachrach and Mukerji, this volume; Baker et al., 1999; Carr et al., 1998; Steeples et al., 1999) as well as the development of seismic cone penetration tests (SCPT) and other improved geotechnical methods (e.g., Crouse et al., 1993; Boulanger et al., 1998) have begun to permit the application of seismic attribute analysis to shallow soils. Unfortunately, the laboratory studies required for establishing relations between the physical properties of the soils and their seismic properties have not been extended to the low pressures analogous to these shallow depths, so the effects of lithology, fluid saturation, and compaction on seismic (P- and S-wave) properties of shallow soils are largely unknown.
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Near-surface geophysics uses the investigational methods of geophysics to study the nature of the very outermost part of the earth’s crust. Man interacts with this part of the earth’s crust: he walks on it; he drills and excavates into it; he constructs structures on and in it; he utilizes its water and mineral resources; and his wastes are stored on and in it and seep into it. The very outermost part of the Earth’s crust is extremely dynamic-in both technical (physical properties) and nontechnical (political, social, legal) terms-which leads to both technical and nontechnical challenges that are much different than the challenges faced by “traditional” applications of geophysics for regional geologic mapping and for oil and gas exploration (see Chapter 2).