Seismic methods are geophysical techniques that involve the generation and recording of seismic waves for the purpose of mapping the subsurface. Each method is based on the propagation of waves from an artificial source to a set of receivers, followed by an analysis of the recorded wavefield in terms of subsurface properties. Although seismic methods are conceptually not limited to any particular macroscopic scale, the emphasis here is on source-receiver separations that range from a few meters to a few hundred meters, and on depths of investigation that fall approximately within the same range. These linear dimensions define the domain of near-surface seismology, and focus attention on a portion of the uppermost crust that is of great importance to other geologic disciplines, especially geotechnical engineering and hydrogeology. Site characterization and the delineation of aquifers constitute the primary practical applications of near-surface seismology, thereby explaining the many references to engineering and groundwater methods in the applied geophysical literature.
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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).