Near-surface seismology derives much of its identity from the physical characteristics of the near-surface environment. Natural materials encountered at shallow depths possess exceptionally diverse mechanical properties as documented by the classification schemes of soil and rock mechanics. Geologic boundaries across which mechanical properties undergo large and rapid changes are commonly present, most notably the water table and the soil-bedrock interface. Porosity occurs in many forms and at a wide variety of scales, and tends toward relatively high values because of low confining pressures. Water, air, biogenic gases, and fluid contaminants occupy the pore space in spatially varying proportions. Near-surface stress increases very rapidly with depth, but the principal stresses may not align with vertical and horizontal directions. At depths where soils and rocks are saturated with groundwater, significant pore water pressure acts on the solid frame. All of these physical characteristics combine with the nature of the seismic source to determine the near-surface seismic wavefield.
Figures & Tables
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).