Seismic Methods
Seismic geophysical methods can cost-effectively image the subsurface over a large area and have been extensively used in deep earth studies and natural resource exploration. Observations of the propagation of the seismic energy generated by earthquakes spurred the inception of seismic geophysics and helped determine many critical deep earth characteristics. Seismic technologies advanced with the exploration of hydrocarbons, which typically requires subsurface information from tens to hundreds of kilometers in depth.
Seismic methods in environmental investigations are relatively novel and collect higher resolution images of the shallow subsurface from tens to hundreds of meters below land surface. Though depth and objective may differ, all seismic methods consider the density contrasts that can occur with material contrasts and result in variations of seismic energy propagation velocity. As such, seismic surveys can be effective at mapping geologic layers/structure and determining the types of subsurface materials.
In typical active-source seismic surveys, a seismic source (e.g., sledgehammer) is used to transmit seismic energy into the subsurface. As the seismic energy travels through the subsurface, many seismic wave types (e.g., S-waves, P-waves, Raleigh waves, surface waves) move through and interact with the geologic layers. A line array of geophones is used to detect the seismic energy as it refracts or reflects in the subsurface due to the density variations and returns to the surface.
There has been extensive research relating seismic wave velocities (see seismic velocities) to various material types. Some average velocities of common rock-forming minerals are described by Christensen (1990). These velocities are used to interpret the seismic data and determine the subsurface geologic materials and structure. The disbursement of actively- and passively sourced seismic energy and its interactions with geologic materials has led to the development of many seismic methods. Some of these that have been applied to environmental studies are as follows:
References
Christensen, N.I., 1990, Seismic properties of rocks, in Encyclopedia of Earth Science: Geophysics: Boston, MA, Springer, doi:10.1007/0-387-30752-4_133.