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.
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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:
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.
In the early days of the oil and gas industry, exploration was generally conducted by trial and error. Knowledge of subsurface structures was limited to the well being drilled and nearby offset wells. Today, geophysicists bounce seismic waves off strata below the Earth's surface and record seismic reflections to create high-resolution images of the subsurface.
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Seismic wave types include surface waves that travel along the surface of the Earth and body waves—compressional and shear waves—that travel through the Earth. Reflection seismic surveys typically use compressional waves, which are also known as P-waves. P-waves are the fastest waves and the first to arrive and be recorded by instruments. P-waves are also pressure waves; each P-wave cycle contains a compression and extension component. Sound waves are examples of P-waves.
During reflection seismic surveys, seismic waves are generated at or near the Earth surface using a seismic source—dynamite, hammer, vibrator, air gun, water gun or anything that will transmit energy into the subsurface. The seismic waves propagate from the source and travel through geologic layers. At the interface between one type of rock and another, there is a change in physical properties—in particular, density, seismic velocity and their multiplication product, seismic impedance.
At geologic interfaces, some seismic energy reflects, or echoes, back to the surface. At the surface, seismic receivers, which are electromechanical devices—microphones, geophones, hydrophones or accelerometers—detect the reflected energy and convert it into electrical signals. The signals are recorded and later processed into images of the underground geology, which may be interpreted for oil and gas exploration, development and production purposes. E&P geoscientists use these images to map and delineate exploration prospects, to plan placement of wells in reservoir zones and to monitor reservoir changes during hydrocarbon production.
When P-waves travel through geologic layers, only some of the wave energy reflects from lithologic interfaces; the remainder crosses the interfaces. Two physical relations govern the reflection and transmission of waves. Both relations are simplifications of the wave equation, which describes the behavior of sound, electromagnetic radiation and other wave phenomena. The first relation is Snell's law, which describes the directions that reflected and transmitted waves take from an interface.
Incident P-waves strike a geologic interface and reflected P-waves travel away from the interface at the same angle, analogous to a billiard ball banking off the table rail. At an interface, P-waves may also be converted to reflected and transmitted S-waves. Across an interface, physical properties change, and transmitted P- and S-waves cross the interface and bend at angles that depend on the seismic velocity contrasts between lithologies (Figure 1).
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