You are visiting the web site of the EPSCI Program at the U.S. Dept. of Energy's Ames Laboratory, Ames, IA
General Description
Seismic methods of subsurface evaluation involve directing an intense sound source into the ground and spreading receivers called geophones, analogous to microphones, across the ground. The receivers pick up "echoes" that come back up from the ground and record the intensity and time of the "echo" on computers. Complicated data processing is done to turn these signals into images of the geologic structure in the ground.
There are two types of "echoes" produced as the sound waves travel into the ground. "Reflected" waves are just that--waves which travel downward, bounce off a layer or object in the soil or rock and return to the surface. "Refracted" waves are ones which travel downward then turn at a geologic boundary, such as the surface of a rock layer, and travel along it before returning back to the surface.
By their nature, reflected waves have the ability to show more detail about the geology below the ground; however, the complex physics resulting from multiple "echoes" can make reflections very difficult to interpret. Refracted waves are most typically used to determine information such as the nature of the surface of shallow bedrock (e.g., rock less than 100 feet below the ground). Seismic reflection surveys have historically been valuable tools in the exploration for oil since they can provide a wealth of information about deep rock structures. Seismic refraction surveys are more suitable for shallow exploration and definition of geologic surfaces, such as bedrock or the water table.
Technical description
Seismic methods of subsurface evaluation involve the generation of pulses of energy, usually at the ground surface, consisting of compression, shear and surface waves that propagate through the ground and either dissipate, are reflected back toward the surface, or are refracted at and travel along lithologic boundaries. The wave energy which returns to the surface is picked up by receivers (geophones) place in a line on the ground surface. The geophones convert the wave energy to an electronic signal which is recorded on a seismograph.
Refracted waves travel along the interface of lower density material overlying higher density material, for example sediment overlying bedrock. The speed of wave propagation of refracted signals changes along the path, traveling at relatively slow speeds in the sediment, at higher speeds along the bedrock interface, and slow speeds again as they return to the surface. Refracted waves can only be detected at the surface for situations in which higher velocity material underlies low velocity material. Reflected waves travel downward until they meet an interface between layers of differing density. The speed of wave propagation is constant at the speed of the overlying material. In the case of sediment overlying bedrock, the wave will travel at the slower sediment speed. Surface waves are the portion of the seismic energy pulse which travels along the surface of the ground directly to the geophone.
A complex series of data processing steps, particularly for reflection data, are required to convert the raw field records into usable information, including filtering noise, "stacking" data with a common reflecting point, and correcting for phase distortions. Data is most often collected and interpreted in two-dimensional linear profiles. However, three-dimensional images are possible with reflection data collected from a 2-D grid of geophones.
Selecting the optimum seismic technique for a particular site depends on a number of project-specific parameters, such as the desired depth of exploration, the desire resolution of subsurface features, and the funds available for the survey. In general, reflection surveys can be applied to map deeper targets, can produce higher resolution images, and require greater effort in data collection and processing, resulting in higher cost than with refraction surveys.
Contractor: Coleman/Blackhawk - Colorado
Please e-mail comments to: epsciwebkeeper@ameslab.gov.
Last Modified: 1 January 2002 by dave eckels
Expedited Site Characterization: etd/technologies/projects/esc/technologies/seismicrefraction.html
