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Seismic Refraction Profiling:
|In addition to all of this data collection and compilation, we conduct seismic refraction surveys in order to provide greater subsurface detail, particularly in areas where there are no well or test borings. Since many of the areas where we work are remote and subsurface information simply does not exist, these surveys provide the only subsurface information from which to identify the aquifers and aid in the delineation of aquifer boundaries. Also, since seismic refraction profiling is significantly more cost-effective than drilling test borings, it provides the geologist a very important tool in the identification and characterization of sand and gravel aquifers. Figure 1 shows the types of data that we collect and portray on our aquifer and materials maps.|
Seismic refraction is a geophysical technique used to determine the thickness of underlying geologic strata, depth to the water table, and bedrock surfaces. Seismic refraction methods use sound waves to determine the thickness and extent of aquifer materials. A prerequisite for success of this technique is that each major successive underlying geologic layer must increase in density as well as thickness (Figure 2 illustrates this requirement). The principle of seismic refraction is founded on the fact that sound waves travel through different earth materials such as dry (unsaturated) sand and gravel, wet (saturated) sand and gravel, and bedrock at different velocities. The denser the material, the faster the waves travel. These seismic waves can be generated by hitting a metal plate with a hammer (Figure 3), dropping a heavy weight, or by the detonation of buried explosive charges (Figure 4). Energy from these sound waves is transmitted through the ground by elastic waves. These waves are referred to as elastic because as they pass though the geologic formation, particles are momentarily distorted, but immediately return to their original position after the wave passes. Types of waves that can be created include compressional waves, shear waves, and surface waves. The arrival of a seismic wave is detected by geophones, or motion sensitive earth sensors, which are placed firmly in the ground (Figure 5).
Compressional waves are the first to arrive at the geophones and consequently are of the most use in seismic refraction surveys (Figure 6). Typically, the higher the density and elasticity of the earth material, the faster the transmission of the compressional wave. Conversely, the velocity of compressional waves is quickly dissipated in dry, unconsolidated sediments thus providing a lower velocity. This relationship is illustrated in Figure 2. During a seismic refraction survey, the seismograph measures the time the seismic wave takes to reach one or more geophones placed at known distances from the sound source (Figure 7). By plotting the first arrival times of the shock waves arriving at the individual geophones placed along the survey line and making corrections for elevational differences as well as any offsets to the line, it is possible to infer a profile depicting the land surface, water table surface, and bedrock surface (Figure 8). These interpreted cross-sections are made with the aid of a computer program which allows for numerous calculations as certain inputs are adjusted.
Readers interested in learning more about the application of seismic refraction in hydrologic studies are referred to the following U.S. Geological Survey sites:
Driscoll, F G., 1986, Groundwater and wells: H. M. Smyth Co., St. Paul, Minnesota, 1089 p.
Haeni, F. P., 1988, Application of seismic-refraction techniques to hydrologic studies: U. S. Geological Survey, Techniques of Water-Resources Investigations, Book 2, Chap. D-2, 86 p.
Locke, D. B., Neil C. D., Nichols, W. J., Jr., and Weddle, T. K., 1997, Hydrogeology and water quality of significant sand and gravel aquifers in parts of Aroostook, Penobscot, and Washington Counties, Maine: Maine Geological Survey Open-File Report 97-44, 91 p.
Locke, D. B., 1999, Significant sand and gravel aquifers of the Augusta quadrangle, Maine (compiled by C.D. Neil): Maine Geological Survey, Open-File Map 99-33.
Locke, D. B., 1999, Surficial materials of the Augusta quadrangle, Maine: Maine Geological Survey, Open-File Map 99-71.
Tepper, D. H., Williams, J. S., Tolman, A. L., and Prescott, G. C., Jr., 1985, Hydrogeology and water quality of significant sand and gravel aquifers in parts of Androscoggin, Cumberland, and Franklin, Kennebec, Lincoln, Oxford, Sagadahoc, and Somerset Counties, Maine: Maine Geological Survey Open-File Report 85-82a, 106 p.
Originally published on the web as the March 2004 Site of the Month.
Last updated on October 6, 2005
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