Seismic refraction

Seismic refraction techniques can measure the density, thickness, and depth of geologic layers using sound (acoustic) waves transmitted Into the subsurface. These sound waves travel at different velocities In various soils and rock and are also refracted (or bent) at the Interface between layers, thereby affecting their path of travel. The time required for the wave to complete this path Is measured, permitting determination of the number of layers at the site as well as the sound velocity and depth of each layer. The wave velocity In each layer Is related to layer properties such as density and hardness. 

The seismic refraction method Is based on several Important assumptions (1) layer acoustic velocities Increase with depth (2) sufficient velocity contrast exists between layers to discriminate between different strata of interests (3) and layers must be thick enough to permit detection. 

Seismic refraction can be used to define many natural and geohydrologic conditions Including number and thickness of layers, layer composition and physical properties, depth to bedrock or water table, and anamalous feature. 

Several different types of sound energy (waves) are propagated through the earth. Seismic refraction methods are concerned primarily with the compresslonal wave energy, commonly called primary wave or P-wave. Primary waves move through... 

Figure 12. Simplified block diagram of single-trace seismic refraction system.

Currently available geophysical methods most applicable in hazardous waste site investigations include metal detectors, magnetometers, ground-penetrating radar (GPR), electromagnetic induction (EM), resistivity, and seismic refraction. These methods should be regarded as complementary, since no one method... 

Durrheim R. J. and Green R. W. E. (1992) A seismic refraction investigation of the Archaean Kaapvaal craton. South Africa, using mine tremors as the energy source. Geophys. J. Int. 108, 812-832. 

Luosto U., Fliih E. R., Lund C.-E., and Group W. (1989) The crustal structure along the POLAR profile from seismic refraction investigations. Tectonophysics 162, 51-85. 

Klingelhofer E. Geli L., Matias L., Steinsland N., and Mohr J. (2000) Crustal structure of a super-slow spreading centre a seismic refraction study ofMohns Ridge, 72° N. Geophys. J. Int. 141, 509-526. 

Harvey, N., 1977. The identification of subsurface solution disconformities on the Great Barrier Reef, Australia, between 14°S and 17°S, using shallow seismic refraction techniques. Proceedings of the Third International Symposium on Coral Reefs, Vol. 2, pp. 45—51. 

Fig. 1. The Proterozoic Grenville Province of the easternmost part of the North American craton. The Marcy Anorthosite is crossed by the O-NYNEX high-resolution seismic refraction profile. Dashed lines indicate seismic refraction profiles.

Fig. 4. Vp/Vs V. age for the continental crust. Data are from seismic refraction profiles (Barth Klemperer 1993 Musacchio 1993 Walther Flueh 1993 Musacchio et al. 1995, 1997) and teleseismic measurements (Jordan Frazer 1975 Owens 1987 Zandt Ammon 1995 Chevrot van der Hilst 2000) in various areas. Data from some areas (i.e. young island arcs) where composition cannot be reliably inferred from VpjVs ratios have been omitted. The hi VpjVs ratio in the Mid-Proterozoic crust is not found in crust of any other age.

Limitations to consider when evaluating the suitability of the seismic refraction method for a given site include the following ... 

Figure 1. Cross sectional view of seismic refraction survey.

Figure 2. Time vs. distance plot of "first arrivals" for seismic refracted waves at corresponding geophones.

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