Refraction seismics

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Schematic transit time diagram with associated beam paths (red: subcritical reflections, brown: supercritical reflections, blue: direct wave, green: head wave)

The refraction is a method applied geophysics . For this purpose, artificially seismic waves are generated by hammers, drop weights, explosions , vibrators or other sources and the propagation of the wave field along the earth's surface is recorded. For the measurement, sensors ( geophones ) are usually laid out along a profile line. The distances between the geophones can range from less than a meter to several kilometers, but a fixed distance is used on a single profile. The total length of the display is a few tens of meters to more than 100 kilometers, depending on the depth of the structure to be examined. The general rule here is that structures can be examined to a maximum depth of one third of the display length.

In contrast to reflection seismics , where only waves reflected in the subsurface are considered for evaluation and interpretation, refraction seismics use the refracted energy ( head wave or mintrop wave). The evaluation is carried out in a simplified manner by creating runtime diagrams . The result is a model of the subsurface that shows layers with different propagation speeds of the seismic waves.

Principle of refraction seismics: The faster head wave (green) of the lower layer overtakes the direct wave (blue)

Areas of application include geological explorations of the earth's crust into the upper mantle (e.g. determination of the depth of mountain roots ) or near-surface investigations for engineering and environmental geology or to determine the thickness of the weathered layer in connection with seismic reflection measurements.

A disadvantage of this method is that refracted waves are only generated when the speed changes “normally”, i.e. when the speed increases downwards. On the other hand, if the speed decreases downwards at a layer boundary, this layer boundary remains invisible. This can lead to errors in the interpretation.

literature

  • Konstantin Meskouris, Michael Mistler, Christoph Butenweg, Klaus-G Hinzen: Buildings and earthquakes. Vieweg + Teubner / Springer Fachmedien, Wiesbaden 2011, ISBN 978-3-8348-0779-3 , p. 107 f.

Web links

Spektrum.de Lexicon of Physics Refraction Seismics