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PHY 361 GEOPHYSICS II

# The vertical axis for each rock type is intended to

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The vertical axis, for each rock type, is intended to show approximately the relative numbers of samples that would show a given velocity . In dry rocks, the pore spaces are filled with air ( V = 330 ms 1) rather than water. Time averaging cannot be applied quantitatively to gas-filled pores, but dry materials generally have very low P-wave velocities. If they are poorly consolidated and do not respond elastically, they may also strongly absorb S waves. Poorly consolidated water-saturated materials generally have velocities slightly greater than that of water, and the water table is often a prominent seismic interface. Weathering normally increases porosity, and therefore reduces rock velocities. This fact underlies the rippability ranges shown in Figure 1.1. Few fresh, consolidated rocks have velocities of less than about 2200 ms 1, and rocks that are rippable are generally also at least partly weathered.

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8 3.1.4 Ray-path diagrams It is convenient to identify the important travel paths by drawing seismic rays , to which the laws of geometrical optics can be applied, at right angles to the corresponding wavefronts. Ray-path theory works less well in seismology than in optics because the most useful seismic wavelengths are between 25 and 200 m, and thus comparable with survey dimensions and interface depths. Wave effects can be significant under these circumstances but field interpretation can nonetheless be based on ray-path approximations. 3.1.5 Reflection and refraction When a seismic wave encounters an interface between two different rock types, some of the energy is reflected and the remainder continues on its way at a different angle, i.e. is refracted . The law of reflection is very simple; the angle of reflection is equal to the angle of incidence (Figure 1.2a). Refraction is governed by Snell’s law , which relates the angles of incidence and refraction to the seismic velocities in the two media: sin i/ sin r = V 1 /V 2 If V 2 is greater than V 1, refraction will be towards the interface. If sin i equals V 1 /V 2, the refracted ray will be parallel to the interface and some of its energy will return to the surface as a head wave that leaves the interface at the original angle of incidence (Figure 1.2b). At greater angles of incidence there can be no refracted ray and all the energy is reflected. When drawing ray paths for either reflected or critically refracted waves, allowance must be made for refraction at all shallower interfaces. Only the normal-incidence ray, which meets all interfaces at right angles, is not refracted. Figure 1.2 (a) Reflection and (b) refraction. Simple refraction occurs at A, critical refraction at B .
9 . 4.0 Conclusion A seismic wave is properly described in terms of wavefronts , which define the points that the wave has reached at a given instant. However, only a small part of a wavefront is of interest in any geophysical survey, since only a small part of the energy returns to the surface at points where detectors have been placed. 5.0 Summary

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The vertical axis for each rock type is intended to show...

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