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Einflüsse lithologischer Parameter oberflächennaher Sedimente auf Kompressions- und Scherwellengeschwindigkeiten

  • S. Hübner
  • R. Meißner
  • H. Stümpel

Summary

Research was concentrated on the measurement of compressional (P) and shear-waves (S) in sands and boulder clay. Results may be summarized as follows.

S-velocities (Vs) in the upper hundred meters of non-consolidated material are generally three to five times smaller than P-velocities (Vp). In saturated sands and clay extreme values of VP /V s up to 15 have been observed (fig. 4b). The records with horizontally polarized shear-waves (SH) are not seriously disturbed by other wave types. No converted waves are created by SH-waves at horizontal boundaries (fig. 2). No interference takes place between Love-waves and SH-reflections because Lovewaves propagate with a similar velocity as the first arrivals do. Because of their shorter wavelength SH-waves have a better resolution than Pwaves. Correlation of abserved velocities Vp and Vs to lithologic and mechanical parameters (fig. 7) indicate that
  1. (i)

    Vp and Vs in boulder clay are influenced by its porosity, degree P s of Saturation and the properties of grain boundary contact, which is a function of various parameters like clay-calcite content, size, shape, pressure etc.

     
  2. (ii)

    Porosity in boulder clay decreases with increasing pressure and increasing content of CaCO3. Increasing percentage of clay results in an increase of porosity. Vp and Vs decrease with increasing porosity in boulder clay as shown in fig. 7d.

     
  3. (iii)

    The influence of the State of Saturation is most different for Vs and Vp: while both values are about constant up to high values of Saturation, Vs shows an abrupt increase at about 99 % Saturation while Vs shows a small decrease which is caused by changes in density and shear modulus (fig. 7a,b). For boulder clay the decrease in Vs may be attributed to the volume change of the water absorbing clay minerals which reduce the shear modulus and show the strength of the material (fig. 7b).

     
  4. (iv)
    The effective pressure (lithologic-pressure minus pore-pressure) has a strong influence on the grain-boundary contacts (fig. 3). The Vg values in dry and saturated sands as well as in boulder clay follow a relationship like Vp -values follow a similar relationship in dry and partially saturated sands while for Vp in saturated sands and boulder clay a P rather constant value is found for the low pressure conditions of near-surface layers.
     
  5. (v)

    Boulder clay shows a decrease in Vp for an increase of the clay content. This is related to the different elastic properties of the grain boundary contacts of the clay minerals and those of the coarser material. An increasing content in CaCO3, on the other hand, is correlatable with an increase in Vp and Vs . This correlation is possibly due to the cementation and stabilization of the grain-boundary matrix (see figs. 7e and 7f).

     
  6. (vi)

    A correlation seems to exist between the Vg-values and the resistance against penetration, as shown in the regression lines of fig. 6. Further research on this subject is in progress.

     

From the above mentioned items the relationships between Vp, Vs and the pressure and Saturation (especially in sands) are the most solid and general ones. Future investigations have to show whether the other correlations have a local or also a more general character.

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Copyright information

© Springer-Verlag Berlin Heidelberg 1985

Authors and Affiliations

  • S. Hübner
    • 1
  • R. Meißner
    • 1
  • H. Stümpel
    • 1
  1. 1.Institut für GeophysikUniversität zu KielKielGermany

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