Geology Reference
In-Depth Information
of materials at the low strains associated with soil-structure interactions.
Shear wave velocity profiles can be used to predict deformation in response
to earthquakes (including earthquake-induced liquefaction analysis), in soil
compaction control, in locating weak zones in embankments and in classi-
fying soils according to the International Building Code (IBC).
Dispersion is the key to using Rayleigh waves for shear modulus depth
profiling. Longer wavelengths penetrate deeper into the ground and gener-
ally have higher velocities. The velocities at specific frequencies are often
referred to as the phase velocities , since they are manifest in the changing
phase relationships between components of different frequencies. Veloci-
ties, and therefore wavelengths, can be estimated by measuring travel times
between known points at selected frequencies, and the results can be used
to build up wavelength-frequency or phase velocity-frequency dispersion
curves. Depths can then be assigned to each velocity value using a factored
wavelength method (typically by taking the depth of investigation to be equal
to one-third of the wavelength). This may be adequate if only a few data
points are available and there are no abrupt changes in shear wave velocities.
Alternatively, full 1D inversion can be used to fit layered Earth models to
the measured dispersion curves. In some cases, slowness-frequency curves
are used (where slowness is the reciprocal of velocity).
There
are
three
main
steps
in
carrying
out
surface-wave
seismic
surveys:
1.
Collect seismic data with a system designed to detect wavelengths (see
Section 14.1.2).
2.
Identify the Rayleigh wave fundamental mode, determine the variation
in velocity as a function of frequency and present this as a dispersion
curve.
3.
Use the dispersion curve to determine the stiffness structure that matches
the measured dispersion.
14.1.2 Types of survey
Seismic surface waves can be produced using impact sources such as sledge-
hammers, weight drops and vibrators ( active sources) and are measured at
frequencies typically between 3 and 30 Hz. Surface waves are also generated
by random 'natural', 'passive' or 'cultural' sources such as wind, thunder,
site construction activities and vehicular and pedestrian traffic. These can
be merely noise but are also used in some methods as sources of signal.
They can carry sufficient low-frequency energy for results to be obtained to
depths of 100 m. Of the active sources, sledgehammers generate relatively
high frequencies that allow exploration to a maximum depth of 10-30 m,
depending on material type, whereas the Vibroseis trucks used for deep
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