Geology Reference
In-Depth Information
The effect of topography is negligible for surfaces with constant slope but
can be significant if elevations vary by more than 10% of the array length.
2D arrays are recommended by some workers for accurate evaluation and
correction of Rayleigh wave directional effects. It has been claimed that
velocity errors of up to 25% can result if these are not taken into account,
but 2D deployment may not be possible in built-up areas.
14.1.5 Earthquake hazards
The parameter
V
s
is important in standards such as the International Building
Code (IBC), where it is used for ground-motion amplification hazard rank-
ing, and is also widely used in structure design in earthquake prone areas.
Table 14.2 shows a soil hazard ranking scheme based on
V
s
values. Drilling
and logging to the depths required for earthquake hazard investigations is
expensive and often impractical in urban settings and these facts have been
behind much of the development in seismic surface-wave methods during
the past decade.
14.2 Data Processing
Fast Fourier Transforms (FFTs) are used to convert the time-domain field
data into the frequency domain, and derive phase differences. The Rayleigh
wave velocities and wavelengths are then computed using the distances and
phase differences between receivers, via the relationship:
V
R
(
f
)
=
2
π
f
x
φ
where
φ
is the phase difference and
x
is the geophone spacing. The
Rayleigh wavelength is then:
V
R
f
λ
R
(
f
)
=
The f-k (frequency-wavenumber) and p-tau (slowness-intercept time) trans-
forms are effective at isolating fundamental-mode Rayleigh wave energy
from the higher harmonics, body waves and other forms of noise in the
ground roll data.
14.2.1 Dispersion curves
A variety of techniques are available to calculate dispersion curves. The
CSWS method utilises frequency information to plot dispersion curves
of phase-difference against frequency, and wavelength against frequency,
