Geology Reference
In-Depth Information
seismic reflection work can produce usable surface wave data from depths
of as much as 100 m.
Acronyms abound in surface wave studies and seem deliberately designed
to confuse. The simplest possible (Common-Offset Rayleigh Wave) method
uses a hammer source and a single geophone with a resonant frequency
below 5 Hz. The two are moved together to carry out the survey. When the
geophone or accelerometer is left in one place but a number of different
source positions are used, the first acronym (MISW - Multiple Impact Sur-
face Wave) is introduced. Add additional geophones in a linear array and the
possibility of using a weight drop source, and the method becomes SASW
(Spectral Analysis of Surface Waves). Switching to a controlled vibrator
source, operating typically at between 5 and 600 Hz, may also involve a
change of acronym to CSWS (Continuous Surface Wave Seismics). Use of
random sources such as passing vehicles and site activities such as piling or
drilling, with either linear or 2-D arrays of detectors, is implied by the MSM
(Microtremor Survey Method) acronym. The recording times for MSM and
other random-source methods should not be less than half a minute. Add
a hammer wielded at random to increase the high-frequency content or a
weight drop, also untimed, for deeper penetration, and the survey might be
described as ReMi (Refraction Microtremor), although most users would
restrict this term to surveys with linear detector arrays. Use almost every
possible combination of random (untimed) and controlled (possibly timed)
sources, and of linear and 2-D arrays and use also the acronym MASW
(Multi-channel Analysis of Surface Waves). The names are not important,
as long as the limitations are understood.
Surface wave arrivals are also unavoidably recorded in seismic reflection
and refraction surveys, although usually attenuated in reflection surveys by
the use of arrays (Section 12.2.2), and these can be processed and interpreted
to produce shear-wave velocity models. Combination surveys are becoming
increasingly popular.
All of the methods listed have in common the recording of time-domain
seismic data, transformation into the frequency domain, conversion to dis-
persion plots of phase velocity (or wavelength) against frequency, and mod-
elling to produce shear-wave velocity-depth profiles. Differences arise not
only in the field parameters but in the methods used to derive the dispersion
curves and in the modelling techniques used to produce the profiles.
14.1.3 Recording arrays
Linear arrays used for surface-wave studies are similar to those used in
refraction and reflection surveys, but low-frequency geophones are usually
required to reach the required target depths. For combined (ReMi)/refraction
surveys it is usual to set out and switch between two rows of geophones.
