With the artificial excitation of seismic waves near the earth's
surface, both body and surface waves arise as they do with
earthquakes.
Here you can see an example of a seismic profile from oil exploration.
After the significant impact of P-waves, another wave impacts somewhat
later.
These are referred to as surface waves.
They arrive at the geophones later than the body waves and have
relatively high amplitudes.
Surface waves are used to a lesser extent for exploration purposes.
However, they are very effective for studying near-surface structures
or water saturation in the rock.
Hello and welcome.
How can surface waves in the field of applied seismics be used to
explore the subsurface?
I would like to answer this question in this video.
You will learn how surface waves are artificially excited, what
properties they have, and what methods are used to investigate the
subsurface.
In the introductory example, I have shown recordings of vertical
geophone components.
And you have seen that surface waves have larger amplitudes than body
waves.
Within the area of application of reflection and refraction seismics,
surface waves sometimes overlap important body wave phases so that
they must be suppressed during processing.
What is a disadvantage there, however, is an advantage in surface wave
seismics.
Especially these large amplitudes result in relatively clear signals
at a relatively low level of excitation.
The surface waves visible here are so-called Rayleigh waves or, as
they are also known, ground roll.
They are a combination of longitudinal waves and vertically polarized
transverse waves.
In this animation, you can see the elliptical movement of this kind of
surface wave.
Near the surface, this usually occurs retrograde, in other words,
counterclockwise, whereas deeper down it occurs prograde or clockwise.
The amplitudes of the surface waves decrease exponentially with depth.
Already at a depth of only one wavelength, their amplitude is very
small.
The second type of seismic surface wave is the so-called Love wave,
which is formed as a result of the interference of horizontally
oscillating transverse waves.
The animation shows the shear movement transverse to the direction of
propagation.
Unlike the Rayleigh waves, Love waves propagate only in an earth
layer, which in depth is delimited from another layer with a higher
propagation velocity.
Both Rayleigh and Love waves decrease in amplitude with depth.
Since the propagation characteristics of both types of surface waves
are primarily determined by shear wave velocity, they are particularly
useful for shallow seismic determination of shear wave velocity.
Surface wave analyses are therefore useful for seismic hazard
assessment, the examination of foundation soils or the detection of
water saturation in a subsurface.
In addition, it is also possible with the help of surface waves to
study structures below hard layers such as, for example, asphalt.
Both types of surface waves can be produced by hammer impacts for
shallow seismic surveys.
For initiating Rayleigh waves, a vertical hammer impact on a flat
metal plate is adequate.
For artificially created Love waves, a horizontal impact on, for
example, a vertical metal plate can be applied.
A so-called Galperin source allows hammer impacts on obliquely
standing metal plates so that both wave types can be excited.
This source is anchored in the ground with steel pins so that
horizontal impacts can also be transmitted to the ground.
The surface waves generated in this way are guided along the free
surface to be recorded by geophones.
Depending on the direction of their oscillation, Rayleigh waves are
recorded in their vertical and horizontal components.
Love waves, however, can only be recorded in the horizontal direction.
A special feature of surface waves is that, in contrast to body waves,
they are dispersive in stratified media.
This means that their propagation velocity is frequency-dependent.
Low frequencies reach the geophones before the higher frequencies.
In surface wave seismics, this property is used to determine the shear
wave velocity as a function of depth.
The frequency dependence of the propagation velocity is typically
illustrated using a dispersion curve.
Here you can see one example measured to the south of Karlsruhe, which
was used to explore the shallow subsurface.
It illustrates the phase velocity of the Rayleigh waves as a function
of the wave frequency.
The phase velocity is the speed with which the waves of the same
oscillation phase propagate.
With surface waves in stratified media, as in the case of an
oscillating string, oscillations occur with different modes or
overtones.
For one and the same oscillation frequency, different modes with
different phase velocities may occur at the same time.
In the dispersion curve, the red areas represent high amplitudes,
while the blue areas represent low amplitudes.
The striking red area corresponds to the fundamental mode, that is,
the root in musical terms of the Rayleigh wave.
Waves with frequencies of 5 Hz thus show phase velocities of about 300
m per second, while higher frequency components of 40 Hz are much
slower paced at 150 m per second.
Other areas with increased amplitudes here correspond to higher modes
or overtones with higher propagation velocities.
There are several methods that can be used to generate such curves.
The methods are based on different mathematical transformations from
the time domain of the seismogram to the frequency domain.
The complex formalism of this will not be examined in depth here.
This example here shows the result of a radon transformation.
How can a model of the subsurface be calculated from these types of
dispersion curves?
Different wave frequencies are synonymous with different wavelength
and penetration depth.
Surface waves penetrate up to approximately one wavelength into the
earth's interior.
This means that high-frequency areas of the dispersion curve
correspond to propagation velocities for shallower areas, while lower
-frequency areas are indicative of deeper layers.
Through inversion of the dispersion curve, this correlation can be
used to calculate a model of the subsurface.
At the same site near Karlsruhe, a so-called Fourier Bessel expansion
was used as a dispersion curve.
There, the phase slowness is shown in dependence of the frequency.
The slowness is merely the reciprocal of the phase velocity.
For this reason, the fundamental mode is here at the top of the
diagram.
The higher modes are found in the lower part.
The dispersion curve can be inverted to form a model of the shear wave
velocity.
For this purpose, there are different methods.
The goal of all methods is to seek out a one-dimensional shear wave
velocity model as a function of depth that best explains the observed
dispersion curve.
The differences lie in the data preparation and the method by which
the differences between the observed and model dispersion curves can
be minimized.
Here you can see the result of an inversion for the example just
shown.
In this case, the data of the dispersion curve were supplemented with
the P-wave arrival times.
Through this, both the longitudinal P and the transverse S-wave
velocity could be modelled.
The one-dimensional model shows slightly increasing shear wave
velocities down to nearly 7 meters in depth, at which point there is a
slight jump.
The P-wave velocity there shows an even greater difference.
In the original publication by Lisa Gross, this boundary is
interpreted as a groundwater table, since an increased water
saturation reduces the shear wave velocity more than it reduces the P
-wave velocity.
The synthetic dispersion curve forward calculated from this model
depicts the characteristic elements of the observation well and
underscores the result of the inversion.
In this video, I have provided you with a brief overview of the use of
surface waves in seismics.
Here, special forms of excitation, such as the Galperin source for
love and Rayleigh waves, are used.
In the case of a source near the earth's surface, high-amplitude
surface waves arise.
These are recorded using geophones.
Surface waves in stratified media are dispersive.
This dependence of the propagation velocity on the wave frequency can
be illustrated using dispersion curves.
The inversion of these curves allows the deduction of a one
-dimensional model of shear wave velocity as a function of depth.