Sebastian Białas
PhD student, Institute of Geophysics
Polish Academy of Sciences
* Surface waves are not a new type of waves but only a result of
interference of body waves. In prospective seismic aqusitions
and processing flows, that waves are attunated as much as
posible because they cover the useful signal on the seismic data
* Can we use this type of waves for any other puproses and use
this results for seismic procesing?
* If the velocity depends on frequency the dispersion phenomena
occurs, that can be used to study the medium through which the
waves have propagated
* And then the near surface model can be calculated..
1. Theory and history of MASW technique
2. Example of MASW on enginereeing scale
3. Case study of extension MASW methodology on
prospective scale
4. Conclusions
* MASW = Multichannel Analysis of Surface Waves
* MASW method relay on Rayleigh waves
* MASW method base on the dispersion curve of surface waves in
order to describe the stiffness of the surface
* MASW is one of the newest methodology on seismic survey
technique of weathernig layers recognition
* MASW method was devolop for engineering, geotechnical purposes
* The MASW method can be distinguished into active MASW method
and passive MASW method or a combination of both methods can be
used
•
In early 80s, a two-receiver approach was introduced by investigators at the University of
Texas (UT), that was based on the Fast Fourier Transform (FFT) analysis of phase spectra of
surface waves generated by using an impulsive source like the sledge hammer. It then
became widely used among geotechnical engineers and researchers. This method was
called Spectral Analysis of Surface Waves (SASW) (Heisey et al., 1982)
•
In early 2000s, the MASW (Multichannel Analysis of Surface Waves) method came into
popular use among the geotechnical engineers. The term “MASW” originated from the
publication made on Geophysics by Park et al. (1999). The project actually started in mid90s at the Kansas Geological Survey. Knowing the advantages with the multichannel method
proven throughout almost half-century of its history for exploration of natural resources,
their goal was a application of multichannel method to utilize surface waves mainly for the
purpose of geotechnical engineering projects.
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Data acquisition, the a survey depends
on the ability to control three
characteristics of the seismic source:
excitation time, location (azimuth and
distance) relative to the receiver array,
and impact power
Preprocesing: FFT + normalization, field
recorde is decomposed due to FFT into
individual frequency component and then
normalization is applied.
Calculating the experimental apparent
dispersion curve on the basis of the wave
motion measured in field
Calculating the numerical apparent
dispersion curve, based on a preliminary
soil profile
Finding the optimal shear wave velocity
by varying the thickness h, the shear
wave velocity and the Poisson coefficient
and the mass density of the ground
layers, until an optimal match between
the experimental apparent dispersion
curve and the numerical apparent
dispersion curve is obtained
*
A field record of N-channels is in the offset-time domain i.g. surface
wave of arbitrary frequency of 20 Hz within arbitrary phase velocity of
140 m/sec (a) and then is transformed to the time-phase velocity
wavefield by slant stacking (b), next 1-D Fourier transform is applied to
create the slowness-frequency domain
*
The physical meaning of phase velocity tell us how fast the surface of
constant phase, associated to the perturbation is moving
*
Dispersion properties of Rayleigh waves are used to estimate the S-wave
velocities of the near-surface earth materials.
*
According to Achenbach, the speed of propagation of the Rayleigh wave
on the free surface of the homogeneous half-space is slightly lower than
the speed of the shear waves S (between 0.862× Vs and 0.955× Vs) and it
depends on the Poisson coefficient (Achenbach, 1999):
Vr =
Masw processing scheme by Perk (1999)
*
0.862+1.14
1+ 𝜎
+ Vs
By Novotny(1999) the depth of penetration of fundamental modes may be
estimated by the value of
h=
λ
3
=
𝑉𝑟·𝑇
3
where: λ is wavelength, Vr is velocity of Rayleigh wave, T is wave period
*
When more then one phase velocity exist for a given frequency, it is
called multi-modal dispersion. The slowest one in this case is called the
fundamental mode (d) and it will be the main study field on this work.
The next faster modes are called the first higher mode and so on (e)
Goal of the field survey and subsequent data processing before
inversion takes place is to establish the fundamental mode (M0)
dispersion curve as accurately as possible, which has been one
of the key issues with data acquisition and processing in the
history of surface wave applications.
The inversion of the dispersion curve is the result of best curve
estimation to the near-surface model with minimum misfit
error.
During the inversion process theoretical curves of fundamental
mode are calculated for different earth models by using a
forward modeling scheme, and then compared to the measured
dispersion curve
Example of MASW procesing flow on engineering scale
Key acqusition
parameters:
Sampling freq = 4000 Hz
Recording time = 2.5 s
Receiver interval = 4m
Example of MASW procesing flow on prospective scale
Key acqusition
parameters:
Sampling freq = 200 Hz
Recording time = 7.0 s
Receiver interval = 25 m
*
The reflection seismic data acquisition
were carried out on the Braniowo 2D
project in north of Poland in 2014
*
In Braniewo 2D experiment the field
geometry was desired for relfection
seismic purposes
*
*
*
The recevier stations interval was 25 m
*
Standard vibroseis technique was used as
the seismic source.
*
Institute of Geophysics, Polish Academy
of Science ware allowded to deploy
additional sensors along the profile in
order to wide angle seismic refraction
measurements
*
That data was used for MASW processing
The shots interval was 25 m
Total length of the seismic line was
20000 m
Geophone responce curve used for
MASW
Geophone responce curve used for
prospective reflection seismic
In most seismic surveys, more then two-thirds of total seismic energy is transformed into
Rayleigh waves as principal component of ground roll.
On the prospecitve relfection seismic scale all the surface waves are undesirable but they
can used for describe the near surface layers, the only things that must be added are
extra low frequency receivers, deployed with desired distance interval.
The seismic record is created according to common receiver stations technique, 48 shots
were gathered to achieve one seismic record, maximum offset is 1200 m. The multichannels recording used in clasic MASW method is replaced here by multi-source
technique.
* Into a homogeneous half-space the phase velocity of Rayleigh wave
does not depend on frequency or wave number, hence dispersion does
not happen.
The geopsy software packages were used for calculation of dispersion curve, inversion and curve fit
statistic, the neighborhood algorithm is used to solve inversion problem. The initial model for inversion
calculations consist of 5 layers
The Vs range on the both layers vary from150 m/s to 2000 m/s, the Poisson ratio range is: 0.2 - 0.5 and
density range is 1700 kg/m3 to 2300 kg/m3. According to Sheriff (1991) the ordinary values of Poison’s
ratio range from 0 for very stiff solids to 0.5 for fluids .
The MASW provides the
monodimensional Vs profile
by assuming an average of Vs
along the recorded profile
On Braniewo 2D project the
neighbouring records
overlaps each other
Total number of field records
is 48.
Pegs number interval is 3
In order to fold up all
covering records, the
number of single record
channels are interpolated up
to 144 to keep pegs interval
as one.
Then avarage value of depth
and Vs for every single layer
were calculated
All depths boundaries were
interpolated every 10m along
all profile length
The depth boundaries were
fold up according to
coverage, then mean value
was calculated and
interpolated every 10 m
The previous boundaries were
the input for smoothing the
depth boundaries, mean value
of neighbouring 10 cells were
calculated for this model
Final 2D model of MASW methodology
Surface waves are known to be sensitive to the presence of near-surface anomalies such
as near-vertical fractures and voids
The MASW method allows to determine the shear wave velocity profile when dealing with
strong stiffness contrast among layers. The existence of the water is modelled by
assigning a Poisson coefficient comprised between 0.3 and 0.5 depending on the
saturation degree.
The purpose of refraction statics is to compute
weathering statics corrections during the processing
of reflection seismic data by using the travel times
of critically refracted seismic energy (first breaks).
*
The weathering layer may be the most
variable of all layers yet in seismic
processing it is taken to be either
uniform in thickness or velocity.
*
Datuming through an incorrect
weathering model can corrupt the
stack and can introduce false
structure into deep reflectors. It is
important to correct for the effect of
variable thickness and lateral velocity
variation of the weathering layer.
*
The MASW method is very similar to
the seismic refraction and seismic
reflection methods, because they both
are based on the measurement of the
wave motion on the free surface of
the ground. With respect to the
refraction method, the MASW method
offers the advantage of overcoming
some problems, for example caused
by the existence of soft layers
between stiffer ones, or stiff layers
trapped between softer ones,
* Classic data processing of MASW methodology on
engineering scale can be applied on the bigger scale
too. Dispersion curve of surface wave is clearly visible
on every seismic data recorded by low frequency
geophones. This is a fundamental point for the next
stages of data processing and optimization of final
output 2D model.
* With respect to the other geotechnical methods used
to determine shear wave velocity profile, such as
cross-hole, down-hole, up-hole, the MASW methods
needs less time and money.
* MASW method offers the advantage of providing
averaged information within the whole region of
ground that has been investigated, without the need
of repeating the test at several points
The work was funded by National Science Center (NCN)
Grant Number DEC-2012/05/B/ST10/00052
The Braniewo 2D experiment required considerable
investment from PGNiG S.A. and would not been possible
without the support and commitment of senior
management at PGNiG S.A company. I would like to thank
PGNiG S.A. for permition to present these results
Choon B. Park, Richard D. Miller, Jianghai Xiam, 1997, MASW,
summary report of technical aspects, experimental results,
and prospective.Kansas Geological Survey
Choon B. Park, Richard D. Miller, Jianghai Xiam, 1999,
Multichannel analysis of surface waves
Sheriff, R. E., 1991, Encyclopedic dictionary of exploration
geophysics: Society of Exploration Geophysicists
Novotny, O. 1999, Seismic surface waves
Geopsy pakages software, www.geopsy.org, www.masw.com,