this is dissertation report of MASW study in IIT Bombay

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M.Sc. Dissertation Report
on
Multichannel analysis of Surface Wave
Submitted in Partial fulfillment of the requirements
Of the degree of Master of Science
By
(Bhupal Mani)
(Roll no. 165320012)
Supervisor: Prof. G. Mohan
Department of Earth Sciences
INDIAN INSTITUTE OF TECHNOLOGY BOMBAY
2018

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APPROVAL SHEET
This M.Sc. project report entitled “Multi-channel analysis of
Surface Wave” prepared by Bhupal Mani (165320012) is
hereby approved for submission.
Prof. G.Mohan Prof. E.Chandrashekhar Prof. K.H.singh
Supervisor Examiner 1 Examiner 2
DATE- 11 April 2018
PLACE-MUMBAI

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Declaration
I Declare that this written submission represents my ideas in my own words and where
other’s ideas or words have been included, I have adequately mentioned and referenced
the original sources. I also declare that I have adhered to all principles of academic honesty
and integrity and have not misrepresented or falsified any idea/data/source in my
submission. I understand that any violation of the above will be cause for disciplinary
action by the institute and can also evoke penal action from the sources which have thus
not been properly cited or from whom proper permission has not been taken when
needed.
Signature
Bhupal Mani
Date-11 April 2018 Roll no.(165320012)

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ACKNOWLEDGEMENTS
I am very grateful to my supervisor, Prof. G. Mohan for all useful discussions , especially
during the difficult conceptual development stage. His guidance has been most helpful,
mentoring in both the topic and beyond.
I would like to thank Dr. B. Sairam, Senior Scientist at Institute of seismological research
for his guidance and suggestion.
I would also like to thank Prof. Bharath Shekhar, Mr. Rajesh Manjrekar, technical supdt.,
Mr. Saju D S, research scholar, IIT Bombay, for their valuable support during data
acquisition at gymkhana ground.
At last but not the least, I would like to thank my classmates for motivating me in all the
aspects of the projects.

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Table of contents
1. Chapter-1 introduction………………………………..………………………………………………………….…..7
1.1. Background………………………………………………………………………………………………………………………..7
1.2. Objective…………………………………………………………………………………………………………………………..8
1.3. Overview…………………………………………………………………………………………………………………………..9
2. Chapter-2 Seismic Wave…………………………..………………………………………………………………………….11
2.1. Types of seismic waves……………………………………………………………………………………………………..11
2.1.1. Body wave………………………………………………………………………………………………………………..11
2.1.1.1. Primary wave……………………..………………………………………………………………………11
2.1.1.2. Secondary wave…………………………………..……………………………………………………..12
2.1.2. Surface wave…………………………………………………………………………………………………………….12
2.1.2.1. Rayleigh wave…………………………………………..……………………………………………….12
2.1.2.2. Love wave …………………………………………………..……………………………………………..12
2.2. Importance of surface waves…………………………………………………………………………………………….13
3. Chapter-3 Surface wave analysis………………………….…………………………………………………………….14
3.1. MASW method………………………………………………………………………………………………………………….14
3.2. MASW standard workflow………………………………………………………………………………………………..15
3.3. Advantage of MASW…………………………………………………………………………………………………………15
4. Chapter-4 Field measurement………….……………………………………………………………………………….16
4.1. Active Source MASW………………………………………………………………………………………………………..16
4.2. Passive source MASW………………………………………………………………………………………………………16
4.3. General procedure…………………………………………………………………………………………………………..17
4.4. Acquisition geometry……………………………………………………………………………………………………….17
4.5. Equipments………………………………………………………………………………………………………………………18
4.6. Acquisition parameter……………………………………………………………………………………………………..19
4.7. Data acquired……………………………………………………………………………………………………………….….19
5. Chapter-5 Dispersion analysis…………………………………………………………………………………………..20
5.1. Dispersion methodology…………………………………………………………………………………………………..20
5.1.1.Stretch function………………………………………………………………………………………………………..20
5.1.2.Dispersion: phase-shift method………………………………………………………………………………..21
6. Chapter-6 Inversion…..…………………………………………………………………………………………………….22
7. Chapter-7 Result and discussion……………… …………………………………………………………………….23
7.1. Dispersion curve…………………………………………………………………………………………………………….…23
7.2. Initial model……………………………………………………………………………………………………………………..25
7.3. Final model…………………………………………………………………………………………………………………….…25
7.4. Discussion………………………………………………………………………………………………………………………...26
8. Chapter-8 Conclusion…………………………………………………………………………………………………………27
9. Chapter-9 References …………………..…………………………………………………………………………..........28

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List of Figures
Figure-1: field site “Gymkhana ground IIT Bombay” in Mumbai where MASW data have
been acquired………………………………………………………………….………………….…08
Figure-2: different types of seismic waves…………………………………………………………12
Figure-3: typical representation of 1-D MASW………………………………………………….14
Figure-4: Active source MASW………………………………………………………………………..…16
Figure-5: passive source MASW data acquisition………………………………………………..16
Figure-6: Acquisition geometry for 24 channel active source MASW………………….17
Figure-7: Equipments used at field site ………………………………………………………………18
Figure-8: Data acquired at Gymkhana ground, IIT Bombay…………………………………19
Figure-9: A shot gather obtained using a sledgehammer as the source……………….20
Figure-10: A surface wave of an arbitrary frequency of 20Hz………………………………21
Figure-11: MASW inversion flowchart………………………………………………………………….22
Figure-12: picked dispersion curve of gymkhana ground MASW data………………….23
Figure-13: dispersion curve of gymkhana ground MASW……………………………………..24
Figure-14: initial model of gymkhana ground MASW……………………………………………25
Figure-15: final Vs model of gymkhana ground MASW…………………………………………25

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Chapter-1
1. Introduction
1.1. Background
Today knowledge of subsurface properties is very important not only for constructing dams,
buildings, roads, tunnels but also for understanding the phenomenon of landslide etc. Earth
is Consider to be homogenous layered spherical model with increasing density towards the
center, this is the ideal consideration for the simplicity of the mathematics and process. But
actually it is not so, earth’s actual is very complex, density increases and decreases laterally
and vertically.
There are several methods to deal with the subsurface properties and are used for different
purposes based on their capabilities, for example seismic method deals in kilometers depth
and mainly used by oil and gas exploration industries.
Surface wave method is one of very non-destructive and cost effective method for
measuring the shear wave velocity of subsurface. In seismic survey when compressional
wave source is used most of the energy generated is imparted into surface wave. There are
two types of surface wave Rayleigh wave and Love wave. Surface wave is generally
considered as Rayleigh wave because Love wave generates only when there is layered
medium with high velocity layer followed by low velocity layer. The main property of the
surface wave that is useful is its dispersion or in other words its nature of velocity variation
with frequency, this is the fundamental of this study, the whole methodology is based on
this property. The dispersive nature of Rayleigh waves in a vertically heterogeneous
medium provides key information regarding the stiffness properties of near-surface
materials. The basis of all the surface wave analysis methods is depends upon correct
determination of the frequency-dependent phase velocity of fundamental mode Rayleigh
waves (Park, Miller, & Xia, 1999), i.e. the experimental fundamental mode dispersion curve.
Since we know that the higher frequency attenuate faster than the low frequency wave
that’s why lower frequency wave penetrates deeper into the surface while high frequency
wave penetrates to shallow depth. Each frequency component wave gives the unique
information about the subsurface property i.e. shear wave velocity.

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1.2. Objective
The objective of this study “Multichannel Analysis of Surface Wave” is to obtain the 1-
Dimensional shear wave velocity structure with depth. An overview of test site where
MASW field data have been acquired in figure 1.2.
Figure 1.2. field site “Gymkhana ground IIT Bombay” in Mumbai where MASW data have
been acquired.
These are the steps in data analysis to estimate the shear velocity structure beneath the
Gymkhana ground IIT Bombay.
1. To acquire field data containing surface wave using 48-channel geode seismograph.
2. To obtain dispersion curve from acquired field data.
3. Inversion of dispersion curve to get final shear wave velocity model.

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1.3. Overview
This report consist of six chapters, each chapter describes briefly about the topic of the
chapter.
Chapter 1 gives some basic introduction about the seismic waves i.e. body wave and surface
waves, how they are called so body and surface waves, what are the properties of the body
and surface waves .Types of body and surface waves and their importance in different
method to deal with subsurface properties discussed briefly.
In chapter 2 some methods of surface wave analysis discussed briefly. In this chapter also
contains the advantages and disadvantages of methods, its uses in different fields, also
about principles in which that method works.
Chapter 3 gives some idea about the field related problems i.e. what is the procedure for
field measurement, what should be the geometry of the acquisition, what are the
equipment that used in it and how it works. It also include the data acquired by us with
configuration and necessary parameters.
Chapter 4 contains the methodology of the dispersion analysis, how to pick the dispersion
curve.
Chapter 5 briefly introduces about the inversion analysis
Chapter 6 is final chapter of this report and it conclude with result and discussion.

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Chapter-2
2. Seismic waves
Seismic waves are the energy that travels through the earth’s layer, and causes tectonic
activity such as earthquake, volcanic eruption, landslides. The travelling seismic waves
velocity depends on density and elasticity of the medium. Velocity of the seismic waves
increases with depth as density generally increases with depth.
2.1. Types of seismic wave
There are two types of seismic waves, Body and surface waves.
2.1.1. Body waves
Body waves travels through the interior of the earth along path controlled by the material
property in term of density and elasticity. This property vary according to temperature,
composition, and material phase. Two types of particle motion in the body waves resembles
two types of body waves.
2.1.1.1. Primary waves
Primary waves are waves of energy that travels through earth by causing particles in rocks
to compress and stretch apart in the direction of the wave or in other words its particle
motion is along the propagation of the wave. This type of wave also called longitudinal or
irrotational waves.
2.1.1.2. Secondary waves
Secondary waves or S-waves are shear waves that are transverse in nature and arrives after
the primary wave (lower velocity than primary wave). Particle motion of secondary wave is
perpendicular to the direction of propagation of the wave. Also called transverse or
rotational or equivoluminous wave. S-wave can travel only through solids, as fluids and gas
do not supports shear stresses.

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2.1.2. Surface wave
Seismic surface waves travel along the Earth's surface. They can be classified as a form
of mechanical surface waves. They are called surface waves, as they diminish as they get
further from the surface. They travel more slowly than seismic body waves (P and S). In
large earthquakes, surface wave can have an amplitudes of several centimeters. Surface
wave further divided into two types.
2.1.2.1. Rayleigh wave
Rayleigh waves, also called ground roll, these are the combination of P-wave and vertical
component of S-wave and travel as ripples with motions that are similar to those of waves
on the surface of water (note, however, that the associated particle motion at shallow
depths is retrograde, and that the restoring force in Rayleigh and in other seismic waves is
elastic, not gravitational as for water waves). They are slower than body waves, roughly
90% of the velocity of S waves for typical homogeneous elastic media. In the layered
medium the velocity of the Rayleigh waves depends on their frequency and wavelength.
2.1.2.2. Love wave
Love waves are the waves with combination of P-wave and horizontal component of S-
wave. These types of wave generated under some conditions that the medium should be
layered and important one is that there should be high velocity layer followed by low
velocity layer. They travels slightly faster than the Rayleigh wave and its velocity is
approximately 90% of the S-wave velocity.
Picture credit: park seismic LLC
Figure 2.1 different types of seismic waves

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2.2. Importance of surface wave
Surface wave is as much important as the body waves. It contains some properties of the
subsurface that others(body wave) cannot. Surface wave can be used to obtain Vs profiles
for followings:
 Earthquake site response
 Seismic microzonation
 Liquefaction analysis
 Soil compaction control
 Mapping subsurface stratigraphy

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Chapter-3
3. Surface wave analysis
Shear wave velocity is an important parameter for estimating how the earth will behave
during an earthquake. Shear wave velocities are difficult to measure directly due to low
signal-to-noise ratio, the bulk of the noise coming from higher amplitude P-waves. Surface
waves, as the name suggests, travel at or near the surface of the ground. They are
characterized by low velocity, low frequency, and relatively high amplitude. They are often
referred to as “ground roll” and are a source of noise in shallow seismic reflection. Surface
waves are dispersive; different frequencies travel at different velocities in a manner similar
to light. As it happens, surface wave phase velocities –the velocities of different
frequencies- are a useful proxy for shear wave velocity, and because of their high
amplitude, are relatively easy to measure. They are easily generated by both active and
passive sources.
3.1. MASW method
The multichannel analysis of surface waves (MASW) method is one of the seismic survey
methods evaluating the elastic condition (stiffness) of the ground for geotechnical
engineering purposes. MASW first measures seismic surface waves generated from various
types of seismic sources—such as sledge hammer—analyzes the propagation velocities of
those surface waves, and then finally deduces shear-wave velocity (Vs) variations below the
surveyed area that is most responsible for the analyzed propagation velocity pattern of
surface waves. Shear-wave velocity (Vs) is one of the elastic constants and closely related
to Young’s modulus. Under most circumstances, Vs is a direct indicator of the ground
strength (stiffness) and therefore commonly used to derive load-bearing capacity. After a
relatively simple procedure, final Vs information is provided in 1-D, 2-D, and 3-D formats.
Picture
credit: park seismic LLC
Figure 3.1. typical representation of 1-D MASW

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3.2. MASW standard workflow
3.3. Advantages of MASW
 It gives direct measurement of shear wave velocity
 It is very cost effective than conventional seismic survey
 Works good in small Spread length as compared to the conventional seismic survey
 Non destructive method can be perform even near the logistics
 Also useful in construction of big structures i.e. Dams, buildings etc.
Data acquisition Dispersion analysis inversion

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Chapter-4
4. Field measurement
4.1. Active source MASW
In active source MASW the active source is used such as hammers, accelerated weight drop.
It can be 1-D or 2-D depends if data acquired with only one shot then it is called 1-D active
source MASW and if data acquired with multiple shot then it is called 2-D active source
MASW.
Picture credit: park seismic LLC Figure 4.1. Active source MASW
4.2. Passive source MASW
In passive source MASW the natural source or background noise is used such as vehicle
passing, wind, raining, ocean tidal activity of earth etc. these are often less than 1 Hz.
Picture credit: park seismic LLC Figure 4.2. passive source MASW data acquisition

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4.3. General procedure
The MASW field layout is similar to that of seismic refraction technique. Twenty four or
more, geophones are laid out in a linear array with 2 m spacing and connected to a
multichannel seismograph as shown in figure 4.1. This technique is ideally suited to Vs
imaging, with data collected in a roll along manner similar to that of the seismic reflection
technique. The source is offset at a predetermined distance from the near geophone.
4.4. Acquisition geometry
Following acquisition geometry was used while acquiring data at Gymkhana ground, IIT
Bombay
Figure 4.4. Acquisition geometry for 24 channel active source MASW

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4.5. Equipments
The following field equipments were used while acquiring the data in Gymkhana ground
Geode is a multichannel seismograph manufactured by Geometrix used. It is ideal for
refraction or reflection, downhole or VSP - even tomography surveys.
4.5 Hz vertical sensor geophone is used in order to record the Rayleigh wave as discussed
before that Rayleigh wave consists of P and Sv waves. So this vertical component is suitable
for MASW (4.5 Hz is the peak frequency response).
As a active source accelerated weight drop is used. It works automatically and hit the
ground attached with a metal plate.
Figure 4.5. Equipments used at field site

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4.6. Acquisition parameters
These are the following data acquisition parameters while acquiring the data at Gymkhana
ground, IIT Bombay
• Channel line direction is North-South
• Number of channels = 48
• Spacing of channels = 2 m
• Length of spread = 96m
• Total number of shots = one shot for 1-D and 49 shots for 2-D MASW
• Shot spacing = 2m
• Shot location = -1 to 95m
• Total number of folds= 24
• Record length =0.5 seconds
• Numbers of shot per stack = 2 shots
• Depth of investigation = half of the spread length (48m)
4.7. Data acquired
The following data was acquired with above configurations
Figure 4.7. Data acquired at Gymkhana ground, IIT Bombay

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5. Dispersion analysis
Dispersion analysis is one of the most important part of the MASW study.
5.1. Dispersion methodology
In order to get the dispersion curve (plot between frequency and amplitude) it is necessary
to first change the raw data to its different frequency components, this can be done by
using stretch function.
5.1.1. Stretch function
An impulsive record R(t) obtained by using a source such as a sledgehammer or weight drop
can be transformed into the swept-frequency record Rs(t) by convolution of R(t) with a
stretch function S(t) (Coruh, 1985):
Rs(t) = R(t)*S(t)
where ∗ denotes the convolution operation. The stretch function S(t) is a sinusoidal function
with changing frequency as a function of time. A linear sweep similar to those commonly
used in Vibroseis surveys (Waters, 1978) is a good choice for S(t):
S (t) = sin(2π f1 𝑡 + π( f2 − f1) 𝑡2
/T)
where f1, f2, and T are lowest, highest, and length of s(t). These parameters can be
optimized using the previously outlined procedure.
Figure 5.1.1. (a) A shot gather obtained using a sledgehammer as the source and (b) its 3-s-long
swept-frequency display after transformation using a stretch function. Picture credit: Park et al

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5.1.2. Dispersion : phase shift method
An N-channel record 𝑚𝑟 𝑁 is defined as an array of N traces collected by one of the aforementioned
acquisition methods: 𝑚𝑟 𝑁=𝑟𝑖 (i=1, 2, …, N) with its frequency-domain representation of
𝑀𝑟 𝑁(ꙍ)= 𝑅𝑖(ꙍ)=FFT[𝑟𝑖] (i=1,2,3,….,N). Then, Ri(ꙍ) can be written as a product of amplitude, Ai(ꙍ),
and phase, Pi(ꙍ), terms: Ri(ꙍ) = Ai(ꙍ) Pi(ꙍ). Ai(ꙍ) changes with both offset (i) and angular
frequency (ꙍ) due to spherical divergence, attenuation, and source spectrum characteristics. Pi(ꙍ)
is the term that is determined by phase velocity(c) of each frequency:
Pi(ꙍ)=e−jΦi(ꙍ)
,
Where
Φi(ꙍ)=ꙍxi/c = ꙍ {x1+ (i-1) dx}/c
Consider one specific frequency of Ri(ꙍ). Its time-domain representation will be an array of
sinusoidal curves of the same angular frequency, but with different amplitude and phase.
Since the amplitude does not contain any information linked to phase velocity Ri(ꙍ) can be
Normalized without loss of significant information:
Ri,norm (ꙍ) =Ri(ꙍ)/| Ri(ꙍ)| =Pi(ꙍ).
Fig. 5.1.2a shows an array of normalized sinusoid curves for an arbitrary frequency of20 Hz
propagating at another arbitrary phase velocity of 140 m/sec. Sinusoid curves in the figure
have the same phase along a slope (S0) of the phase velocity, whereas they have different
phase along the slopes of other phase velocities, as indicated in the figure.
Picture credit: Park et al
Figure 5.1.2(a) A surface wave of an arbitrary frequency of 20 Hz with an arbitrary phase
velocity of 140 m/sec, and (b) curves of summed amplitudes for different number of trace

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Chapter-6
6. Inversion
Inversion of dispersion is as important as dispersion itself. The Vs profiles are calculated
using an iterative process that requires the dispersion data and estimation of Poisson’s ratio
and density. For the method used here, only Vs updated after each iteration, with Poisson’s
ratio, density, and model thickness remaining unchanged throughout the inversion.
An initial earth model needs to be specified as a starting point of the iterative inversion
process. The earth model consists of velocity (p and s –wave velocity), density, and
thickness parameters. Among these four parameters, Vs has the most significant effect on
the reliable convergence of the algorithm. After the theoretical model the non-linear least
square method were used for iteration and theoretical model changes accordingly until
the error minimized upto certain value then we can say that this is the near to true model
whose dispersion curve matches with original curve.
Figure 6. MASW inversion flowchart

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Chapter-7
7. Results and discussion
These are the result of the Multichannel analysis of surface wave at Gymkhana Ground, IIT
Bombay.
7.1. Dispersion curve
Dispersion curve obtained for 1-D masw data of Gymkhana ground, the fundamental mode
was picked as shown in red dots near to the maximum normalized amplitude. The two blue
straight line showing in the dispersion curve showing that beyond these line dispersion
curve showing no surface wave in other words these lines are window in which the picking
have to be done.
Figure 7.1.a picked dispersion curve of gymkhana ground MASW data

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Figure 7.1.b dispersion curve of gymkhana ground MASW

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7.2. Initial model
Figure 7.2. initial model of gymkhana ground MASW
7.3. Final model
Figure 7.3. final Vs model of gymkhana ground MASW

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7.4. Discussion
From the final model of shear wave velocity with depth that contains five layers may be of
following type:
We can consider layer 1 and 2 as a single layer because there is slight difference in S-wave
velocity between them. It extends upto 10 meters and upper part Vs is near to 300 m/s
which may be the velocity of the soil with vegetation and lower part Vs is nearly 375 m/s
which may be due to compacted soil.
Similarly for layer 3 and 4 can be considered as a single layer extends from 10m to at a
depth of 30m. Upper part of the layer showing Vs close to 820m/s and lower part Vs
920m/s that is close to the velocity of weathered basalt. After depth of 30m basement rock
basalt starts with shear wave velocity close to 1540m/s. standard shear velocities of
different soil and rock given below.
Ground profile name Average shear wave velocity (m/s)
Soft soil 0 < Vs ≤ 180
Stiff soil 180 < Vs ≤ 360
Very dense soil and soft rock 360 < Vs ≤ 760
Weathered Rock 760 < Vs ≤ 1500
Hard Rock Vs >1500
Source:https://asa.scitation.org/doi/abs/10.1121/1.3385162

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Chapter-8
8. Conclusion
According to the shear velocity of different soil and rock type given in previous page, the
final conclusion is that at first 10 m depth there is stiff soil then from 10-30 m depth it is
weathered rock and after depth of 30m un-weathered /hard rock continues.

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Chapter-9
9. References
 Choon B. Park, Richard D. Miller, Jianghai Xia,GEOPHYSICS, VOL. 64, NO. 3 (MAY-
JUNE 1999); P. 800–808, 7
 Choon B. Park, Richard D. Miller, Jianghai Xia ,GEOPHYSICS, VOL. 64, NO. 3 (MAY-
JUNE 1999); P. 691–700, 8
 http://www.masw.com/DataAcquisition.html
 Park, C.B., Miller, R.D., and Xia, J., 1999, Multichannel analysis of surface waves
(MASW): Geophysics, 64, 800-808.
 http://www.masw.com/files/DispersionImaingScheme
 https://en.wikipedia.org/wiki/Seismic_wave
 SeisImagerSW_Manual_v3.0
 John N.louie, shear-wave velocities from refraction micrometer, Seismological
society of America , feb 27,2001, P.1-13
 Jianghan Xia, Richard D. Miller, Choon B. park, and Julian ivanov, The Leading Edge,
August 2004, P.753-759.
 https://asa.scitation.org/doi/abs/10.1121/1.3385162