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Why Ricker Wavelets Are Successful in Processing Seismic Data: Towards a Theoretical Explanation

Afshin Gholamy

Department of Geological Sciences University of Texas at El Paso El Paso, TX 79968 afshingholamy@gmail.com

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1. Outline

• Formulation of the problem
• Seismic waves and the empirical success of Ricker wavelets
• Analysis of the problem
• Main result
• Conclusions and future work

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2. Outline

• Formulation of the problem
• Seismic waves and the empirical success of Ricker wavelets
• Analysis of the problem
• Main result
• Conclusions and future work

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3. Seismic Data Is Very Useful

• Already ancient scientists noticed that earthquakes gen-

erate waves which can be detected at large distances.

• These waves were called seismic waves, after the Greek

word “seismos” meaning an earthquake.

• After a while, scientists realized that from the seismic

waves, we can extract: – not only important information about earthquakes, – but also information about the media through which these waves propagate.

• Different layers reflect, refract, and/or delay signals dif-

ferently.

• So, by observing the coming waves, we can extract a

lot of information about these layers.

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4. Seismic Data is Very Useful (cont-d)

• Since earthquakes are rare, geophysicists set up small

explosions to get seismic waves.

• The resulting seismic information helps:

– geophysicists, petroleum and mining engineers, to find mineral deposits; – hydrologists to find underground water reservoirs; – civil engineers to check stability of the underground layers below a future building, etc.

• In particular, computational intelligence techniques are

actively used in processing seismic data.

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5. Ricker Wavelet: Reminder

• We need to describe how the amplitude x(t) of a seismic

signal changes with time t.

• In 1953, N. Ricker proposed to use a linear combination
• f wavelets of the type

x0(t) =

• 1 − (t − t0)2

σ2

• · exp
• −(t − t0)2

2σ2

• .
• Different wavelets correspond to:

– different moments of time t0 and – different values of the parameter σ describing the duration of this wavelet signal.

• The power spectrum X0(ω) of this wavelet has the form

X0(ω) = K · ω2 · exp(−c · ω2), where c = σ2.

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6. Ricker Wavelets Are Empirically Successful

• Since the original Ricker’s paper, Ricker wavelets have

been successfully used in processing seismic signals.

• In particular, Ricker wavelets are used with computa-

tional intelligence techniques,

• The power spectrum S(ω) of the seismic signal is rep-

resented as a linear combination of Ricker spectra: S(ω) ≈

n

• i=1

Ki · ω2 · exp(−ci · ω2).

• This description requires 2n parameters Ki and ci.
• Often, this approximation is the most accurate of all

approximations with the fixed number of parameters.

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7. Need for a Theoretical Explanation

• Empirical fact: Ricker wavelets, in general, lead to a

better approximation of the seismic spectra.

• Problem:

– there are many possible families of approximating functions, and – only few of these families were actually tested.

• Natural question:

– are Ricker wavelets indeed the best or – they are just a good approximation to some even better family of approximating functions?

• What we show: Ricker wavelets are the best.

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8. Outline

• Formulation of the problem
• Seismic waves and the empirical success of Ricker

wavelets

• Analysis of the problem
• Main result
• Conclusions and future work

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9. Wavelets as a Way to Describe Seismic Waves

• Waves are ubiquitous in nature, from the ocean waves

to electromagnetic waves.

• In most physical examples, waves last many periods.
• Such waves are well described by the traditional sine

and cosine functions – or by their linear combinations.

• In contrast, earthquakes usually have a reasonably short

duration.

• Thus, the related seismic waves also have a short du-

ration.

• Such short-duration waves also happen in other phys-

ical situations.

• To describe such “small waves” (“wavelets”), researchers

use special functions called wavelets.

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10. Geophysical Applications of Ricker Wavelets: classification

• Ricker wavelets x0(t) are used in all aspects of seismic

data processing. 1) Ricker wavelets x0(t) are used to simulate: – the original earthquakes, – the explosions set up by the geoscientists, and – the seismic waves generated by these earthquakes and explosions. 2) Functions x0(t) are used to process the seismic data x(t), by representing x(t) as a linear comb. of x0(t). 3) Ricker wavelets x0(t) are also used to process other geophysical signals.

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11. Ricker Wavelets are Useful in Simulating the Original Earthquakes

• Often, Ricker wavelets x0(t) provide the best descrip-

tion of the near-fault ground motion.

• In Apostolou et al. 2007 and Gerolymos et al. 2005,

x0(t) simulate the effect caused by earthquakes: – on “block-size” structures, in which the height is commeasurable with width, and – on tall structures – like high-rise building and tall bridge piers.

• Zania et al. 2008 uses Ricker wavelet to simulate the

effect of earthquakes on waste landfills.

• This is important: e.g., 1994 Northridge and 1995 Kobe

earthquakes caused serious damage to landfills.

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12. Ricker Wavelets Are Useful in Simulating Source Signals

• Ricker wavelets are also often the best in simulating the

source signal, i.e., the explosion set up by geoscientists.

• In Chakraborty et al. 1995 and Zhang et al. 2002, such

a simulation is used: – to select the best method – for processing seismic data.

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13. Ricker Wavelets Are Useful in Simulating Seis- mic Waves

• Linear combinations of Ricker wavelets x0(t) provide a

good description of the actual seismic signals.

• As a result, such linear combinations are used to select

the best methods for processing seismic data.

• For example:

– McCormack et al. 1993 use x0(t) to select an algo- rithm for picking the first-break refraction event; – Barnes et al. 1993 use x0(t) to select signal pro- cessing techniques in reflection seismology.

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14. Ricker Wavelets Are Useful in Processing Seis- mic Signals

• Ricker wavelets are also effectively used in processing

seismic signals.

• Specifically, a signal is approximated as a linear com-

bination of Ricker wavelets.

• Liu and Fomel 2004 show that this improves joint in-

version of P-wave and S-wave seismic data.

• Wolfe et al. 1988 shows that the visualization of this

approximation helps seismologists’ understanding.

• This is illustrated on the example of the “Golden Block”

– an oil-rich area in the North Sea near Scotland.

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15. Geophysical Applications Beyond Seismic Waves

• Ricker wavelets are very useful in analyzing the signal

recorded by the Ground Penetrating Radar.

• Economou et al. 2012 showed that the use of Ricker

wavelets leads to better results in: 1) construction of public buildings, to locate possible fracture zones and voids; example:

• a university building in Crete, where the main

geology is karstic carbonates; 2) road monitoring, to measure the thickness of diff. lay- ers and to look for possible defects; examples:

• an airport auxiliary road in Greece
• a highway connecting Central Greece with At-

tica (region containing Athens).

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16. Ricker Wavelets Are Often Empirically the Best

• Several papers provide an explicit comparison between

Ricket wavelets and other techniques.

• These papers show that Ricker wavelets are indeed the

best.

• For example, Deng et al. 2007 and Liu et al. 2004 show

that: – we need fewer parameters to represent a seismic signal as a linear combination of Ricker wavelets – than to get a similarly accurate representation in terms of wavelets of other type.

• A similar conclusion is made in Economou et al. 2012

for representing the Ground Penetrating Radar data.

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17. Resulting Question

• Empirical studies show that Ricker wavelets, in gen-

eral, lead to a better approximation of seismic data.

• However, in principle, there are many possible families
• f approximating functions.
• Only few of these families were actually tested.
• So, a natural question arises:

– are Ricker wavelets indeed the best – or are they just a good approximation to some even better (not yet known) family of functions?

• This is the question that we will try to answer in this

thesis.

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18. Outline

• Formulation of the problem
• Seismic waves and the empirical success of Ricker wavelets
• Analysis of the problem
• Main result
• Conclusions and future work

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19. How Each Propagation Layer Affects the Seis- mic Signal

• Layers are not homogeneous; as a result:

– the same seismic signal, when passing through dif- ferent locations on the same layer, – can experience different time delays.

• Thus, a unit pulse signal at moment 0 is transformed

into a signal m(t) which is distributed in time.

• A signal x(t) can be represented as a linear combina-

tion of pulses of amplitudes x(si) at moments si.

• Each such pulse is transformed into m(t − si) · x(si).
• So, each layer transforms the original signal x(t) into

the new signal

• m(t − s) · x(s) ds.

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20. What is the Joint Effect of Propagating the Signal Through Several Layers?

• A signal x0(t) passes through the first layer, and is thus

transformed into x1(t) =

• m1(t − s) · x0(s) ds.
• The signal x1(t) passes through the second layer, and

is, thus, transformed into y(t) =

• m2(t − s) · x1(s) ds.
• Substituting the expression for x1(s), we conclude that

y(t) =

• m(t − u) · x0(u) du, where

m(t) =

• m1(s) · m2(t − s) ds.
• The formula is known as the convolution of two func-

tions m1(t) and m2(s) corresponding to the two layers.

• In general, the joint effect of several layers is a convo-

lution of several functions mi(t).

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21. How to Describe Convolutions of Several Func- tions?

• A similar problem appears in probability theory.
• If independent random variables xi have pdf’s ρi(xi),

then the pdf ρ(x) of x = xi is a convolution: ρ(x) =

• ρ1(x1) · ρ2(x − x1) dx1.
• According to the Central Limit Theorem:

– if we have a large number of small independent ran- dom variables, – then the distribution for their sum is close to Gaus- sian (normal).

• Different layers are independent.
• Thus, the joint effect of several layers is described by

the Gaussian formula m(t) = C · exp

• − t2

2σ2

• .

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22. Fourier Transform Helps

• We know that y(t) =
• m(t − s) · x(s) ds.
• If we know n values of m(t) and x(t), we need n2 com-

putations to follow this formula.

• FFT computes Fourier transform

ˆ f(ω)

def

=

• exp(−i·ω·x)·f(x) dx in time O(n·ln n) ≪ n2.
• In terms of Fourier transforms, ˆ

y(ω) = ˆ m(ω) · ˆ x(ω).

• For Gaussian m(t), its Fourier transform is also Gaus-

sian, so ˆ y(ω) = const · exp

• −1

2 · σ2 · ω2

· ˆ x(ω).

• We are interested in the power spectra X(ω)

def

= |ˆ x(ω)|2 and Y (ω)

def

= |ˆ y(ω)|2, so Y (ω) = const · exp(−α · ω2) · X(ω), where α

def

= σ2.

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23. Analysis of the Problem

• We want to select a function F(ω) that describes ob-

served power spectrum of the seismic signal x(t).

• By definition, power spectrum X(ω) is always non-

negative, so we require that F(ω) ≥ 0.

• A single seismic signal quickly fades with time.
• It is known that when a signal x(t) is limited in time,

its Fourier transform ˆ x(ω) is differentiable.

• So, we require that F(ω) be smooth.
• Thus, its power spectrum X(ω) = ˆ

x(ω)·(ˆ x(ω))∗, where z∗ means complex conjugation, is also differentiable.

• A seismic signal can have different amplitude: if x(t)

is a reasonable signal, then C · x(t) is also reasonable.

• If F(ω) is a good approximation to spectrum X(ω),

then for K · X(ω), it is reasonable to use K · F(ω).

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24. Analysis of the Problem (cont-d)

• So, we look for an approximating family {K · F(ω)}K.
• Some seismic events are faster, some slower:

– if x(t) is a reasonable seismic signal, – then x(t/c) is also reasonable.

• For x(t/c), the spectrum is F(c · ω).
• Thus, we look for a family {K · F(c · ω)}K,c.
• We want to approximate observed energy spectra

Yi(ω) = const · exp(−αi · ω2) · X(ω); when α1 < α2: Y2(ω) = exp(−α · ω2) · Y1(ω), where α

def

= α2 − α1.

• So, if X(ω) is a reasonable power spectrum, then the

function exp(−α · ω2) · X(ω) is also reasonable.

• It is thus reasonable to require that exp(−α·ω2)·F(ω)

have the form K · F(c · ω) for some K and c.

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25. Outline

• Formulation of the problem
• Seismic waves and the empirical success of Ricker wavelets
• Analysis of the problem
• Main result
• Conclusions and future work

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26. Main Result

• Let F(ω) ≥ 0 be infinitely differentiable.
• We say that a family {K · F(c · ω)}K,c is propagation-

invariant if for every α, there exist K(α) and c(α) s.t. exp(−α · ω2) · F(ω) = K(α) · F(c(α) · ω).

• Every propagation-invariant family corresponds to

F(ω) = ω2n · exp(−ω2) for some n = 0, 1, . . .

• The simplest case n = 0 correspond to a propagation
• f a simple pulse.
• Thus, the case n = 0 does not reflect the shape of the
• riginal signal.
• The simplest non-trivial case is n = 1, which is exactly

the Ricker wavelet.

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27. Outline

• Formulation of the problem
• Seismic waves and the empirical success of Ricker wavelets
• Analysis of the problem
• Main result
• Conclusions and future work

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28. Conclusions

• A natural way to process dynamic signals is to approx-

imate them by functions from an appropriate family.

• In this thesis, we consider the problem of processing

seismic data.

• For this problem, we formulated reasonable require-

ments for approximating functions.

• We showed that the simplest family of functions satis-

fying these requirements is the family of Ricker wavelets.

• This theoretical result is in good accordance with em-

pirical findings: that in many cases, – for a given accuracy, – the use of Ricker wavelets enables us to use fewer parameters.

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29. Future Work

• In many cases, Ricker wavelets provide a very good

approximation for seismic data.

• However, sometimes, the approximation quality of Ricker

wavelets needs improvement.

• Thus, it is not always sufficient to use the simplest

possible approximate family of functions.

• More complex approximating functions are sometimes

needed.

• It is therefore desirable:

– to find the best of such more complex approximat- ing families, – similar to how we found that the best of the sim- plest approximating families consists of Ricker wavelets.

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30. Acknowledgments

• First, and foremost, I would like to express my pro-

found gratitude to my mentor Professor Laura Serpa.

• I thank you for all your support, guidance, and friend-

ship over the last two years.

• You took a chance on me when I was anything but a

sure bet. For that, I am forever in your debt.

• I wish to thank the members of my committee, Dr. Aaron

Velasco and Dr. Vladik Kreinovich, for their support.

• I would like to thank all the faculty, staff, and students
• f the Department of Geological Sciences.
• My acknowledgements also goes to Dr. Reza Ashtiani

from Civil Engineering Dept. for his invaluable help.

• Last but not least, I would like to thank my family for

the endless support throughout my life.

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31. Appendix: Proof

• We require that exp(−α·ω2)·F(ω) = K(α)·F(c(α)·ω).
• We know that all the functions F(ω), K(α), and c(α)

are differentiable.

• Thus, we can differentiate the above equality, and get

−F(ω) · exp(−α · ω2) · ω2 = K′(α) · F(c(α) · ω) + K(α) · F ′(c(α) · ω) · c′(α) · ω.

• For α = 0, we use K(0) = c(0) = 1 to get

−F(ω)·ω2 = k·F(ω)+F ′(ω)·c·ω, where k

def

= K′(0), c

def

= c′(0).

• Moving all terms ∼ F(ω) to the left-hand side, we get

F · (−k − ω2) = c · dF dω · ω.

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32. Proof (cont-d)

• Let us move all the terms dF and F to the right-hand

side and all the other terms to the left-hand side: 1 c · −k − ω2 ω = dF F , i.e., − k c · 1 ω − c · ω = dF F .

• Integrating both sides, we get

C − k c · ln(ω) − c 2 · ω2 = ln(F).

• By exponentiating both sides, we conclude that

F(ω) = A·ωb·exp(−B·ω2), w/A = exp(C), b = −k c, B = c 2.

• The requirement that F(ω) is infinitely differentiable

for ω = 0 implies that b is a natural number.

• The requirement that F(ω) ≥ 0 means that b is even:

b = 2n for some natural number n. Q.E.D.