Deba Tg Pr1

NAÏVE EFFORT OFA DAREDEVIL

UTTERLY & TRULY A

NOT-THAT-TECHNICAL PRESENTATION

ON

SEISMIC WAVES:

Theory & Implications

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Propagation of seismic wave in the ground and the effect of presence of land mine.

PLEASE,DO GIVE IT A SHOT.

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EXPERIENCING THE GEOSCIENCE FIELD

EFFECT

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GET SEISMIC!!!

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Seismic waves are waves that travel through Earth, most often due to natural causes, sometimes from anthropogenic ones.

‘ TIS WAVE ?! “heavily overloaded with meanings” "stuff that wobbles in space and time“ ( This is from a PF Mentor)

P- and S-waves sharing with the propagation

Waves can be represented by simple harmonic motion.

ATTENTION PLEASE!!! THERE ISTRANSFERANCE OF ENERGY!!!

WHAT???

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ELECTROMAGNETIC SCATTERING Here two charged particles approach each other and scatter by exchanging a virtual photon. Animation reproduced by kind permission of (c) J. Eric Slone www.FeynmanOnline.com.(http://www.egglescliffe.org.uk/physics/particles/parts/

parts1.html)

Could you please define ENERGY?

In My Book, The Most Elusive Stuff In The Universe!

ENERGY THE GREATEST MAKER!!!THE GREATEST BREAKER!!!

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The elastic wave equation in three dimensions describes the propagation of waves in an isotropic homogeneous elastic medium. As most solid materials are elastic, seismic waves that travel through earth can be described by this equation:

(2μ+λ) (   .ψ)–μ   ×(   ×ψ)=ρ(∂2ψ/∂t2) … … … (1)

Where:

λ and μ are the so-called Lamé parameters describing

the elastic properties of the medium,ρ is density, ψ is the source function (driving force),

This equation must account for both transverse and longitudinal motion. Here, both force and displacement are vector quantities. That’s why this equation is sometimes known as the vector wave equation.

Elastic Wave Equation in Three Dimensions

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‘TIS SEISMIC DELICACY!

Seismic Waves come chiefly in TWO FLAVORS

SO APPETIZING!!!!!

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In an infinite homogeneous

isotropic medium, only body waves exist.

BODY WAVES

BODY WAVES

THIS STUFF ALSO COMES IN

TWO FLAVORS.

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Putting ( × ψ)=0 in the equation number 1

we get P wave.

This type of seismic wave is also variously known as a dilational, longitudinal, irrotational, compressional waves.

It Involves alternatingly Compression

and dilation.

P WAVE

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Copyright 2004. L. Braile.

P WAVE

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Putting ( . ψ)=0 in the equation

number 1 we get S wave.

This type of seismic wave is also

variously known as a shear, transverse, rotational waves.

It Involves Alternating transverse

motion.

S WAVE

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Copyright 2004. L. Braile.

Transverse particle motion shown here is vertical but can be in any direction. However, Earth’s layers tend to cause mostly vertical (SV) or horizontal (SH) shear motions.

S WAVE

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Velocities of P wave and S wave can be formulated as the following way:

α={( λ+2μ)/ ρ}1/2

β=(μ/ρ)½

Where α and β stand for velocities of P wave and S wave respectively. As the elastic constants are positive, α is greater than β. Writing γ for the ratio β/ α we see that

γ2= β2/α2= μ/( λ+2μ)= (1/2-σ)/(1-σ)As σ decreases from 0.5 to 0 γ increases from 0 to its maximum value 1/21/2.

Therefore, the velocity of the S wave ranges from 0 to 70% of the velocity of the P wave.

VELOCITIES OF P AND S WAVES

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S-waves do not pass through the earth's core, forming a Shadow Zone on the earth's side opposite from the epicenter. There is another shadow zone for P wave which is formed due to refraction.

SHADOW ZONE

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SURFACE WAVES

When the medium does not

extend to infinity in all directions, surface waves are produced.

Surface Waves

Surface waves come in different flavors including Rayleigh, Love, Tube, Stoneley and Channel waves.

We, however, confine ourselves to Rayleigh and Love waves for this presentation.

Propagates along the free surface of a semi-infinite medium.

For a Poisson ratio of σ =1/4,the Rayleigh wave velocity is 0.9194 times the S wave Velocity.

It can be thought of as special case of Stoneley wave( wave along a interface).

RAYLEIGH WAVE

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Particle motion near the surface is elliptical and retrograde i.e., the particle moves opposite to the direction of propagation at the top of its elliptical path in the vertical plane containing the direction of propagation.

Its amplitude decreases exponentially with depth, and the elastic properties to a depth of about one wavelength determine its velocity.

RAYLEIGH WAVE

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FIG. R-3. Rayleigh-wave motion. (a) Theoretical Rayleigh wave has retrograde elliptical motion at the surface. (b) Hodograph of particle velocity involved in ground roll.

(From Howell, 1959, 80.) (c) Experimental measurements from a small explosion measured with buried geophones showing the change from retrograde to prograde motion

with depth. (After Dobrin, 1951).

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FIG. The dependence of Rayleigh wave velocity VR on Poisson’s ratio σ. The P-wave velocity is α and the S-wave velocity β.

RAYLEIGH WAVE

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Copyright 2004. L.

Braile.

RAYLEIGH WAVE

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LOVE WAVE

This is a seismic channel wave associated with

a surface layer that has rigidity, characterized by horizontal motion perpendicular to the direction of propagation with no vertical motion. It may be thought of as channel waves in the upper layer.

Earthquake love waves have velocities up to

4.5 km/s, faster than Rayleigh waves.

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It can travel by different modes, designated by the number of nodal planes within the layer. However, usually only the zero mode is observed.

The dispersion of love waves can be

used to calculate the thickness of the surface layer.

LOVE WAVE

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FIG. Earthquake seismogram schematic (a) The vertical component; (b) one horizontal component

LOVE WAVE

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Copyright 2004. L.

Braile.

LOVE WAVE

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John Lahr demonstrates seismic waves on slinkies.URL:www.exo.net/.../summer_day10waves/wavetypes.html

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SEISMIC SOURCE

HOW???

Then Where do they come from?

Sorry! Sorry! Sorry!

It’s not from

Heaven or Hell!

They have

Mundane Sources.

AGAIN TWO FLAVORS!!!

A Seismic Source generates seismic energy.

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Sh!

Mom

Is

Furious

!!!!

NATURAL SEISMIC SOURCES

Natural seismic sources include the following ones:

EARTHQUAKES

VOLCANOES

INFRASOUND FROM THUNDERSTORM

Thunder is the sound made by the lightning

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ARTIFICIAL SEISMIC SOURCESAn artificial seismic source generates

controlled seismic energy that is used in both reflection and refraction seismic surveys. A seismic source signal has the following

characteristics: generated as an impulsive source band-limited the generated waves are time-varying

The generalized equation that shows all above properties

where fmax is the maximum frequency

component of the generated waveform.

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Hey

Kids!

No

More

Pranks

!!!

An air gun seismic source (30 litre).

The Artificial Seismic Sources:

EXPLOSIVES

AIR GUN

PLASMA SOUND SOURCE

THUMPER TRUCK

VIBROSEIS SOURCES

BOOMER SOURCES

ARTIFICIAL SEISMIC SOURCES

Plasma sound source fired in small swimming pool.

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WHY??????

THEN, WHY SHOULD WE BOTHER TO TEASE OUR EXTRAORDINARY BRAINS STUDYING ALL THESE

STUFFS!?!?!?!?!?

‘CAUSE WE KNOW THAT’S NOT A WASTE.

This is worth investing

our money, time and effort.

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HERE, WE WILL FOCUS ON THE FOLLOWING ONESTO HAVE AN IDEA. First: SURVIVING THE HARSHNESS OF MOTHER NATURESecond: WHY DO INDUSTRIES CARE?

SURVIVING THE HARSHNESS OF MOTHER NATURE

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To Fight the EARTHQUAKES a complete grasp of SEISMIC WAVE is INDISPENSABLE.

IF YOU CARETHE SHAKE, YOU HAVE GOT TO CARE ABOUT IT.

OTHERWISE YOU NEED NOT!!!!!!!!!!

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SURVIVING THE HARSHNESS OF MOTHER NATURE

TSUNAMIS AND EARTHQUAKES SHARE A COMMON ORIGIN. THEREFORE, TO SURVIVE TSUNAMIS SEISMIC WAVES HAVE GOT TO SURVIVE.

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SURVIVING THE HARSHNESS OF MOTHER NATURE

THEREFORE, GET SEISMIC AND HE COULD NO LONGER SAY:

I LAVA YOU!

AT TIMES,STUDYINGSEISMIC WAVESWE CAN PREDICT VOLCANIC ERRUPTION.

WHY DOES INDUSTRY CARE?

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SEISMIC METHOD IS VIRTUALLY ALL SEISMIC WAVE. I KNOW YOU MUST KNOW THAT!!! AND

WHEN PEOPLE MEAN

HYDROCARBON INDUSTRIES,

WE -- GEOSCIENCE

FOLKS -- MEAN

SEISMIC METHODS.

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WHY DOES INDUSTRY CARE?

The SEISMIC METHODS are by far the most important geophysical techniques in terms of expenditures and number of geophysicists involved.

METHOD MEASURED PARAMETER

‘OPERATIVE’ PHYSICAL PROPERTY

Seismic Travel time of reflected/refracted seismic waves.

Density & elastic moduli which determine the propagation velocity of seismic waves.

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WHY DOES INDUSTRY CARE? APPLICATION APPROPRIATE

SURVEYMETHOD

Exploration for fossil fuels (oil, gas, coal)

S, G, M, (EM)

Exploration for metalliferous mineral deposits

M, EM, E, SP, IP

Exploration for bulk mineral deposits (sand & gravel)

S, (E), (G)

Exploration for underground water supplies

E, S, (G)

Engineering/construction Site investigation

E, S, (G), (M)

G: gravity; M: magnetic; S: seismic; E: electrical; SP: self potential; IP: induced polarization; EM: electromagnetic; Subsidiary methods in parentheses.

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WHY DOES INDUSTRY CARE?SEISMIC REFLECTION SURVEYING

In Seismic Reflection Surveys the travel times are

measured of arrivals reflected surface interfaces between media of different acoustic impedance.

Reflection surveys are most commonly carried out in

areas of shallowly dipping sedimentary sequences.

In such situations, velocity varies much more as a

function of depth, due to lateral facies changes within the individual Layers. For the purposes of initial considerations, the horizontal variations of velocities may be ignored.

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WHY DOES INDUSTRY CARE?

SEISMIC REFRACTION SURVEYING

The seismic refraction survey method utilizes seismic

energy that returns to surface after traveling through the ground along refracted ray paths.

The method is normally used to locate refracting

interfaces (refractors) separating layers of different seismic velocity, but the method is also applicable in cases where velocity varies smoothly as a function of depth or laterally.

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Refraction seismology is applied to a very wide range

of scientific and technical problems, from engineering site investigation surveys to large scale experiments designed to study the structure of the entire crust or lithosphere.

Refraction measurements can provide valuable velocity

information for use in the interpretation of reflection surveys, and refracted arrivals recorded during land reflection surveys are used to map the weathered layer.

This wide variety of applications leads to wide variety

of field survey methods and associated interpretation techniques.

WHY DOES INDUSTRY CARE?

REFERENCESW.M. Telford et al., APPLIED GEOPHYSICS, 2nd EditionP. KEAREY et al., AN INTRODUCTION TO GEOPHYSICAL EXPLORATIONhttp://en.wikipedia.org/http://www.psc.edu/science/2006/inprogress/images/quake_fig2.jpghttp://www.mysupplychain.co.uk/images/mainpic4_uncertainty.jpghttp://www.teachingboxes.org/earthquakes/lessons/rev-Squiggles_clip_image001.gifhttp://web.ics.purdue.edu/~braile/edumod/waves/WaveDemo.htmwww.matter.org.uk/.../shadow_zone.html http://iri.columbia.edu/~lareef/tsunami/

http://segdl.org/dictionary/ AND

Other sources which cannot be enumerated here. GEOSCIENCE CARES DEBA_PR142

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PRESENTER DEBAJYOTI BASU SARKAR JN 024 DEPT. OF GEOLOGY UNIVERSITY OF DHAKA

F

R

E

S

H

M

A

N

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LOTS AND LOTS AND LOTS OF

FOR YOUR TIME & EFFORT!!!!!!!!