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EARS2232EARS2232

Exploration Exploration SeismicsSeismics

2006/20072006/2007

Semester 1Semester 1

Teaching Team

Dr. Sebastian Rost (Module Leader)

Dr. Graham Stuart (Seismic Interpretation)

Ben Dando (Demonstrator)

Objectives

On completion of this module students should be able to:

1. Understand the physical principles underlying the applicationof the seismic refraction and reflection techniques to thedetermination of shallow structure and the exploration forhydrocarbons

2. Appreciate the techniques and equipment used to undertake exploration seismic surveys on land and sea

3. Understand techniques for the processing and interpretationof seismic refraction data

Books

An introduction to GeophysicalExploration

Kearey, Brooks and Hill

Blackwell publishing

• Covers more than Seismology• Good introductory textbook• Easy to understand• not very mathematical

~32 £

Exploration Seismology

Sheriff and Geldart

Cambridge

• Very complete • Graphics a bit outdated• Mathematical background• reprinted in 2006

~45 £

Seismic Data Analysis

Yilmaz

SEG

• Way over the top for thiscourse

• Probably most complete• Must have if you stay in

the field

~150 - 290 $ (really US $)

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3D Seismic Interpretation

Bacon, Simm and Redshaw

Cambridge

• Good for seismic interpret.• Acquisition• Processing• last ¼ of course

~80 £

What is Exploration Seismology ?

“Exploration seismology deals with the use of artificially

generated elastic waves to locate mineral deposits

(including hydrocarbons, ores, water, geothermal

reservoirs, etc.), archeological sites, and to obtain geological

information for engineering.”

(Sheriff and Geldart, 1995)

Mintropkugel in Göttingen(first used in 1908 – L. Mintrop)

Wiechert Vertical Seismometer

http://www.erdbebenwarte.de

Seismological exploration stops long before unique answers arefound…

⇒ Additional (better methods: drilling wells etc)

Techniques are exchanged between exploration seismology and global seismology

Basic techniques: measuring travel times from seismic time series

Simple concept:

Seismic waves are generated at a source such as an explosion, these waves propagate through an elastic medium by reflection and refraction and are recorded at a receiver.

Amount of time taken and intensity (amplitude) holds information about both the source and the medium through which the wave has travelled.

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Wave types

BODY WAVES

Seismic waves which travel through the body of the medium

divided into P- and S- waves

SURFACE WAVES

Seismic waves which travel along or near the surface of a body

And the motion decays rapidly with distance from the surface.

Body Waves

P waves compressional waves,

particle motion in direction of propagation

S-waves transversal/shear waves

particle motion perpendicular to direction

of propagation

ρ

µκ3

4+

=pv

ρ

µ=sv

κ = bulk modulus = incompressibililtyµ = shear modulus = rigidityρ = density

SV-wave:

S wave energy polarised so the the motion is in a vertical

(saggital) plane which also contains the direction of wave

propagation –P and SV solutions are coupled. recorded on the radial component.

SH-wave:

S-wave which has only a horizontal component of motion

SH waves are mathematically decoupled from P-SV solutions

Important points:

• Elasticity increases at a greater rate than density so velocity (in general) increases with depth

• No shear waves in a fluid

• for perfectly elastic solid: SP VV ⋅≈ 3

0=SV

Surface waves

Seismic waves which travel along or near the free

surface of a body and the motion or energy of the

wave decays rapidly with distance from the surface.

Surface waves travel with slower velocities than

body waves

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Rayleigh wave

A surface wave whose particle motion is elliptical and retrograde in the vertical plane containing the direction of wave propagation.

Its amplitude decreases exponentially with depth.

Particle motion retrograde ellipse.

In exploration know as Ground Roll are important as obscure signals of interest.

In a layered Earth they are dispersive.

Love waves

A surface wave associated with the surface layer which is characterised by horizontal motion perpendicular to the direction of propagation with no vertical motion.

They can be thought of SH waves trapped in a surface or channel; must have at least one layer to exist.

They are dispersive and travel faster than Rayleigh but slower than S-waves

Dispersion

Variation of velocity with frequency.

Dispersion of a body wave is usually small* but surface waves show considerable dispersion.

Group velocity refers to the velocity of energy propagation. Phase velocity refers to the velocity of a particular phase e.g. peak/trough

*Typically just a few % difference between 10s of Hz and 10s of kHz

A dispersed Rayleigh wave generated by an earthquake

in Alabama near the Gulf coast, and recorded inMissouri.

Phase and Group velocities

Sheriff and Geldart, 1995

P-wave velocitiesUnconsolidated material: Dry sand 0.2 - 1.0 km/s

Wet sand 1.5 - 2.0 km/sClay 1.0 - 2.5 km/s

sedimentary rocks: Tertiary sandstone 2.0 - 2.5 km/sCarbon. sandstone 4.0 - 4.5 km/sChalk 2.0 - 2.5 km/sLimestone 3.4 - 7.0 km/sSalt 4.5 - 5.0 km/s

Igneous/metamorphic rocks: Granite 5.5 - 6.0 km/sGabbro 6.5 - 7.0 km/sGneiss 3.5 - 7.5 km/s

Air: 0.33 km/s; Water: 1.43-1.54 km/s; Petroleum: 1.3-1.4 km/s

P-velocities (cont.)

• Lithology - most obvious factor to control velocities

• Porosity: very important, depends on depth and pressure

• Velocity lowered, when gas/petroleum present

• More sensitive: Vp/Vs ratio

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Basic definitions

Frequency - number of times a wavelet repeats a second; measured in hertz (Hz = s-1)

Period - time between peaks, troughs or zero crossing on a waveform; measured in seconds (s)

Frequency = Period-1

A wavelet with a duration between peak and trough of 25mshas a period of 50 [ms] = 0.05 [s]

Frequency = 1/0.05 [s] = 20 [s-1] = 20 [Hz]

Wavelength - distance between peak or troughs on theGround - measured in meters (m)

Wavelength = Velocity / Frequency

A 50Hz wave traveling with a velocity of 4000 m/shas a wavelength of 4000/50 = 80m

Amplitude - measure of the intensity of the wave ~ energy

Wavefront: a surface over which the phase (travel-time) of a traveling wave is e e.g. one ripple on a pond

Ray: the raypath is the direction of energy transport. In isotropicMedia the ray is perpendicular to the wavefront.

Travel time: the time for a wave to travel from one point toanother along a ray path.

As a seismic wave propagates through regions of changing velocity its ray direction will change.

This is known as refraction.

Rays will refract towards regions of lower velocity and away from high velocity regions.

In general velocity increases with depth. As a result seismic energy will turn as it propagates in to the Earth eventually arriving at the surface again (turning waves).

It can be shown that a linear velocity gradient results in a ray path which is an arc of a circle.

When a seismic wave crosses a

boundary between two media the wave changes direction such that

the horizontal component of 1/velocity is conserved It is easy to

prove using Fermats principle

Snell’s law

Snell’s law: ratio of sine of angle of incidence and refraction angle

are equals the ratio of velocities

2

1

2

1

sin

sin

β

β

α

α==

r

i

pv

r

v

i==

21

sinsin

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Reflection/Transmission

Reflection:

a) normal incidence b) inclined incidence

Reflection coefficient

(normal incident):2211

1122

vv

vv

A

AR

I

R

ρρ

ρρ

+

−==

incident ray reflected ray

A A

transm. rayA

I

T

R

v1,ρ1

v2,ρ2

Acoustic Impedance

VZ ⋅= ρ

= density x velocity

Note: when ρρρρ1V1 < ρρρρ2V2 then R is negative, i.e. the

Reflected wave will undergo a phase change by ππππ

⇒⇒⇒⇒ The polarity of the wavelet will undergo sign change

Normal Incidence!!

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The simple normal-incidence relations are a special case of the more

complex equations which describe the reflection and transmission

coefficients for elastic waves with arbitary angle of incidence form

the boundary – known as Zoeppritz equations (1919)

http://www.crewes.org/Samples/ZoepExpl/ZoeppritzExplorer.html

Karl Zoeppritz

Head waves

Rays which enter or leave a high velocity medium at

a critical angle are known as head waves or

refracted waves

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Using rays: assumption: reflection in one point.

reality: waves – energy turning from

large area

= Fresnel Zone

F.Z. : area from which reflected energy arriving at the station

has phase difference of less than half cycle

⇒ energy interferes constructively

2

2

0

2/1

0

hS

hnRn

πλ

λ

≈∆

≈

λ/4 criterion

2

2

0

2/1

0

hS

hnRn

πλ

λ

≈∆

≈

Radius of FZ

Area of annular ring

Diffraction occurs at abrupt discontinuities

or structures whose radius is shorter than a

wavelength

Cause: Huygens principle

Diffracting edge

Huygens’ Principle every point on an advancing wavefront can beregarded as the source of a secondary wave and that a later

wavefornt is the envelope tangent to all the secondary sources