ears2232 exploration seismics dr. sebastian rost...
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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• Very complete • Graphics a bit outdated• Mathematical background• reprinted in 2006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