chapter 2 - basic principles of the seismic method 01
TRANSCRIPT
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Overview ta3520Introduction to seismics
• Fourier Analysis• Basic principles of the Seismic Method• Interpretation of Raw Seismic Records• Seismic Instrumentation• Processing of Seismic Reflection Data• Vertical Seismic Profiles
Practical:• Processing practical (with MATLAB)
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Types of seismic waves
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Types of seismic waves: nomenclature
P-waves:
- compressional waves
- longitudinal waves
- Push-waves
- Primary waves
S-waves:
- shear waves
- transverse waves
- Shake waves
- Secondary waves
(physics)
(mathematics)
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Types of seismic waves: nomenclature
P-waves: S-waves:
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Seismic Waves
Medium 1
Medium 2(faster)
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Huygens’ principle
Wavefront at time t
Wavefront at time t + Δt
ray
ray
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Huygens’ sources
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Snell’s Law (at boundary)
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Snell’s Law for P- and S-waves
boundary
θtrans,P
θrefl,Pθinc,P
θtrans,S
θrefl,S
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Different arrivals
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Seismic Waves
Medium 1
Medium 2(faster)
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Seismic Waves
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Different arrivals
- Direct wave- Reflected wave- Refracted wave / Head wave
Derive each traveltime eq.
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Different arrivals: arrival times
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Arrival times: time pointing downwardsdistance x
traveltime
direct
refraction
reflection
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The seismic record
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Examples of records
On land At sea
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The wave equation (for P-waves)
Consider mass with volume ΔV
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Desired:
Equations in terms of pressure (p) and particle velocity (v)
(pdf-file with eqs)
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Different arrivals at surfaceDirect waves:
- Direct P-wave in solid- Direct S-wave in solid- Direct P-wave through air (“air wave”)- Direct surface wave
..
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Surface wave
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Surface waveRayleigh wave: Love wave:
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Different arrivals at surfaceDirect waves:
- Direct P-wave in solid- Direct S-wave in solid- Direct P-wave through air (“air wave”)- Direct surface wave
In same medium:P-wave always faster than S-waveS-wave always faster than surface wave
So: cP > cS > cSurf
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Different arrivals at surface
- Reflected waves:- Down-going P-wave and up-going P-wave- Down-going P-wave and up-going S-wave- Down-going S-wave and up-going P-wave- Down-going S-wave and up-going S-wave
- Refracted Waves / Head waves- Down-going P-, refracted P- and up-going P-wave- Down-going P-, refracted S- (If cS2 > cP1) and up-going P-wave- Down-going P-, refracted S- (If cS2 > cP1) and up-going S-wave-