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3 Seismology and Earth Structure

• Determine Velocity structure– Travel time, Dispersion, Eigen frequencies …

• Refraction Seismology• Reflection Seismology• Seismic Waves in a spherical earth• Body wave travel time studies• Anisotropic earth structure• Attenuation and anelasticity• Composition of mantle and core

3.2 Refraction Seismology

• Flat layer method• Dipping layer method• Advanced analysis methods• Crustal structure• Rocks and minerals

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3.2.1 Flat layer method

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Figure 3.2-4 Mohorovičić’s results showing distinct mantle and crust

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3.2.2 Dipping layer method

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3.2.3 Advanced analysis method

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3.2.4 Crustal structure

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Crustal-scale seismic experiment in Korea

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Portable Seismometers

Data acquisition Parameters

Number of seismic stations

Distance between stations Average 1.5 km

170 Vertical-Component 28 Three-Component Total 198 Stations

Sampling rate 120 Hz

Record length 120 sec

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Record section of shot at Seosan

(2-15hz Bandpass filtered)

Record section of shot at Yeongdong

(2-15hz Bandpass filtered)

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Ray trajectory and corresponding synthetics (Seosan)

Ray trajectory and corresponding synthetics (Yeongdong)

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Final layered velocity model

Geology and tectonic provinces of the southern Korean Peninsula

Gyeonggi massifGyeonggi massif

Gyeongsang

Okcheonfold belt

Yeongnammassif

Gyeongsangbasin

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3.3.1 Travel time curves for reflections

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002 sin)/(sin iTvviTvx jjjjjj Δ=Δ=

∑

∑=

Δ

+=Δ==

n

nn

n

jjj

TTVxdT

tVxxTTvxv

idxdT

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2222

0

2

0

0

/)2(

/)(,)2/(sin

∑∑

∑

==

=

ΔΔ=

Δ=⇒=

n

jj

n

jjjn

jjjn

n

tTvV

TTvVTVdx

00

22

0

222

)/()(

/)2(

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3.3.2 Intercept-slowness formulation for travel times

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3.3.3 Multichannel data geometry

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3.3.4 Common midpoint stacking

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Common midpoint (CMP) stacking

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3.3.5 Signal enhancement

Eliminate head waves and surface waves to isolate reflections.

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(Remove signals with p > 650 micro-s/m)

3.3.6 Deconvolution

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3.3.7 Migration

Migration tries to find surfaces in the case of non-horizontal layers –to remove the effects of point diffractors

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Bright spot:

A strong reflectionat the interface between

a gas layer andunderlying oil or water

in hydrocarbon reservoir

Horizontal or “flat spot”

Detection depends onppull-up or pull-down effects caused by variability in

the thickness or velocity of layers above.

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Reflection seismology and the search for hydrocarbons

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