Seismogram

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Seismogram

• Relation between seismic trace and geology
• Wavelets
• Multiples
• Resolution
• Vertical resolution
• Horizontal resolution

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Seismogram of an earthquakemid atlantic ridge
measured in Brasil magnitude 5 (sept 2002) 3000km
Unfiltered
Filtered
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From geology to seismogram
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Waves
Amplitude
t or x
p
1
2
t
Period
---
------

w
f
l
Wavelength
Phase
p
t
2
Period
Wavenumber k
---

l
l
f
Velocity v

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Important wave forms
Zero-Phase
Minimum Phase
0 time
0 time
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Minimum phase and zero-phase wavelet
Minimum phase wavelet
Zero phase wavelet
Energy is maximally front-loaded (Energy does not
arrive before zero time)
Symmetric with respect to zero time and peaks at
zero time (Energy arrives also before zero time)
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Important waveforms
Minimum Phase
Zero Phase
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Display of Seismogram
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Example of a shot
Reynolds, 1997
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Multiples
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Air gun record
Keary and Brooks
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Surface
Riverbed
Sediment
Bedrock
Reynolds
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Resolution

• Resolution refers to the minimum separation
  between two features such that we can tell that
• there are two features rather than only one.
• Vertical resolution
• Horizontal resolution

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Comparison of resolution of different data sets
30 Hz wave
gamma log
Big Ben (ca. 123 m)
through oil field
at 2000 m/s
(Emery and Myers)
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Vertical resolution
Rayleighs-criterion
Vertical resolution depends on
- Frequency
- Velocity
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Resolution of two boundaries depends on wavelength
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l/412 ms
R12/R23 pos.
R12/R23 neg.
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Horizontal resolution
z
w
for zgtgtl
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Sampling theorem
At least two samples per (horizontal) apparent
wavelength must be obtained in order to
recognize Features (remember the time sampling
theorem!!). For example, to recognize a stream
channel on a horizontal slice generally requires
bin sizes no larger than 1/3 or ¼ the channel
width.