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Ref Seis > Uge 35 > Flashcards

Uge 35 Flashcards

1
Q

What is seismic resolution

A

The ability to distinguish seperate features

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2
Q

What can we say about low frequency seismic wavelets?

A
  • long pulse length
  • give poor seismic resolution = less detailed
  • see greater depth
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3
Q

What can we say about high frequency seismic wavelets?

A
  • good seismic resolution = more detail
  • see shallow depth
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4
Q

What are the manmade seismic sources?

A

(hight to low frequency)
Ekkolod
Pinger
Boomer
Sparker
Luftkanon
Vibroseis
Sprængning

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5
Q

What are some natural seismic sources?

A
  • P and S waves from Earthquakes
  • surface waves from Earthquakes
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6
Q

What characterises explosives as a seismic source?

A
  • old method
  • pressure wave generated by shaking during explosion
  • clean sharp pulse
    (- near-offset to wide-angle setups)
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7
Q

What characterises vibroseis as a seismic source?

A

Vibrating “car” - creating pressure waves
- makes seismic sweep (low to high frequency)
- 1-2 km

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8
Q

What characterises an airgun as a seismic source?

A

Usually used at sea
- pressure wave generated by released bubbles of compressed air
- depth 1-10km

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9
Q

What characterises a sparker as a seismic source?

A
  • pressure wave generated by electricity via sparks - creates a vapour bubble that collapse and make sound
  • lies on the surface - take waves into account
  • 100-200m
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10
Q

What characterises a boomer as a seismic source?

A
  • pressure wave generated by electricity via spiral cord (spole der vipper) (moves the water)
    -100-200m
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11
Q

What are the 2 kinds of receivers?

A
  • Geophones (land)
  • Hydrophones (marine)
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12
Q

What are the 2 kinds of receivers?

A
  • Geophones (land)
  • Hydrophones (marine)
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13
Q

What is special about hydrophones?

A
  • Used at sea
  • In a cable (a streamer)
  • can be analog or digital streamer
  • single or multichannel streamers
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14
Q

What is a bird?

A

A device to adjust the depth of the streamer

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15
Q

What characterises a Pinger (sub-bottom profiling) as a seismic source?

A
  • transmitter and receiver in one
    • normal incidence reflections
  • pressure/sound wave generated by electric signals
  • sends out pings at a fixed frequency
  • Marine
  • 10-50m
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16
Q

A Pinger (sub-bottom profiler) sends out what?

A

2 high frequency signals send out, producing one low frequency sound wave

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17
Q

What characterises Chirps as a seismic source?

A
  • Pressure/soundwave generated by electrical signals
  • Sweep of frequency
  • transmitter and receiver in one
  • marine
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18
Q

What characterises echosounders as a seismic source?

A
  • only marine surface
  • high frequency sound generated by electrical signals
  • transmitter and receiver usually in one
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19
Q

What is the CMP technique?

A

(Common mid point)
Processing: Sorting of data so you get it from a single point and stacked afterwards, to get simple data
- Reduces noise (noise ratio: S/N)

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20
Q

What is fold coverage?

A

The number of ‘hits’ for each CRP (Common reflection point)
Also = the number of traces stacked together in CMP
- Covers less at the ends

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21
Q

What is some random noise?

A

Wind
Humans/traffic
Rain
Waves
Ship noises
Tail buoy noise

22
Q

What is some organised (coherent) noise?

A

Direct P-waves
Surface waves
Diffractions/reflections
Multiples (reverberation/ringing)
Linear noise (ex. waves within streamer)

23
Q

What is the goal of processing?

A

Simplify the data
Remove noise

24
Q

What should you consider when processing?

A

…

25
Q

In what ways can you decrease noise under possessing?

A

CMP
Configuration of receivers
……

26
Q

What is the difference between 3D and 2D seismic?

A

….

27
Q

Hvad afhænger resolution af?

A

Pulslængde (lamda)
- og derfor frequency (f) (eller periode (T)) og velocity (v)

28
Q

Hvad er NMO?

A

Normal move out
- tidsforskel der afhænger af afstanden mellem geofonen og kilden, samt de seismiske lag

29
Q

Hvad er afgørende for bjergarters seismiske velocity?

A

Deres rock strength, porøsitet og densitet

30
Q

Hvad er rock strenght?

A

En funktion af konsolideringsgraden
- afhænger af cementering og kompaktering

31
Q

Hvordan hænger denisitet sammen med seismisk velocity?

A

“stærkere” bjergarter har oftest højere densitet og giver derfor højere velocity

32
Q

Hvornår er porøsitet særligt vigtigt og hvad betyder det?

A

Det er særligt vigtigt for sedimentære bjergarter
- de samme bjergarter viser forskellig velocity med forskellig porøsitet.

33
Q

Hvordan ændre seismisk velocity sig generelt nedad?

A

Den stiger
- pga. cementering og kompaktion

34
Q

Hvad afhænger porøsitet af?

A
  • Sortering (depo. miljø)
  • cementering (post depo.)
  • kompaktion (post depo.)
35
Q

Hvad gør gas ved velocity?

A

Reducere den
(3-5% gas -> ca. 30% reducering)

36
Q

Hvad er huygens princip?

A

At hver punkt på en bølgefront kan betragtes som en kilde til nye, sekundære bølger der gensidigt forstyrre, og den samlede bølgefront på et senere tidspunkt er tangent til de sekundære bølger.

37
Q

Hvad bruges Huygens princip til i seismik?

A

Til at forudsige bølgefronten ud fra den sekundære - vi kan se mere end vi burde (ift.resolution)

38
Q

Hvordan sker der dæmpning af bølger? (P-bølger)

A
  • Sfærisk divergens
  • Absorption
  • Transmissionstab ved laggrænser
39
Q

Hvad er den sfæriske divergens?

A

Den samme energi fra bølgen bliver spredt ud tyndere og tyndere (lavere energi)
(cirklen udvider sig)

40
Q

Hvordan adskiller overflade bølger sig bl.a fra P- og S-bølger?

A

De har generelt en markant højere amplitude

41
Q

Hvad er absorption?

A

Energitab pga. friktion under bevægelse frem og tilbage.
- samme fraktion energi tabt, derfor overlever lavfrekvente bølger længere (mere afstand)

42
Q

Hvad er transmissionstab?

A

Vi ser kun den energi der bliver reflekteret

43
Q

Hvad er akustisk impedans (A)?

A

Beskrivelse af et materiales akustiske egenskaber f.eks. hvordan seismisk energi bliver transporteret igennem.

  • stiger normalt med dybden
44
Q

Hvad er refleksions koefficienten (R)?

A

Forskellen mellem incidens amplituden (bølgen) og den reflekterede (altså hvor meget bliver reflekteret).
- Måles ud fra kontrasten af akustisk impedans i lagene

R= Ar/Ai = (Z2-Z1) / (Z2+Z1)

45
Q

Hvad betyder en henholdsvis positiv eller negativ refleksions koefficient?

A
  • Pos: nedadgående stigning i akustisk impedans (Z1 < Z2)
  • Neg: nedadgående fald i akustisk impedans (Z1 > Z2)
46
Q

Hvad forstås ved gode og dårlige reflektorer?

A

Dårlige: Lav (A) kontrast og lave R
- mindre refleksion, mere energi forsætter og transmittere dybere

Gode: Høj A kontrast og høje R
- mere refleksion, mindre dybde

47
Q

Hvad sker der mellem hav overfladen og luften?

A

Meget høj (A) kontrast
- betyder at der kun kommer en refrakteret bølge

48
Q

Hvordan kan man beskrive en seismisk bølge?

A

Som summen af et antal sinus og cosinus svingninger, hver beskrevet af en frekvens, amplitude og fase

49
Q

Hvilken type bølge ønsker vi at producere?

A

Zero-phase - Ricker wavelet(/bølge)

50
Q

Hvad er Nyquist frekvens (fny)?

A

Den højeste frekvens der kan blive rekonstrueret over bestemt tidsinterval

51
Q

Hvad er vigtigt når der skal rekonstrueres seismiske signaler?

A
  • mindst 2 prøver pr. periode (T)
  • korrekt prøve afstand - for lang afstand = for få prøver
    -fny må ikke være lavere end fs (frekvensen af signalet)

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