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Marine Geomatics > Marine Seismic Profiling > Flashcards

Marine Seismic Profiling Flashcards

1
Q

What is marine seismic profiling also called?

A

Sub Bottom Profiling (SBP)

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2
Q

What does marine seismic profiling do?

A

Investigate the sub bottom conditions of seas, lakes and waterways

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3
Q

Give 3 applications of SBP

A
  1. Determining the seabed lithology
  2. Determining the depth of a fluid mud layer
  3. Determining the existence of buried wrecks/obstructions
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4
Q

In water there is what type of waves?

A

P waves (longitudal waves)

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5
Q

In solids there is what type of waves?

A

P and S waves (longitudal and shear waves)

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6
Q

Where does reflection of sound energy usually occur?

A

At the boundaries between two different geological layers

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7
Q

Which technique uses P waves?

A

Reflection

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8
Q

Which technique uses P and S waves?

A

Refraction

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9
Q

Give 3 points on reflection of seismic waves

A
  1. They are reflected from beneath the seafloor and recorded
  2. The amplitude of the seismic arrivals are plotted against time
  3. The reflection profile is 2D and continuous
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10
Q

What doe the seismic reflection technique measure?

A

The time lapse between transmission and reflection (p wave)

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11
Q

What does the seismic refraction technique measure?

A

The differences in first and later arrival times (p and s waves)

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12
Q

What 3 things does the reflection amplitude depend on?

A
  1. The reflection coefficient of the reflector
  2. Power/Intensity of the source transmission
  3. Proximity of the source to the seabed
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13
Q

Why are hydrophone lengths varying with depth?

A

To allow for slower returns as the vessel is moving

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14
Q

What happens to shear waves in the refraction technique?

A

They travel along geological boundaries before being refracted back

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15
Q

Give 4 requirements of seismic data acquisition

A
  1. Seismic source
  2. Position of vessel and towed elements
  3. A means of detecting seismic signals
  4. A method of recording and displaying in a way it can be interpreted
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16
Q

Give 5 seismic sources

A
  1. Boomer
  2. Sparker
  3. Chirp
  4. Air Gun
  5. Water Gun
17
Q

How does a boomer work?

A

Metal plates sit below the surface, which contract and return slowly whilst pushing water away rapidly creating a temporary vacuum in which the surrounding water fills rapidly creating a boom noise

18
Q

What is a chirp suited to surveying and why?

A

Shallow features such as pipelines – high frequency and resolution and poor penetration

19
Q

What is a boomer suited to surveying and why?

A

Smooth material – high frequency

20
Q

What is a sparker suited to surveying and why?

A

Mapping deeper buried features such a channels in water

21
Q

What is a air gun suited to surveying and why?

A

Geophysical exploration in deep water oil and gas fields – low frequency and high penetration

22
Q

Where did the Uni conduct a survey using a boomer do look at the largest urban landslide in Europe?

A

Ventor, Isle of Wight

23
Q

What is a number of joined hydrophones called, what are they separated by and what are the benefits?

A

Streamers
Birds
Good coverage

24
Q

How long can hydrophone arrays be?

A

60km

25
Q

What is the SPI?

A

Shot Point Interval – the half way distance between the hydrophone receiver channels in which air guns are fired at the set distance over the ground

26
Q

What is the CMP?

A

Common Mid Point – The ½ way point

27
Q

What is Stacking?

A

Combining multiple data for each CMP to enable the signal: noise ratio to be reduced providing a clearer image of the seabed

28
Q

Give 4 limitations of SBP

A
  1. Acoustic Noise (Ships Noise, Sea Clutter, Ambient Noise)
  2. Positioning of the towed source/detector
  3. Multiple reflections (often mistaken for sediment layers)
  4. Hard to choose equipment with varying offshore conditions
29
Q

What raw data do you get from SBP?

A

Time sequence and intensity

30
Q

What 3 things may be needed for interpreting a SBP record?

A
  1. TWTT -> depth conversions
  2. Ground Truthing (Grabs and Corers)
  3. Borehole Data
31
Q

In SBP, what is is used for deepwater O&G exploration due to its high penetration qualities?

A

Air Gun

32
Q

In SBP, what is used for mapping buried features in

A

Sparker

33
Q

In SBP, what is used for mapping smooth material due to high resolution?

A

Boomer

34
Q

In SBP, what is used for surveying shallow features such as pipelines because of the good resolution and frequency?

A

Chirp

35
Q

In SBP, what is used for inspection and geological surveys?

A

Pingers

Marine Geomatics (11 decks)

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