Marine Seismic Profiling

Question 1

What is marine seismic profiling also called?

Answer 1

Sub Bottom Profiling (SBP)

Question 2

What does marine seismic profiling do?

Answer 2

Investigate the sub bottom conditions of seas, lakes and waterways

Question 3

Give 3 applications of SBP

Answer 3

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

Question 4

In water there is what type of waves?

Answer 4

P waves (longitudal waves)

Question 5

In solids there is what type of waves?

Answer 5

P and S waves (longitudal and shear waves)

Question 6

Where does reflection of sound energy usually occur?

Answer 6

At the boundaries between two different geological layers

Question 7

Which technique uses P waves?

Answer 7

Reflection

Question 8

Which technique uses P and S waves?

Answer 8

Refraction

Question 9

Give 3 points on reflection of seismic waves

Answer 9

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

Question 10

What doe the seismic reflection technique measure?

Answer 10

The time lapse between transmission and reflection (p wave)

Question 11

What does the seismic refraction technique measure?

Answer 11

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

Question 12

What 3 things does the reflection amplitude depend on?

Answer 12

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

Question 13

Why are hydrophone lengths varying with depth?

Answer 13

To allow for slower returns as the vessel is moving

Question 14

What happens to shear waves in the refraction technique?

Answer 14

They travel along geological boundaries before being refracted back

Question 15

Give 4 requirements of seismic data acquisition

Answer 15

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

Question 16

Give 5 seismic sources

Answer 16

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

Question 17

How does a boomer work?

Answer 17

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

Question 18

What is a chirp suited to surveying and why?

Answer 18

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

Question 19

What is a boomer suited to surveying and why?

Answer 19

Smooth material – high frequency

Question 20

What is a sparker suited to surveying and why?

Answer 20

Mapping deeper buried features such a channels in water

Question 21

What is a air gun suited to surveying and why?

Answer 21

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

Question 22

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

Answer 22

Ventor, Isle of Wight

Question 23

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

Answer 23

Streamers
Birds
Good coverage

Question 24

How long can hydrophone arrays be?

Answer 24

60km

Question 25

What is the SPI?

Answer 25

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

Question 26

What is the CMP?

Answer 26

Common Mid Point -- The ½ way point

Question 27

What is Stacking?

Answer 27

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

Question 28

Give 4 limitations of SBP

Answer 28

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

Question 29

What raw data do you get from SBP?

Answer 29

Time sequence and intensity

Question 30

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

Answer 30

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

Question 31

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

Answer 31

Air Gun

Question 32

In SBP, what is used for mapping buried features in

Answer 32

Sparker

Question 33

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

Answer 33

Boomer

Question 34

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

Answer 34

Chirp

Question 35

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

Answer 35

Pingers