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Marine Geomatics > Marine Magnetic and Gravity Surveys > Flashcards

Marine Magnetic and Gravity Surveys Flashcards

1
Q

The Earth’s magnetic field varies with what 2 factors?

A
  1. Location

2. Time of Day

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2
Q

What is the Earth’s magnetic field strength measured in?

A

nanoTeslas (nT) … 1 nT = 10 to the power of -9 T

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3
Q

What is the Earth’s magnetic field strength in Northern Canada?

A

60,000 nT

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4
Q

What is the Earth’s magnetic field strength in Brazil?

A

24,000 nT

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5
Q

What may the Earth’s magnetic field strength be influenced by?

A

Ferrous material on or under the seafloor

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6
Q

What is the difference between the effects of geology and objects?

A

Geological effects are small and gradual, whereas, objects are more pronounced

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7
Q

What must be considered when planning a magnetic survey?

A

The time as there may be differences in the magnetism strength in the morning and afternoon

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8
Q

What impact do solar storms have on background magnetic strength?

A

Cause Large Changes

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9
Q

What does a magnetometer measure?

A

The spatial variations in the magnetic field

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10
Q

How is a magnetometer deployed?

A

Towed behind the vessel at a distance that won’t be impacted by the vessels magnetic properties

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11
Q

Is a magnetometer passive or active?

A

Passive

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12
Q

What is the length:width ratio for detecting objects with a magnetometer?

A

1:1

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13
Q

What is detected by a magnetometer?

A

Anomalies and object location and size

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14
Q

Can a magnetometer detect distortions in the magnetic field and why?

A

Yes – it isn’t a directional instrument

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15
Q

What impact do surface ships have during a magnetic survey?

A

They give the same record as for a submarine – can lead to confusing interpretations

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16
Q

What is the magnetic anomaly formula?

A

T2D = M/(rxr) where T = magnetic anomaly, M = magnetic moment of the object and r = distance of the object from the magnetometer

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17
Q

Give 4 applications of magnetic surveys

A
  1. Geological Mapping
  2. Site Investigations for identifying Hazardous Objects
  3. Detecting subsea pipelines and cables
  4. Marine archaeology/detecting wrecks
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18
Q

What is the oldest type of magnetometer with an example?

A

Fluxgate (e.g. Barrington Mag-03RC)

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19
Q

What is the accuracy of a Fluxgate magnetometer?

A

+/- 1 nT

20
Q

What is required when using a Fluxgate magnetometer?

A

Accurate Orientation

21
Q

What type of magnetometer used Kerosene (with an example)?

A

Proton (e.g. Geometrics G-877)

22
Q

What is the accuracy of a Proton magnetometer?

A

+/- 0.1 nT

23
Q

What is the most commonly used type of magnetometer (with an example)?

A

Ceasium (Geometrics G-882)

24
Q

What is the accuracy of a Ceasium magnetometer?

A

+/- 0.001 nT

25
Q

Is a Ceasium magnetometer sensitive to sensor movement?

A

No

26
Q

What is the newest type of magnetic survey technique?

A

Magnetic Gradiometers

27
Q

Why are 2 sensors often used in magnetic surveys?

A

To provide a method of measuring the gradient

28
Q

What are the advantages of the Gradiometric method?

A

Provides better resolution and is able to detect smaller objects

29
Q

What does the Quasi-Analytic Signal do?

A

Has 2 magnetometers in simultaneous mode/removes background noise

30
Q

Give 2 advantages of magnetic surveys

A
  1. Fast Survey Method

2. Can be used on small vessels in shallow waters

31
Q

Give 4 disadvantages of magnetic surveys

A
  1. Only ferrous material can be detected
  2. Subtle seabed features may be obscured by highly magnetic geology or modern materials
  3. Large tow distance makes positioning difficult
  4. Reduced resolution with target depth below seabed
32
Q

Give 2 disadvantages of gravity surveys

A
  1. Coarse

2. Limited Accuracy

33
Q

What does gravitational attraction cause?

A

Sea Level to bulge over sea mounts and to dip over trenches

34
Q

What does satellite altimetry to?

A

Give an indication of subsea topography

35
Q

What 6 things impact the height of the ocean surface?

A
  1. Tides
  2. Currents
  3. Waves
  4. Geoid
  5. Density Fluctuations
  6. Atmospheric Pressure
36
Q

Over what distance are satellite radar altimeter measurements made and why?

A

2-12km (to even out errors)

37
Q

Give 4 applications of marine gravity surveys

A
  1. Offshore Oil and Gas Exploration
  2. General Bathymetric Charting
  3. Detecting Subsea Features
  4. Geological Mapping
38
Q

In geophysical terms, what is gravity?

A

The force due to the mass of the Earth, which acts on the measuring instrument

39
Q

What is the gravity at the poles?

A

9.83 ms-2

40
Q

What is the gravity at the equator?

A

9.77 ms-2

41
Q

What happens to the density distribution within the Earth’s crust?

A

It is uneven, which causes gravity to vary from the expected value as the measurement position changes

42
Q

What can gravity anomalies be used for?

A

Mapping of detailed structures and geology of the Earth

43
Q

What are +ve gravity anomalies?

A

Higher than average density bodies

44
Q

What are -ve gravity anomalies?

A

Lower than average density bodies

45
Q

How can marine gravimeters be deployed to the seabed?

A

ROVs and AUVs

Marine Geomatics (11 decks)

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