Marine Magnetic and Gravity Surveys Flashcards
The Earth’s magnetic field varies with what 2 factors?
- Location
2. Time of Day
What is the Earth’s magnetic field strength measured in?
nanoTeslas (nT) … 1 nT = 10 to the power of -9 T
What is the Earth’s magnetic field strength in Northern Canada?
60,000 nT
What is the Earth’s magnetic field strength in Brazil?
24,000 nT
What may the Earth’s magnetic field strength be influenced by?
Ferrous material on or under the seafloor
What is the difference between the effects of geology and objects?
Geological effects are small and gradual, whereas, objects are more pronounced
What must be considered when planning a magnetic survey?
The time as there may be differences in the magnetism strength in the morning and afternoon
What impact do solar storms have on background magnetic strength?
Cause Large Changes
What does a magnetometer measure?
The spatial variations in the magnetic field
How is a magnetometer deployed?
Towed behind the vessel at a distance that won’t be impacted by the vessels magnetic properties
Is a magnetometer passive or active?
Passive
What is the length:width ratio for detecting objects with a magnetometer?
1:1
What is detected by a magnetometer?
Anomalies and object location and size
Can a magnetometer detect distortions in the magnetic field and why?
Yes – it isn’t a directional instrument
What impact do surface ships have during a magnetic survey?
They give the same record as for a submarine – can lead to confusing interpretations
What is the magnetic anomaly formula?
T2D = M/(rxr) where T = magnetic anomaly, M = magnetic moment of the object and r = distance of the object from the magnetometer
Give 4 applications of magnetic surveys
- Geological Mapping
- Site Investigations for identifying Hazardous Objects
- Detecting subsea pipelines and cables
- Marine archaeology/detecting wrecks
What is the oldest type of magnetometer with an example?
Fluxgate (e.g. Barrington Mag-03RC)
What is the accuracy of a Fluxgate magnetometer?
+/- 1 nT
What is required when using a Fluxgate magnetometer?
Accurate Orientation
What type of magnetometer used Kerosene (with an example)?
Proton (e.g. Geometrics G-877)
What is the accuracy of a Proton magnetometer?
+/- 0.1 nT
What is the most commonly used type of magnetometer (with an example)?
Ceasium (Geometrics G-882)
What is the accuracy of a Ceasium magnetometer?
+/- 0.001 nT
Is a Ceasium magnetometer sensitive to sensor movement?
No
What is the newest type of magnetic survey technique?
Magnetic Gradiometers
Why are 2 sensors often used in magnetic surveys?
To provide a method of measuring the gradient
What are the advantages of the Gradiometric method?
Provides better resolution and is able to detect smaller objects
What does the Quasi-Analytic Signal do?
Has 2 magnetometers in simultaneous mode/removes background noise
Give 2 advantages of magnetic surveys
- Fast Survey Method
2. Can be used on small vessels in shallow waters
Give 4 disadvantages of magnetic surveys
- Only ferrous material can be detected
- Subtle seabed features may be obscured by highly magnetic geology or modern materials
- Large tow distance makes positioning difficult
- Reduced resolution with target depth below seabed
Give 2 disadvantages of gravity surveys
- Coarse
2. Limited Accuracy
What does gravitational attraction cause?
Sea Level to bulge over sea mounts and to dip over trenches
What does satellite altimetry to?
Give an indication of subsea topography
What 6 things impact the height of the ocean surface?
- Tides
- Currents
- Waves
- Geoid
- Density Fluctuations
- Atmospheric Pressure
Over what distance are satellite radar altimeter measurements made and why?
2-12km (to even out errors)
Give 4 applications of marine gravity surveys
- Offshore Oil and Gas Exploration
- General Bathymetric Charting
- Detecting Subsea Features
- Geological Mapping
In geophysical terms, what is gravity?
The force due to the mass of the Earth, which acts on the measuring instrument
What is the gravity at the poles?
9.83 ms-2
What is the gravity at the equator?
9.77 ms-2
What happens to the density distribution within the Earth’s crust?
It is uneven, which causes gravity to vary from the expected value as the measurement position changes
What can gravity anomalies be used for?
Mapping of detailed structures and geology of the Earth
What are +ve gravity anomalies?
Higher than average density bodies
What are -ve gravity anomalies?
Lower than average density bodies
How can marine gravimeters be deployed to the seabed?
ROVs and AUVs
