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Magnetometers and Gradiometers
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Table of Contents
Introduction to Magnetometers
Types of Magnetometers
Applications
Construction
Deployment
Sealink Software
Operation Of SeaSpy
Maintenance
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Introduction to Magnetometers
The magnetometer is a device that measures localized distortions in the
earth’s magnetic field caused by the presence of ferrous material. It will only
detect iron or steel. Materials such as gold, silver, copper or bronze cannot
be detected. The primary advantage the magnetometer has over other
detection technologies is its passive design that relies on the earth’s natural
magnetic field as the detection medium. Because of this, detection is omni-
directional and is unaffected by other materials. Shipwrecks can be located
through layers of sedimentation or coral overgrowth as easily as if they were
not covered by anything.
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Pulse Induction Sensors
Very Short Range (perhaps 2-3m)
Poor Sensitivity (around 1 nT)
Slow update rate (2 second cycle time)
• Will detect any conducting metal
Pro’s
Con’s
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Magnetometers
• Higher sensitivity (theoretically 0.02nT)
• Faster cycle rates (Up to 10Hz)
• Longer range detection
Pro’s
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Magnetometers
• Large distant targets mask small local targets.
• Difficult to pick out small target due to
background noise.
• No sense of direction of target on single pass.
• Subject to diurnal variations in the earth’s
magnetic field.
Con’s
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….the targets…what are we looking for?
Large WW11 Sea Mine
• Perhaps 500kg of ferrous material
• Relatively large target
• Range which gives a 1 nT deflection <31m
• Range which gives a 5 nT deflection <18m
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….the targets…what are we looking for?
250kg HE Bomb
• Perhaps 120kg of ferrous material
• Relatively large target
• Range which gives a 1 nT deflection <19.5m
• Range which gives a 5 nT deflection <11.5m
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….the targets…what are we looking for?
Hand Grenade
• Perhaps 400g of ferrous material
• very small target
• At 1 nT deflection the range is only 2.9m
• Range which gives a 5 nT deflection <1.7m
(Note at 4 kts you travel at 2m/sec therefore a high update rate is desired for small munitions detection)
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Current Technology Available
Higher sensitivity magnetometer sensors
Low noise digital transmission.
Tow platforms such as the Focus that allow fixed height and accurate line
spacing.
3D software modelling tools such as Geosoft Oasis montaj.
Towed Gradiometer Platforms.
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105 10-5 10-3 10-1 101 103 107
Search Coil (AC fields only !)
Alkali Vapor
Flux-gate
Nuclear Precession
SQUID
Hall Effect
MRS
Earth’s Field
109
Magnetic Field Strength (nT)
Magnetometer sensors
MAGNETOMETERS TYPES AND RANGES
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Proton Precession
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Measure Precessing Protons
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Proton Precession Theory
A standard proton precession magnetometer uses hydrogen atoms to generate precession signals. Liquids such as kerosene and methanol are used because they offer very high densities of hydrogen and are not dangerous to handle.
A polarizing DC current is passed through a coil that is wound around the sample. In a magnetometer this creates a high-intensity magnetic field of over 100 Gauss.
Protons in this field are polarized to a stronger net magnetization corresponding to the thermal equilibrium of stronger magnetic flux density. When the auxiliary flux is released, the "polarized" protons precess to re-align themselves with the "normal" magnetic flux density. The frequency of the precession relates directly to the magnetic field strength.
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Overhauser Sensor Picture
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Overhauser Theory
The Overhauser Effect is a nuclear method that takes advantage of a "quirk" of physics that affects the hydrogen atom. This effect occurs when a special liquid (containing electrons) is combined with hydrogen and then exposed to a radio frequency (RF) magnetic field.
RF fields are ideal for this type of application because they are transparent to the Earth's DC magnetic field and the RF frequency is well out of the bandwidth of the precession signal (i.e. does not contribute noise to the measuring system).
The unbound electrons in the special liquid (normally a mixture of free radicals) transfer their excited state (i.e. energy) to the hydrogen nuclei (protons). This transfer of energy alters the spin state populations of the protons and polarizes the liquid - just like in a proton magnetometer - but with much less power and to greater extent.
The proportionality of the precession frequency and the magnetic flux density is linear and can be described through a simple equation.
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Alkali Vapour Theory
Optically pumped magnetometers use gaseous alkali metals from the first column of the periodic table, such as cesium and potassium. That means that the cell containing the metal must be continuously heated to approximately 45 degrees C.
First, a glass cell containing the gaseous alkali metal is exposed (or pumped) by light of a very specific wavelength - an effect called light polarization. The frequency shift of light is specifically selected and circularly polarized for each element to shift electrons from level 2 to the excited state 3.
Electrons at level 3 are not stable, and these electrons spontaneously decay to both energy levels 1 and 2. Eventually, the level 1 is fully populated (i.e. level 2 is depleted). When this happens, the absorption of polarizing light stops and the vapour cell becomes more transparent.
This is when RF depolarization comes into play. RF power corresponding to the energy difference between levels 1 and 2 is applied to the cell to move electrons from level 1 back to level 2 (and the cell becomes opaque again). The frequency of the RF field required to populate level 2 varies with the ambient magnetic field and is called the Larmor frequency.
The effect of polarization and depolarization is that the light intensity becomes modulated by the RF frequency. By detecting light modulation and measuring the frequency, we can obtain a value of the magnetic field.
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Alkali Vapour Pictures
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Earth Magnetic Background
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Typical Gamma Readings
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Geometrics Magnetometers
MSD 101 Basic Engineering Course
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BASIC G-880 SYSTEM
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G-881 MARINE MAGNETOMETER
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G-881Tow Fish Components
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G-881Tow Fish Components
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G-881Tow Fish Components
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G-881Tow Cable Installation
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G−882 MARINE MAGNETOMETER
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G882 Accessory Kit
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G882 Soft Tow Cable
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G877 Geometrics Magnetometer
The G–877 Proton Precession Magnetometer is a
digital device and requires that a customer or
Geometrics supplied portable laptop or desktop
computer be used for recording and display of the
data.
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Nose Tow Assembly
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Geometrics Junction Unit
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SeaSpy Magnetometers
MSD 101 Basic Engineering Course
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SeaSpy System Components
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FSME Standard Connection
Iowin Setup Annotation SeaSpy - Magnetometer Out
Auxiliary- SeaSpy Magnetometer
Position Outputs- NMEA Output
Output Data
to Logging
PC
TowFish Winch
SeaSpy Transceiver
PC
Position Loop Back
Ethernet
24V DC
Run Starfix Time and Automatic Update PC clock
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Towfish Connector
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SeaSpy Towfish
SeaSPY has a modular construction that allows for quick and easy connection and dis connection of all
components and parts. For normal use, the only connection you will have to think about is the main brass
tow connector. Sometimes it becomes necessary to access the internal components of the towfish
To open the towfish, remove the four brass holding screws near the nose of the
towfish, as indicated in the above figure. All of the towfish internals are fastened to a rack that is bolted to
the nose section. Once the screws have been removed, pull out the nose section to remove the internal
assembly.
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SeaSpy Explorer
Transceiver
•TowFish
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SeaSpy Towfish Internal Structure
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SeaSpy Maintenance
A SeaSPY System is designed to withstand years of use in harsh
marine environmental conditions. If some simple procedures are
observed when deploying and storing the instrumentation, your
SeaSPY system will continue to deliver high quality performance.
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SeaSpy Deployment and Storage
When connecting the main tow connector, ensure that the alignment slot is properly inserted into the groove, and that the male connector is fully inserted.
Tighten the holding nut firmly, making sure that any air pressure inside the connector is completely overpowered.
Use a tow speed and cable length combination that keeps the towfish submerged at least 1m below the surface, and as far below waves and swell as possible if the water is rough. Other than this, there is no restriction on tow speed.
Do not, under any circumstances, exceed the maximum rated operating depth of the towfish. In some cases, permanent damage may occur to certain components (such as the pressure sensor) if the towfish’s rated depth is exceeded by even a small amount.
Rinse the towfish with fresh water after removal from salt water. Surface corrosion of the brass fittings and screws will only significantly take place after exposure to atmospheric oxygen in the presence of salt water. Rinsing with fresh water will keep the brass fittings clean and shiny.
Blow out the pressure sensor hole with compressed air after removal of the towfish from salt or fresh water. Stagnant water in the pressure sensor hole can cause pitting corrosion of the pressure sensor after long-term use.
Do not store the towfish in direct sunlight, and keep it away from very hot environments. The operating and storage temperature range for a towfish is –40C to +60C, but a secluded, sunlight area in a tropical location can easily exceed +60C. Keeping the towfish stored in moderate temperatures will prolong the lifetime of the seals and the internal electronics.
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Calculating Towing Depth
Controlling the depth of the SeaSPY towfish during a survey is essential to obtaining
good results. The following factors will influence the depth of the towfish while towing.
1. Survey speed (slower=deeper)
2. Deployed tow cable length (longer=deeper)
3. Weight of tow cable (heavier=deeper)
4. Weight of towfish (heavier=deeper)
The above may seem obvious, but it is important to note that they are the only factors
that will affect towfish depth. Manipulation of these four variables is the only way to
regulate the depth of the towfish.
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Gradiometers
MSD 101 Basic Engineering Course
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Traditional Detection Methods….
Pulse Induced Metal Detector
• Towed Magnetometer
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Description of a Towed Gradiometer
Two or more synchronised high sensitivity sensors
arranged in a fixed geometry array.
This array can be configured to act in a transverse,
vertical, longitudinal manner or a combination of the
above.
The gradient value is derived by comparing the field
values from the relevant sensors.
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Different Gradients
Vertical Gradient – One sensor mounted above the other to enhance
detection of objects directly below.
Transverse Gradient – Sensors mounted side by side. Enhances
detection to either side of the array.
Longitudinal Gradient – One sensor behind the other. Enables a
long baseline between the sensors (perhaps 20m+).
Total Field Gradient – A gradient derived from summation of the
above.
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Advantages of using a Gradiometer
Distant large targets are ignored.
Diurnal variations are irrelevant.
It is possible to “focus” the direction of maximum
sensitivity.
External Noise is automatically filtered when the
gradients are calculated.
By this noise reduction the effective sensitivity is
increased.
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That was the theory, in practice consider the following
Attitude of the array must remain stable.
A rigid frame must resist vibration.
As far as practical keep a fixed altitude.
Individual sensors may give slight linear offsets from one
another.
Frame gives high drag which makes deep towing tricky.
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Marine Magnetics Corporation SeaQuest Gradiometer
Platform
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Development of the GSE Gradiometer
Had to be able to use readily available sensors – MMC Explorer
Overhauser
Lightweight and compact.
Low vibration.
Flexible configuration - transverse or a vertical gradiometer.
By the addition of an interface bottle this system had to be able to be towed
behind a Klein 3000 digital sidescan.
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Sensor Orientations for Gradient Mode
The operator carries two mobile sensors separated by a vertical distance of usually one half or one meter perpendicular to the ground (for upper northern and southern latitudes). Typically the bottom sensor is referenced as the total field sensor and the top sensor is referenced as the gradient sensor. After data is collected a built-in program is used to subtract lower sensor data from the upper sensor. This resulting positive or negative value will be the most accurate method in eliminating most atmospheric noise. The data will be expressed in gammas or nanoTesla per meter or half meter (dependent on the distance separating the sensors)
Gradient Advantages Gradient Disadvantages
Automatic correction for atmospheric
disturbances
Detection limits slightly shallower than total
field mode
Both sensors carried by operator - no base
station needed
More expensive to purchase than total field
systems
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Gradiometer configurations
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GSE Rentals Gradiometer
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Acquisition Software
Real time scrolling display.
Logs raw sensor data as well as calculated gradient values.
Synchronisation of the Overhauser Sensors.
Flexible interfacing capabilities.
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Gradiometer display Magnetometer Display
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• What size and type of target do you have to detect?
• What is the water depth and the seabed topography like?
• What vessel can you use? Is it magnetic and can you deploy the system far enough away?
• Is the survey location in a difficult magnetic environment i.e. a harbour or near a platform?
Some obvious things to consider when planning the
survey…..
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• Pulse induced systems are only really effective in small areas where range is not an issue or not desirable (canal, small harbours). This has to be the system of choice for non ferrous conducting targets.
• Single magnetometers may be used to find large targets at
relatively good range but have difficulty in discriminating small targets against background noise.
• Gradiometers are a particularly effective tool for the detection
of small ferrous targets.
To Conclude
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