SeaQuest2 Operating Manual Introduction 6
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1.4 Understanding the System Components
1.4.1 Overhauser total field sensors
Marine Magnetics supplies a separate document titled the SeaSPY Technical Application Guide containing an in-depth
description of how the Overhauser sensors work as well as general guidelines for estimating the size of magnetic
anomalies produced by a given amounts of ferrous iron. Marine Magnetics provides this document to anyone free of
charge, so please contact us if you do not already have a copy.
All SeaQuest gradiometers are supplied with omnidirectional sensors that are completely isotropic with respect to
magnetic field direction. The only restriction that must be observed is that SeaQuest2 must not be oriented vertically
with the nose facing upwards. This is a restriction with respect to the direction of gravity, not magnetic field.
The Overhauser sensor measures magnetic flux density, the unit for which is the Tesla (T). Magnetic flux density on the
surface of the Earth typically varies between about 18T to 70T, depending on location. The flux density at any fixed
location on the Earth’s surface also varies with time of day, due to diurnal effects, which include influence from the Sun
(mainly the interaction of the Sun’s radiation with Earth’s atmosphere, which is subject to regular and random
changes), and movement of the Earth’s molten interior.
One often speaks of a magnetometer as measuring magnetic field instead of flux density, since the two values are
directly related given an environment of constant magnetic permeability (such as air or water). Some materials will
distort the surrounding magnetic flux density by ‘amplifying’ or adding to the ambient magnetic field. Such objects are
known as paramagnetic. Some materials (such as iron, nickel, cobalt and alloys containing these materials) exhibit this
effect very strongly, and are known as ferromagnetic. Objects made from these materials are very easily detectable by
a magnetometer. Most building materials, especially those used to build modern boats and ships, contain iron alloys
and are therefore magnetic. Some stainless steels (austenitic alloys such as 316) are only weakly ferromagnetic, but will
become more strongly magnetic if their microstructure is disturbed by annealing, welding, machining or severe
stressing. In addition to ferrous construction materials, there is a number of naturally-occurring minerals (e.g. iron
oxides) that can be strongly magnetic. Such minerals are commonly found in igneous and volcanic rocks, as well as
pottery, and any accumulations of gravel and boulders derived from igneous rocks. Surveying in environments
dominated by igneous or volcanic bedrock can present challenges to the data analyst, and requires especially careful
control of survey altitude.
Carbonate-based rocks, on the other hand, do not contain any magnetic minerals, and areas dominated by limestone
or sedimentary bedrock do not usually create a strongly-variable magnetic background, but tend to be affected by
diurnal variation to a greater degree. For this reason the use of a base station magnetometer is especially
recommended in non-magnetic bedrock environments.
When an object of high magnetic permeability distorts the flux density around it, it creates a magnetic gradient that is
proportional to the magnitude of its permeability. If the magnetic gradient through the volume of the magnetometer
sensor is too great, the sensor will not operate correctly. For this reason, massive magnetic objects must be kept away
from the sensor. Do not expect the magnetometer to produce good results on the deck of a ship, or inside a building,
or next to a rocky outcrop containing igneous or volcanic rocks, any more than you would expect a high-powered
telescope to see distant stars in the middle of the day or through a dirty window.
For more information on magnetic fields and operation of the Overhauser magnetometers, please refer to the SeaSPY
Technical Application Guide. This document can be obtained from Marine Magnetics.
1.4.2 Leak Detector
Each SeaQuest is equipped with a leak sensor that triggers a warning in BOB when water is present inside the
electronics pod. The message “WARNING! WATER DETECTED IN CENTER POD” will also appear in the command
window every 10 seconds. Even a small drop of water will activate the leak sensor. In the event of a leak warning, the
SeaQuest should be retrieved immediately, as it is very likely that a leak has developed in the electronics housing.
Water inside the electronics pod may damage the electronics module and Overhauser sensors.
