GSM-19M Shallow Tow Marine Magnetometer
Sensitivity = 0.01 nT
Advanced Overhauser Effect
The GSM-19M Marine Magnetometer
utilizes Overhauser effect sensors to create a shallow
tow magnetometer system that is extremely sensitive, yet
easily deployed. The Overhauser type sensor delivers
sensitivities an order of magnitude better than standard
proton sensors. It also is much more efficient,
requiring only a fraction of the power required by
proton sensors. The proton sensor uses a large DC
magnetic field to polarize the sensor, whereas the
Overhauser sensor is polarized with a low power, high
frequency magnetic field.
Overhauser sensors can measure the
magnetic field while polarizing. As a consequence the
GSM-19M magnetometers can sample at one sample per
second with virtually no loss of sensitivity. Even at
rates of four samples per second the noise envelope is
typically less than 0.1 nT.
Overhauser sensors do not differ
from the Heading Errors typically associated with Cesium
magnetometers. With cesium type magnetometers the sensor
must be oriented properly so that "dead zones" are
avoided. Even when oriented correctly Cesium
magnetometers have a heading error of about 0.25 nT that
cannot be eliminated. The GSM-19M does not have this
problem. The sensor is completely omni-directional, even
in low latitudes.
The GSM-19M magnetometers consists
of a surface console with rechargeable battery, a 100
meter tow cable, and a fish housing the sensor. The
console is used to communicate with the magnetometer,
and to display and store data. Only the sensor is
contained in the fish. Should the fish be lost,
replacement costs are lower than systems that contain
both electronics and the sensor in the fish.
The GSM-19M console features a real
time graphic display of the current profile. In addition
digital display of the current reading, current
position, and warning messages are provided. The console
design, with internal rechargeable battery pack, allows
the unit to be completely sealed against the elements.
Tuning is automatic worldwide, with
provision for manual override. In high gradient
conditions the GSM-19M monitors the signal decay rate
and displays a warning message when the gradient becomes
too great. Filters for rejection of 50 or 60 Hz noise
The RS-232 port on the GSM-19M will
output data as it is collected. This allows interface to
GPS loggers that will accept RS-232 data. The standard
GSM-19M may be operated in a remote mode via computer.
Memory storage is 512 K in the standard unit, and may be
upgraded to 2 MB.
With the GPS Log Option the GSM-19M
magnetometers will display and store GPS data using
standard NMEA format. Position accuracy is dependant on
the user's DGPS system.
Also offered is an internally
mounted GPS board that may be integrated with radio
modem for DGPS mode. A range of GPS boards are offered
to meet customer specified accuracy. These are quoted on
a case by case basis to take advantage of current
technology. Complete systems, with base station, and
DGPS software are provided.
Cable Weak Link Option
The cable may be installed with a
weak link at the shipboard end of the cable, and a buoy
attached. In case of a snag the buoy will support the
end of the cable, and recovery of the fish is probable.
The GSM-19M Overhauser system is a
robust package, free of heading errors, that is ready
for surveys anywhere in the world. Its 0.01 nT
resolution and high absolute accuracy deliver data
quality that can be relied on, without worry of heading
error corrections. For diurnal corrections (base
stations) on offshore surveys the GSM-19 Overhauser
should be considered.
||20,000 to 150,000nT
|Operating Modes, Single Reading:
||1) contact cl. 2) ext. trigger CR (ASCII)
||2 channels (prog. range)
||4 x 20 dot matrix LCD
||24V, 30mA stb, 0.2A max current
||-40°C to +60°C
||9cm dia 60cm long
||7 x 22.5 x 24cm
Fish sensor, console, 100m cable
and instruction manual.