Geometrics Inc. G-882 Cesium Marine Magnetometer Page 2
1.1 How it Works
The G−882 magnetometer employs an optically pumped Cesium-vapor atomic
resonance system that functions as the frequency control element in an oscillator
circuit. The frequency of the magnetometer’s electrical oscillator is known as the
Larmor frequency. In Appendix A we give a more complete description of the
internal working of the sensor, but here it suffices to say that this oscillation
varies with the external ambient magnetic field. We measure those variations and
send them to a computer for display and recording.
Where there are materials that have iron in them (like cables, pipelines, different
types of rock, small artifacts like nails or big items like a steel ship) the earth’s
field distorts about the object and the cesium magnetometer sees this distortion
as an increase or decrease in earth’s field intensity. In the northern hemisphere,
we will typically see an “anomaly” over a ferrous object which presents dipole
structure with a magnetic high to the south of the object and a magnetic low to
the north. This phenomenon is fully described in our Applications Manual for
Portable Magnetometers (AMPM) which is available on the Magnetometer CD
that comes with the system. It is also available on our website
(www.geometrics.com).
The very high sensitivity of the cesium magnetometer allows it to detect small
targets at quite large distances. For localized objects, the magnetometer can
sense anomalies of 1 ton (1000 kgs) of iron or steel at 100 feet (30m) or more,
250 lbs (100kgs) at 50 ft (15m) and 30 lbs (15kgs) at 25 feet (10m) or more
depending on the background magnetic noise level of the earth.
Survey design is crucial to generating a data set that gives you the answers you
want. In general the height of the magnetometer above the sea floor must be
controlled to enable detection of the survey objective. Survey line spacing
should be approximately equal to twice the detection distance to the search
object for 100% coverage. The AMPM manual will give additional information on
survey design for different applications such geological survey and object
location. In addition, we are available to consult with you via phone or email to
help you design your survey for maximum productivity.
2.0 Quick Start Hardware Guide
The information in this and the following section is provided to help you get the
magnetometer set up and running quickly and ensure that it is working properly.
It can be used prior to any survey as a system check-out.
The cesium-vapor sensor is located inside the magnetometer pressure vessel
and is situated at back of the tow fish in the cylindrical body that forms a ‘T” with
the long axis of the vessel. (see figure #1). The sensor is internally connected to
the sensor-driver electronics and Larmor frequency counter circuit module
To Next Page
Loading...
Need help?
General