Selecting an
Overhauser Magnetometer: Points to Remember
Francisco Lopez and Elena Khakimova (GEM Systems, Inc.,
Markham, Ontario)
Overhauser magnetometers take advantage of key
phenomena the ability to use radio frequencies (RF)
signals to excite both protons and electrons
contained in a mixture of proton-rich solvent and
electron-rich free radicals.
When
excited, the electrons add their net (higher) energy to that
of protons with the result that signal to noise is increased
by approximately 5000 times. The result is very high quality
results with low power consumption.
When choosing an Overhauser magnetometer for your
application, it is important that you identify:
1. What type of unit do I need: Rover or Base?
For
normal ground operations, a rover and a base station are
required for drift corrections. However, it is also possible
to conduct field operations with just a rover by
re-occupying selected stations repeatedly through the survey
and using these to establish drift corrections.
For
airborne operations, Overhauser is available in base station
configuration. Overhauser offers one of the lowest-noise,
high accuracy systems available.
2. What type of sampling best meets my requirements:
Continuous or Discrete?
Overhauser offers both walking (i.e. nearly continuous)
and mobile (i.e. discrete or standard) survey operations.
Overhauser sample rates (i.e. 5 Hz) are higher than for
Proton Precession magnetometers and gradiometers (i.e.
maximum of 1 sample every 3 seconds).
Sample rates almost match Optically Pumped Caesium systems
(i.e. 5 Hz compared with 10 Hz. 5 Hz is more than adequate
for most ground surveys. It also enables you to perform
vehicular surveys over various types of surfaces (ex. snow
or regular ground) as needed.
3. What should I expect in terms of data quality?
The
Overhauser magnetometer and gradiometer has a number of
built-in features that ensure high data quality. These
include sensor design and processing algorithms.
These systems also have minimal heading error. In
comparison, Optically Pumped Caesium systems can have up to
2 nT heading error. Overhauser has one relatively narrow
spectral line which minimizes heading error. In contrast,
Caesium systems use lumped spectra to reduce errors due to
changes in coupling angle with the ambient magnetic field.
4. What sensitivities are appropriate?
When considering sensitivity of systems, it is important to
remember that Overhauser has, in general, higher
sensitivities than Proton Precession and similar
sensitivities to Caesium, and is available at lower cost
than Caesium.
Sensitivities for Overhauser are on the order of 0.015 pT /
√Hz for Standard (i.e. mobile) mode at 0.33 Hz (discrete)
and for Walking mode at 1 Hz. This is comparable to Caesium
at this sample interval.
5. What are the power requirements and operating
characteristics?
The
GSM-19 Overhauser has the lowest power consumption in
comparison with Proton and Optically Pumped systems; a
consideration that is important for daily productivity and
operator convenience.
Power
consumption is as low as 1W for walking operation. The
reason is that RF polarization requires less power than the
DC current used for polarization in Proton Precession, and
is less power hungry than Optically Pumped magnetometers and
gradiometers which require dedicated heating circuits, etc.
There is no warm-up time in contrast with Optically Pumped
devices, which translates into faster survey initiation.
Overhauser magnetometers operate in either pulsed or
continuous modes. With pulsed modes, the sensor can be shut
down to save power. With continuous reading magnetometers,
power consumption is still minimized as only a few watts are
required to operate the system.
6. What GPS options should I consider: Time and Position?
For stationary (i.e. base station) applications, time-only
GPS is appropriate for synchronizing to the rover prior to
survey initiation.
For walking or mobile surveys, GPS is available to
several levels of precision. <1.5 m circular error
probability is standard for most units which feature an
internal GPS. Higher levels of precision can also be
provided using an external GPS (or using post-processing
algorithms that are available commercially).
It is also feasible to interface to a customer-supplied GPS
on a custom basis. For details, contact GEM.
7. What are the conditions for operating the magnetometer /
gradiometer?
The Overhauser features higher gradient tolerance than
Proton Precession systems for efficient operation in the
vicinity of magnetic rocks or cultural artifacts. Overhauser
signals are stronger and take longer to decay; hence, the
signals are easier to read in regions of high magnetic
gradients.
Environmental parameters for the GSM-19 Overhauser
magnetometer and gradiometer include operation between -40
and +50 degrees Celsius.
8. What if I have more questions?
GEM is more than happy to answer your specific technical or
other questions regarding its families of Overhauser,
Optically Pumped Potassium and Proton Precession units.
Please feel free to send an email to
info@gemsys.ca
or visit
www.gemsys.ca
for more details on the products and capabilities in which
you are most interested.
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