Geometrics, Inc. G-824A Magnetometer Manual
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Occasionally this automatic hunting is not desirable. Perhaps the survey wants to skirt as
close as possible to the dead without having the hunt mode come on prematurely. Or
perhaps an powerful EM transmitter signal is going to quench the larmor signal while
sensor will only be operating in only one hemisphere. In this case the hemisphere
hunting circuitry could delay the reemergence of the larmor signal after the EM transmit
pulse is gone. Therefore there are two other switch positions to lock the G-824 into only
one hemisphere of operation (North or South).
Note that this is a mechanically locking switch to prevent accidental changes in switch
position. To change the switch setting you must pull out the toggle bar first before
changing the switch position.
If want to lock the hemisphere:
1)
If the sensor is mounted vertically with the cable on top, then in the Earth’s
northern hemisphere you would set the switch to the right as shown in the picture
above. In the Earth’s southern hemisphere the switch would be set to the right.
2)
If the sensor is mounted vertically with the cable down then the switch settings
will be opposite to 1) above.
In practice it may be faster to just mount the sensor and set the switch left or right. If
there is signal fine. If not just switch to the other hemisphere locked postion.
8.0 G-824B MAGNETOMETER
The model G-824B magnetometer is a variant of the G-824A intended for use as a high-
performance base-station instrument. This model has the same dimensions and
performance as the G-824A with the exception of its heading error. G-824B heading
error is larger than the “A” sensors and not specified. As with the G-824A, the G-824B is
shipped with a 25ft external signal/power interface cable, a power/data junction box, and
an RS-232 data cable. These components are used to connect the magnetometer to the
user’s 30VDC power source and logging computer.
When used as a base station, Julian date/time are available for recording and the standard
sensitivities and measurement rates of the CM-321 counter apply. In this application the
A/D converters installed in the CM-201 can be used to convert and transmit other analog
signals (i.e., barometric altitude). Recording rates from once each 1 second to 100 times
per second may be selected. Higher sample rates may be possible, but will depend on the
number of transmitted characters, format, and baud rate.
The 25ft power/data cable included with G-824B will permit the logging computer to be
located far enough from the sensor to avoid interference. Base-station data will also be
more noise free if the sensor is positioned in a spot where the gradients are relative low.
Such a spot may be located by casual surveying of the area selected for the base station
site and identifying the spot with the most uniform field. Selecting this spot will also
