Geometrics Memory-Mag G-856AX, Руководство по эксплуатации

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Geometrics, Inc
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G-856AX Operation Manual
30
If the local gradient is very high the magnetometer will not be able to measure with its
maximum precision. This is because the sensor signal collapses, or dies, before the count
time of the measurement circuit has ended. The operator is alerted to this condition by a
series of 5 quick beeps from the console and the operator will also observe that the
measurement displayed is truncated: the magnetometer will not show the least significant
digit on the display. Depending on the resolution you need, this may not be a problem. If
high gradient is the cause, there is no need for 0.1 gamma resolution because the local
magnetic field is changing very rapidly with position. A similar effect may be observed
in very low fields. You can usually improve the signal strength by shortening the count
time and/or lengthening the polarization time. (See the Appendix for information on the
console’s Internal Programming Switches for information on polarization time.)
Magnetic environment
Objects carried by the operator or that have been added to the magnetometer system can
noticeably affect the total magnetic field measurement. Objects suspected to be magnetic
should be checked in the following manner:
1. Mount the sensor on its staff, place the suspected article far away from the sensor,
and take several measurements. Each measurement should repeat to ±1 nT.
2. Now place the suspected article fairly close to the sensor (about ½ meter) and
again note the measurements.
3. Remove the article and again take several measurements.
4. If no diurnal shift is present then the first and third group of measurements will be
within 1 nT of each other. You can assume that the test article is magnetic if the
first group and the second group of readings varied by more than 1 nT.
The magnetometer will not operate reliably in areas that are near sources of strong radio
frequency energy, where power lines and transformers are nearby, in most buildings, or
near highly magnetic objects. The sensor should always be placed on the staff above the
ground, or in the "backpack" if the console harness is so equipped. The sensor will NOT
operate properly when placed directly on the ground.
Magnetic surveying
Standard operation
The typical survey geometry consists of a series of measurements equally spaced along a
series of parallel lines. Usually the lines are positioned so as to form a rectangular area.
This survey design insures that the measurement points (stations) will define a grid that
samples the magnetic field strength at a uniform spatial density. An example of this
survey geometry is shown in Figure 21.