Geometrics, Inc. G-822/G-823 Magnetometer Manual
35
Note: It is helpful to have CsAz running on a computer while studying the
following material.
The first piece of information required is the earth's magnetic field inclination (dip angle)
in your survey area. Refer to Figure 4 and obtain this angle from the map. For instance,
the dip angle in San Francisco is 62 degrees North, Chicago is 72 degrees North, Sao
Paolo is 30 degrees South and Singapore is 18 degrees South.
Next enter a N or S for the hemisphere (no enter key required here).
The first time, we recommend that you just press enter twice for no tilt and no rotation.
After a few moments you will hear a beep when the computations are completed. Press
any key to see the graphical display of the signal to noise ratio of the sensor in that
orientation at that dip angle.
The circumference of the graphic represents the compass directions in degrees, i.e. the
survey direction. The white portion of the graphic emanating from the center is the
proportional signal-to-noise ratio that would be obtained if the survey were run in that
direction. The black regions represent survey directions (approximately a 30 degree
cone) in which the earth’s magnetic field vector would be in the dead zone of the sensor.
The region of the display that is white and outside of the 15 degree circle represents
survey directions in which the earth’s magnetic field vector would be in the sensor’s
active zone and provide good data. The longer the white line, the better the response of
the Mag in that direction.
If the white line terminates inside the 15 degree circle, then you will get no signal
traveling in that direction. If it terminates just outside the 15 degree circle, then the
installation will tolerate very little in the way of yaw, pitch or roll (depending on
orientation, but most likely yaw) of the sensor before the earth's field vector goes into the
sensor dead zone. Longer lines are better.
Next, press escape and reenter the dip angle and hemisphere information. Now you can
tilt and rotate the sensor to see the effect on the signal versus survey direction. We have
found that tilt angles of 0, 45 and 90 cover all realistic installations and dip angles.
Rotating the sensor often has the effect of changing the optimum direction of survey.
Try a dip angle of 0 degrees with a tilt of 90 degrees. Try rotating 45 degrees. Even in a
horizontal field, one can survey in both north-south and east-west directions. Geometrics
sensors are fully automatic hemispheric switching allowing this versatility.
Try a dip angle of 70 degrees (Indiana) with a tilt of 45 degrees and a rotation of 90
degrees. In some areas, maximum signal cannot be achieved in all survey directions, but
any signal-to-noise lines extending out to 20 and beyond will produce excellent data in
those survey directions even in turbulent air.
