Geometrics Memory-Mag G-856AX, Operation Manual

Page: 38 / 60

Geometrics, Inc
.
G-856AX Operation Manual
32
Gradiometer operation
The survey layout and measurement accuracy considerations for a gradiometer survey are
the same as for a standard, single sensor survey. The typical sensor separation between
the two staff sections will be 4 ft. - comprising two staff sections. In this configuration
the top sensor will be 8 ft above grade and the lower 4 ft above grade. This arrangement
places the lower sensor closer to the operator and console than is the case with a standard
single sensor survey. Because of this, we recommend that the gradiometer staff be held
at a full arm’s length when taking a measurement. This will position the lower sensor
further from the console and operator and help to reduce their residual magnetic influence
on the gradient measurement.
The G-856AX gradiometer operates in sequential mode. That is, a reading is first
obtained from one sensor and then a reading is obtained from the other sensor. These
two readings are separated by approximately three seconds and it is important to hold the
gradiometer staff steady during measurement. Except in cases of local thunderstorm
activity, there will be no temporal variation in the magnetic field that are detectable with
the G-856AX over this three second delay period.
For gradiometer operation the G-856AX console should be configured for normal
polarize, normal gate, 9600 Baud data transfer, and disable 3-read averaging by setting
switches 1 through 4 to "off" and 6 through 8 to "on" on the G-856X CPU board. See
Appendix section on Internal programming switches for details.
Base station operation
The typical location for a base station is near or within the survey area. The base station
should be placed away from buildings, traffic, and power lines to provide the best results.
Base station setup using Geometrics tripod is shown in Figure 16.
Base station recording is started by entering Automatic Mode. To do this press:
AUTO
,
SHIFT
‘n’,’n’,’n’ ,
ENTER
To stop recording press:
AUTO
,
CLEAR