G-858BP Backpack Mag
Operators Manual
Magnetic Surveying Checklist
Here we give a quick checklist for performing a magnetic survey. Please see Appendix 1 if you
are unfamiliar with magnetic surveying. Subsequent chapters will explain in detail the operation of the G-
858BP during the survey and of use with the Novatel GPS (SmartAntenna or Smart V1 - See Chapter 9).
GPS data is automatically stored and later interpolated to provide a position for each magnetometer
reading. The following sections refer to operation in X-Y positioning mode only. X-Y data positioning can
be used as a backup to GPS positioning.
1. Setting up the survey grid
a)
Designate an individual to be responsible for making a sketch of the survey site, with
notes and comments on all relevant objects such as power lines, fences, pipes, and
surface debris.
b)
Establish a baseline, which will provide the start or end points for all profile lines.
Designate the left hand corner of the baseline to be (0,0) for the X and Y axes. Note that
the survey lines can run in any direction, but if a choice exists, the preferred direction
would be North and South.
c)
Using the smallest search target anomaly size, determine the separation of the profile
lines. For small targets such as a one pound ferrous mass with worst case shape and
orientation, lines spaced 2 meters apart is a good initial choice. For mining and
geologic surveys, line spacing may be 50m to 100m or more. See Geometrics Video
Library movies for more information.
d)
Place non-magnetic, brightly colored markers at the start and stop of each profile. If the
lines are long or require irregular walking speeds, place a marker or transverse rope at
regular intervals (perhaps each 20 or 50 meters) along the profile. These will become
the fiducial or waypoint entries in the data file. You will press the green Mark button
on the console at every waypoint.
e)
If the survey is to be broken into separate but adjacent areas, it will help you to stitch
the sections together if there is at least one profile line of overlap. Also insure that the
profiles extend beyond the actual survey boundary by at least 2 or 3 times the estimated
target depth.
f)
Locate the survey area corners and reference them to other surface objects. If the site
will be relocated in the future, it may be useful to permanently mark the corners with an
iron stake (re-bar) driven down to ground level. Note that these corner stakes are the
only magnetic objects that are used on the entire survey site. Flags, cones, stakes and
other markers must be carefully inspected to be non-magnetic.
g)
MagMap2000 includes a facility to “remap” the data to another datum or transform the
data to another surface. If the positions of the corners of the survey area are recorded in
GPS, this information may be used to transform the data set to this coordinate system.
In this way magnetic data may be output in local coordinate systems such as State Plane
Coordinates. See the MagMap2000 manual for more information.
2.
Determine whether you will operate the G-858BP with the sensor mounted above the shoulder on
the back pack (G-859 style, see Figure 1) or with the sensor mounted on the staff (G-858 style, see
Figure 2). For searching for deeper objects with longer spatial wavelengths (larger bombs, storage
tanks, pipelines and geological survey) it is common practice to mount the sensor higher, away
from ground clutter. This may be counter intuitive, but deeper targets require the sensor be
removed from surface sources of magnetic contamination such as magnetic debris (fragments of
exploded ordnance), magnetic rocks or other man-made artifacts. The sensor should be carried
lower (and out in front of the operator) when searching for small targets (small UXO,
archaeological survey, forensic survey). The choice is left up to the operator which will become
an easy decision with experience.
3.
Note that if the G-858BP is to be operated in gradiometer mode (two sensors in vertical or
transverse gradiometer configuration) the sensors must be mounted on the staff or on a non-
magnetic cart only. Gradiometers cannot be deployed on the backpack.
4.
Turn on and warm up the G-858BP (about 2 – 10 minutes depending on ambient temperature),
using the procedure described in Chapter 2. Select the Search Mode and adjust the sensor and
clamp for the proper operating orientation, i.e. there should be continuous signal and correct
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