G-858BP Backpack Mag
Operators Manual
Having determined the optimum line spacing, layout the survey area in a square or rectangular
format with the lines running N to S if possible. Use a Theodilite if available, otherwise use a
measurement tape (non-stretch) of the required length. Designate one side of the area to be the “baseline”
and layout and mark on this line each survey profile. Note that any stakes driven into the ground must be
non-magnetic. (Wooden stakes driven into the ground will work well but may not be visible at distance.)
If the site will be reoccupied in the future, it may be desirable to drive a one-meter iron rod (re-bar) into
the ground at each corner of the survey area as a permanent marker. Since the profile line markers may not
be visible at a distance, a method must be found that will allow the Magnetometer operator to locate and
follow the line: a light cord or rope stretched on the ground between the beginning and end of each line; a
long PVC pipe or other colored marker to be held at the end of each line and then sequentially moved; use
of spray paint to mark a series of dashes along the path of each line, etc.
If the terrain is rough or bushy, ground markers along each line will be essential. Otherwise, if the
terrain is flat and each end of the line is readily visible, a marker at each end and in the center may suffice.
(The operator should have at least two markers to line up on when starting a line.) Whatever method is
chosen, it must be completely non-magnetic, positive, easily moved in a coordinated way, and must give
the operator a precise direction.
Note that coordination between the people moving the markers is sometimes difficult and
frequently a source of error. In addition, if the survey lines are closely spaced, e.g., one- or two-meter
separation, the Magnetometer operator may have trouble distinguishing between which marker to head for.
The most certain and positive method in all cases is to mark the path by stretching a light string or rope the
entire length of the pathway; or, to paint or otherwise mark the ground at short intervals.
If the area to be surveyed is larger than say 100 x 100 meters or difficult to walk through, then the
area should be broken up into convenient sub-sections which overlap by at least one profile line. If some
sections of line are not passable, then provision should be made for the operator to detour around them but
only after establishing a procedure to stop/start (pause) and annotate the data. (This must be foolproof,
simple and fully coordinated with the data processing.) Note that the Magnetometer operator must be
aware of what line he is on at all times and the line number must agree with the line number recorded in the
data. This is once again a frequent source of error and should be double-checked by another person.
In no case should a survey be started until all lines have been laid out and marked, and all aspects
of the survey carefully re-checked. A few hours more or less at this stage means very little. What is crucial
is to prevent major errors (or even minor ones) that may cost extra days of time and effort.
Diurnal Correction
There are many types of surveys that do not require correction for time varying field errors
(diurnal). Generally they involve large magnetic targets such as pipelines and tanks, or coarse line spacing,
or where the survey may be accomplished in a very short period of time. In these cases, once it is
determined that a severe magnetic storm is not in progress, the survey can proceed without the normal base
station correction and with good reliable results.
Note that the high measurement rate of the G-858BP allows a rapid walking speed along the
profile line. Thus, even large target anomalies are covered within tens of seconds, reducing the potential
Diurnal effects.
Surveys that involve very close line spacing, small or subtle targets or where the maximum
accuracy is desired require diurnal correction from a base station installed close to the survey area. Ideally,
equal sensitivity and measurement rates to the field instrument should be employed in the base station. In
general practice, whatever instrument is available is used. In most cases this works well even during
periods of relative high field activity.
It should be remembered that "diurnal error" will have the same effects in the mapping process as
"location error", and that without correction, low amplitude/high frequency anomalies could appear in the
measured data that would seem to be targets but would in fact not be real. In addition, if the survey has
been broken up into blocks that are acquired on different days, the blocks will not fit smoothly together
unless a diurnal correction is made and their D C level shifted.
There are also other types of local magnetic field disturbances that can seriously effect the map
accuracy and quality. These include ground currents and other local AC or DC fields from power lines or
urban electric trains or trolleys. Of these, electric trains or trolleys are the more serious as their effects may
88
