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Survey do’ s and don’ ts
ENVI-MAG Manual
B3-5
Ex
ample
The magnetometers must be synchronized to the same time to allow for
proper corrections when removing the time-based variations. If the two
magnetometers are not properly synchronized, you may end up effectively
adding noise to the corrected survey data. This synchronization should be
done at the beginning of every survey day.
Once the magnetometers are synchronized and the base-station started,
then the survey can be carried out. The base station is taking measurements
independently of the survey magnetometer. Therefore, the survey can
proceed without any undue time constraints, nor are you required to follow
a specific line and station pattern to cover the grid. We shall see in the next
section that this is not always the case for tie-point corrections methods.
Tie-point Line and Loop mode corrections
If you want to remove the time-based variations, but only have one magne-
tometer, you could choose to carry out either a tie-line survey in line or loop
mode. If you a fairly small area to survey, i.e. can be done quickly, then the
loop method is adequate. For larger grids, the line mode is recommended.
However, it should ne noted that neither of these methods are as accurate or
precise as the base-station correction. A technical paper on this subject
(
Magnetic Correction Techniques ) is available from SCINTREX.
The tie-point line method uses data collected along the base line (or rarely,
tie-lines) as reference points to correct for the diurnal drift of magnetic data.
The following figure (Figure B-21 on page B3-6) illustrates a typical tie-point
line method survey.
The basic sequence is to survey the base-line as quickly as possible, taking
readings at every line that crosses the base-line. This data is entered into a
special memory section when recorded with the “TIE-PT” key. You then
proceed to survey the grid on a systematic basis. As you proceed along the
grid, you will then be occasionally taking readings at known stations (those
collected with the “TIE-PT”). This repeating of measurements at certain
stations is then used by the correcting procedures (performed after the
survey) to remove any diurnal variations detected.
If you will be surveying a large grid, that may take more than one day (or
more than one instrument’s memory capacity), then you
must record all of
your tie-line data before any other grid survey data is collected. This ensures
that the data from the second and subsequent days (or second memory full)
are reduced to the
same set of correction values.