Geology Reference
In-Depth Information
or suspected magnetic sources. Unless the grid is already well mapped, the
notebook should also contain enough information for the line locations to
be verified using maps or air-photos.
At the end of the day, a final reading should be made at the base first
occupied. This should again be timed to coincide with a reading of the
diurnal magnetometer. If the field readings are being recorded manually, it
is good practice to transcribe the diurnal values for the times of the field
readings into the field notebook, which then contains a complete record of
the day's work.
3.4.4 Standard values
A diurnal curve records the way in which field strength has varied at the
fixed base, and data processing is simplified if this base is at the same point
throughout a survey. A
standard value
(SV) must be allocated to this point,
preferably by the end of the first day of survey work. The choice is to some
extent arbitrary. If the measured value varies between 32 380 and 32 410 nT,
it could be convenient to adopt 32 400 nT as the SV, even though this was
neither the mean nor the most common reading.
In large survey areas it may be necessary to establish sub-bases (see
Section 1.6) and determine their SVs. The underlying principle is that if,
at some given time, the base magnetometer reading is actually equal to the
base SV, then identical instruments at all other bases and sub-bases would
record the SVs at those points. The field readings are then processed so that
this is also true of the values assigned to all survey points.
3.4.5 Processing magnetic data
During a survey, bases or sub-bases should be occupied at intervals of not
more than 2 hours, so that data can be processed even if the diurnal record
is lost or proves faulty. The ways in which such readings might be used to
provide diurnal control, with or without an automatically recorded diurnal
curve, are shown in Figure 3.6. The diurnal correction at any time is simply
the difference between the SV at the diurnal station and the actual diurnal
reading, but magnetic data can be corrected in two different ways using
this fact. The more straightforward is to determine, by interpolation when
necessary, the diurnal value at the time of a given field reading and to subtract
this from the reading. The diurnal station SV can then be added to give the
SV at the field station. If a computer is available, the whole operation can
be automated.
This method is simple in principle and provides individual values at all
field points but is tedious and error-prone if hundreds of stations have to
