Geology Reference
In-Depth Information
3.4.2 Monitoring diurnal variation
Diurnal corrections are essential except in differential or gradient surveys.
If only a single instrument is available, corrections have to rely on repeated
visits to a base or sub-base, ideally at intervals of less than an hour. A more
complete diurnal curve can be constructed if a second, fixed, magnetometer
is used to obtain readings at 3 to 5 minute intervals. This need not be
of the same type as the field instrument. A cheaper proton magnetometer
can provide adequate diurnal control for surveys carried out using a (more
expensive) caesium vapour instrument.
In principle, frequent base reoccupations are unnecessary when an auto-
matic base station is operating. It is, however, poor practice to rely entirely
on the automatic record, since the field data will then be difficult, if not
impossible, to correct if the base instrument fails. Problems can occur even
when a base instrument is manually operated, but they are especially likely
if it is unattended. The battery drain is rather high in automatic instruments
and the transition from operational to unworkable can occur suddenly and
without warning. Readings already stored are preserved, but diurnal control
is lost for the rest of the day.
Obviously, bases should be remote from possible sources of magnetic
interference (especially temporary sources such as traffic), and should be
describable for future use. Special vigilance is needed if field and diurnal
instruments are later linked by a data-exchange line and corrections are made
automatically. Unless the diurnal curve is actually plotted and examined,
absurdities in the diurnal data (as might be caused by an inquisitive passer-by
driving up to the base) may appear, in reverse, as anomalies in the field data.
3.4.3 Field procedures - total field 'point' surveys
Except in gradiometer surveys, a diurnal magnetometer should be set up at
the start of each day. The first reading of the field magnetometer should be at
a base or sub-base, and should be made at the same time as a reading, either
automatic or manual, of the diurnal magnetometer. This does not necessarily
require the two instruments to be adjacent.
High-sensitivity magnetometers, with their 0.1 second reading times, can
be used to obtain essentially continuous profiles, but proton magnetometers
require the operator to stop for each reading. All such readings should be
repeated, and the two should differ by not more than 1 nT. Greater differences
may indicate high field gradients, which may need further investigation.
Large differences between readings at adjacent stations call for
infill
at
intermediate points. It is obviously desirable that the operator notices this,
and fills in immediately.
At each station the location, time and reading must be recorded, and also
any relevant topographic or geological information and details of any visible
