Schmucker, 1970 (Scripps) - MTNet

2. INS'THUMENTS AND FIELD OPERATIONS
2.1 Askania Variograph
The survey was conducted with six Askania variographs, model Gv3. The
variograph records continuously on 16 mm film the three components of the
transient geomagnetic variations: D (positive magnetic eastward), H (positive
magnetic northward), Z (positive downward). The film speed is 5 mm/hour,
hence a lOO-ft roll is sufficient for about eight months of record.
Providing that the optical system is occasionally readjusted the sharpness
of the traces allows at least 12 -fold magnification, thereby yielding a time
resolution of 1 min/mm in Z. The variometers are of the classical type and
employ suspended magnets in proper orientation. They are mounted inside a
temperature insulated cylindrical container, and an intricate mirror system
gives an effective optical lever of about three meters.
At field sites where AC power is available the variograph can be operated
with thermostat control, requiring 50 watts. If the variograph has to be
operated on batteries, requiring 6 ampere hours/day, temperature compensation
and additional temperature protection of the instrument are essential
when the evaluation of long periodic variation is desired.
Prior to the actual field operations the original sensitivities of the Dvariometer
were doubled to match those of the H-variometer.. For that
purpose a pair of compensation magnets were installed near the D-system to
bias the horizontal magnetic field intensity. Since transient fluctuations in Z
would be of particular importance during the survey, the sensitivity of the
Z -variometer was also increased by adding a small coil of thin copper wire
as trimmer weight to the system. This trimmer weight lifted the center of
mass of the suspended Z-system closer to its axis of rotation, thereby making
the system less stable.
2.2 Intercalibration of the Variographs
The detection and analysis of anomalous variations requires a precise intercomparison
of magnetograms from different instruments at different sites.
Therefore the six variographs were carefully intercalibrated by operating
them side by side for about four months at Scripps Institution, La jolla. This
test has been conducted twice, at the beginning and in the middle of the program.
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