Schmucker, 1970 (Scripps) - MTNet

14 Bulletin. Scripps Institution of Oceanography
earth is, as a rule, small in comparison to their spatial nonuniformity. As
a consequence, the downward attenuation of F and thereby the correlation
between Fa and F' are independent of the "wave number" of the incident variation
field as is evident from the pertinent relations of sections 5.2 and 5.3 •
. It remains to verify the truly internal origin of the induction anomaly which
these transfer functions describe in a normalized form. To this end we apply
separation methods to their spatial distribution for each resolved frequency
component, thereby eliminating unwanted contaminations of external origin.
In summary, the successive steps of the data reduction are (a) digitalization
of the magnetograms within selected time intervals, (b) separation of
anomalous and normal parts of the. observed variations, (c) cross correlation
between the anomalous and normal parts in the frequency domain, (d) sepa- .
ration of external and internal parts of the thus normalized anomalous part.
3.2 Selection of Time Intervals for Analysis
Three months of field operations with six Askanias gave about sixty days of
usable simultaneous records from all stations. From these eight quiet days
were chosenfor the study of the slow diurnal variations and about thirty hours
of vigorous magnetic activity for the analysis of fast variations. We can
choose here between isolated events of short duration and long-lasting disturbances
or storms. Plates I and II show typical examples.
Single events, in particular so-called bays, provide an excellent signal
which clearly stands out from the general background activity of quiet nights,
even though it contains only few usable frequencies. Numerous events in
various frequency groups can be evaluated without involving an unreasonable
amount of scaling. Storms permit us to analyze a wide frequency range concurrently,
but the overhead storm field is of greater complexity than the
smooth field of bays in mid-latitudes. This impairs the proper identification
and analysiS of induction anomalies. Furthermore, suitable storms of moderate
intensity are rare events and the chance to record one successfully at
all survey stations is not very large.
In short, the forthcoming deductions are derived primarily from the
anomalous behavior of single events. In one instance thus obtained results
were checked against those from a magnetic storm and found to be comparable
(fig. 23).
3.3 Scaling and Scaling Errors
The traces in D, Hand Z have been scaled either on photographic prints,
12 -fold enlarged from the 16 mm film, or with the aid of a Kodagraph microfilm
reader with a 20-fold magnification. For prints the reSUlting time resolution
was 1 min/mm and the scale values were roughly 3y fmm in D and H,
1. 5y /mm in Z. Individual readings of the traces relative to the base line
were-made at fixed time intervals as described in sections 3.6 and 3.7.