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Long range shock waves from a large explosion underwater: experiment and data
spikes of which the amplitude varied enormously. A significant proportion
of the spikes in the digital files were dropouts to zero. This procedure
was chosen as the best able to maintain any trends in the data without
distorting the background noise level or losing sgnificant signal
amplitude. The data files are available on magnetic tape both with and
without spike removal.
7.3.2 Zeroing
The PUSS digital files were already normalised to zero when received.
The files from the BGS package and land stations were not, each channel
having an offset to a greater or lesser degree. The procedure here was to
take the average of all points up to the arrival of the seismic signal and
to subtract this value from all data points in the file.
7.3.3 Low Gain
The gain on the system at Tentsmuir appeared to have been set too low such
that the signal was buried in system noise generated during digitisation.
Fortunately, the frequencies in the noise tend to be higher than those in
the data, allowing some improvement to be brought about by filtering. For
display purposes only, the Tentsmuir data were filtered at SHz before
plotting (Figs. Fl.S and Fl.l6). The data files on the magnetic tape are
not filtered. Since the original analogue signal is clean, but of low
amplitude, it is hoped to be able to achieve better recovery by
transcribing the source tape at a higher gain. The graphs of the
Tentsmuir data should therefore be regarded as preliminary only.
7.4 PRELIMINARY ANALYSIS
7.4. I Travel time determinations
A program was written to pick the arrivals from the sea bed instruments
and the BGS vertical hydrophone string for both events. The arrival times
for both seismic and acoustic signals are listed in Appendix H. Travel
times were calculated using the shot times determined at the Goosander
(Table 5.1). The travel times are plotted against distance from the shot
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