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Abstract

A reversed seismic profile has been recorded across northern
Tasmania using mine blasts as the western source and several marine
blasts as the eastern source. The initial west to east profile was
recorded using a 24-channel exploration seismograph which was adapted for
crustal studies by the construction of magnetic recording and timing
systems to allow digital frequency filtering and stacking of the data
as well as correction for undesirable instrument characteristics.
During the profile reversal the 24-channel system was used in conjunction
with five 3-channel systems designed using parameters determined from the
initial profile. Plots of the logarithm of the instantaneous seismic
"energy" incident at a recording site were confirmed as a reliable
indicator of arrival times.

The Tasmanian crust has an average P1 velocity of 5.86 km/sec and
overlies a mantle with an average Pn velocity of 7.95 km/sec. A combination
of near vertical-incidence and wide-angle reflection data shows three
crustal layers, the upper layer being consistent with the granitic and
Precambrian rocks near the sources. This thin crust is underlain by a
layered mantle. Strong reflections from the Mohorovicic discontinuity
on near vertical-incidence recordings correlate with wide-angle reflection
arrivals and indicate a crustal thickness varying from 22.3 kilometres to
27.4 kilometres. An asymmetric depression four kilometres below the
average level of the Moho is the expression of a major fracture zone
between northeast and northwest Tasmania.

Arrivals from the mine blasts recorded at the fixed stations of
the Tasmania University Seismic Net indicate a Q value for the crust
between 114 and 328, corresponding to absorption coefficients between
15.0 x 10-3/km and 5.2 x 10-3/km. Arrivals from these mine blasts
recorded along the profile have arrival amplitudes in the first five
seconds that are expressed by
A= 4.6 x 10-5C0 ·6Δ- 1 exp(-7.3 x 10- 3Δ) m/sec
where C is the charge weight in kilograms and Δ is the source
seismometer distance in kilometres. Arrival amplitudes from the marine
shots are given by
A= 3.3 x 10-3C0·33Δ-1exp(-7.3 x 10- 3Δ) m/sec
and within the charge range used are at least 17 times more efficient
than the mine blasts. 1he lower exponent of C in the latter case results
from the use of blow-out to reduce bubble-pulse effects from the marine
shots.

Item Type:	Thesis - PhD
Authors/Creators:	Richardson, R. G.(Robert George)
Keywords:	Seismology
Copyright Holders:	The Author
Copyright Information:	Copyright 1980 the author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).
Additional Information:	Thesis (Ph.D.) - University of Tasmania, 1981. Bibliography: l. R.1-7
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