| Popis: |
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 ¬¨‚àë6˜ívÆ- 1 exp(-7.3 x 10- 3˜ívÆ) m/sec where C is the charge weight in kilograms and ˜ívÆ is the source seismometer distance in kilometres. Arrival amplitudes from the marine shots are given by A= 3.3 x 10-3C0¬¨‚àë33˜ívÆ-1exp(-7.3 x 10- 3˜ívÆ) 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." |
