The determination of crustal structure in the Adelaide Geosyncline using quarry blasts as seismic sources

Abstract

This thesis describes a seismic experiment, the aim of which was to determine a model of the crust for South Australia in the region of the Adelaide geosyncline, in terms of the variation of seismic velocity with depth, down to the Mohorovicic Discontinuity. Chemical explosions at three large open cut mining operations were used as Sources of seismic energy for recording along three linear profiles directed along and approximately transverse to the north-south axis of the geosyncline. The first chapter contains a review of seismic experiments conducted previously in South Australia, together with a resumé of other geophysical experiments which have bearing on the crustal-mantle structure in the Adelaide geosyncline. Chapter two describes the experimental procedures which were adopted for the present seismic survey. This includes descriptions of the recording equipment, the choice of recording sites, the method of determining the times of blasting at each quarry and a description of each blast source. Stations of the permanent seismic network, which is operated by the University of Adelaide, recorded seismic arrivals from the three blast sources. The travel times of seismic waves from the sources to the permanent stations are discussed in chapter three. An attempt is made to obtain a model of the crust based on these travel times to the permanent stations, for comparison with crustal models deduced by earlier workers from essentially the same station network. The results of the present study are significantly different from crustal models determined previously from earthquake data. Chapter four contains the experimental results obtained from the three seismic profiles, and their interpretation. The resultant crustal model, which is the simplest model consistent with the observed travel times, consists of two essentially homogeneous crustal layers overlying the mantle. The average P wave velocities in the upper and Iower crustal layers are 5.95 km/s and 6.46 km/s respectively, with the boundary between these layers occurring at a depth of approximately 18 km. The P wave velocity in the upper mantle is 7.97 km/s. The mean thickness of the crust is 39 km. Both the intermediate and Moho discontinuities vary by up to 5 km from their mean depths. Shear waves have velocities of 3.43 and 4.45 km/sec in the upper crustal layer and the upper mantle respectively. Appendix A reviews the theoretical basis of seismic interpretation in large scale refraction surveys, and briefly considers alternative approaches to the seismic inversion problem. Appendix B contains the travel times of seismic phases observed on the temporary station networks.