The first full-time university lecturer in Australia in exploration geophysics was Dr Henry Ivison Shipley Thirlaway, who was appointed to the position in the University of Sydney's Department of Geology in 1949. Dr Alan Day (1966) reports that the only teaching in geophysics before this was in 1931, when E. H. Booth gave a series of lectures, demonstrations and three days field work in geophysical prospecting 'under the auspices of the Extension Board of Sydney University' 1 .
Summary
The gravity measurements made by members of the Department of Geodesy and Geophysics of Cambridge University in the Welsh Borders from 1947 bo 1950 are described and the geological results discussed. The methods of measurement and the calculation of the gravity anomalies are not given in detail since they have already been discussed elsewhere. The data given in the paper in the form of a map of the Bouger anomalies are probably consistent to within ±0–25 mgl.
At the Malvern Hills the Bouguer anomalies change rapidly from about —25 mgl. on the east to about -(-5 mgl. on the west. A probable interpretation is that there is a deep basin east of the Malverns, filled mostly with Triassic rocks, reaching a depth of nearly 10,000 feet near Worcester and bounded on the west by a surface sloping down at about 45° from the Malvern Hills. It is suggested that the basin was formed by faults, the movement along which began in late Carboniferous times and ended in late Triassic times.
Large variations of anomaly in the Welsh Borders appear to have no relation to the known geology and are probably due to heavy Pre-Cambrian masses, particularly in the Black Mountains and near Leominster. The Church Stretton disturbance appears to be marked only by a slight maximum anomaly.
Before 1946 four explosions with yields of a kiloton or more were recorded by seismographs up to ranges of 10°. After the first motion, the seismograms were not obviously different from those of earthquakes at similar distances. In 1946 Bikini Baker was recorded out to distances of 80° with amplitudes equivalent to an earthquake of magnitude
m
51/2. Since then numerous explosions have been recorded at teleseismic ranges along nine different transmission paths with signal/noise ratios large enough to enable details of the
P
wavetrain to be analysed. Between 30 and 90° these explosion seismograms are characterized by a large, relatively simple onset pulse followed by 20 to 30 s of low amplitude oscillations. Variations of waveform across a large array of seismometers are shown to be due to signal-generated noise. The time-shifted and array-sum records of a given explosion are almost identical. Differences between explosion seismograms are a function of focal depth and local geology. A good working hypothesis is that the deep mantle is very homogeneous. Waveforms of the majority of shallow earthquakes recorded under these experimental conditions are dependent upon azimuth and range. Signals related to focal depth may account for only a small proportion of the observed
P
energy. The conversion of
SV
waves qualitatively explains the seismograms. The observations are consistent enough to make it worth while taking stock of the capacity of the international network to study earthquake mechanism.
Summary
Gravity measurements in the Bristol and Somerset coalfields are described and discussed. Accounts are given of the methods of measuring gravity, of calculating the gravity anomalies and estimating the densities of rocks. It is shown that the variations of gravity correspond in a general way to the geological structure of the coalfields but that discrepancies in detail indicate that the anomalies are partly due to structures lying below the coalfields which may have controlled the development of sedimentation and deformation in the region. Detailed studies were made in the neighbourhoods of Bitton, Chew Stoke, Ston Easton and Avonmouth with a view to assisting the planning of a boring programme for the development of the coalfields in these areas.
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