During 1949 and 1951, whilst investigating possible underground water supplies, staff of the Bureau of Mineral Resources surveyed two east-west gravity traverses near Watheroo Observatory (120 miles north of Perth) and Bullsbrook (20 miles north-north-east of Perth). Results of these investigations suggested the presence of a major fault a few miles to the east of both places and gave evidence of a large thickness of sediments underlying the coastal plains. It was decided, therefore, to extend the gravity investigations to a reconnaissance survey of the whole of the coastal region between Geraldton in the north and Cape Leeuwin in the south, extending inland from the coast for distances of 100 to 150 miles. This bulletin gives results of the survey, which was made in 1951 and 1952 over an area of about 43, 000 square miles and comprised gravity observations at 650 stations. The survey revealed a negative gravity anomaly of large magnitude and areal extent. Bouguer anomaly values, with a minimum of more than 130 milligals, form a trough about 400 miles long and up to 55 miles wide. Steep gravity gradients on the flanks of the anomaly suggest the existence of faults which have resulted in relatively light sediments lying adjacent to denser basement rocks. Such steep gradients indicate that the major part of the anomaly is due to an accumulation of sediments which at their maximum development may be about 30,000 feet thick. The Darling and Dunsborough Faults show up prominently on the gravity pattern, but other faults deduced from geological evidence are not apparent probably because the vertical movement on these faults has been too slight to have affected the gravity pattern. On the other hand, the gravity results indicate the presence of other, faults previously undetected. Two minor synclines are indicated, diverging from the main synclinal axis, and the importance of anticlinal axes associated with these synclines is stressed. The gravity pattern indicates that although the Perth Basin narrows to the north, between converging faults, it continues in that direction, to join the Carnarvon Basin. Recommendations are made for more detailed gravity surveys to be made in certain localities and for seismic surveys of selected traverses across the basin.

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As a contribution towards the assessment of the oil potential of the Canning Basin, the Bureau carried out a seismic survey in July, 1955 along a traverse to the north-west of Lansey's Cross ing in the Kimberley Division of Western Australia. There is a considerable thickness of sedimentary rocks, probably exceeding 20,000 feet, in the nearby Fitzroy Basin, but little is known of the structure of the Canning Basin. The object of the survey was to obtain information on the thickness of the sedimentary rocks in this part of the Basin and on thc geologicul structure of the sedimentary rocks at depth. The results show that near Langey's Crossing the thicknes s of the sedimentary rocks probably exceeds 16,000 feet and may exceed 20,000 feet. The traverse was oblique to the assumed. direction of the axes of the major structure in the basin and results indicate that it crosses the axis of what appears to be a broad syncline of low relief.

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These documents have been scanned by the GA Library. Please refer to the document for contents.

These documents have been scanned by the GA Library. Please refer to the document for contents.

A seismic reflection survey was made to the south-west, south and east of the Poole Range Structure, Kimberley Division, W.A. as an extension of a previous seismic survey. The results obtained confirm the surface information and show the existence of an anticlinal structure l the axis of which plunges to the east and whose axial plane probably dips to the south. A total thickness of sediments of the order of 20,000 feet is shown. Some slight evidence of deep faulting down to 10,000 feet was recorded, but is not conclusive. Insufficient evidence was obtained to select a site for a test bore, but further seismic work is not recommended at present.

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