The investigation of deep leads by the seismic refraction method

Deep alluvial leads of Recent, Pleistocene, and Tertiary age have been of considerable economic importance in Australian mineral production. Numerous basalt-covered and alluvium-covered auriferous leads have been worked in New South Wales and Victoria, but are quite rare in other states. However, important stanniferous deep leads have been worked at Herberton in North Queensland. Several geophysical methods can be applied to the investigation of alluvium-covered leads, the seismic refraction method being generally the most suitable. In the case of basalt-covered leads the geophysical problem is more difficult, although satisfactory results have been obtained by the magnetic method at Gulgong, New South Wales, and at Herberton. More recently (1948-49) the seismic refraction method has been applied to problems of this nature at Kalgoorlie, Western Australia, and Wellington and Ardlethan in New South Wales. At Kalgoorlie, geological evidence indicates that deep auriferous leads may extend for some distance to the south of the main Kalgoorlie lodes. At the request of the Western Mining Corporation, seismic refraction profiles were obtained along two 7-mile traverses. The first traverse (A) was about 15 miles south of the mines and the second (B) about 2 miles to the south of the "golden mile". The refraction profile revealed two deep channels crossing traverse A and two channels crossing traverse B; the depth to bedrock on both traverses was, 'however, much greater, than anticipated. For some years alluvial gold has been dredged along the banks of the Macquarie River near Wellington, New South Wales. In order to plan the course of the dredge, a considerable amount of advance drilling and sampling has to be done by, the company concerned. It was considered that the seismic method might be employed in order to reduce the amount of test drilling which has to be done. In 1949, the method was tried in this area but it was found to be too costly in this -application, due to the poor transmission qualities of the overburden which necessitated the use of -large charges or the drilling of deep shot holes. However, the limited amount of work which was done did indicate the unexpected presence of deep ground in a part of the area which had not been tested by boring. In the past, small stanniferous lodes have been worked about 6 miles to the north-west of Ardlethan. The geological evidence suggested that alluvial tin, shed from the Bygoo deposits, might be found at shallow depth. The seismic refraction method was used once again, but it was found that the depth to bedrock was much greater than had been anticipated. However, the presence of a deep gutter, running in a north-easterly direction from the old workings, was fairly well established and drilling recommendations were made to the company concerned.