geophysical survey to assist with resource definition at a gravel quarry
In 2021, Fender Geophysics were approached by R.W. Corkery www.rwcorkery.com.au to undertake a geophysical survey to assist with resource definition at a gravel quarry in central NSW. The quarry extracts both gravel and pebbles which are dominantly used for landscaping purposes in the region. The pebble and sand layers are generally flat-lying but have highly variable lateral extent. These pebble and sand layers are overlain by a brown to grey clayey sand or sandy clay. The areas currently approved for extraction were close to being exhausted and although additional areas for resource extraction had been identified, they required further investigation to determine their economic value.
Estimates of quarry reserves have relied upon extensive drilling activity on the property. Drilling indicates that the depth to the economically important gravel ranges from 3 to 20m depth. The thickness of the unit ranges from 1-15m. The gravel overlays regional volcanic and granitic rocks. Accurately defining of the thickness of the top-most clay and the underlying sand and gravel was crucial for future planning at the quarry. R.W. Corkery asked Fender Geophysics to use a geophysical survey to map the extent of these units. The results of which could be correlated against adjacent drilling and provide some insights into the distributions of the variation sediment units that would not be resolved by drilling alone. They survey would also provide a ‘proof of concept’ for applying geophysics to mapping and resource estimation in a quarry.
INNOVATIONS AND APPROACH
A resistivity survey was chosen as the most suitable method for the survey as the uppermost clays could have possibly limited use effectiveness of an electromagnetic survey. Fender used a 64 channel ZZ Universal resistivity meter to acquire approximately 1km of data using a ‘roll along’ dipole-dipole survey. The 64 channels with a 3m electrode spacing provided a high-resolution survey with 100% coverage down to a depth of +30m. The interpretation of the resistivity data was aided the presence of nearby drill holes that mapped the depth to sands and gravels and down to the top of the underlying granite.
The high-resolution resistivity survey was shown to correlate will with existing geological data from drilling and indicated that the method would be useful for further assessment of quarry resources. The lobes in the resistivity data are likely to be preserved palaeo-channel deposits from an ancient river systems and additional survey work is required to map the extent of these features. Follow-up survey work indicated that the thickness of the upper clay unit increased further to the north.
The final resistivity dataset for the quarry delineates two sets of features. There was an upper, laterally extensive, low resistivity unit that ranged in depth from 2.5 – 5 m. Below this is a unit consisting numerous alternative ‘lobes’ of moderate and comparatively high resistivity. The lobes extend vertically through the resistivity survey. The upper low resistivity unit correlated well with clay in the adjacent drill sites, the underlying ‘lobes’ correlated with sands and gravels.