Marine seismic profiling and shallow marine sand resistivity investigations in Jervis Bay, NSW, Australia*
Julian Vrbancich 1 4 Robert J. Whiteley 2 Don W. Emerson 31 Defence Science and Technology Organisation (DSTO), 13 Garden Street, Eveleigh, NSW 2015, Australia.
2 Coffey Geotechnics Pty Limited, 8/12 Mars Road, Lane Cove West, NSW 2066, Australia.
3 Systems Exploration (NSW) Pty Ltd, Box 6001, Dural Delivery Centre, NSW 2158, Australia.
4 Corresponding author. Email: julian.vrbancich@dsto.defence.gov.au
Exploration Geophysics 42(2) 127-138 https://doi.org/10.1071/EG10037
Submitted: 30 November 2010 Accepted: 19 May 2011 Published: 28 June 2011
Abstract
A marine continuous seismic profiling (CSP) study and a resistivity study of vibrocore samples of shallow marine sands were undertaken in Jervis Bay, NSW, Australia, to characterise the seabed. The CSP study also included Crookhaven Bight, adjacent to Jervis Bay facing north-east to the ocean. The overall aims were: (i) to estimate sediment thickness, and surficial sediment resistivity (upper 3 m) at selected sites, to explore the features of the bedrock topography; and (ii) combined with upper layer (sea water) parameters that can be accurately measured, to provide a simplified geo-electrical model of the ground (sea water/seabed) to support interpretation of airborne electromagnetic survey data. The results of the CSP studies indicate very dense sands (tighter packing) and variably weathered sandstones, suggesting variable geological conditions below the seabed. The bedrock surface was also highly irregular in places suggesting an erosional pulse(s) under rapidly falling sea levels at various times in the past. Bedrock was found to form the sea floor in some locations and is deeply incised by palaeochannels extending to 62 m below sea level in the Crookhaven Bight and Jervis Bay entrance areas.
The laboratory resistivity values, obtained from four electrode measurements on seawater saturated subsamples, averaged 0.88 Ωm (20°C). Inter- and intra-site changes in mineralogy (shell/sand), grain packing, grain size, grain shape, cohesion, and inferred porosity were thought to be responsible for minor variations in resistivity. Archie Equation plots showed some scatter, but the data indicate a cementation factor of ~1.6 and an average formation factor of 4.3 for the suites of sands. These values are consistent with values cited in the literature.
Key words: bedrock topography, Jervis Bay, marine seismic, sediment resistivity, sediment thickness.
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