Marine seismic profiling and shallow marine sand resistivity investigations in Broken Bay, NSW, Australia

Abstract

A marine continuous seismic (CSP) profiling study and a resistivity study of vibrocore samples of shallow marine sands were undertaken in Broken Bay, NSW, Australia, to characterise the seabed. The acoustic impedance contrast between deeper sediments and the anticipated sandstone was variable suggesting very dense sands, evident as layered sediments owing to regression and transgression of sea level, and variably weathered sandstones. Interpreted bedrock levels vary considerably across the survey area and represent drowned river valleys of the inner continental shelf. A broad deep channel representing a high-energy paleo-fluvial drainage system in the Hawkesbury outreaches is identified, extending to ~ -80 m below sea level (BSL). Another area is revealed with a dentritic fluvial pattern extending to ~ -70 m BSL. A moderately narrow paleovalley extending to -90 m BSL either side of the Palm Beach tombolo is clearly identified. The sampled sands had very low magnetic susceptibilities, thus corroborating the visual impression of low ultrafines content. However, a variety of grainsizes, grainshapes, colours and cohesions were noted. These factors, together with variations in inferred porosity, gave rise to a range of resistivities, which, when temperature corrected, plotted with some scatter about appropriate Archie Equation curves. Resistivity values recorded ranged from 1.3 to 0.6 O m (20°C) for 64 samples obtained from 17 sites.