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

Abstract

A marine continuous seismic profiling (CSP) study and a resistivity study of vibrocore samples of shallow marine sands were undertaken in Broken Bay, NSW, Australia, to characterise the seabed. The overall aims were to provide an estimate of the sediment thickness using CSP and an estimate of sediment resistivity in the upper 4 m of the seabed at selected sites. This information can provide a simplified geo-electrical model to assess the accuracy of interpreted seabed and bedrock depths obtained independently from AEM data and to assist the calibration of AEM instrumentation for bathymetric surveys. The acoustic impedance contrast between deeper sediments and the anticipated basement sandstone was variable suggesting irregularly layered sediments. These sediments contained very dense (tightly packed) sands, owing to marine regression and transgression of sea level, and variably weathered underlying 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 palaeo-fluvial drainage system in the Hawkesbury River outreaches was identified; it extended to approximately –80 m AHD. Another area revealed a dendritic fluvial pattern extending to approximately –70 m AHD. A moderately narrow palaeovalley extending to –90 m AHD either side of the Palm Beach tombolo was clearly identified. This same feature was also clearly identified from airborne electromagnetic data which, when interpreted, showed very good agreement with seismic depths. Sand samples from 17 sites were obtained from vibrocores with a penetration depth of 4.5 m. The laboratory resistivity values, obtained from four-electrode measurements on 64 seawater saturated subsamples ranged from 1.3 to 0.6 Ω m (20°C); 0.92 Ω m average. Inter- and intra-site changes in composition (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 4.2 for the suites of sands. These values are consistent with values cited in the literature for similar lithologies.