Brendan Brooke, Geoscience Australia - CERF Surrogates Program

Satellite-derived bathymetry data was produced for the shallow inner shelf around Lord Howe Island, one of the Marine Hub’s field sites. These data will enable the Hub’s surrogacy research to extend into the shallow sections of the shelf by providing high-resolution bathymetry coverage of areas that cannot otherwise be mapped.

The new satellite-derived data complement and extend the multibeam sonar data collected during the Hub’s survey on RV Southern Surveyor in 2008 (with University of Wollongong), as well as the Navy’s LADS bathymetry coverage of a section of the shelf. A QuickBird satellite image of the island and shelf was used in this work. The image was acquired on the 3rd August 2002, has a 2.4m horizontal pixel resolution and comprises 4 spectral bands (Blue 450 - 520 nm; Green: 520 - 600 nm; Red: 630 - 690 nanometers; Near-IR: 760 - 900 nm).

To derive bathymetry data from the QuickBird image, a physics-based bathymetry mapping approach was employed that was recently developed by CSIRO’s Land & Water Group in Canberra. Called SAMBUCA (Semi-Analytical Model for Bathymetry, Un-mixing, and Concentration Assessment), this approach uses an objective and repeatable algorithm and optimisation routine to extract depth, water column constituents and substrate variables on a pixel-by-pixel basis from the spectral bands. The basis of this approach is the expression of the subsurface reflectance as a set of variables, and a parameterisation of this model to define it as a function of a set of environmental variables. An optimisation routine is used to minimise the difference/error between the measured subsurface reflectance and the modelled reflectance by adjusting the variables and retrieving the environmental parameters, including water depth, that correspond to the lowest error between measured and modelled reflectance values. The output for the Lord Howe work is a 2.4 m bathymetry grid that covers the lagoon and fringing reef, the inshore zone around the island and extensive shallow areas that extend from Lord Howe Island to the Admiralty Islands.

Satellite Bathymetry Lord Howe Rise. Photo: Geoscience Australia
Images: Geoscience Australia
Lord Howe Rise. Photo: Geoscience Australia