Aliwal Shoal

The Marine Geoscience Unit of the Council for Geoscience (ex. Geological Survey) has over the past 3 years been mapping the marine geology of the Aliwal Shoal and the surrounding shelf area. The project forms part of an intensive research study of the area lead by Charl Bosman. The aims of which are to: accurately map the marine geology of the Aliwal Shoal shelf area using various geophysical techniques in order to, determine the seafloor types (reef, sediments), topography (related to depth) of the area and to understand the dynamics (sedimentary and oceanographic) and evolution of Aliwal Shoal.

Aliwal Shoal

Geophysical techniques employed have been side-scan sonar, single beam echo sounding and seismics. Side-scan sonar allows the mapping of the seafloor surface using sound frequencies in the order of 100 - 500kHz. Acoustic images (sonographs) are produced of the seafloor from the reflected sound energy. These are mosaiced together to produce a continuous record of the seafloor, similar to an aerial photograph. Researchers are then able to interpret the seafloor types from the side-scan sonar mosaics. Groundtruthing via Scuba diving is also carried out to verify interpretations and to collect samples.

Echo-sounding measures the return time of a sound signal propagated to the seabed, from which the depth of to the seafloor can be determined. Several hundred thousand soundings have been collected in the study area and have resulted in the compilation of detailed bathymetric charts of the area. Three-dimensional modeling of Aliwal Shoal has been made possible from this very high-resolution data set.

Seismics allows for the collection of sub-surface geological information (the prime tool used in oil and gas exploration). A high energy, low frequency sound source is emitted towards to seafloor, much of which penetrates beyond the seabed. The downward transmitting sound energy gets reflected back towards the surface where it encounters any interface between medians/geological layers that have a density contrast. Return signals arriving at slightly different times, each carry information about the interface layer that reflected them. From this a two-dimensional slice/cross section through the seafloor is build up. Seismics data will aid in unraveling the evolution of the continental shelf around Aliwal Shoal since the break up of Gondwana, which started about 180 million years ago.

In addition, ten in-situ rock samples weighing between 50-80 kg each, were extracted from various depths (- 8 to -34m) using hammer and chisel and brought to the surface using lift bags. These samples are in the process of being dated using various radio-isotope dating techniques and will finally yield the geological age/s of formation of the sandstone that makes up Aliwal Shoal. It is expected that these results will reveal a complex evolutionary history for the Aliwal Shoal, resulting from many osscilations in sea level heights.

The attached figure illustrates the extends of the Aliwal Shoal study area, where geophysical and dive sampling has been undertaken. In addition the boundaries of the Aliwal Shoal Marine Protected Area (MPA) and the Aliwal Shoal Restricted Area are given. Note the Restricted Area around the wreck of the Produce has been omitted due to its small size on this map scale.