Swath bathymetry sidescan (SBS) sonar and the angle-of-arrival processing that underlies these systems has the capability to produce much higher resolution three dimensional imagery and bathymetry than traditional beamformed approaches. However, the performance of these high resolution systems is limited by signal-to-noise ratio (SNR) and they are also susceptible to multipath interference. This thesis explores two methods for increasing SNR and mitigating multipath interference for SBS systems. The first, binary coded pulse transmission and pulse compression is shown to increase the SNR and in turn provide reduced angle variance in SBS systems. The second, transmit beamforming, and more specifically steering and shading, is shown to increase both acoustic power in the water and directivity of the transmitted acoustic radiation. The transmit beamforming benefits are achieved by making use of the 8-element linear angle-of-arrival array typical in SBS sonars, but previously not utilized for transmit. Both simulations and real world SBS experiments are devised and conducted and it is shown that in practice pulse compression increases the SNR, and that transmit beamforming increases backscatter intensity and reduces the intensity of interfering multipaths. The improvement in achievable SNR and the reduction in multipath interference provided by the contributions in this thesis further strengthens the importance of SBS systems and angle-of-arrival based processing, as an alternative to beamforming, in underwater three dimensional imaging and mapping. / Graduate / 2015-04-15 / 0544 / 0547 / mark.butowski@gmail.com

Swath bathymetry sidescan (SBS) sonar and the angle-of-arrival processing that underlies these systems has the capability to produce much higher resolution three dimensional imagery and bathymetry than traditional beamformed approaches. However, the performance of these high resolution systems is limited by signal-to-noise ratio (SNR) and they are also susceptible to multipath interference. This thesis explores two methods for increasing SNR and mitigating multipath interference for SBS systems. The first, binary coded pulse transmission and pulse compression is shown to increase the SNR and in turn provide reduced angle variance in SBS systems. The second, transmit beamforming, and more specifically steering and shading, is shown to increase both acoustic power in the water and directivity of the transmitted acoustic radiation. The transmit beamforming benefits are achieved by making use of the 8-element linear angle-of-arrival array typical in SBS sonars, but previously not utilized for transmit. Both simulations and real world SBS experiments are devised and conducted and it is shown that in practice pulse compression increases the SNR, and that transmit beamforming increases backscatter intensity and reduces the intensity of interfering multipaths. The improvement in achievable SNR and the reduction in multipath interference provided by the contributions in this thesis further strengthens the importance of SBS systems and angle-of-arrival based processing, as an alternative to beamforming, in underwater three dimensional imaging and mapping. / Graduate / 0544 / 0547 / mark.butowski@gmail.com

The use of iceberg-keel ploughmarks as proxy indicators of past and present iceberg morphology, keel depth and drift direction has seldom been approached in the southern hemisphere. Using high-resolution multi-beam swath bathymetry of the mid-shelf Pine Island Trough and outermost Weddell Sea shelf regions of Antarctica, detailed analysis of >13,000 iceberg-keel ploughmarks was undertaken. By considering the draft of icebergs calved from Antarctica today, calculated from detailed satellite altimetric datasets by this work, almost all observed ploughmarks were interpreted to be relict features. In Pine Island Trough, ploughmark planform parameters and cross-sections imply calving of a large number of non-tabular icebergs with v-shaped keels from the palaeo-Pine Island-Thwaites ice stream. Geological evidence of ploughmark form and modern water depth distribution indicates calving-margin thicknesses (949 m) and subaerial ice cliff elevations (102 m) equivalent to the theoretical threshold predicted to trigger ice-cliff structural collapse and calving by marine ice-cliff instability (MICI) processes. Thus, ploughmarks provide the first observational evidence of rapid retreat of the palaeo-Pine Island-Thwaites ice stream, driven by MICI processes commencing ~12.3 cal ka BP. On the Weddell Sea shelf, ploughmark morphologies imply considerable variation in palaeo-iceberg shape and size, most likely reflecting calving from multiple source margins. In turn, an absence of grounded ice on the Weddell Sea shelf and a palaeo-oceanographic regime comparable to today are implied at the time of formation. Analysis of a 3D seismic cube of the Sea Lion Field area of the North Falkland Basin reveals iceberg-keel ploughmarks incised into the modern- and palaeo-seafloor, formed by icebergs of varying shape and size that most-likely calved from the Antarctic Ice Sheet during three past glacial periods (estimated ages ~18 - 26.5 ka BP, ~246 ka BP, ~9.8 Ma BP). Despite illustrating the possibility of iceberg drift into the North Falkland Basin today, the relict ploughmark age implies little risk to any seafloor structures in the area, which might be required for hydrocarbon production. By these analyses, the significance of iceberg-keel ploughmarks as indicators of palaeo-glaciology and palaeo-oceanography at the time of formation is emphasised.