Characterising the subtropical front and associated water masses offshore Otago using seismic oceanography

• Main Author: Cooper, Joanna Kathleen
• Other Authors: Gorman, Andrew R, Vennell, Ross
• Format: Thesis
• Language: English
• Published: University of Otago 2021
• Subjects: Subtropical Front; seismic oceanography; Otago; New Zealand; seismic; Antarctic; Antarc*
• Online Access: http://hdl.handle.net/10523/10906

The Subtropical Front is a global ocean boundary separating warm, salty Subtropical Water from relatively cool, fresh Subantarctic Water. Near Dunedin, on the east coast of the South Island of New Zealand, the front is located in the vicinity of the continental shelf break, just 20–40 km offshore. At this boundary, mixing processes are important in the transfer of heat, salt, and nutrients between the two water masses. Seismic oceanography involves the acquisition of marine seismic reflection data normally used for subseafloor imaging, repurposed to image the water column. Seismic reflections come from temperature and salinity contrasts within the ocean. Four different applications of the seismic oceanography method were examined in this study, including the first dedicated seismic oceanographic cruises in Australasia. First, seismic data acquired over decades of petroleum exploration were reprocessed, revealing significant seismic reflectivity associated with Subtropical, Subantarctic, and Antarctic Intermediate Waters. The reflectivity patterns were interpreted based on synthetic seismograms calculated from historical oceanographic data, and the location of the Subtropical Front was confirmed using satellite sea-surface temperatures as well as near-surface temperatures calculated from the seismic data themselves. High-frequency electro-acoustic seismic data were acquired along with conductivity-temperature-depth profiles (CTDs) on four cruises along the historically well-studied Munida Transect. While the seismic data did not produce discernible water-column reflections above the background noise field, synthetic seismograms produced from the CTDs contributed to a better understanding of the water masses in the region, including an examination of temporal variability in reflectivity. A larger-scale cruise was also carried out, where high-frequency generator-injector air gun seismic data were acquired, accompanied by expendable temperature-depth probes. These data produced seismic images with reflections clearly associated with the temperature gradients at the Subtropical Front. Repeat acquisition over a period of days showed the dynamic nature of the boundary. Finally, CTD data were collected by a chase boat during the acquisition of a petroleum industry 3D seismic survey. These data definitively connect strong seismic reflections in the seismic image to offshore waters with high temperatures and salinities, and particularly allow for the interpretation of a lens-like reflective feature as an eddy. The swath seismic data also allow the three-dimensionality of reflections to be examined. This study demonstrates the feasibility of investigating oceanographic features in this region using seismic oceanography and provides a methodological comparison to guide future projects. Legacy seismic data are a vast data source, well-suited for mapping water mass boundaries throughout the water column, helping to determine the regional distribution and variability of oceanographic features. High-resolution seismic acquisition produces cross-sectional subsurface images of the Subtropical Front at scales typically unachievable using conventional oceanographic methods. These images reveal characteristic differences in the expression of the front at the surface compared to the subsurface. Combining seismic images with in-situ oceanographic data corroborates the identification of high-temperature, high-salinity waters found well offshore of the Subtropical Front, masked by the presence of a surface mixed layer. These waters appear in features resembling meanders and eddies that could represent a significant mechanism for the mixing of water masses.