Abstract
Seismic oceanography generally makes use of multi-channel seismic reflection data sourced by air gun arrays and long hydrophone streamers to image oceanographic water masses and processes—often piggybacking on surveys that target deeper geological features below the seafloor. However, due to the acquisition methods employed, shallow (upper 200 m or so) regions of the ocean can be poorly imaged with this technique, and resolution is often lower than desirable for imaging fine-structure within the water column. In 2012, we collected a set of higher-resolution seismic lines off the southeast coast of New Zealand, with a generator-injector airgun source and hydrophone streamer configuration designed to improve images of shallower water masses and their boundaries. The seismic lines were acquired with coincident expendable bathythermograph deployments which provides direct ties between physical oceanographic data and seismic data, allowing for definitive identification of the Subtropical Front and associated water masses in the subsurface. Repeat acquisition along the same transect shows significant temporal variability on the scale of hours, illustrating the highly dynamic nature of this important ocean boundary. Comparisons to conventional low-frequency seismic data in the same location show the value of high-resolution acquisition methods in imaging the near-surface of the ocean and allowing subsurface features to be linked to their expressions at the surface.
Highlights
The Subtropical Front (STF) is a global ocean boundary separating warm, salty Subtropical Water (STW) from relatively cool, fresh Subantarctic Water (SAW) (e.g., Orsi et al, 1995)
The major differences in water-column imaging between the lines are a result of the vastly different acquisition parameters, with CB82-94 acquired using a large low-frequency airgun array compared to the small single G/I gun of KAH1201-5, and a longer streamer containing many more receivers (3 km and 120 channels vs. 300 m and 24 channels)
The CB82-94 line has a larger gap in the shallow part of the image (∼60 m vs. 40 m) due to direct-arrival interference and muting, but shows significant reflectivity at greater depths; the synthetic seismograms suggest that this is the transition between SAW and underlying Antarctic Intermediate Water (AAIW), expected at depths between 500 and 1,000 m, and not imaged in the KAH1201 data
Summary
The Subtropical Front (STF) is a global ocean boundary separating warm, salty Subtropical Water (STW) from relatively cool, fresh Subantarctic Water (SAW) (e.g., Orsi et al, 1995) This global front primarily lies between 30 and 45◦S, but in the vicinity of New Zealand (Figure 1) it is deflected further south by the continental landmass (e.g., Heath, 1981; Smith et al, 2013). Burling (1961) described the Southland Front as being characterized by steeply sloping isotherms and isohalines at 8–9◦C and 34.5– 34.6 at depths > 70 m At this boundary, mixing processes are important for the transfer of heat, salt, and nutrients between the two water masses (e.g., Chiswell, 2001). The Subantarctic Water of the Southern Ocean south of the Subtropical Front represents a significant potential carbon sink (e.g., Currie and Hunter, 1998); this involvement in the global carbon cycle highlights the importance of studying temporal changes in oceanographic properties including circulation and oceanatmosphere interaction in this region (e.g., Chiswell et al, 2015)
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