What a Difference a Swath Makes!

Abstract:

Since the introduction to the academic community of multibeam sonar swath mapping techniques in the late 1970s, the ability to collect high-resolution bathymetry and seafloor imagery has steadily increased. Early systems provided a limited number (e.g., 16) of relatively broad (2-4 degree) beams and swath widths of approximately one times the water depth, whereas current systems offer hundreds of narrower (0.5 to 1 degree) beams over swath widths of five to seven times water depth. Innovations like chirped pulses and high-density beam forming (multiple depth solutions per beam footprint) have also pushed the resolution of modern systems. Despite these improvements, the resolution achievable is fundamentally limited by trade-offs between beam width, bandwidth, transducer size and propagation, resulting in the need for short ranges to the seafloor to achieve the highest levels of resolution. In the deep sea this means delivery of high-frequency, high-resolution mapping systems close to the bottom by means of towed or autonomous vehicles, the trade-off here being limited areal coverage due to narrow swaths and slow mapping speeds. Despite these constraints, we cannot forget that only a small fraction of the deep ocean has been mapped by any sort of swath mapping system (10-20%) and that even a single well-calibrated and properly operated multibeam swath collected at high-speed during a long transit can offer a remarkable amount of insight into seafloor processes. We will illustrate this through the exploration of a single transit by the R/V KILO MOANA (using a Kongsberg EM120 MBES) from Hawaii to Kodiak Alaska. When compared to the best available global bathymetric compilations using both satellite derived bathymetry and existing bathymetric data (e.g., Global Topography of Smith and Sandwell, GMRT, or ETOPO-1), the single swath offers detailed views of deep-sea channels, seamount and guyots, tectonic structure and volcanic processes that are totally absent in the compilations. Given the difference that a single swath can make in interpreting deep-sea processes, imagine what we would learn from complete coverage – even at 100 m pixel resolution - of the deep ocean seafloor.