AUV-based swell characterization

In this study we show that ocean surface wave swell can be measured from a subsurface transiting AUV. Data were obtained onboard an extended REMUS 100, 2m long, which carried a RGL turbulence package and a SeaBird CTD cantilevered off of the port and starboard sides of the bow, respectively. The data reported in this manuscript was obtained in the summer of 2005, in northeast Monterey Bay, as part of the Layered Organization of the Coastal Ocean (LOCO) experiment. The AUV traveled at 1.2 m/s, sampling at 10 m depth, in the region of the 20 m isobath. In the study region, surface wave swell have a wavelength and phase velocity an order of magnitude greater than the AUV length and transit speed, respectively, with swell wave height often no larger than a meter. A simple model for the T-REMUS response to swell displacement is presented. The model allows use of pressure and vertical acceleration measured on board T-REMUS as input parameters. Four legs, both crossing and following the bathymetry, were performed. We observed a well defined peak in the estimated spectra at .08-.09 Hz (T ~ 11.8 s), typical of swell. A secondary peak also occurs at ~.17 Hz (T~ 5.9 s), twice the swell frequency, and corresponds to the AUV pitch response. For ground truth, swell observations where made with a pressure sensor at a fixed station within 2 km of the AUV transit. Comparison between these two data sets showed excellent agreement at the swell peak frequency.