Subsurface Observations of Surface Waves From an Autonomous Underwater Vehicle

In this study, we will show how ocean surface wave displacement can be estimated from sensors onboard a subsurface moving autonomous underwater vehicle (AUV). The approach is to use a high-resolution vertical accelerometer along with a highresolution pressure sensor mounted coincidently on an AUV. We apply this approach to data collected in summer 2005, in northeast Monterey Bay, CA, during an engineering experiment as a part of the Layered Organization in the Coastal Ocean (LOCO) program. The AUV used was the School for Marine Science and Technology, University of Massachusetts Dartmouth T-REMUS vehicle. Data were collected while the vehicle traveled at a constant speed 1.2 m/s and a constant depth of 10 m in water of depth 19.5 m. The local wind-generated surface wave field was relatively weak and dominated by surface wave swell of frequencies of 1/12 and 1/6 Hz, respectively. Surface waves of these frequencies in this depth of water have wavelengths of 150 and 55 m, respectively, both values being much larger than the length of the T-REMUS, which is 2 m. This condition, along with the fact that the AUV is moving over an order of magnitude slower than the phase speed associated with these surface waves, allows estimation of the frequency spectrum of surface waves by sensors onboard the AUV as well as interpretation of how the AUV responds to the surface wave field. From our estimation technique, we have confirmed that the two frequencies of 1/12 and 1/6 Hz were the dominant surface wave frequencies present and that the AUV-based estimated spectral values agreed very closely with in situ observations made by a fixed slow rise profiler located 200 m away. Pitch spectra indicated that the AUV responded to the higher frequency swell component of 1/6 Hz by adjusting its depth to try to follow the surface-wave-induced pressure. At the lower frequency, the AUV tended to follow the surface wave coherently.