Physical Ocean Lunch Seminar, 26 Apr 2023

Wednesday, April 26, 12:30 PM to 01:30 PM PDT

Physical Oceanography Lunch Seminar
Observational frontiers in physical oceanography enabled by fiber-optic sensing

Ethan Williams
UW Earth and Space Sciences

12:30 P.M., 123 Marine Sciences Bldg

ABSTRACT:

Distributed acoustic sensing, or DAS, converts ordinary fiber-optic cables into dense arrays of environmental sensors, capable of detecting both strain and temperature perturbations at meter-scale resolution across distances as large as 100 km. In this talk, I will discuss the opportunities and challenges offered by DAS for physical oceanography, utilizing pre-existing optical fibers in seafloor telecommunication and power cables:

(1) Surface waves. In shallow water, ocean surface gravity waves dominate the near-bottom pressure field and can be recorded by DAS. With a 4.5-day dataset from the Strait of Gibraltar, I quantitatively demonstrate that DAS strain is proportional to surface gravity wave pressure, and qualitatively demonstrate that DAS can record both wind waves and swell in a similar manner to wave buoys. Array-processing methods enable measurement of small perturbations in the dispersion relation, including variations associated with water depth and mean-flow velocity. Integrating these measurements with a simple model of wave-current interaction, I extract a spatio-temporally continuous slice of the depth-averaged flow velocity along the cable path. With further in-situ calibration to constrain the amplitude response and directional sensitivity of DAS to surface gravity waves, deploying DAS on the vast network of existing seafloor telecommunication cables promises to dramatically densify the network of near-shore wave and current observations.

(2) Internal waves. Where fiber-optic cables are unburied and exposed at the seafloor, DAS is also sensitive to temperature fluctuations from internal wave and tide dynamics in the bottom boundary layer, a region of enhanced ocean mixing but scarce observations. First, I showcase temperature transients up to 4 K along a power cable in the Strait of Gibraltar, associated with passing groups of large-amplitude internal waves propagating on the near-surface thermocline. Then, I show how temperature variability of about 2 K at 1-km depth decreasing to 0.2 K at 2.5-km depth on the slope of Gran Canaria, an island off the coast of west Africa, reveals the bore-like propagation of the nonlinear internal tide at locations where the slope is near critical. While the temperature sensitivity of DAS is advantageous for observational oceanography, it also presents a significant challenge for long-period seismology and seafloor geodesy on unburied cables, as temperature fluctuations associated with internal waves and boundary-layer flows may overprint elastic strain from fault creep or slow earthquakes.