For decades, scientists have used a technique called seismic reflection profiling to image underground rock layers. Sound waves produced at the surface reflect off different layers at different times, revealing their positions. In recent years, use of this same technique has expanded to imaging water layers of different temperature and salinity in the ocean.
A new study by Dickinson et al. investigates how seismic reflection profiling can be used to quantify the rate of mixing between layers of water at different depths in the ocean. Known as diapycnal diffusion, this small-scale mixing plays an important role in global ocean circulation, with significant implications for predictive climate models.
The research team exploited a two-dimensional, 175-kilometer-long seismic reflection profile from the northern Gulf of Mexico that was acquired by ION Geophysical. It was captured by a ship equipped with an air gun to generate sound waves. Water depth along the length of the profile varied from 200 to 3,000 meters, but the profile captured detailed data only for the upper 1,000 meters, where there were large changes in temperature and salinity with depth.
After processing the sound wave data, the researchers identified 1,171 separate reflections in the image, representing many layers of differing temperature. In the upper 300 meters of deeper waters, some reflections extended horizontally for as much as 80 kilometers, whereas reflections were shorter and more disrupted closer to the continental shelf and at greater depths, suggesting enhanced mixing. The reflections also revealed the presence of other features, including a subsurface eddy.
The researchers then used the reflection data to mathematically quantify diapycnal diffusivity throughout the profile, combining a variety of previously developed methods. This enabled them to assign a diffusivity to each of the 1,171 reflections. Consistent with reflection lengths, diffusivities were lower in the upper 300 meters of deeper waters, whereas higher diffusivities were seen near the seabed.
Overall, these findings suggest that strong density stratification suppresses turbulent mixing in the upper 800 meters of the northern Gulf of Mexico. Mixing is enhanced above rough seabed topography and by lower stratification at greater depths.
Most previous work in the Gulf of Mexico has focused on large-scale circulation, so this study makes a significant contribution to the understanding of small-scale mixing in the region. The authors also performed a thorough error analysis and comparison of the methods they used to determine diapycnal diffusivity. This comparison could help inform and enhance the use of quantitative analysis in future seismic oceanography research. (Journal of Geophysical Research: Oceans, https://doi.org/10.1002/2017JC013352, 2017)
—Sarah Stanley, Freelance Writer