When to be Suspicious of an Area’s Sound Velocity Structure
By: Chris Malzone, VP of Sales & Marketing & Jehan Zouak, Marketing Manager - AML Oceanographic
The ocean is a dynamic environment and there is not one comprehensive model that you can apply to dictate when and how frequently you should sample.
In our last blog, we attempted to answer the age old question of, “How often should I take a profile?” For starters, if you took the time out of your busy schedule to read the entire post, thank you!
To recap, my response to the question I posed was the one kind that I can’t stand; the kind that comes not just in the form of a question but several questions. However, when it comes to surveying, the correct answer is often situation-specific. The ocean is a dynamic environment and there is not one comprehensive model that you can apply to dictate how and when you should sample. In this post we take it one step further to examine a few survey settings where it’s next to impossible to construct a viable sampling strategy.
Inland Surveys
Inland surveys, particularly in estuaries, may encounter a plethora of variable oceanographic conditions. Estuaries are regions in which exchange between inland freshwater systems and marine environments take place. In many estuaries the dominant components in the salt balance are:
- Decreases in density/salinity by the freshwater discharge and ebbing tides
- Increases in density/salinity by flooding tides
These processes lead to salinity distributions that may take the form of a tidal prism (commonly known as a salt wedge). Partially mixed or stratified systems are often characterized by frontal zones, separating the less dense and fresher river waters from the more dense seawater.1 The salt wedge is not a static feature; it can vary in both size and extent as the tide ebbs and floods (see image below).
Another issue with estuaries is that they are regions of high sediment transport, which for the purpose of hydrographic surveying means that the depth in any one location is varying constantly as is the sound velocity. Knowing the sound velocity in these regions is critical due to the varying bathymetry. To further complicate matters, rainfall, runoff, tidal cycles, storm surge, and other events all impact the physical oceanography – and hence the sound velocity – of an estuary. The Port of Rotterdam, one of the busiest ports in the world, is situated in an estuary. For this reason, authorities such as the Royal Netherlands Navy sample sound velocity continuously during survey operations in order to remove any uncertainty in the quality of their data.
The fact remains, most of our largest ports – including London, Hamburg, Vancouver, New York City, and many more – are located in estuaries. Similar issues will be present regardless of location, and knowing exactly what is happening and when remains unseen without consistent sampling.
The estuarine environment that encapsulates the Port of Rotterdam estuarine location makes for dynamic conditions.1
Offshore Surveys
So you read the above section and thought, “But I’m surveying offshore so I’m all good, right?” Not so fast.
So you’re not surveying in an estuary and are currently located offshore. Further, you’re a bit of a nerd on these sorts of things (present company included), and have recently read John Hughes Clarkes’ 2017 paper on coherent refraction of “noise” in multibeam data due to oceanographic turbulence2 and have come to learn that the two biggest external inputs that create a density difference in the ocean are solar heating and freshwater inflow. Even further, you’ve discovered that a shear zone or layer of turbulence can form due to the imbalance between mixing at the surface currents (caused by wind) and bottom currents along the lower seabed (caused by friction).
Two features form in the water column that can impact your multibeam data either separately or together; Kelvin Helmholtz (KH) waves (aka instability) or internal waves. Both form due to mixing at the pycnocline.
WHAT IS THE PYCNOCLINE?
The pycnocline is the point at which density shows a rapid change (increase or decrease). In most areas it is contained in the first few hundred meters of water.
- KH waves form in the presence of shear (two bodies of water moving in different directions) which can take place at a variety of scales. Due to the fact that they can form regardless of bottom morphology (ie. pretty much anywhere), it’s next to impossible to predict when KH waves will form and dissipate. What is consistent is their impact on the bathymetry, which manifests as an increased roughness of your overall data.
- Internal waves form when ocean currents push cold, dense bottom water over complex geomorphology – such as a series of seafloor ridges – and set up a disturbance. They have wavelengths of hundreds of meters and amplitudes of tens of meters. Internal waves will show up as “undulations” along the seams of adjacent swaths.
Regardless of whether it’s KH or internal waves or both, density driven features in the water column are not entirely resolved even by using a Moving Vessel Profiler (MVP) to continuously collect and incorporate sound velocity data in real time. However, the higher sampling frequency enabled by such a system will drastically improve your overall data quality.
High Latitudes
Imagine you are in the northern latitudes and there is a significant shoal that you’ve been contracted to survey due to its reputation as a fruitful and well-exploited fishing ground. It’s wide open ocean so you think to yourself, “I’ve got this.” Then one day you’re looking at a satellite image of the region and there are clear changes in color – most likely some level of upwelling – which you learn is due to significant tidal mixing. It’s late spring and your understanding of oceanography tells you that the increased sunlight over the region combined with low wind conditions will lead to less mixing and more stratification. The latest satellite run also shows there may be significant variations of these layers throughout the tidal cycle. So… what now?
GEORGES BANK
Did that last scenario sound familiar? It played itself out during a survey over Georges Bank spearheaded by John Hughes Clarke back in 1999, and the research that came out of it has laid the groundwork for understanding the full impact of sound velocity on multibeam data.
Georges Bank is not only known for showing higher stratification during the spring due to increased solar radiation but as summer arrives, a strong thermocline is established that constantly migrates back and forth in response to tidal variations. The migration sets up a region of shear along the pycnocline which further increases during tidal cycles. The perturbation of the pycnocline over the bank results in large internal waves which slowly propagate along the thermocline, ultimately forming a front at the shelf-slope break. This front of tidal mixing tends to migrate back and forth over a spatial extent of roughly 8 km during the tidal cycle.
Despite having a graduate degree in Oceanography and a willingness to accept that there are assumptions that must be made in any oceanographic data collection (eg. not being able to completely resolve the issues associated with KH and internal waves), you have doubts about your ability to construct a viable sound velocity sampling strategy. You’re now willing to take on more sampling, but how much more?
Stay tuned for my next post, where I’ll be dissecting a data set to demonstrate what happens when you don’t collect enough sound velocity data.
1 De Nijs, M., Wintwerp, J.C., Pietrzak, J. [2010]. The Effects of the Internal Flow Structure on SPM Entrapment in the Rotterdam Waterway. Journal of Physical Oceanography Vol. 40, pp. 2357-2380.
2 Hughes Clarke, J.E. [2017]. Coherent refraction “noise” in multibeam data due to oceanographic turbulence. Paper Presented at U.S. Hydro 2017, Galveston, TX, 20-23 March. Retrieved from http://ushydro2017.thsoa.org/wp-content/uploads/2017/04/JHC_USHC_2017_paper_format.pdf
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