How Often Should I be Taking a Sound Velocity Profile?
By: Chris Malzone, VP of Sales & Marketing & Jehan Zouak, Marketing Manager - AML Oceanographic
Following the completion of a well done installation, sound velocity has the most drastic impact on the quality of your survey data. In this post, we begin the discussion on profiling frequency.
This is a question that has been posed to me more times than I can count! I started “doing multibeam” back in the late ‘90’s as a Field Engineer for RESON, Inc. Back then, we didn’t have the luxury of automated profiling as AML provides with the Moving Vessel Profiler (MVP), but rather I had to fall back on my MS in Oceanography to explain to new surveyors why they had to stop in the middle of their survey to lower this obscure 1m long cylinder over the side. Keep in mind that many people new to survey are just that, new! They come from a variety of backgrounds, but I have found that there is a commonality. Regardless of education and training, there is often a lack of understanding around the impact that sound velocity can have on survey data and why more data is always better.
What's Sound Velocity got to do with it?
Following the completion of a well done installation, sound velocity has the most drastic impact on the quality of your survey data. Since multibeam echosounders utilize the transmission and reception of acoustic pulses, knowing the exact path each wave front takes on its round trip journey to the seafloor and back is crucial. There are three primary influences that impact this pathway: conductivity, temperature and pressure (depth).
How much of an influence? To give you an idea:
As these three factors vary, so does the velocity. These changes bend the “beam,” causing it to change direction. When measured incorrectly, not frequently enough or in the wrong location, you get what are called Refraction Errors. Refraction errors tend to worsen the further away you are horizontally or off-nadir from the center of the sonar. When the measured velocity is higher than it should be, a flat surface will bow upwards (eg. ‘smile’) and when it’s too low, a flat surface will bow downwards (eg. ‘frown’).
In 10 m of water, the error at 10 m off-nadir on the seafloor is 4.6 cm1. This may not seem like much, but it becomes significant when scaled up to 100 m and out to the full coverage of your multibeam.
When the measured sound velocity is higher than it should be, a flat surface will bow upwards (eg. ‘smile’) and when it’s too low, a flat surface will bow downwards (eg. ‘frown’).
Is there a standard for accuracy?
I know that I still haven’t answered the question. How often should I take a profile? The short answer… as frequently as possible! The long answer is more complicated. The ocean is highly dynamic and the amount of factors that can impact changes in temperature and salinity in general are too numerous to list.
I have listed below questions and factors that merit consideration when determining how often one needs to take a sound velocity profile in order to get accurate survey results.
Sound velocity is particularly variable in coastal waters, where internal and external forces create dynamic oceanography conditions.2
KEY QUESTIONS
- Are you surveying in an estuary which has both fresh and saltwater?
- If you’re surveying offshore, are you close to an estuary?
- Are you surveying near a river mouth?
- Is there any glacier or snow run-off near your survey location?
- Is it windy?
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Are you surveying near any major oceanographic feature such the Gulf Stream, California Current, Kurishio Current, Loop Current, etc?
If you answered “yes” to any of the above questions, the answer to “How often should I be taking a sound velocity profile?” is “continuously.”
FUNDAMENTAL FACTORS
- Fresh water vs. salt water
- Tide patterns
- Tidal prisms
- Longshore currents
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Internal waves
How do these elements affect your survey results? Find out in the third installment of this series.
1 Brennan, C.W. [2009]. Basic Acoustic Theory. R2Sonic LLC Multibeam Training, p. 4.
2 Kammerer, E. [2000]. New Method for the Removal of Refraction Artifacts in Multibeam Echosounder Systems (PhD thesis). Université de Bretagne Occidentale. Retrieved from http://www.omg.unb.ca/omg/papers/kammerer.pdf
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