SV vs CTD
Why Your Sound Speed Measurement Method Matters
By: Jehan Zouak, Marketing Manager & Chris Bueley, VP of Projects & Services - AML Oceanographic
It’s an age old question (Okay, maybe not that old): Should I collect sound speed data for my hydrographic survey with a CTD or a Sound Velocity (SV) profiler? What’s the difference?
How does a CTD profiler measure sound velocity?
A CTD profiler measures three parameters: conductivity, temperature, and pressure (depth). These parameters are considered fundamental because they form the foundation upon which seawater can be characterized.
Measurement of these parameters enables the calculation of a host of characteristics, most notably density and salinity. Another quality that can be obtained from conductivity, temperature, and pressure data is sound speed which is calculated through an empirically derived formula.
EMPIRICALLY DERIVED FORMULA
An empirically derived formula is created from correlations observed in data rather than from proven theoretical derivations.
How does a SV profiler measure sound velocity?
AML Oceanographic (then Applied Microsystems Ltd.) released the first time-of-flight (TOF) sound velocity sensor in 1995. Unlike CTDs, which rely on an empirically derived formula to calculate sound speed, the measurement methodology of a TOF sound velocity sensor is based on physics and first principles. TOF sound velocity sensors have improved in form and function over the years, but the core physics behind the technology remain the same.
To measure sound speed, the sensors time an acoustic pulse as it transits a known, fixed distance. Speed of sound in seawater (or in any fluid) is then determined just as one would calculate the average speed of your morning run: speed = distance/time.
FIRST PRINCIPLES
First principles are theories supported by established science. Associations are made through derivations rather than assumptions made through observations as they are in empirically derived formulas.
Here’s how you’d determine speed on a morning run:
And here’s how you’d measure sound speed using the same principles on a TOF sensor:
Learn how MVP can make a difference for you.
AML’s Moving Vessel Profiler is proven to remove both the technical and financial unpredictability associated with survey operations. With over 130 systems sold, MVP is the market leader in underway profiling systems, and is backed by 20 years of experience and thousands of successful surveys.
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