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.
What is an empirically derived formula?
An empirically derived formula is created from correlations observed in data rather than from proven theoretical derivations.
How does an 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.
What are 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:
You’ll notice that the only value being measured is time; not conductivity, not temperature, not pressure. The sensor relies on first principles rather than an empirically derived formula. For this reason, TOF sensors are fluid agnostic – they work in any liquid be it orange juice, tomato soup, or vodka (Yes, we’ve tried this).
Which is more accurate?
CTD-derived sound speed measurements have four primary sources of error. The individual measurements of conductivity, temperature, and pressure (depth) each have respective error margins. Those inaccuracies, combined with the limitations of the empirically derived formulas used to calculate sound speed from CTD data, result in a generally agreed upon accuracy limitation of 0.25 m/s. In addition to having their own accuracy specifications, each parameter has a different response time which can further exacerbate the issue.
Typical CTD Error Margins
A: Conductivity measurement = 0.003 mS/cm
B: Temperature measurement = 0.005 °C
C: Pressure measurement = 0.05 %FS
D: CT&P conversion to Salinity = 0.01 ppt
E: CTD-derived Sound Velocity accuracy: 0.25 m/s
Sound velocity sensors measure directly, rather than an amalgamation of different measurements. If you remember from the previous section, sound velocity sensors use a basic distance/time relation to calculate speed. This relation assumes of course, that total distance is known.
It should be noted that the total path length of a given sensor is not known at the time of manufacture. For a sensor to produce accurate results, the path length must be known to an extremely high accuracy; an accuracy which cannot be achieved by direct ruler measurement. To put this into context, a path length measurement error on the order of half the width of a human hair would yield a sound velocity measurement error well beyond the sensor’s accuracy specification.
It's all about the calibration.
The path length of a given sensor is determined during its calibration process. Calibration is done by submerging the sensor in a fluid in which sound speed is known to a very high accuracy, referred to as a reference fluid. In our case, the reference fluid is distilled water. We use distilled water because the relationship between sound speed and temperature is described to a very high accuracy by the work of Bilaniuk and Wong2. With baths of distilled water at a known pressure, sound velocity can easily be calculated as a function of temperature. With this process, we back-calculate the path length. This provides the unique path length for each sensor which is then programmed into the sensor’s memory.
The accuracy of modern time-of-flight sound velocity sensors is limited largely by the available reference fluids. The best equation to date yields an SV reference accuracy of 0.02 m/s2. As it is not possible for a sensor to have an absolute accuracy that exceeds (or even exactly meets) its reference standard, sound velocity sensors have a generally accepted accuracy of 0.025 m/s. This accuracy specification encompasses the reference fluid’s accuracy limitation plus a bit of ‘wiggle room.’
Measuring Flight Time
As the method’s name implies, the time it takes an acoustic pulse to travel over a fixed path length – the flight time – is a key element of time-of-flight sound velocity. Time is measured in nanoseconds to achieve the required precision.
How does the sensor measure time with such precision?
The timing technology behind modern sound velocity sensors is adopted from GPS applications. As would be required for any technology launched into space and expected to provide accurate data for years, the timing technology is extremely precise and stable.
Contrary to popular belief among some CTD manufacturers, CTDs are not used as reference standards during the calibration of sound velocity sensors. Why? Simply put, they aren’t good enough. Why would you use a reference that can only provide accuracy to 0.25 m/s when there is a reference that is ten times more accurate?
So, which should I use?
For hydrographic surveying, TOF sound speed sensors are the way to go. Beam steering and refraction correction demand highly accurate sound velocity. CTDs can’t compete with the direct measurement of TOF SV for accurate sound speed measurement.
Another important, but less discussed factor, is robustness. Even if CTD-derived sound velocity data were as accurate as time-of-flight sound velocity, CTDs are generally not suitable for measuring sound speed at the multibeam transducer face. Why? They’re too fragile. The sensing structures of CTDs are typically made of glass or ceramic; far too delicate to withstand the rigors of being installed on the underside of a ship hull. A TOF sound speed sensor, in contrast, is typically made of titanium and carbon fibre.
Most users install their sound velocity probe at the multibeam transducer face, but some prefer to install their probe within a sea chest. This facilitates easy installation and removal, and also protects the probe from the aforementioned impacts to which probes installed on the hull are vulnerable.
Installing your sound speed probe in a sea chest has trade offs, however:
- The water velocity inside the chest will be different than outside the vessel. This can create a thermal lag between sound speed seen at the sonar acoustic face and that measured by the probe.
- The water temperature inside the chest is quite often warmer than outside the vessel due to thermal shedding. A small change in temperature will result in a large change in sound velocity.
- Sensors in a sea chest are more susceptible to fouling due to the reduced flow of water.
There are, however, a few reasonable arguments for using a CTD for water column profiles:
1 – Oceanographic data: If you are collecting data for more than hydrography, you probably need a CTD. Remember, Conductivity, Temperature, and Pressure (Depth) are the parameters that form the foundation from which a long list of characteristics can be calculated. CTD is the basis for all TEOS10 equations.
2 – Continuity: With CTDs having been the best tool for sound velocity measurement for decades before TOF SV instrumentation came along in 1995, continuing to use CTDs allows data to merge with decades of historical data.
3 – Backscatter data: Accurate salinity calculation is required to determine absorption coefficients. Salinity is most often calculated from CTD measurements.
There is one small yet important fact I have not mentioned: We’re not in the ’90s anymore!
Profilers equipped with both CTD and TOF Sound Velocity sensors have been on the market for years now and they become more compact and easier to use with every generation. So even if you do need CTD data for scientific applications or historical continuity, it doesn’t have to be at the expense of hydrographic performance. Use a combined CTD-SV instrument and satisfy all requirements.
1 Chen, C-T, and Millero, F.J. . Speed of sound in seawater at high pressures. The Journal of the Acoustical Society of America Vol. 62, No. 5, pp. 1129-1135.
2 Bilaniuk, N. and Wong, G. S. K. . Speed of sound in pure water as a function of temperature. The Journal of the Acoustical Society of America Vol. 93, No. 3, pp. 1609-1612.