Sound Speed

= Seeking Contributors =

This section is under development. If you have expertise in sound speed profiling and processing, please reach out to the admins at [mailto:omcadmin@ccom.unh.edu omcadmin@ccom.unh.edu] to become a contributor today!

= Overview =

The sound speed environment fundamentally impacts the operation of a multibeam system. This environment is always changing, and care must be taken to sufficiently capture its variability.

Data quality depends directly on acquiring and applying correct sound speed information at the transducer (or the 'surface') and in the water column. There are no replacements for in situ measurements.

= Transducer =

Sound speed at the transducer face directly affects the beamforming and beamsteering capabilities of a multibeam echosounder. Most, if not all, multibeam echosounders require transducer sound speed information to enable transmission or allow acquisition. A fixed value should never be used during normal survey operations.

This is typically measured directly (e.g., Reson SVP-70 probe) or calculated from temperature and salinity data (e.g., Seabird SBE45 thermosalinograph). The sensor is typically mounted in one of two configurations:


 * 1) near the transducer to provide near-real-time sound speed at the face, or
 * 2) in a flow-through system with an intake near the transducer.

Where possible, in situ measurement near the transducer face is preferable for quickly and accurately capturing transients in the upper surface layers. The flow-through approach may be necessary in some cases (e.g., icebreakers), at the risks of temperature changes between intake and measurement and the associated time lag, depending on layout of the intake system and speed of flow.

Increasing insulation and reducing residence times can improve the flow-through measurements. Conversely, mixing in a large-volume sea chest (especially with any outflows nearby) may delay and alter the flow-through measurement to the point that it is no longer applicable for multibeam operations.

In all cases, the intake or sensor must be exposed to the same water flowing over the transducer. This is generally not a concern with a well-mixed upper layer extending below the hull draft, but becomes important when transmitting and sampling at significantly different depths / hull locations across steep temperature and/or salinity gradients between upper surface layers.

Tips
Recommend cleaning the probe before deployment--if the probe is not measuring sound velocity accurately, it can have a cascading effect. SIS will reject the probe if the values are out of range (1400-1700m/s). The RESON SVP 70, for example, flatlines at 1350 when the probe is covered in growth! Cleaning is, of course, dependent on logistics and diver availability if the probe is not otherwise accessible.

[Add additional topics for transducer sound speed]

= Profile =

A profile of the sound speed structure between the transducer and the seafloor is required to compensate for refraction effects (&quot;raytracing&quot;) and correctly georeference each bottom detection. It is assumed that the profile represents a uniform local structure across the swath.

However, oceanographic conditions are always changing, requiring new profiles over the course of survey operations (e.g., typically every 1-6 hours, depending on conditions and refraction artifacts observed in the swath data). It is also not uncommon to observe variation in sound speed profiles across the swath, such as internal waves, causing potentially major swath artifacts that cannot be corrected with single profiles.

Profiles are typically collected with a variety of sensors:


 * expendable bathythermographs (XBTs)
 * conductivity-temperature-depth sensors (CTDs)
 * underway CTDs (UCTDs)

While XBTs and UCTDs are intended to be used while the vessel is moving, CTD profiles generally require the vessel to stop. CTDs attached to remotely operated vehicles (ROVs) can also provide profiles that are useful for mapping operations.

For instance, a ROV-mounted CTD profile collected during ascent from the seafloor could be processed and applied for mapping shortly after ROV recovery. This reduces the need for XBTs and provides updated salinity data for processing XBTs later in the mapping watch.

Suggestion: add a table overview of all profiling methods with general pros, cons, relative costs (up-front vs. long-term), risks, and examples of ships with installations of each.

XBT
[XBT topics]

Calibration
XBTs are not 'calibrated' in the conventional sense, but systems can be routinely checked against another source to ensure data quality. For instance, conducting a simultaneous (or nearly-so!) cast with both XBT and other profiler (e.g., CTD) can verify that the data from the XBT matches (gives you confidence) the other, more sophisticated sensor with a documented calibration history. Differences of more than 1 m/s between the final SV profiles should be investigated.

XSV and XCTD
[XSV and XCTD topics]

CTD
[CTD topics]

Calibration
CTDs are typically calibrated once per year. Be sure to keep track of the calibration information!

Underway CTD and SV sensors
Underway CTD and sound velocity (SV) sensors can save significant time by eliminating the requirement to stop the ship for profiling. Underway systems can improve data quality by providing salinity and temperature (CTD) or the in situ sound speed directly (SV) as opposed to processing XBT temperature data with assumed salinity profiles.

Underway CTD and SV options include:


 * Teledyne rapidCAST SV https://www.teledynemarine.com/brands/rdi/rapidcast

Tips
= Management =

Sound Speed Manager
Sound Speed Manager is an open-source package used widely to manage sound speed information during acquisition, processing, and reporting.

Tips
This is a very useful tool to collate all profiles from all devices used onboard. Some vessels have only one method of profiling, while others may have several (e.g. XBT and CTD). Being able to bring data into one program helps streamlines the process of editing and transmitting casts to the multibeam. This program also enables you to use synthetic profiles when it is not possible to collect your own.