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This section is under development. If you have expertise in sound speed profiling and processing, please reach out to the admins at [email protected] to become a contributor today!

Overview

The oceanographic environment impacts multibeam data quality through several fundamental processes related to sound speed:

1. Beam forming and spacing at the transducer
2. Beam steering and stabilization at the transducer
3. Refraction correction along ray path of each beam
4. Range calculation along the ray path of each beam

Many of the properties that control sound speed also factor into acoustic attenuation and, consequently, backscatter / reflectivity measurements from the seafloor and water column.

This environment is always changing and there are no replacements for in situ measurements. Care must be taken to sufficiently capture its variability at the transducer and in the water column in order to correctly position and characterize acoustic returns across the entire swath.

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 transient conditions in the upper surface layers, where the impacts on beamforming, beamsteering, and refraction correction are most significant. 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chest (especially with any outflows nearby) may delay and/or alter the flow-through measurement to the point that it is no longer applicable for multibeam operations.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

Probes should be cleaned routinely and before deployment for mapping missions. If the probe is not measuring sound speed accurately, it can have a cascading effect. For instance, SIS will reject the probe data if the values are out of range (1400-1700 m/s) and the RESON SVP 70 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. It is strongly recommended to have backup probes in place or available to swap out (without hauling out). The importance of transducer ('surface') sound speed cannot be overstated for multibeam mapping, and incorrect values can easily invalidate an entire survey without recourse in post-processing.

[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 ("raytracing") and correctly georeference each bottom detection. It is often assumed that the profile represents a uniform local structure; however, oceanographic conditions are always changing and it is common to observe variability across the swath (e.g., internal waves between oceangraphic layers of different density).

This variability cannot be captured by single-point profiles, and can manifest in a variety of time-varying swath artifacts (e.g., 'ripples' or 'waves' in the swath) as the vessel moves through a non-uniform sound speed environment.

Profiles are typically collected at single-point locations with XBTs and CTDs, with more frequenty profiling through underway systems such as UCTDs and MVPs. Continuous, broad-scale sound speed environment monitoring is an area of active research, such as using autonomous underwater vehicles to conduct frequent profiles in the vicinity of a multibeam survey, and assessing real-time water column data from multibeam and fishery echosounders to identify and track mixed layer depths.

XBT

[XBT topics]

Tips

Recommend conducting a simultaneous (or nearly-so!) cast with both XBT and other profiler (e.g. CTD) to compare data--to verify that the data from the XBT matches (gives you confidence) the other, more sophisticated sensor.

CTD

[CTD topics]

Tips

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

UCTD

[UCTD topics]

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.