This section is under development. If you have expertise in water column mapping, please reach out to the admins at [email protected] to become a contributor today!

Water column (or midwater) mapping involves recording backscatter information during the propagation time between transmission and seafloor detection.

This recording capacity enables acoustic imaging of discrete targets (such as fish and plankton) and oceanographic phenomena (such as thermohaline stratification, turbulent mixing, mixed layer depth, and the oxic-anoxic interface).

Water column logging is possible with practically any sonar, including single-beam, split-beam, and multibeam echounders, as well as subbottom profilers and seismic systems. This is a continually developing field, with manufacturers providing different levels of support for water column logging, processing, and visualization. Acoustic target strength calibration methods are available for some but not all of these systems.

Recent advances in hardware have opened up a wide array of transmission capabilities (and related logging options), vastly improving range resolution and frequency-response discrimination among targets.

This section is intended to help users consider their options for data acquisition, calibration, and processing to push these new boundaries of water column mapping while supporting meaningful comparison with historic datasets.

Water column backscatter data can be acquired with a variety of systems, each with specific advantages and drawbacks.

Discussion and examples of typical acquisition settings and their purposes.

Advantages and tradeoffs of acquisition settings:

1. Continuous waveform (CW) vs frequency-modulated (FM)
2. low, standard, and high resolution
3. logging complex samples / raw stave data
4. recording range and ping rate
5. other factors?
6. application of sound speed
   1. surface / transducer sound speed
   2. sound speed profiles

The EM series multibeam systems by Kongsberg discovery record data in specific datagrams encoded as binary files in the old *.all format, or the new *.kmall format. Water-column data are recorded in dedicated datagrams that may be saved with the other datagrams, but are most commonly recorded as a separate *.wcd (or *.kmwcd) file. Thus, processing Kongsberg multibeam water-column data usually relies on processing .all/.wcd pairs or .kmall/.kmwcd pairs.

Water column sonars provide a wide range of transmission options and log received signals nearly continuously. These realities can present several challenges for operating in tandem with each other and other acoustic systems, namely:

1. Interference of water column TX on other mapping systems
2. TX from other acoustic systems on water column data

There are various strategies for synchronizing sonars to manage these interference issues, and each must be considered carefully to maintain data quality and achieve ping rates suitable for the mission.

Water column data processing options vary widely, reflecting the many uses of the data and the ongoing development of new visualization tools.

While many fisheries/acoustics groups use their own MATLAB regimes to process EK data, commercial software developers continue to integrate water column visualization and target detection into 'standard' ocean mapping workflows.

Several packages are available for processing EK .raw files:

1. ESP3
2. EchoLab
3. Echoview

[Seeking WC expert input on additional options, strengths / tradeoffs of each package, etc.]

Multibeam water column data is typically processed with special modules built into larger bathymetry processing packages:

1. BeamworX - water column support added as of v2023.2
2. CARIS HIPS/SIPS - bathymetry and water column data processing
3. QPS FM Midwater - standalone water column data processing
4. QPS Qimera - bathymetry and water column data processing

There are also some open-source packages or software for multibeam water column data processing:

1. CoFFee - MATLAB toolbox including functions for water column data visualization and processing
2. Espresso - standalone Windows application with graphical user interface, based on CoFFee.
3. themachinethatgoesping - Python package. See tutorials.

Water column data rates can far exceed other typical data streams during a cruise. This section provides some examples of how plan and manage the data flow and storage.

Impacts of acquisition settings on data rates, with recommendations for weighing the tradeoffs. Options to downsample / reduce data rates?

Who wants the data, and where should it go?

Target strength calibration can significantly increase the utility of backscatter data from any echosounding system. This process corrects for beam pattern effects and hardware-level biases that may stray from the modeled beam pattern.

For midwater mapping systems, calibrated target strength information improves the reliability of species identification, bubble size estimation and cross-calibration with other systems, such as for seafloor backscatter.

A variety of methods are in development for multibeam backscatter calibration, where some of the main challenges are related to positioning of calibration targets within narrow beamwidths. [List/link various methods: tank, extended target, cross-calibration, NEWBEX, etc.]

Single-beam and split-beam scientific (fishery) echosounders have traditionally been calibrated for target strength using a reference sphere with known acoustic properties. The sphere suspended on a series of lines and moved throughout the echosounder field of view; the measured and modeled TS values are then compared to quantify in situ biases across the beam pattern.

For example, the CRIMAC SFI survey reports from 2023 and 2024 provide in-depth discussions of calibration procedures, noise assessments, and the corresponding improvements in data quality with a full suite of EK80 systems.

The following documents provide guidance on calibration procedures:

1. Simrad EK calibration procedure
2. Others? [Oden / Okeanos / OceanXplorer documents?]

Calibration results depend heavily on:

1. local oceanographic parameters, such as
   1. temperature impacts on hardware sensitivity
   2. salinity/turbidity impacts on acoustic attenuation
2. hardware conditions, such as:
   1. transducer element health
   2. external impedance impacts, such as paint or biofouling
   3. other attenuation factors, such as ice protection windows

When TS information is critical, calibration should be completed:

1. at least annually for a system in the same general water mass
2. at least once prior to data collection in each distinct water mass
   1. for instance, one for Gulf of Mexico and one for the New England seaboard
3. after data collection for verification

The following tables outline typical Simrad EK60/80 TX calibration parameters and their purposes

The following pulse parameters and sphere properties have historically been used for fisheries research.

Regardless of other modes and intended uses on a given mission, these standard CW, 1-ms, max-power parameter are broadly considered the 'base' parameters across all frequencies that facilitate comparison to historic datasets. These modes should be prioritized for TS calibration in most cases.

[Note: seeking input from WC experts on the most common uses for each frequency in CW. Add other CW parameters that are important!]

Frequency	Pulse length	Pulse form	Power	Purposes	Sphere
12 kHz1	1 ms	CW	Max	Fisheries / seeps / deepwater	?
18 kHz	1 ms	CW	Max	Fisheries / seeps	63-64 mm Cu
38 kHz2	1 ms	CW	Max	Fisheries standard / seeps
70 kHz	1 ms	CW	Max	Fisheries / seeps
120 kHz	1 ms	CW	Max	Fisheries / upper WC structure
200 kHz	1 ms	CW	Max	Fisheries / upper WC structure
333 kHz	1 ms	CW	Max	Fisheries / upper WC structure

1. Not widely installed
2. 38 kHz transducers are widely installed; this is a standard for fisheries research

In addition to the standard fisheries research parameters above, non-standard CW pulses that have been used for specific purposes are outlined below. [Seeking input from WC experts!]

Frequency	Pulse length	Pulse form	Power	Purposes	Sphere
18 kHz	8 ms	CW	Max	Deep seeps / icebreakers1	63-64 mm Cu

1. A tradeoff of reduced resolution to help overcome attenuation from long range and ice windows

New wideband transceiver hardware supports a massive range of CW and FM pulse parameters. The following table outlines the FM parameters used most commonly, with the understanding that many programs and platforms may have their own set of FM pulses for specific purposes. This table provides an opportunity to update and discuss these many options.

[Note: examples below from WC research aboard SVEA; seeking input from WC experts on additional FM params of interest.]

Frequency	Pulse length	Pulse form1	Power	Ramping	Purposes	Sphere
32-45 kHz	[pending]	LFM Up	Max	Fast	WC structure	[pending]
45-90 kHz	[pending]	LFM Up	Max	Fast	WC structure	[pending]
95-140 kHz	1 ms	LFM Up	Max	Fast	WC structure	[pending]
160-250 kHz	1 ms	LFM Up	Max	Fast	WC structure	25 mm WC2
275-450 kHz	1 ms	LFM Up	Max	Fast	WC structure	25 mm WC2

1. LFM = linear frequency modulation; Up or Down describes direction of frequency change; Up or Down calibrations are separate
2. WC = tungsten carbide

25 mm Tungsten Carbide used but not recommended for all systems

Some systems are not authorized or recommended for FM use (e.g., ES18-11 transducer) as these frequency ranges may cause damage. In some cases, FM use is recommended only after reducing power from 2000 W to 1000 W. Seek guidance from Kongsberg / Simrad for FM limitations with specific hardware.

Questions:

1. What FM params are becoming standard for fisheries?
2. How does sweep direction impact these choices?
   1. Must make sure upsweep or downsweep