Water Column Mapping

= Seeking Contributors =

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

= Overview =

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.

= Data acquisition =

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

= Simrad split-beam =

Parameters and purposes
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
 * 7) surface / transducer sound speed
 * 8) sound speed profiles

= Kongsberg multibeam =

= Reson multibeam =

= Synchronization =

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.

= Data processing =

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.

EK60/80 processing
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 processing
Multibeam water column data is typically processed with special modules built into larger bathymetry processing packages:


 * 1) CARIS HIPS/SIPS - bathymetry and WC processing
 * 2) QPS FM Midwater - dedicated WC processing
 * 3) QPS Qimera - bathymetry and WC processing

= Data management =

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.

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

Repositories
Who wants the data, and where should it go?

= Target strength calibration =

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.

Calibration resources
The following documents provide guidance on calibration procedures:


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

When to calibrate
Calibration results depend heavily on:


 * 1) local oceanographic parameters, such as
 * 2) temperature impacts on hardware sensitivity
 * 3) salinity/turbidity impacts on acoustic attenuation
 * 4) hardware conditions, such as:
 * 5) transducer element health
 * 6) external impedance impacts, such as paint or biofouling
 * 7) 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
 * 3) for instance, one for Gulf of Mexico and one for the New England seaboard
 * 4) after data collection for verification

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

CW pulse parameters
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!]


 * 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!]


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

FM pulse parameters
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.]


 * 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

Warnings
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?
 * 3) Must make sure upsweep or downsweep