Detecting schools on multibeam data

Before you begin

Overview

For an overview of multibeam school detection, see:

• About 3D school detection
• Multibeam School Detection module

Match your data to a 3D school detection algorithm type

About scanning and About school detection algorithms discuss multibeam data under a number of platform and multibeam ping mode arrangements.

3D school detection algorithm options include:

	3D school detection algorithm*
Ping mode restrictions/caveats	Cruise scanning	By ping	Instrument scanning
Multibeam	Y	Y	N
Multibeam H-mode	Y	Y	Y
Multibeam V-mode	Y	Y	Y
Multibeam S-mode	Y	Y	N

*The 3D school detection algorithms may apply further restrictions on H- and V-mode tilt and bearing.

Target-locked 3D school detection is only available for target-locked data.

This is how to prepare your instrument scanning data for school detection.

Narrow down school data

There are a variety of multibeam operators available for preparing data. They may help to significantly improve the schools you detect.

• Use noise removal operators to identify and remove noise from your data.
• The Multibeam Background Removal operator may be able to detect and threshold static data samples.
• Surfaces can be used to exclude above and exclude below surfaces under a 3D school detection. This can decrease the detection time. The Analysis page of the Variable Properties dialog box allows you to specify Exclude above and Exclude below surfaces, surface offset and minimum range exclusion. Such surfaces enable the detection to avoid data like ring down noise or data beyond a specified range. Alternatively, the Crop samples operator removes samples (relative to range, depth or sample number).
• Apply a Minimum data threshold (and optionally a Maximum data threshold) on the Data page of the Variable Properties dialog box until the data representing the school(s) are clearly visible and distinct from the background noise on the multibeam echogram.
• Create a virtual echogram using operators, such as the beam closing filter, XxYxZ Statistic operator or processed data operator if the data representing the school(s) contains a lot of holes or can be excluded with a surface. See also Virtual variable applications.
• See Multibeam convolution operators for examples of processing effects, and refer to bitmap and mask operators that support multibeam data.

Optimizing pings

Ideally, multibeam data along a cruisetrack would consist of a series of parallel ping sectors, where ping samples represent the acoustic return for a unique volume of water. However, vessel instability and vessel motion, for example, when the vessel turns, can cause ping sectors to overlap, resulting in data duplication as the samples overlap. To deal with the issue of overlapping samples, Echoview pre-processes ping data and then applies 3D school detection. Even with such handling, the multibeam school intersections on the multibeam echogram may be challenging to interpret. You can define bad data regions on a cruisetrack or use transects to isolate pings that may cause problems during multibeam school detection.

Figure 1: 4D View window, overlapping sectors on a cruisetrack.

Figure 2: 4D View showing a loop in a cruisetrack and challenging intersections between multibeam sectors and school regions.

Caveats

Exclude above or Exclude below surfaces and Bottom elimination for 3D school detection settings are not applied if there is no GPS fix associated with a ping.

By ping detection is not attempted when Region width is less than the corresponding Minimum shortest dimension.

Different detections

Consider creating separate region classes to distinguish schools detected using different multibeam school detection configurations.

Different multibeam detection configurations for different hydroacoustic conditions can be created. The selected configuration and the amount and type of data being processed can affect computation time and the resulting school detections.

You may wish to experiment with different thresholds, operators and school detection settings then compare the results in 3D space. Echoview allows you to assign colors and transparency levels to schools so you can compare multiple schools in the one 3D scene, see About 3D school detection for an example. You apply color and transparency settings on the Display page of the (3D Object) Properties dialog box.

Detect schools

To detect schools on multibeam data:

1. Display the multibeam echogram from which you wish to detect schools.
2. On the Echogram menu, click Detect Schools....
3. On the Detect 3D Schools dialog box, specify the aspects of the detection:
   1. Specify the Pings.
   2. Select a Detection algorithm suited to your data. If using the By ping detection, specify a Region width.
   3. Select a region class from the Assign class list. You can use different region classes to distinguish the results of different detections.
   4. Choose whether to select Delete existing 3D regions with this class first. When selected, existing 3D regions with the assigned class within the specified ping range are deleted before detection.
   5. Consider if you want to use or clear the setting for Save vacuoles.
   6. Specify a multibeam school detection configuration from the Detection configuration list.
4. Click Detect.
   Note: The detection process can take quite a long time. When it is complete, a message is displayed to notify you of how many schools were detected.
5. One or more new 3D regions (possibly with vacuoles) are created (one for each detected school).

Notes:

• To view a 3D region you must add it to a scene. See the Objects page of the Scene Properties dialog box for more information.
• Region-ping intersections can be displayed on multibeam echograms.
• Each 3D region has its own (3D Object) Properties and is listed on the Regions window.
• 3D regions may be integrated.
• You can view details about vacuoles in a 3D region and delete vacuoles from a 3D region on the Vacuoles page of the (3D Object) Properties dialog box.
• Size, shape and position in a scene are calculated using geographic coordinates that have been corrected for transducer geometry.

Scripting support

Refer to About region detection configurations: What changed for scripting with COM and the console?

Detect schools in this ping

To detect schools on the displayed ping:

1. Display the ping of interest on the multibeam echogram.
2. On the Shortcut menu (right-click), click Detect Schools In This Ping.

This command immediately applies the By ping algorithm to the displayed multibeam ping using stored detection settings. These settings include Region width, Delete existing 3D regions with this class first, Assign class, Save vacuoles, and Detection configuration.

To store specific settings for Detect Schools In This Ping:

1. On the Echogram menu, click Detect Schools....
2. Specify a Ping range containing one ping and select By ping as the Detection algorithm.
3. Specify the Region width and other settings to use for this type of detection.
4. Click Detect.

See also

About 3D schools detection
Creating 3D regions
3D school detection algorithms
Preparing instrument scanning data for 3D school detection
About surfaces