Miscellaneous

Can DIDSON shadows be used to derive target shape and depth?

Ensonified objects cast shadows on the background in the same manner as objects that are illuminated by a light source. Depending on the angle of the incident sound, the position of the target and the angle and position of the background, the shadow can reveal features of the object’s shape in planes that are not resolved in the primary acoustic image (i.e. the image that is created by echoes reflected by the object itself). This can potentially be useful for fish identification as many distinguishing features of fish (fins, head and body profile) lie in the vertical plane and thus generate echoes that, in a side-looking configuration, are poorly resolved in range. We have conducted a series of experiments to test the extent to which target shape and depth can be derived from shadow images collected with DIDSON. The technique requires an even background plane of sufficient reflectivity, tilted at a known angle and marked with a defined ruler, and a known transducer position. The shadows are well-defined when the objects are close to the backplane but blur quickly when the distance between the object and the background increases.

The image below shows the shadow of a 24-inch fish model projected on a 45-degree plane (relative to horizontal). Unlike the primary echoes (bright line) at 1.6 m range, the shadow provides a good view of the profile shape.

Shadow of 24-inch fish model.

24-inch fish model: silhouette cut from plywood.

When the relative geometry of the transducer and the background plane is known, one can correct the distortion of the shadow that is introduced by its projection.

Shadow correction for 7-inch fish model.

Shown here is the shadow of a 7-inch fish model projected on a 24-degree plane (top), and the corrected image of the shadow (center). The shape of the corrected image is a good reproduction of the shape of the original silhouette model, which is shown at the bottom. Note, because the background plane is tilted at a shallower angle, the uncorrected shadow of the small fish model is more distorted than the shadow of the large fish model shown above.

3D-model of the bathymetry and submerged trees in Thurmond Reservoir

In an effort to offset the loss of baitfish that are killed when the reversible turbines of Richard B. Russell Dam (Savannah River, Georgia) are operated, the Army Corps of Engineers has agreed to install an aeration system in Lake Thurmond. Currently, as oxygen is depleted during hot weather, striped bass and their prey (herring and shad) are forced to stay close to the tailrace of the dam. The aeration system will release oxygen from submerged pipes and thus increase the water volume that can be used by the fish. In many areas of the reservoir, trees remaining from the time before the area was flooded are still standing on the bottom. These ghost trees make it difficult to install pipes for the aeration system. Data from mobile acoustic surveys were used to delineate areas that are clear of obstacles that would interfere with the placement of the pipes. The image on the left shows a 3D-model of the bathymetry and tree coverage (in green). The image on the right shows an echogram from an area with trees.

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DIDSON footage of alewives avoiding a predator

This clip comes from a demonstration of the DIDSON system at a hydroelectric station on the Kennebec River, Maine. The client is interested in testing the effectiveness of a by-pass that has been built to allow fish to pass the dam without going through the turbines. In the course of the demonstration we observed several schools of young-of-the-year alewives migrating downstream. On some occasions, one could see the school react to the attacks of a predatory fish that was holding in the vicinity of the guide curtain (intended to guide the fish towards the by-pass), which is visible in the upper right corner. Note the predator as it separates from the school and becomes clearly visible in the center of the upper edge of the image.

DIDSON footage of eel backing away from trashrack

This footage comes from the same demonstration as the clip above. The DIDSON system is aimed along the upstream edge of the trashrack, which is visible as a diagonal stationary line in the lower left corner of the image. An eel can be seen approaching from upstream (right side of image) and quickly turning back as soon as it encounters the face of the trashrack.

Split-beam angle echograms

Angle echograms are visual representations of the angle data derived from split-beam transducers. As in the more conventional TS and Sv echograms, data are displayed in the 2 dimensions of range and time. Each data point on an angle echogram represents a position within the acoustic beam, measured by the angle in the direction of the minor or major axis of the transducer (or a combination of the two angles). Angle echograms can be a powerful tool for fish track editing and determining fish behavior, especially when data are collected with stationary transducers (e.g. in-river applications that estimate fish passage). Angle echograms allow the user to:

• interpret the relative position and movement of targets quickly and intuitively; an entire page of the echogram can be assessed at one glance and without the need to draw selection boxes;
• distinguish the echotraces of multiple fish that swim head-to-tail from interrupted echotraces left by a single fish; synchronized with a single target echogram, the angle echogram is a valuable aid in editing fish tracks;
• delineate echogram areas that show distinct fish behavior (e.g. upstream, downstream movement, up/down in water column, milling): regions can be drawn directly on the angle echogram and be used to exclude data or apportion biomass (or fish passage) estimates;

The image below shows synchronized horizontal and vertical angle echograms of milling schools of pink salmon in the Ksi X'anmas River, collected with a stationary split-beam transducer aimed along the bottom and across the river. In the horizontal angle echogram (top half of image) red is downstream, blue is upstream of the transducer. In the vertical angle echogram (bottom half of the image) red is towards the river bottom, blue is towards the river surface. Note the two big schools of fish, one swimming upstream (individual fish change red to blue on the horizontal echogram), the other swimming downstream (individual fish change blue to red on the horizontal echogram). At the beginning of the echogram, the school of fish traveling upstream is spread through the middle and upper portion of the water column (green and blue in vertical angle echogram). As the two schools pass in front of the transducer, the upstream swimming fish rise towards to the surface, while the downstream swimming fish descend in the water column.

Click on image to enlarge