Under-Ice Sonar Visualization

The Naval Undersea Warfare Center provides naval and environmental scientists with an integrated 3-D display of under-ice sonar information.
Understanding High Frequency Under-Ice Acoustic Scattering

Understanding the behavior produced by the scattering of sound energy in complex environments, such as under the Arctic Ocean pack ice, is an area of great interest to the US Navy and other navies. Insight into this complex acoustic environment is aided greatly by the simultaneous visualization of the in-water acoustic reverberation and the associated acoustic scattering from the ice keel.

Acoustic reverberation, which also could be called unwanted sound noise, simply is the re-echoing caused by sound bouncing off surfaces in all directions. In the case of the under-ice environment, these surfaces are the ice canopy and ice keels (Figure 4). Because sound energy bounces off objects three-dimensionally, it can be represented as a volume. This volume is referred to as the reverberant volume and can be represented in 3-D by individual volume elements, called voxels. Each voxel is color-coded to match the intensity level of the sound energy reaching it. Similarly, the intensity of the sound energy bouncing directly off the embedded ice blocks, called acoustic scattering, also is color-coded for intensity. The central idea behind the UEV software is to create an animated display that enables the user to interpret better the behavior of ice block scattering, as well as some of the space-time properties of the reverberant volume.

Figure 4. 3-D Sound Behavior

Displaying High-Frequency Under-Ice Acoustic Scattering

Individual nodes within the scenegraph represent the 3-D display of the information of interest, that is, the acoustic scattering from the surfaces of the ice blocks. Each block is formed as a six-faced polygonal surface, with each face colored to represent the target strength of the acoustic scattering from that face. To conserve memory and decrease rendering time, only those surfaces above a predetermined threshold are lit for any given acoustic scatter time interval. Turning the faces of the keel on or off is accomplished through the use of switches attached to each facial node. The reverberation associated with a given acoustic scatter interval is represented by a color-coded volume consisting of thin stacks of voxel volumes representing the reverberation for a given water depth. Again, these component reverberation volumes are addressed individually as nodes within the scenegraph. Figure 5 graphically illustrates the code snippets for the nodal structure of the ice facets and reverberation volume given in Listing 2. The advancement or regression of the acoustic scatter-reverberation display is controlled by the bezel. The display can be set to either continuous update mode or manual step mode. For any given time interval, the user can view any combination of ice block scattering and reverberation information, including blocks ensonified, lit within the entire ice keel; only the ice blocks of interest lit; the entire reverberation volume; or a user-selected depth slice of the reverberation volume.

Figure 5. Graphic Illustration of Display Options Provided by Nodal Structure

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