Documentation of the U.S. Geological Survey Oceanographic Time-Series Measurement Database

Imagenex Sonars

Two types of sonars made by Imagenex (http://www.imagenex.com/) have been used to document changes in the sea floor over time. Each sonar emits a pulse or beam of high frequency sound over the sea floor and the sound reflects off objects and topographic features. The length of time required for reflected sound to return to the sensor is used to map sea-floor bedforms like sand waves and ripples. The 881A imaging sonar (fig. 28A) samples along a line perpendicular to the head to profile the height of the sea floor along the scan. This style sonar may be attached to a motor that rotates through a preprogrammed series of radial steps to obtain multiple linear scans at known angular offsets. These scans may be combined to compute sea-floor height in a circular area beneath the head instead of only along a line. The 881 fan sonar (fig. 28B) samples radially at an oblique angle to the head as it rotates through a preprogrammed arc, allowing a "snapshot" image of the sea floor within 3 to 5 m of the head to be created. A high-frequency version of the 881 imaging sonar may be deployed to obtain higher resolution images within a 10-m radius. All three sonars may be configured to collect a number of images over some duration, which may be averaged to produce a single sonar image. Due to power constraints, the sonars are programmed to only sample every 3–8 hours, depending on the planned deployment duration.

Photographs

Click on figures for larger images.

Figure 28A. An Imagenex 881A imaging sonar in a laboratory test tank.

Figure 28B. An Imagenex 881 fan sonar.