Dynamic sediments and sediments with active biogeochemistry often generate a variety of chemical compounds, some of which will be gaseous. This inverted echo-sounder image from 30 October 2011 at the Delta Dynamics Laboratory in the Strait of Georgia at 108m water depth near the mouth of the Fraser River has captured numerous clouds of rising bubbles. Such bubbles have several distinct characteristics.
First, we can see the uniform rise velocity, suggesting nearly constant bubble size. Bubbles have been detected that dissolve with height (common in unsaturated water conditions, as the bubbles shrink they slow and the traces arc to the right) or bubbles that grow as a result of reduced hydrostatic pressure (the traces curve upwards). This image coincides with low tide, another condition that is known to encourage gas release as the water pressure is at a minimum. We have also detected bubbles rising from schools of fish.
One final observation: the ensonified volume of water is an 8 degree cone expanding upwards from the transducer head. The bubbles may appear in the acoustic back-scatter at any height as they enter the beam, and with the strong tidal currents in the Strait of Georgia, they can easily be advected horizontally both into and out of the beam, thus the random appearance of bubble clouds into and out of the ensonified volume.