Bubble populations and acoustic interaction with the gassy floor of Eckernfِrde Bay

Anomalies in data taken with acoustic profiling systems often have been interpreted as indications of the widespread occurrence of free gas (gaseous state compounds) in the continental margins of the world’s oceans. Direct demonstration of the correlation between seafloor free gas and such acoustic anomalies has been rare. Interpretations have relied on occasional measurements of gas concentration in recovered seafloor samples, indirect indicators of in situ seafloor free gas and presumed analogous dynamic response of bubbles in sediments to the response of gas bubbles in water. Here, examples are provided of the measurement of free gas bubbles under in situ conditions for samples from the floor of Eckernförde Bay on the Baltic coast of Germany. The occurrence of this population of sediment gas bubbles has been related to the measured acoustic response of the region’s seafloor via model calculations. Indications of volume scattering of the acoustic energy by bubbles in a buried gassy layer are contrasted with evidence of possible gas bubble returns from a thin surficial gassy zone.