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

Anomalies in data taken with acoustic profiling systems often have been int erpreted as indications of the widespread occurrence of free gas (gaseous s tate compounds) in the continental margins of the world's oceans. Direct de monstration of the correlation between seafloor free gas and such acoustic anomalies has been rare. Interpretations have relied on occasional measurem ents of gas concentration in recovered seafloor samples, indirect indicator s of in situ seafloor free gas and presumed analogous dynamic response of b ubbles in sediments to the response of gas bubbles in water. Here, examples are provided of the measurement of free gas bubbles under in situ conditio ns for samples from the floor of Eckernforde Bay on the Baltic coast of Ger many. The occurrence of this population of sediment gas bubbles has been re lated to the measured acoustic response of the region's seafloor via model calculations. Indications of volume scattering of the acoustic energy by bu bbles in a buried gassy layer are contrasted with evidence of possible gas bubble returns from a thin surficial gassy zone. (C) 1998 Elsevier Science Ltd. All rights reserved.