Systems and methods are described for non-invasively acoustically monitoring containers to distinguish gas contents from liquid contents. In some embodiments the container is part of a buoyancy tank (132) in a subsea pipeline network (102) used to transport production fluid from a subsurface wellhead (112) to surface facilities, and the systems and methods are used to detect water ingression into the buoyancy tank (132) and to transmit alert signals to the surface relating thereto.