Herman Medwin
Physics Department
Naval Postgraduate School
Monterey, California
408-625-1775
Popular version of paper 3aPA1
presented Wednesday morning, November 29, 1995
Acoustical Society of America, St. Louis, Missouri
In the early 1960s an oceanographer at the Navy Electronics Laboratory wrote a provocative internal memo, "Do Invisible Microbubbles Exist at Sea?". The oceanographer dared to question the usefulness of physics laboratory measurements which had "proven" that bubbles cannot persist in water. The implications of a "yes" answer would be of major interest to oceanographers and meteorologists. As it turned out, the near-surface ocean is an ever-changing rich pudding of microbubble holes. Bubbles are a major factor in the dispersal of impurities and bacteria at sea. The hydrosols that they produce when they burst at the surface are a major impediment to the use of lasers over the sea surface.
It was the direct ocean application of standard laboratory physical acoustics techniques that provided a resounding "yes" to the oceanographer's question. From 1964 to 1974 a series of M.S. students at the Naval Postgraduate School published the first microbubble density distributions at sea. "Unbelievably large" numbers of bubbles of radius 15 to 30 microns were found (one micron is about 40 millionths of an inch). The acoustically-determined bubble densities were finally believed in 1989, when a cumbersome laser holographic device measured one million bubbles per cubic meter of radius between 15 and 16 microns in near-calm seas. The easily used acoustical techniques have found that the number depends on depth below the surface, geographical location, time of day, season of the year, wind speed, and even the presence of sea- slicks on the ocean surface. The bubbles left by the wake of a passing ship can be identified for almost an hour after the event.
Acoustical methods have a mammoth advantage over electromagnetic devices such as photography. The technical reason is that bubbles are themselves acoustic resonant systems. The radius is inversely proportional to the bubble resonance frequency so that a bubble's response to sound is distinctive and, very large, only at that frequency. As a scatterer, a resonating bubble has an acoustical cross section that is hundreds of times greater than its geometrical cross section and billions of times greater than that of a rigid particle. On the other hand a body or bubble of the same radius would have an optical scattering cross section that is, at most, equal to its geometrical cross section.
The density of microbubbles is determined from either the sound backscattered from the bubbly water, or the change of speed of sound, or the change of sound attenuation, compared to laboratory values. One of the in-situ techniques uses a simple devices in which a 0.1 millisecond pulse if radiated from a flat source to a flat reflector two or three feet away. The multiple echoes are used to find the excess attenuation, the speed of travel, and the backscatter as a function of the sound frequency. This information is then inverted to yield the bubble density as a function of radius. Another technique interprets the change of the amplitude and phase of the harmonics of a saw-tooth signal propagating between two separated hydrophones; this yields the speeds and the attenuations, which are interpreted as bubble numbers at radii corresponding to the resonance frequencies. A third technique which is now being employed by several research groups, is based on a one-dimensional acoustic resonator which is simply a one foot diameter source spaced a few inches from a one foot reflector; the weakening of the resonator's standing waves, when there are bubbles in the water, allows one to calculate the bubble densities at the various standing wave frequencies.
The predominant microbubbles which acoustical techniques have identified throughout the upper ocean have radii from about 15 to 200 microns. They have their origins in a galaxy of physical, chemical and biological processes which include breaking waves, rainfall, continental aerosols, photosynthesis, decaying matter and various biological activities.
The 1990s have seen a renaissance in the field of ocean bubble studies. The spatial and temporal variation of bubble numbers have been investigated, and acoustical oceanographers now use bubbles as tracers to determine turbulent processes near the ocean surface.
Recently, sea state noise and rain noise have both been definitively ascribed to the radiation from huge numbers of newly-created "infant" microbubbles. These interpretations follow the passive listening and dissecting of the ocean noise into its components. Indeed, the "noises" have now become "signals." The underwater sound of breaking waves has been inverted to yield the spectrum of ocean wave height. The distinctive underwater sound of rainfall is inverted to reveal the real-time rainfall drop size distribution. Because large raindrops cause large microbubbles, there is a particular spectrum of underwater sound during rainfall which allows one to describe the clouds from which the rain has fallen. Large raindrops at sea have been shown to produce both measurable numbers of hydrosols, which when carried aloft become nuclei of rainclouds, and measurable numbers of bubbles which rise to the surface to produce hydrosols. The private world of the microbubble and hydrosol turns out to be a long-lasting cycle of reincarnation, in which large raindrops produce bubbles, which burst at the surface and create hydrosols, which fall back into the sea and generate more bubbles. Much of this understanding has come from the effective application of classic acoustical techniques to the largely unknown ocean.