Abstract:

To make a scientific analogy between imaging with ocean-acoustic ambient noise and imaging with daylight, one must preserve the ratios of relevant physical scales. For example, the roughly meter-scale objects to be imaged by the ``Acoustic Daylight Ocean Monitoring System'' of Buckingham et al. [Sci. Am. (February 1996)] are millions of wavelengths across optically, but only tens to hundreds of wavelengths across in underwater sound even at up to 100 kHz. People commonly see meter-scale objects in normal diffuse daylight. When imaged with underwater sound, however, meter-scale objects behave as airborne dust particles behave in real daylight. This is because the dust particles are tens to hundreds of wavelengths across optically. Everyday, experience teaches us that dust particles are invisible in normal diffuse daylight and only become visible in special cases where they are cross illuminated by highly directional light beams. This is due to the heightened effect of diffraction and forward scattering for objects with diameters so close to the wavelength. The ``acoustic daylight'' analogy also breaks down because the signal-to-noise or mean-to-standard-deviation ratio, which sets image stability, is millions of times larger for typical optical systems in natural light than that possible in any ocean-acoustic ambient-noise imaging system.