Computed narrow-band azimuthal time-reversing array retrofocusing in shallow water

The process of acoustic time reversal sends sound waves back to their point of origin in reciprocal acoustic environments even when the acoustic envir onment is unknown. The properties of the time-reversed field commonly depen d on the frequency of the original signal, the characteristics of the acous tic environment, and the configuration of the time-reversing transducer arr ay (TRA). In particular, vertical TRAs are predicted to produce horizontall y confined foci in environments containing random volume refraction. This a rticle validates and extends this prediction to shallow water environments via monochromatic Monte Carlo propagation simulations (based on parabolic e quation computations using RAM). The computational results determine the az imuthal extent of a TRA's retrofocus in shallow-water sound channels either having random bottom roughness or containing random internal-wave-induced sound speed fluctuations. In both cases, randomness in the environment may reduce the predicted azimuthal angular width of the vertical TRA retrofocus to as little as several degrees (compared to 360 degrees for uniform envir onments) for source-array ranges from 5 to 20 km at frequencies from 500 Hz to 2 kHz. For both types of randomness, power law scalings are found to co llapse the calculated azimuthal retrofocus widths for shallow sources over a variety of acoustic frequencies, source-array ranges, water column depths , and random fluctuation amplitudes and correlation scales. Comparisons are made between retrofocusing on shallow and deep sources, and in strongly an d mildly absorbing environments. (C) 2001 Acoustical Society of America.