Acoustic wave propagation in air-bubble curtains in water; Part II, Field experiment (1982)

Abstract A field experiment consisted of hydrophone recordings in a pond, 25 ft deep, of signals transmitted through air-bubble curtains from a water gun source. The air curtains issued from one to 13 pipes (20 ft long and spaced at 1.67 ft intervals). Air pressures used in the pipes were 15, 25, and 50 psi. Length and complexity of the signals indicate that reverberations occurred to an increasing extent as the number of consecutive air curtains was increased. Analysis of the first pulse in the recorded signals, after approximate removal of hydrophone and recorder response, indicates that the reverberations occur principally in the bubble-free corridors between air curtains. This pulse broadens and its peak amplitude is delayed linearly as the number of successive air curtains is increased. The peak amplitude is decreased substantially by the first air curtain and thereafter remains between 0.1 and 0.2 of the amplitude without air curtains. The time delay increases measurably, whereas the amplitude appears insensitive to an increase in air pressure.Width of the bubble-free corridor, velocity in the air curtains, and reflection coefficient at the air curtain/corridor interface were determined for each of the three air pressures from signal onset times and delay time of the first pulse peak amplitude. The corridor width was approximately three times the air curtain width and did not appear to vary with air pressure. Traveltime in the air curtain, however, increased with air pressure and was from three to four times the traveltime in the corridor. Reflection coefficients ranged from about 0.75 at 15 psi to 0.82 at 50 psi. These data were used to predict, successfully, times of multiple reflections between the outer interfaces of the outermost air curtains.Plane-wave synthetic signals, based on absorptionless models simulating the air curtain configurations and velocities, correspond satisfactorily to recorded signals for the successive-pipe sequence. As for the recorded signals, peak amplitude of the first pulse is decreased substantially by a single air curtain and not appreciably more by additional air curtains. Recorded-signal amplitudes, however, exceed synthetic-signal amplitudes, possibly due to inadequacy of the plane-wave models and to backscattered signals within the pond.The dominant reverberations prevented meaningful measurements of the frequency-dependent absorption in the air curtains. Theoretical absorption values were obtained after synthetically eliminating the bubble-free corridors by expansion of the air curtains. Absorption as a function of air curtain width was determined for each of the three air pressures and for the extremes of possible bubble radii (0.002 to 0.014 ft). Similar to reduction of the first pulse peak amplitude on recorded signals, amplitude of synthetic signals is decreased substantially by the air curtain from a single pipe and at a much lower rate as the air curtain width increases. Frequency-dependent absorption for the smaller bubble radius (0.002 ft) is substantially greater and increases with air curtain width at a greater rate.