A 2-FREQUENCY HYDROACOUSTIC SCATTEROMETER FOR BUBBLE SCATTERING INVESTIGATIONS

High-frequency bubble layer scattering investigations require the meas urement of the intensity of backscattered sound and the corresponding depth of the scatterers below the moving surface. Especially at high s ea state conditions and high acoustic frequencies, bubbles acousticall y mask the surface, i.e., the surface return cannot be detected. Howev er, this environmental condition is the most interesting one in bubble scattering investigations and a reliable method is required to determ ine the range of the scatterers to the surface displacement. A method for the determination of the vertical profiling of acoustic scattering in the presence of bubbles at high sea state conditions is presented. It is based on the transmission of a low-frequency signal alternately to the high-frequency signal at which the scattering investigations a re performed. The only information that is extracted from the low-freq uency echo is the onset of the surface return. It is used to compute t he true depth of scatterers at the high frequency. Experiments were co nducted to determine the optimum low frequency at which the detection of the surface onset in the presence of a high bubble concentration is ensured. A screening ratio is defined to give a measure of the acoust ic masking of the sea surface. It is depicted for an extreme wind cond ition (20 m/s) for the frequency range of 5-25 kHz and as a function o f wind speed for 50 kHz measurements. Selected results of subsurface b ubble scattering at 50 kHz from experiments under open sea conditions are presented for the wind speed regime from 9 to 22 m/s. Additionally , the two-frequency scatterometer is used to measure sea state charact eristics simultaneously to the scattering investigations by remote sen sing techniques.