Low-frequency Arctic reverberation. III: Measurement of ice and bottom backscattering strengths from medium-range bottom-bounce returns

Abstract

Backscattering strength measurements are usually extracted from short-range reverberation data using direct-path returns or from long-range reverberation data for which the dominant source of reverberation (e.g., a bottom seamount) is known. The small area ensonified by short-range direct paths provides only a relatively small data sample for backscattering strength estimates. While long-range data cover a much larger area, the returns from the surface and bottom are not always easily identified in the data. In the Arctic both the surface (ice) and bottom contribute significantly to the reverberation at low frequencies. The contributions of ice and bottom must be separated by signal processing means before their backscattering strengths can be measured separately. In this paper a method to separately measure the surface and bottom backscattering strengths using medium-range (5- to 20-km) reverberation data is investigated. The method of separation is based on the time versus vertical arrival angle contours associated with bottom-bounce propagation paths. Separation of surface and the bottom backscattering returns is most straightforward for a vertical array. It is noted that a (low tilt) vertical array has been shown to be a practical tool for collecting reverberation data in the Arctic. By extending analysis to medium-range reverberation data, backscattering data samples can be significantly increased. The ice backscattering strengths are estimated at 23 Hz from the CEAREX 89 reverberation data and the results are compared with the Burke–Twersky model, in which surface roughness is modeled by elliptic half-cylindrical protuberances on a pressure-release surface. Bottom backscattering strengths are also obtained at 23 Hz, and are found to be consistent with Lambert’s law within the experimental errors.