Geoacoustic Inversion of Airgun Data Under Influence of Internal Waves

In this paper, broadband airgun data from the 1995 Shallow Water Acoustics in Random Media (SWARM'95) experiment are analyzed to study the influence of internal waves on sound propagation and modal behavior, and the subsequent effects on geoacoustic inversion. Modal behavior and fluctuation due to sound intensity focusing and defocusing, when the leading front of internal waves is parallel to and approaches or passes through the acoustic propagation track, are presented. A modal filtering technique, the time-warping transform, is applied to the airgun data to resolve dispersed modes. The extracted modes are used to estimate seabed model parameters by geoacoustic inversion. Two scenarios, defocusing and focusing periods containing 14 events, are selected for study. Seven data sets for each type (defocusing or focusing) are analyzed to extract dispersion curves and carry out geoacoustic inversion. The inversion results for each type of the events are similar with a small standard deviation from the mean. However, the mean values and standard deviations for the defocusing and focusing types are quite different. The impacts of internal waves on inversion results are presented and discussed.