Notes on broadband seabed geoacoustic inversion in shallow water

With increasing interest in shallow-water environments, where bottom interaction is a dominant effect, there is a need to acquire accurate information about seabed geoacoustic parameters. Due to the difficulties and high costs of directly measuring seabed parameters at low frequencies (LF) in shallow water (SW), seabed geoacoustic inversion from long-range propagation has become an active research area. Much progress on this subject, notably on inversion methods, has been made in the last two decades, mainly through the use of powerful numerical codes and data processing tools. Despite this, seabed geoacoustic inversion often fails to yield systematic LF seabed parameters in a broad band (such as speed/attenuation vs frequency) that are required for seabed geoacoustic modeling or for sonar performance prediction in the SW environment. This paper will discuss several basic concepts and technical issues related to SW geoacoustic inversion, including filter phase shifting, speed-attenuation coupling, hidden depth, and sea-surface effects. It shows that inverting the broadband seabed acoustic parameters is a delicate task that can often be subject to errors. These issues should be considered in designing experiments that test seabed geoacoustic inversion methods or in interpreting experimental results. [Work supported by ONR and NNSF of China.]