Error analysis on normal incidence reflectivity measurement and geoacoustic inversion of ocean surficial sediment properties

Abstract

Normal incidence echo data (bottom reflection) can provide acoustic reflectivity estimates for seabed classification and predicting sediment properties with the use of seabed sediment models. However, the accuracy of normal-incident reflection coefficients significantly affects the geoacoustic inversion result. In this study, a multi-core platform with an acoustic recording system, which can perform a near-scope photographical survey of the seafloor, was used to acquire sound data and collect surficial sediment samples in the southwestern sea off Taiwan. The acquired acoustic data was used to analyze the potential errors in acoustic reflectivity estimation, which were induced by erroneous assumptions in acoustic propagation and sea surface reflection. The subsequent data analysis led to the following conclusions. First, theoretical spherical spreading loss compensation is appropriate in seabed reflection coefficient estimates, and relatively small uncertainties in propagation occur. Second, the assumption of perfect reflection of the sea surface is invalid for seabed reflection coefficient estimates. The resulting errors caused by sea-surface effects are described in detail. Moreover, the collected seabed reflection coefficients were used to invert the sediment properties, which were verified using core samples and camera images.