Validity of the frozen-surface approximation for large time-bandwidth signals

Abstract

The physical modeling of underwater acoustic propagation, scattering, and reflection, and the signal processing associated with these processes, usually rely on the frozen-surface approximation. This longstanding approximation, which dates back to Carl Eckart’s seminal paper (The Scattering of Sound from the Sea Surface, 1953) assumes that the ocean surface can be modeled as being frozen in time throughout the entire pulse duration. The approximation is valid for short-duration pulses typically used by sonars in the decades following that paper; however, its applicability to present-day high duty cycle sonars is questionable. Although the assumption is based on the physics of the problem, it can have a profound effect on the signal processing, especially for large time-bandwidth signals. In this paper some experimental results will be presented to provide examples of how the approximation fails for a long duration, wideband pulse. Using this as motivation, the authors will explore some areas where incorrectly employing the approximation can introduce errors in the expected signal processing gain. While no attempt will be made to correct shortcomings in the approximation, it is hoped that the discussion may motivate renewed interest in this issue. Funding from ONR and ONR Global is gratefully acknowledged.