Using relative amplitude and travel times from geometric acoustics to determine nocturnal effective sound speeds.

Abstract

On clear dry nights over flat land, a temperature inversion and stable nocturnal wind structure lead to an acoustic duct in the lowest few hundred meters of the atmosphere. An impulsive signal undergoes strong dispersion during propagation and is received at long ranges from the source as an extended wave train consisting of a series of distinct arrivals followed by a low frequency tail. The leading distinct arrivals have been shown to coincide with the direct and single reflection geometric ray paths. At a range of 2 Km from the source, these ray paths propagate through the lowest 50–70 m of the atmosphere while larger range arrivals reach higher into the atmosphere. Using the solutions of the eikonal and transport equations, travel times, amplitudes, and caustic structures of the arrivals can be determined. Arrival details can then be used to fit a low order Taylor series expansion of the effective sound speed profile. Approximations of the atmospheric conditions in the duct will be presented along with both predicted and measured arrival waveforms and meteorological data from the time of propagation.