The rate of energy transport and direct measurements of group velocity

Abstract

The Intensity Vector Autonomous Recorder (IVAR) is a bottom deployed system measuring both particle velocity and pressure (combined sensor). Results using IVAR in the Sediment Characterization Experiment (SCE17) conducted off New England (spring 2017), are presented. Data originate both from ship noise involving the closest point of approach (CPA) of a large transport vessel, and experimental SCE17 signals relating to Signal Underwater Sound (SUS) MK-64 explosive sources. Several vector and scaler metrics emerge based on different combinations of second-order acoustic fields; one is the rate of energy transport (U) to be emphasized here. For the CPA data, estimates of U versus frequency represent the speed of the net transport of acoustic energy [D’Spain et al., J. Acoust. Soc. Am. 89, 1991], and additional interpretations are presented. For the SUS data, arrivals from individual modes are resolved in time-frequency analysis. Vertical intensity is shown to be much less than horizontal, and estimates of U are identified with modal group velocity with some caveats to be discussed. For example, the group velocity associated with the Airy-phase for mode-2 (frequency ~29 Hz) is U ~ 1370 m/s. This estimate emerges directly from the analysis without recourse to range divided by travel time.