The vertical structure of shadow-zone arrivals at long range in the ocean

Abstract

Multimegameter-range acoustic data obtained by bottom-mounted receivers show significant acoustic energy penetrating several hundred meters into geometric shadow zones below cusps (caustics) of timefronts computed using climatological databases [B. D. Dushaw et al., IEEE J. Ocean. Eng. 24, 202-214 (1999)]. This penetration is much larger than predicted by diffraction theory. Because these receivers are horizontal arrays, they do not provide information on the vertical structure of the shadow-zone arrivals. Acoustic data from two vertical line array receivers deployed in close proximity in the North Pacific Ocean, together virtually spanning the water column, show the vertical structure of the shadow-zone arrivals for transmissions from broadband 250-Hz sources moored at the sound-channel axis (750 m) and slightly above the surface conjugate depth (3000 m) at ranges of 500 and 1000 km. Comparisons to parabolic equation simulations for sound-speed fields that do not include significant internal-wave variability show that early branches of the measured timefronts consistently penetrate as much as 500-800 m deeper into the water column than predicted. Subsequent parabolic equation simulations incorporating sound-speed fluctuations consistent with the Garrett-Munk internal-wave spectrum at full strength accurately predict the observed energy level to within 3-4-dB rms over the depth range of the shadow-zone arrivals.

Deep seafloor arrivals: An unexplained set of arrivals in long-range ocean acoustic propagation

“…The first strategy was compressional wave modeling without bottom interaction ͑keep the bottom properties the same as the water above it but add strong attenuation so that no energy is returned from the seafloor or sub-seafloor͒. This strategy, without including bottom interaction, has successfully predicted long-range, ocean acoustic propagation in the past Dushaw et al, 1999;Heaney et al, 1991;Van Uffelen et al, 2009;Wage et al, 2005;Xu, 2007͒. This seemed like a good initial strategy since bathymetry along the whole 3200 km long geodetic is everywhere deeper than 4400 m and for most of the propagation path is deeper than 5000 m ͑Fig.…”

“…Two goals of LOAPEX were to understand the role of bottom interaction in long-range, lowfrequency acoustic propagation, and to understand the physical mechanisms responsible for the so-called "deep shadow zone arrivals" observed by Dushaw et al ͑1999͒. Deep shadow zone arrivals occur when acoustic energy is scattered vertically many wavelengths below a turning point ͑Van Uffelen et al, , 2009 and examples will be shown below. The 2004 LOAPEX experiment was well suited to address these issues in three respects.…”

Analysis of Deep Seafloor Arrivals observed on NPAL04

“…b) Most of the major events on DVLA-4250 are predicted by the PE. The later arrivals (in blue) at T2300 and T3200, and weakly at T1600, correspond primarily to deep shadow zone (DZA) arrivals, the evanescent tails beneath ray turning points (Van Uffelen et al, 2009), as demonstrated in Figures 4 and 5 of Stephen et al (2009). c) On the south OBS there is an early, small amplitude, arrival at T1600 and T2300 that is predicted by the PE solution (line C, see Figure 5 of Stephen et al (2009)).…”

“…Deep shadow zone arrivals (DSZA, (Dushaw et al, 1999;Van Uffelen et al, 2008a;Van Uffelen et al, 2009;Van Uffelen et al, 2006)) can be ruled-out as an explanation for deep seafloor arrivals by the analysis shown in Figure 1 for T1600 ( Figure 4 of the 2009 paper). Although some energy arrives at the OBSs at times corresponding to caustics of the PE predicted time fronts, there are large amplitude, late arrivals that occur between caustics and even after the PE predicted coda.…”

“…Prior to September 2012, the physical mechanism responsible for DSFAs was unexplained. There were five candidate explanations: a) Deep shadow zone arrivals (DSZA) which occur when acoustic energy is scattered vertically many wavelengths below a PE-predicted turning point (Dushaw et al, 1999;Van Uffelen et al, 2008a;Van Uffelen et al, 2009;Van Uffelen et al, 2006), b) Bottom-reflected surface-reflected paths along the source-receiver geodesic (In-plane BRSR), c) Bottom-diffracted surface-reflected paths along the source-receiver geodesic (In-plane BDSR), d) PE-predicted propagation to a near-by out-ofplane seamount with conversion to a seafloor interface wave (Out-of-plane IW), e) PE-predicted propagation to a near-by out-of-plane seamount with conversion to a BDSR path (Out-of-plane BDSR). Before addressing each of these in turn let's review the fundamental observations and present the procedure for identifying the out-of-plane conversion point on Seamount B (triangulation).…”