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On whether azimuthal isotropy and alongshelf translational invariance are present in low-frequency acoustic propagation along the New Jersey shelfbreak
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Image of FIG. 1.
FIG. 1.

Panel (a) SW06 area east of New Jersey, USA. Panel (b): Positions of Mobile Acoustic Source (OMAS) vehicles (thinner blue [OMAS1] and red [OMAS2] lines) and sonobuoy receivers (thicker red and blue lines) for September 8, 2006 transmissions during SW06. Position of the shelfbreak front, as determined by a Scanfish towed conductivity-temperature-depth (CTD) survey, is shown by the black dashed line. Positions of SW06 oceanographic and acoustic moorings are shown by colored circles, triangles, and squares for reference. Panel (c): Reconstruction of the two OMAS tracks overlaid on an objective map of sound speed at the vehicle operating depth (40 m). Note that both vehicles operated in generally horizontally isovelocity (c  1490 m/s) sound speed conditions except when they passed through the shelfbreak front (indicated by a rapid increase in sound speed from west to east) at true bearings from approximately 060° to 180°. Also note that the radius of each circle was about 7.5 km and the distance between circle centers was about 12.5 km.

Image of FIG. 2.
FIG. 2.

Panels (a) and (b): A visualization of the (a) temperature and (b) salinity fields from August 25, 2006. The sections which appear are based on objective mapping of the multiple (along-isobath) transects using the towed Scanfish. At the bottom of the 3D slices are the temperature and salinity at 44 m depth to show the cross-shelf structure more clearly. The shoreward displacement of the front by the warm core ring is clearly seen in the 44 m slice. Panels (c) and (d) show: the (c) temperature and (d) salinity fields from September 9, 2006. Again, the bottom of the 3D slices indicates the fields at a depth of 44 m. In both cases the sampling was done with a cross-shelf orientation and with objectively mapped fields.

Image of FIG. 3.
FIG. 3.

(Color online) Azimuthal TL (dB re 1 m) variability measured from the OMAS operation on September 8th, 2006 during the SW06 experiment. Data at two frequencies (900 Hz in the top row, panels (a)–(d), and 600 Hz in the bottom row, panels (e)–(h) from two different OMAS vehicles (OMAS1 on the left side, panels (a), (b), (e) and (f) and OMAS2 on the right side, panels (c), (d), (g) and (h) are shown. Two different azimuthal average apertures (15° in panels (a), (c), (e) and (g), and 5° in panels (b), (d), (f) and (h) are applied. The raw data are shown as small dots, and the average values are shown as lines with circular markers showing the individual mean TL data points. The shaded portion of the plots indicates the approximate bearings at which the source is considered to be in the shelfbreak front. All source and receiver depths are 40 m and 61 m, respectively.

Image of FIG. 4.
FIG. 4.

(Color online) Histograms of the differences between the individual measured TL data points (small points in Fig. 3) and the azimuthal means (15° and 5°, shown by lines in Fig. 3). The figures are ordered as in Fig. 3. Each figure has the number of TL measurement samples (N) and the corresponding standard deviations (σδμ) given.

Image of FIG. 5.
FIG. 5.

(a): 15° TL (dB re 1 m) averages of OMAS1 (red) and OMAS2 (blue) at 900 Hz (left) and 600 Hz (right). (b): 5° averages at 900 Hz (left) and 600 Hz (right). Again, OMAS1 is shown in red and OMAS2 is shown in blue. (c): Histogram of angular peak widths versus number of occurrences for the combined 600 Hz and 900 Hz OMAS runs at the two sites. Though sampled number of peaks is small, there seem to be more occurrences at small angles (less than 15°) and at medium angles (around 20°–25°). More data, and/or fully 3D computer simulations are needed to better understand the angular widths and their azimuthal positions.

Image of FIG. 6.
FIG. 6.

Modeled 900 Hz TL vs range in the region of OMAS1. The fluctuations are large for 900 Hz CW transmissions (blue), but fluctuations are greatly reduced in the 200 Hz broadband averaged TL (shown in red). The green line shows the broadband averaged TL from 7.1 to 8.6 km, the range variation of OMAS1.

Image of FIG. 7.
FIG. 7.

(a) Full-field OMAS1 to OMNI1 900 Hz TL model output for one of twelve modeled azimuthal source/receiver positions over the 360° of the circle. (b) and (c) OMAS1 to OMNI1 900 Hz TL model results. Third-octave range averaged TL vs. range for twelve source/receiver positions are shown. Note the dashed red line denoting the intended source to receiver range of 7.5 km. Source and receiver were at depths of 40 m and 61 m, respectively.

Image of FIG. 8.
FIG. 8.

(a) Satellite synthetic aperture radar (SAR) image on July 23, 2006 which shows internal wave activity in the region of the OMAS acoustic transmissions. (b) Angular spectrum of nonlinear internal wave directions during the SW06 experiment. (c) Horizontal acoustic beams in the refracted/reflected region, resulting from a 3D sound propagation model.

Image of FIG. 9.
FIG. 9.

(a) Fully 3D adiabatic propagation over real finescale bathymetry, showing beaming of energy in horizontal. (b) N × 2D calculation with same bathymetry as panel “a”, but not showing beaming, thus indicating effect is fully 3D. (c) Smoothed bathymetry with full 3D calculation, showing that lowpass filtering of bathymetry can eradicate this finescale horizontal beaming effect. (d) and (e) Fully 3D calculations of horizontal beaming at 600 Hz due to bathymetry, showing that this effect increases with increasing mode number.

Image of FIG. 10.
FIG. 10.

(a) Tracks of alongshelf August 28, 2006 OMAS transmissions across an intrusion. OMAS tracks are shown in red, with the receiver shown in green. Both are overlaid on a color plot of water temperature at 30 m depth. (b) Transmission loss curves for the OMAS transmissions across an intrusion, showing a sharp drop off at the range of the intrusion.

Image of FIG. 11.
FIG. 11.

(a) Equidistant paths, one crossing and the other not crossing an internal wave soliton. (b) Various paths crossing an internal wave soliton.

Image of FIG. 12.
FIG. 12.

(Color online) (a) OMAS deployment in the deep (1350 m) waters of the “Tongue of the Ocean” in the Bahamas on April 21, 2008. (b) OMAS Source Track, circle radius 2.5 km. (c) Azimuthal TL variability, computed from the peak of the matched-filter output, measured by the OMAS operations on April 21, 2008. Individual TL data points are shown as dots. A 5° bearing sector average is shown as a dotted line, with the points showing the mean value of all data within each sector plotted at the sector center point.


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‘Out-of-the-front’ (ϕ from 180° to 60°, clockwise) TL data mean (μ) and standard deviation (σ) for individual data points, 5° and 15° sector averages.

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σ is calculated directly from the data, and from the square root of the sum of the variances of the sector averages and the differences of the measured data about the average.

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Translational invariance: mean and standard deviation of differences Between OMAS1 and OMAS2 (OMAS1–OMAS2) 5° and 15° mean TL vs bearing for 900 Hz and 600 Hz, for out-of-the-front data.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: On whether azimuthal isotropy and alongshelf translational invariance are present in low-frequency acoustic propagation along the New Jersey shelfbreak