A short-range, shallow-water bottom loss experiment was conducted in the South Lincoln Sea, under an irregular rough ice canopy. The experiment was carried out using broadband signals generated by imploding light bulbs at depths of 15, 30 and 50m and distances ranging from 15 to 150 m from a vertical hydrophone array. Previous experiments in the vicinity of this site have provided limited seabed information. Preliminary modeling of the acoustic propagation at this location using a ray-based model is used to determine the bottom loss as a function of grazing angle for frequencies between 10 and 1000 Hz. This report will review the measurements, and discuss the challenges associated with the modeling, in view of the experimental uncertainties associated with this complicated environment.
Résumé
Une expérience portant sur les pertes au fond à courte portée en eau peu profonde a été menée dans le sud de la mer de Lincoln, sous une couverture irrégulière de glace rugueuse. Dans cette expérience, on a utilisé des signaux à large bande émis par l’implosion d’ampoules placées à des profondeurs de 15, 30 et 50 m, à des distances allant de 15 à 150 m par rapport à un réseau d’hydrophones vertical. Des expériences antérieures réalisées à proximité du site ont permis de recueillir des renseignements limités relatifs au fond marin. On a recours à une modélisation préliminaire de la propagation acoustique basée sur les rayons à cet endroit pour déterminer les pertes au fond en fonction de l’angle d’incidence à des fréquences comprises entre 10 et 1 000 Hz. Le présent rapport traite des mesures et des défis associés à la modélisation, du point de vue des incertitudes expérimentales associées à cet environnement complexe.
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Executive summary
Introduction
A trial was conducted on shore-fast ice west of CFS Alert, on the north coast of Ellesmere Island, in March-May 2002. Using the ice as a platform, a bottom loss experiment was carried out using broadband signals generated by imploding light bulbs.
Results
Gathering geo-acoustic data in an Arctic setting is difficult at the best of times, and this is reflected in the small amount of data available in the literature. This paper reviews some of the geo-acoustic information available for the area. The results from the bottom loss experiment are presented and put in context of the wider body of information available.
The shape of the bottom loss curve and the frequency dependence of the bottom loss indicate a thin layer of low density material overlays a denser sediment layer. This result agrees with core samples and seismic measurements from the region.
Uncertainties associated with isolating and identifying distinct rays makes the analysis of the data difficult. Unknown ice thickness and limited bathymetric information for the area introduced uncertainties in ray path, as well as the unknown tilt on both the vertical line array and the bulb breaking apparatus. These ray path uncertainties translate into errors in bottom loss estimates, though these errors were limited by careful analysis of travel times to eliminate out-of-bound data.
Military significance
Over the past few years, the Arctic environment has been slowly changing; an increasing amount of open water has been reported during the summer months. If the present trend continues, it is possible that an ice-free channel could develop, opening the Arctic Archipelago to shipping traffic for at least part of the year.
For reasons of Canadian Sovereignty, it is in the interest of the Canadian military to collect tactical sonar information in Arctic waters. A good knowledge of the geo-acoustic properties of the seabed can support the effective use of either passive or active sonar to detect surface and/or underwater targets navigating through the area. This paper collects a significant amount of information about the Arctic environment near CFS Alert.
DRDC Atlantic CR 2003-240 iii
Future plans
There is no immediate plans for DRDC Atlantic to revisit the Arctic regions, however, the results of this analysis will be valuable for future operations near CFS Alert.
Au cours des mois de mars à mai 2002, on a effectué des expériences sur la glace de rive à l’ouest de la SFC Alert, sur la côte nord de l’île d’Ellesmere. En utilisant la glace comme plate-forme, on a mené une expérience sur les pertes au fond en se servant des signaux à large bande émis par l’implosion d’ampoules.
Résultats
La collecte de données géo-acoustiques en milieu arctique présente des difficultés même dans les meilleures conditions, ce qui explique le peu de données disponibles dans la documentation. Le présent document porte sur l’information géo-acoustique disponible pour la région. Les résultats obtenus à la suite d’expériences sur les pertes au fond sont présentés et replacés dans le contexte de l’ensemble plus étendu d’information disponible.
La forme de la courbe des pertes au fond et la dépendance fréquentielle des pertes au fond indiquent qu’une mince couche de matériau de faible densité recouvre une couche sédimentaire plus dense. Ce résultat est en accord avec les observations effectuées sur des carottes prélevées dans la région et les mesures sismiques prises aussi dans la région.
L’analyse des données est difficile en raison des incertitudes associées à l’isolement et à l’identification de rayons distincts. Comme on ne connaissait pas l’épaisseur de la glace et qu’on ne disposait que de données bathymétriques limitées pour la région, et qu’en plus on ne connaissait pas l’inclinaison du réseau vertical ni celle de l’appareil utilisé pour faire éclater les ampoules, des incertitudes existent quant à la trajectoire des rayons. Ces incertitudes sur la trajectoire des rayons, même si elles sont limitées grâce à une analyse minutieuse des temps de parcours visant à éliminer les données hors limite, entraînent des erreurs sur les estimations des pertes au fond.
Intérêt militaire
Au cours des dernières années, le milieu arctique s’est modifié lentement : on a noté une quantité croissante d’eau libre pendant l’été. Si la tendance actuelle se poursuit, il est possible qu’un chenal libre de glaces puisse se former, rendant l’archipel Arctique accessible à la navigation pour au moins une partie de l’année.
Pour des raisons de souveraineté canadienne, il est dans l’intérêt des Forces canadiennes de recueillir de l’information tactique de sonar dans les eaux de l’Arctique. Une bonne connaissance des propriétés géo-acoustiques du fond marin peut appuyer l’utilisation efficace du sonar passif ou du sonar actif pour détecter les cibles naviguant dans la région à la surface et/ou sous l’eau. Le présent document
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rassemble une quantité importante de données sur le milieu arctique à proximité de la SFC Alert.
Recherches futures
RDDC Atlantique ne prévoit pas de nouvelles visites des régions arctiques dans un avenir immédiat, mais les résultats de cette analyse seront précieux pour les opérations futures qui seront menées à proximité de la SFC Alert.
Abstract........................................................................................................................................ i
Executive summary ................................................................................................................... iii
Sommaire.................................................................................................................................... v
Table of contents ...................................................................................................................... vii
List of figures .......................................................................................................................... viii
1. Introduction ......................................................................................................................... 1 1.1 Theory .................................................................................................................. 1
1.1.1 Direct and bounce rays......................................................................... 2 1.1.2 The reflection coefficient (R)............................................................... 2
Distribution list ......................................................................................................................... 28
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List of figures
Figure 1. IceShelf 2002 camp location (red box), core sample site from 1988 (blue circle), sediment sample site from 1990 (filled green squares), 1991(pink triangles), and seismic experiment locations from 1988 (blue x), 1989 (green square), and 1992 (pink lozenge).. 6
Figure 2. Vertical Line Array (red x) and source locations (.) identified by ice-hole number. Range is in meters. .............................................................................................................. 7
Figure 3. Temperature and sound speed profiles taken April 12-18 (green), 22 (red) and 24(blue). Profile taken on the 24th measured at bottom loss hole 6.................................... 9
Figure 4. Ray paths for bottom loss hole 10, horizontal distance 149 m from the VLA. Source depth (red *) is 30 m, receiver depth (blue O) 20 m. Direct ray (magenta), bottom bounce (red) surface bounce (blue) and a multi-bounce (gray) path are shown.............................. 9
Figure 5. Difference between arrival times determined using geometry (equation 1.1) and BELLHOP......................................................................................................................... 12
Figure 6. Observed relative arrival times minus times predicted using Bellhop. ..................... 13
Figure 7. Uncertainty in arrival times due to uncertainty in source (x) and receiver (.) depth, and horizontal range (+). ................................................................................................... 14
Figure 8. Direct arrivals isolated on the VLA as a function of receiver range (upper) and receiver depth (lower). ...................................................................................................... 16
Figure 9. Time series of recorded signal for a direct (blue) and bounce path ray (red) as determined using the correlation (a) and first and second arrivals separated by less than 15 ms (b). .......................................................................................................................... 17
Figure 10. Time series of recorded voltage from each channel of the VLA for a source depth of 15 m and a range of 31 m. Detected (red) and predicted (green) arrivals are shown. . 18
Figure 11. Bounce-path arrivals isolated on the VLA as a function of receiver range (upper) and receiver depth (lower)................................................................................................. 19
Figure 12. Time series (a) of recorded light bulb implosion at depths of 15 (blue), 30 (red), and 50m (green) depth. SSL (blue) of an implosion signal at 50 m depth, with 95% confidence interval (shaded). ............................................................................................ 20
Figure 13. Peak SSL (a) and frequency at peak SSL (b) plotted versus implosion depth. Mean confidence interval at each depth are shown by the red line (a), and the range of three frequency bins centred at the frequency of peak SSL(b). Blue dots are results using the direct arrivals, red dots using one bottom bounce rays. .................................................... 21
viii DRDC Atlantic CR 2003-240
Figure 14. Bottom loss (dB) using ray angle as a proxy for grazing angle (degrees) for all (blue), 1-bottom bounce (red), and top bounce only (red) rays at 600 Hz. ....................... 22
Figure 15. Bottom loss for one bottom bounce rays plotted against grazing angle (degrees) for 120 (blue), 520 (red), and 1000 (green) Hz....................................................................... 23
Figure 16. Modeled bottom loss for parameters in Table 6..................................................... 24
Figure 17. Modeled bottom loss using a thin (1m) layer (Eq. 1.3) at various frequencies...... 25
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List of tables
Table 1. Summary of sediment size from Lincoln Sea core (1988) ........................................... 4
Table 2. Summary of sediment size from selected locations (1991) .......................................... 4
Table 3. Summary of geo-acoustic parameters........................................................................... 5
Table 4. Source range and bearing from VLA, bottom depths and ice thickness at source location holes ..................................................................................................................... 8
Table 5. Arrival times of the first three rays for bottom loss holes 1, 5, and 10. Only selected source depths are included. ............................................................................................... 10
Table 6. Acoustic parameters for the model............................................................................. 25
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1. Introduction
Acoustic observations, both passive and active are an important tool in marine science. Proper modelling of the acoustic environment is essential to the applications of these observations. Arctic marine environments in particular make routine data collection difficult, but have recently become of increased scientific value due to their predicted sensitivities to global warming and are becoming of increased economic value as marine mineral and oil resources become scarce.
In April and May of 2002, a team from DRDC conducted an acoustic transmission experiment at Joliffe Bay, off Ellesmere Island north of Alert, Nunavut. The northern shore of Ellesmere Island borders on the Lincoln Sea. A bottom loss study is presented in this report using recordings from a vertical line array. The acoustic source was imploding light bulbs. Several environmental and geo-acoustic studies have been done in the Lincoln Sea, and near the IceShelf 2002 camp, the results of which are summarised here. These existing geo-acoustic estimates are used to model reflection losses and are compared to the acoustic bottom loss data.
1.1 Theory
For receivers located in the water column, acoustic transmission can be adequately modelled using ray-theory and the speed of compressional sound waves (cp) in the bottom layer(s). To model the reflection coefficient (R) or the bottom loss (BL=20*logR) at the ocean bottom, the density (ρ) of the ocean bottom sediment is also necessary. For a complete geo-acoustic model other parameters are necessary see Hamilton (Reference 4).
For boundary interfaces where the sound speed in the lower medium is greater than that in the upper medium there exists a critical angle at which incident energy is completely reflected into the upper medium when shear components are neglected. Wave fronts travelling at the critical angle excite a head wave at the interface, which generates waves propagating back into the upper media at the critical angle. For rays grazing the boundary at angles greater than the critical angle a portion of energy is reflected in to the upper media, while the remainder is transmitted to the lower layer. This transmitted energy will then either refract due to a gradient in sound speed in the bottom layer and be re-transmitted into the water column, or interact with another boundary interface.
One means of determining cp is by a seismic refraction experiment. An acoustic source and a horizontal line array are used to measure the difference in arrival times of the head wave. Typically this is done using a horizontal array, but a vertical line array can also be used (Reference 6).
To determine the bottom loss coefficient an acoustic reflection loss experiment is conducted. The bottom loss coefficient is determined by isolating the energy of the
DRDC Atlantic CR 2003-240 1
direct and bottom interacting rays. By changing the distance from source to receiver, the bottom loss as a function of grazing angle is obtained. Estimates of the energy at different frequencies gives bottom loss as a function of frequency if a broadband source is used.
The theory section is divided into three sub-sections: ray paths, reflection coefficient model, and head waves.
1.1.1 Direct and bounce rays Rays travelling from source to receiver without reflecting off the surface or bottom (or any sub-layer of the bottom) are called direct rays. When refraction due to the sound speed profile does not occur this direct path is given by the geometry. The time of travel for the direct wave is given by
Td = (r2 + (zH-zS)2)1/2 cw-1, 1.1
where zH and zS are the distances from the bottom to the hydrophones and source, respectively; cw is the speed of sound in water and r is the horizontal separation. When the arrival time is plotted against the zH, the direct arrival times form a hyperbola, with a minimum at zH = zS. Rays may also reflect off the bottom boundary (bottom bounce) or the ice canopy (top bounce) or have multiple bounces off both interfaces (multiple bounces). A ray can have multiple bounces off only one interface if the ray is refracted due to the sound speed profile in the water (cw(z)). The arrival times of predicted rays for this experiment are determined using a ray tracing program (Bellhop, available http://oalib.saic.com/).
1.1.2 The reflection coefficient (R) The amplitude of a reflected ray at an interface is given by A.R, where A is the amplitude of the incident signal and R is the reflection coefficient. The amplitude of the transmitted ray is given by A.T, where T = 1-R. The reflected energy is described by the bottom loss (BL) term, given in dB as 20 log (R). For plane wave reflection the reflection coefficient at the water/ocean bottom boundary is given by (Reference 2)
where the angles are measured from the horizontal, ρ1 and c1 are the density and sound speed of the water and ρ2, c2 the density and sound speed of the bottom.
When a thin layer is present at the ocean bottom, the reflection of rays at the bottom becomes the sum of the energy reflected at the water/thin layer interface and the thin
layer/lower layer interface. The time delay of the ray within the thin layer introduces a frequency dependence. The reflection coefficient at a thin layer interface is given by (Reference 2)
R13 =R12 + R23e
−2iΦ
1+ R12R23e−2iΦ 1.3
where Φ = k2h2cos(θ2), k2 is the wave number in the thin layer, and h2 is the thickness of the thin layer. R12 and R23 are similar to equation 1.2 The definition of the critical grazing angle is given by
θc = arcsin(c1 c2) 1.4
1.2 Previous studies
The Lincoln Sea covers a large area in the arctic bounded by Ellesmere Island to the south, Greenland to the east and by approximately 84°N. The margins of the sea are continental shelf extending 200 km to the north of Ellesmere Island, a shelf break and a deep basin. A broad environmental report by Bucca (Reference 1) describes the circulation, water masses, surface ice condition and bottom types of the sea.
Of relevance to this paper is their description of the Arctic Surface Water. This water mass is cold (< 0 °C) and fresh (< 34.5 PSU). At depths from less than 30 m to 50 m, the layer is well mixed. The sound speed profile of the surface layer has a small positive gradient. The details of the surface water vary spatially within the Arctic Basin.
Sediment in the area is transported from Greenland and Ellesmere Island via ice and consists of clay and silt, as well as coarse sediment ranging from gravel to boulder size. A core sample was taken in 1988, 80 nm north of Alert (shown as blue circle in Figure 1), and sediment samples were taken in 1990 (green circles) and 1991 (magenta circles) along a transect from Stuckberry point north into the Lincoln Sea. Analysis of the core sample from 1988 (Reference 7) gives a bulk density of 2.14 g cm-3, a porosity of 40.7% and a cp of 1794 m/s at the surface of the bottom layer. The core sample penetrated 21 cm, and the grain size data for various depths is summarised in Table 1.
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Table 1. Summary of sediment size from Lincoln Sea core (1988)
SUB-BOTTOM DEPTH
INTERVAL (M)
GRAVEL
(%)
SAND
(%)
SILT
(%)
CLAY
(%)
0.00-0.01 - 5 48 47
0.01-0.02 12 20 48 20
0.08-0.09 22 38 22 18
0.09-0.10 7 57 18 19
0.10-0.11 3 55 34 11
First 5 intervals from Table 54b from Reference 7
Sediment size distribution from the 4 locations nearest the coast taken in 1991 (Reference 1) are summarized in Table 2. The gravel content and clay content vary from 0.2 to 46% and 27 to 71% respectively. Results from these locations show that the surface layer varies considerably but is mainly composed of silt and clay with varying amounts of gravel with no discernable trend in grain size variability.
Table 2. Summary of sediment size from selected locations (1991)
LOCATION
(°N °W)
GRAVEL
(%)
SAND
(%)
SILT
(%)
CLAY
(%)
83 03.51’, 66 20.80’ 0.2 0.7` 25.26 73.83
82 56.97’, 66 32.90’ 7.38 3.96 41.96 46.70
82 56.79’, 66 52.97’ 46.05 11.77 14.60 27.58
82 56.69’, 66 52.83’ 0 1.35 37.21 61.44
Table 8.3 from Reference 1
Seismic measurements collected in 1988 (Reference 8), 1989 (Reference 1), 1990 (Reference 9), 1992 (Reference 3) were all collected near the shelf break in deep water (< 400 m), north of Ellesmere Island (see Figure 1). The results from Reference 8 showed velocities at the surface of the bottom layer of 1980 m/s.
Hunter showed three principle layers at the 1989 site, where the depth of the top layer varied significantly with range from the source. At this location, the cp was estimated at 1820 m/s. His analysis of the data from 1990 showed a surface bottom layer with velocity ranging from 1750 to 2000 m/s.
4 DRDC Atlantic CR 2003-240
Dosso’s analysis of seismic data from 1992 shows that a lower velocity surface layer existed with a cp of 1720 ± 100 m s-1 and a thickness of 10 ± 4 m. The layer below that had a cp of 1900 ± 10 m s-1
Geddes made a geoacoustic model using classical theory of the origin of the continental shelves in the arctic. This model was updated using the seismic measurements from 1989 and the 1988 core sample. He calculated a reflection coefficient of 0.5, or a bottom loss of 6.0 dB, given a bottom water velocity of 1505 m s-1 and a bottom layer velocity of 1738 m s-1.
The results from these studies are summarised in Table 3.
Table 3. Summary of geo-acoustic parameters
AUTHOR YEAR OF DATA
COLLECTION
GEO-ACOUSTIC PROPERTY
EXPERIMENTAL VALUE
Todoeschuk 1988 Cp 1980 +- 10 m s-1
Marsters and Mayers
1988 Cp 1683 m s-1
Bulk density 2.14 g cm-3
Geddes 1988-89 R 0.5
Hunter 1989 Cp 1820 m s-1
1990 Cp 1710 – 2000 m s-1
Dosso 1992 Cp (upper layer) 1720 +/- 100 m s-1
H (upper layer) 10 +/- 4 m
Cp (upper layer) 1900 +/- 10 m s-1
See Figure 1 for locations of experimental site.
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Figure 1. IceShelf 2002 camp location (red box), core sample site from 1988 (blue circle), sediment sample site from 1990 (filled green squares), 1991(pink triangles), and seismic experiment locations from
1988 (blue x), 1989 (green square), and 1992 (pink lozenge).
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2. Experiment
The recordings presented here were made at the IceShelf 2002 research camp, northwest of Alert, Nunavut, on Ellesmere Island, April 24-25 2002 at approximately 65m water depth. The vertical line array (VLA) consisted of four hydrophones at depths of 20, 30, 40 and 50 m. The data from each hydrophone were recorded on one channel of a multi-channel data acquisition system so that the timing of all hydrophones on the array is simultaneous. A light bulb breaking apparatus (Reference 5) was used to control the depth of the implosion. Implosions from ten locations (bottom loss holes) at distances from 15 to 150 m from the VLA were recorded. At each location, the depth of the light-bulb breaker was set to 15, 30 and 50 m. At least three bulbs were used at each depth and location. Figure 2 shows the location of the bottom loss holes used to lower the light bulb breaking apparatus into the water, while Table 4 includes the bearing and distance from the VLA and the ocean depth at each location. The range and bearing were determined using a geodimeter, and are considered accurate within 1 cm.
Figure 2. Vertical Line Array (red x) and source locations (.) identified by ice-hole number. Range is in meters.
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Table 4. Source range and bearing from VLA, bottom depths and ice thickness at source location holes
BOTTOM LOSS HOLE
BEARING RANGE DEPTH ICE THICKNESS
(degrees) (m) (m) (ft)
VLA 0 0 64.92 6
Hole 1 233.55 15.69 65.53 6
Hole 2 242.79 31.30 66.75 6
Hole 3 223.74 46.61 66.14 6
Hole 4 237.15 60.95 67.36 6
Hole 5 234.25 74.74 67.36 6
Hole 6 243.80 89.90 67.67 6
Hole 7 239.56 106.20 67.67 7
Hole 8 241.48 121.31 67.36 7
Hole 9 238.5 136.42 67.67 7
Hole 10 235.23 149.06 67.67 6
Sound velocity profiles (SVP) and temperature profiles were taken at several locations over a two week period at the camp using a profiler made by Applied Microsystems Ltd. There were problems with the casts, with a number of downcasts not showing data consistent with data from the upcasts. Also, the device malfunctioned on the 25th. Figure 3 shows the upcasts from all the successful casts. Profiles taken on the 22nd at holes within 100m of the bottom loss holes are shown in red, and are similar to the profile taken on the 24th (blue) taken at bottom loss hole 6. The profile from bottom loss hole 6 taken on April 24 will be used for the ray path analysis for each hole on both days of the experiment. The depthwise mean cw is 1438 m/s.
All profiles are consistent with the Arctic Surface Water described by Bucca (Reference 1).
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Figure 3. Temperature and sound speed profiles taken April 12-18 (green), 22 (red) and
24(blue). Profile taken on the 24th measured at bottom loss hole 6.
Figure 4. Ray paths for bottom loss hole 10, horizontal distance 149 m from the VLA. Source depth (red *) is 30 m, receiver depth (blue O) 20 m. Direct ray (magenta), bottom bounce (red)
surface bounce (blue) and a multi-bounce (gray) path are shown.
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Ray paths were determined for each unique geometry using Bellhop. Bathymetry was included in the ray path analysis, where the depth is assumed to vary linearly between locations.
Figure 4 shows the rays for hole 10, horizontal distance from VLA of 150 m, source depth of 30 m and receiver depth of 20 m. The sound speed profile from the 24th is included in the figure. It is clear that there is little refraction due to the sound speed profile. From this figure, the effect of bathymetry is also apparent in the arrival of two bottom bounce rays of similar ray angle.
The path length and approximate travel times (using cw = 1438 m/s) of the first three arrivals for bottom loss holes 1, 5 and 10 are given in Table 5. From the values in the table, it is obvious that the time between the direct and second arrival becomes shorter as the distance from source to receiver increases. For bottom loss hole 10, direct rays are distinguishable from the second arrival only at the receivers of depths 30 and 40 m and only for a source depth of 50 m.
Table 5. Arrival times of the first three rays for bottom loss holes 1, 5, and 10. Only selected source depths are included.
HOLE SD RD DIRECT ARRIVAL
SECOND ARRIVAL
GRAZING ANGLE
THIRD ARRIVAL
GRAZING ANGLE
(m) (m) (ms) (ms) (ms)
1 15 20 16.6 38.50 (NaN) 122.81 82.3
1 15 30 21.6 47.79 (NaN) 83.33 78.8
1 15 40 29.6 57.29 (NaN) 73.70 76.8
1 15 50 38.5 63.98 74.8 66.93 (NaN)
5 15 20 74.9 82.57 (NaN) 120.11 47.2
5 15 30 76.2 87.20 (NaN) 161.78 64
5 15 40 78.9 92.70 (NaN) 128.76 52.1
5 15 50 82.4 98.97 (NaN) 98.97 38.6
5 30 20 149.1 152.99 (NaN) 177.39 32.6
5 30 30 149.7 155.81 (NaN) 172.23 29.7
5 30 40 151.1 158.94 (NaN) 167.58 26.8
5 30 50 153.2 162.57 (NaN) 163.02 22.1
10 15 20 149.5 157.28 (NaN) 169.40 29
10 15 30 149.1 160.77 (NaN) 165.10 25.3
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10 15 40 149.5 161.17 22.2 164.59 (NaN)
10 15 50 150.3 157.70 17.1 169.08 (NaN)
10 30 20 151.9 160.37 15.5 160.57 23.9
10 30 30 150.3 157.57 20.9 169.15 (NaN)
10 30 40 149.5 174.08 (NaN) 189.80 39.7
10 30 50 149.1 152.17 11.7 179.58 (NaN)
An angle of NaN means that the ray did not reflect off the bottom boundary
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3. Results
Time series from each hydrophone and each implosion were excluded from this study based on geometric predictions for arrival times and overlapping arrivals (3.1). The effects of receiver depth and separation on the accurate detection of direct arrivals are also presented in 3.1. The remaining time series are searched using a correlation technique for bounce path arrivals (3.2).
Parameters used for the spectral analysis are presented in 3.3 along with a spectral source level and frequency check while bottom loss results are presented in 3.4.
3.1 Array consistency and direct arrival isolation
Since the exact times of the implosions are unknown, the arrival times of the direct rays are estimated via either Bellhop or Equation 1.1 using the known source/receiver separation and the hydrophone depth. Figure 5 shows the difference in arrival times predicted using these two methods for all source and receiver depths, and ranges. The largest difference is 0.2 ms, the mean difference between these methods is -0.01 ms, and the standard deviation is 0.072 ms. From these results, the uncertainty in arrival time is 0.14 ms (2 times the standard deviation), or 20 cm (using a constant speed of sound of 1438 m s-1) . The sampling rate (2560 Hz) sets a time resolution of 0.4 ms, nearly four times greater than the uncertainty of the arrival time, so that any difference between the two methods is beyond the measuring capability of this system.
Figure 5. Difference between arrival times determined using geometry (equation 1.1) and BELLHOP.
12 DRDC Atlantic CR 2003-240
The predicted ray arrival times from Bellhop are used to exclude arrivals which overlap in time. If any ray arrives at a hydrophone within 7.5 ms (half the length of the light bulb signal) of the start of an earlier arrival, the earlier arrival is discarded from analysis. If the earlier arrival is the direct ray, then entire record is discarded. A threshold of 7.5 ms allows for some overlap in signals and is necessary to allow for detections from the bottom loss holes at greater distances from the receiver, where the likelihood of direct and second arrival overlap is high. While any significant distortion of either direct or later arrival caused by the overlapping signal, the selection of bounce path rays via the correlation technique excludes badly overlapped signals from being considered in the bottom loss data (see 3.3). The number of time series discarded using this threshold are 41, 31, 25, and 37 for the hydrophones at 20, 30, 40 and 50 m depth respectively.
An independent measure of time for each implosion is the arrival time of the direct ray at each hydrophone relative to the hydrophone closest to the source. The difference between the measured relative arrival times of the direct ray and the times predicted using Bellhop provides a consistency check of the array geometry (Figure 6). For this experiment there are large (> 2 ms, or more than five times the sampling resolution) differences between observed and predicted arrival times occurring for source ranges greater than 50 m. However the majority of the arrivals occur within 1.5 ms of the predicted arrival.
Figure 6. Observed relative arrival times minus times predicted using Bellhop.
DRDC Atlantic CR 2003-240 13
Following the approach of Todoeschuk (Reference 8), the effects of error in source/receiver location on arrival times are investigated. The horizontal distances involved here are much less than Todoeschuk so direct comparisons to his results are not made. While the location of the holes used to lower the light bulb breaking apparatus into the water are quite accurate (on the order of cm), the currents in the water column are not known, although are presumed to be small (< 10 cm s-1).
Tilts of 5 degrees in the bulb breaker apparatus, which might arise due to currents, would account for vertical displacements in the source depth of 0.1, 0.2, 0.4 m at depths of 15, 30 and 50 m. For the VLA, a tilt of 5 degrees would result in horizontal displacements of 1.7, 2.6, 3.5, 4.4 m for the hydrophones at 20, 30, 40 and 50m respectively and vertical displacements similar to the bulb breaker. The effects of a +/- 2m error in source/receiver depth (. and x in Fig. 7) and an error in range of +/- 2m (+) are shown in Figure 7 for a receiver at 50 m depth and a source at 15 m depth. Note that the x symbols are overlapping the “.” symbols in Fig. 7.
Figure 7. Uncertainty in arrival times due to uncertainty in source (x) and receiver (.) depth, and
horizontal range (+).
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Since current measurements were not taken during the experiment, the exact cause of the differences between the observed arrival times and expected arrival times of the direct rays cannot be verified. However, only four weights of 2 kg each were used on the VLA, one at each hydrophone, so the difference between observed and predicted arrival times which are less than 1.5 ms in magnitude are likely due to a tilt in the VLA/bulb-breaker induced by a current.
Based on Figure 7, differences between predicted and observed relative arrival times greater than 1.5 ms are unlikely, therefore this quantity is used as quality check. Time series where this difference is greater than 1.5 ms are discarded. This quality check results in 4, 3, 6 and 10 times series to be discarded for hydrophones at 20, 30, 40 and 50 m respectively.
The uncertainty in arrival time of 1.5 ms means there is an uncertainty in ray path of 2.2 m. At ranges of 15 m this leads to an uncertainty in spreading losses of 1 dB, and this uncertainty decreases as the path length increases.
Out of a possible 392 time series (4 hydrophones and 93 light bulb implosions) 23 were excluded because of large uncertainties in relative arrival times within the VLA, and 124 excluded because of overlapping direct and second arrivals. Figure 8 shows the number of direct arrivals considered in this study based on receiver range (a) and receiver depth (b). Fewer direct arrivals are accepted as range increases which is consistent with the overlapping arrival times of the direct and second arrival (Table 3). The number of detections of direct arrivals is greater for the hydrophones at mid depths (30 and 40 m) compared to the surface or bottom hydrophones (Figure 9b). This is likely due to the smaller path length differences between the direct ray and surface or bottom bounce ray.
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Figure 8. Direct arrivals isolated on the VLA as a function of receiver range (upper) and receiver depth (lower).
3.2 Signal detection
For each implosion the direct arrival is isolated by searching the output of each hydrophone for the peak voltage. From visual inspection, the beginning of the signal from the light bulb implosion occurred 3.5 ms earlier than this peak. The end of the time series varies with the depth of the implosion, but on average is approximately 11.5 ms after the peak. The total length of the direct arrival light bulb signal is therefore 15 ms, which is consistent with Heard et. al. (Reference 5).
Subsequent arrivals are detected by searching for maxima in the lagged cross correlation function of the direct arrival signal and a portion of the recorded time series beginning 80 ms prior to the direct arrival and ending 100 ms after the direct arrival. Correlations less than R = 0.75 were discarded to avoid signal distortion by overlapping rays. A well isolated direct arrival (Figure 9a, blue), and a bottom bounce arrival (a, red) show high correlation (R = 0.93). A badly overlapped signal, shown in Figure 9b, will not correlate other arrivals.
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Figure 9. Time series of recorded signal for a direct (blue) and bounce path ray (red) as determined
using the correlation (a) and first and second arrivals separated by less than 15 ms (b).
Once distinct arrivals have been isolated they are matched to the ray paths predicted using Bellhop. Observed arrivals within +/- 5 ms of a predicted arrival are matched. If a measured ray matches more than one predicted ray using this method, the measured ray is matched to the predicted array whose arrival time is closest to the measured arrival time.
The time series from each hydrophone for source depth 15 m and range of 31 m are shown in Figure 10 with time along the vertical axis, and hydrophone depth along the horizontal axis. Arrival times, both predicted via Bellhop (green) and those based on correlation with the direct arrival (red) are shown. The predicted arrival time of the direct ray are not shown. The direct ray arrives at the shallow hydrophone first and then at each successive hydrophone. For the receivers at 30 and 40 m depth, the detected (red) and predicted (green) second arrivals concur. The second arrival on the shallow hydrophone is not detected because it occurs within 7.5 ms of the direct arrival, while the arrival of the deeper hydrophone simply lack correlation to the direct signal (most likely because of a third arrival overlapping the second). Later arrivals do not coincide with any predicted arrivals.
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From the 215 time series not excluded in section 3.1, 191 bounce path rays were detected via the correlation technique and matched to a predicted ray. Bounce path rays were not necessarily isolated in all 215 possible time series. Of these 191 bounce-path rays, 81 reflected off the surface only, 55 off the bottom only, and 55 reflected off both the surface and bottom.
The number of bounce-path (n=191) rays observed as a function of receiver range (a) and receiver depth (b) are shown in Figure 11. Again there are fewer isolated bounce rays for receiver ranges greater than 61 m (a), and the hydrophone at depth 40 m has more isolated bottom bounce rays than the other hydrophones.
Figure 10. Time series of recorded voltage from each channel of the VLA for a source depth of 15 m and a range of 31 m. Detected (red) and predicted (green) arrivals are shown.
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Figure 11. Bounce-path arrivals isolated on the VLA as a function of receiver range (upper) and receiver depth (lower).
3.3 Spectral Source Levels and Peak Frequency
Three sample time series are shown in Figure 12a. The signal from a light bulb imploded at 15 m depth (blue), 30 (red) and 50 (green) m are shown. There is an increase in amplitude with implosion depth and a change in frequency content of the signal. Spectral densities for all isolated rays are calculated using a 64 point FFT (6 ms), a Hann window of width 16 points and 50% overlap (zero padded), resulting in 33 frequencies bands and a spectral resolution of 39 Hz. The time series of the bulb implosion is 15 ms long, so that 4 windows are averaged in the source spectral level (SSL). The spectral density of the signal from the 50 m depth implosion is shown in Figure 12b (blue line). The 95% confidence interval is shown by the shaded area, and is approximately +/- 10 dB.
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Figure 12. Time series (a) of recorded light bulb implosion at depths of 15 (blue), 30 (red), and 50m
(green) depth. SSL (blue) of an implosion signal at 50 m depth, with 95% confidence interval (shaded).
For each hydrophone and each light bulb implosion, the spectral energy of background noise is calculated using one second of the recorded time series starting two seconds before the direct arrival and with the same parameters as those for the signal spectral levels. The spectral noise levels are subtracted from the received spectral levels.
To correct for the spreading losses of the direct ray, the path length determined via Bellhop are used. For the bottom bounce rays, the difference in the arrival time of the direct and bounce ray is used to estimate the difference in path length between these two rays. Total path length of the bounce rays is the sum of the direct path determined from geometry plus the path length difference determined from the difference in arrival time. This estimate of path length is used instead of the path length of the predicted bounce ray from Bellhop because matched rays may be mis-timed by as much as 5 ms.
Heard et al. showed the consistency of light bulb implosions, and that the spectral source levels (SSL) and frequency at peak SSL of the implosion increases with depth. To ensure that the direct arrivals are in fact the signal from the imploding light bulbs,
20 DRDC Atlantic CR 2003-240
the maximum SSL and frequency at maximum SSL are plotted versus implosion depth.
The peak SSL of the direct arrivals (blue dots) and all later arrivals (red dots) corrected for spreading losses and gain are plotted versus depth in Figure 13a. The scatter in peak SSL at each depth is less than the mean of the confidence intervals (from Figure 12b) at the frequency of maximum SSL (vertical red line). The maximum SSL increases with the depth of light bulb implosion. Using this, direct arrivals with corrected SSL of less than 15 dB are excluded from the bottom loss analysis. This resulted in three data points for source depth 50 m to be excluded (not shown). The frequency of maximum SSL is plotted against implosion depth in Figure 13b for direct (blue dot) and later (red dots) arrivals. Here the scatter is on the order of the width of two frequency bins (red vertical line). The increased scatter of the SSL is likely a result of the losses at the bottom, while the scatter in frequency is likely an indication that the bottom bounce rays are not entirely well isolated.
Figure 13. Peak SSL (a) and frequency at peak SSL (b) plotted versus implosion depth. Mean confidence interval at each depth are shown by the red line (a), and the range of three frequency bins centred at the frequency of peak SSL(b). Blue dots are results using the direct arrivals, red dots using
one bottom bounce rays.
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Without hydrophone sensitivity, direct comparisons to the results from Heard et. al cannot be made.
3.4 Bottom Loss: Observations and Model
The ray departure angle, measured from the horizontal, is a proxy for grazing angle at both the ice surface and the ocean bottom, assuming both are smooth flat surfaces. The ratio of received energy of the direct and bounce rays at 600 Hz corrected for spreading losses and plotted against this estimate of grazing angle is shown in Figure 14. Rays reflecting only once and only off the ice are shown in red. Rays reflecting only once and only off the bottom are shown in green. All other rays are shown in blue.
Figure 14. Bottom loss (dB) using ray angle as a proxy for grazing angle (degrees) for all (blue), 1-
bottom bounce (red), and top bounce only (red) rays at 600 Hz.
The critical angle, corresponding to 0 dB loss, is between 30 and 40 degrees. The BL increases with grazing angle to approximately 50 degrees and then decreases with grazing angle.
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To better estimate grazing angle, the angle the ray segment makes with the horizontal minus the angle given by the bathymetry at the reflection point is used as the grazing angle in Figure 15 for frequencies of 120 (blue), 520 (red), 1000 (green). In this approach only rays interacting with the bottom are shown.
Figure 15. Bottom loss for one bottom bounce rays plotted against grazing angle (degrees) for 120
(blue), 520 (red), and 1000 (green) Hz.
In both Figures 14 and 15 there are bottom losses which are less than zero. One possibility for this is an overestimate in the path length for the bounce path ray and therefore an overestimate of the spreading losses for that ray.
Bottom loss is modeled using three environments in Figure 16. A single layer bottom of density 2.1 g cm-3 and sound speed of 1800 m s-1 and a single layer with a density of 1.5 g cm-3 and a sound speed of 1500 m s-1 are both shown in gray. Both cases use a single layer reflection loss (equation 1.2). The second set of parameters is more consistent with clay, while the first set of parameters is consistent with the core sample and seismic results. A third environment (coloured lines) has a thin layer with density of 1.5 g cm-3 and a sound speed of 1500 m s-1 over a layer with density 2.1 g cm-3 and sound speed of 1800 m s-1. The thickness of the low density, thin upper
DRDC Atlantic CR 2003-240 23
layer varies (see the legend). In this case, equation 1.3 and a frequency of 500 Hz are used. The very thin layer of 10 cm (blue) is very similar to the single layer case where only the high density sediment is modeled. The low density layer alone has a critical angle much more steep (< 20) than the observed critical angle.
Figure 16. Modeled bottom loss for parameters in Table 6.
The bottom loss for frequencies of 120, 520 and 1000 Hz and the thin layer (1m) environment is shown in Figure 17.
While none of these models matches the observed bottom loss, the frequency dependent behaviour for increasing grazing angle shows that the sediment in the vicinity of the experiment is best modelled by a thin layer of low velocity, low density material overlying a denser higher velocity material.
24 DRDC Atlantic CR 2003-240
Table 6. Acoustic parameters for the model
ENVIRONMENT TYPE
DENSITY CΡ
(g cm-3) (m s-1)
low density 1.5 1500
high density 2.1 1800
thin layer low density over high density
1.5/2.1 1500/1800
Figure 17. Modeled bottom loss using a thin (1m) layer (Eq. 1.3) at various frequencies.
DRDC Atlantic CR 2003-240 25
4. Conclusions
Uncertainties associated with isolating and identifying distinct rays makes analysis of the data difficult. Based on geometric predictions, the second arrival occurs within half a signal length of the direct arrival 41, 31, 25, and 37 times for the hydrophones at 20, 30, 40 and 50 m depth respectively. Uncertainties between the measured relative arrival times of the direct ray on the VLA are larger than 1.5 ms 4, 3, 6 and 10 times for hydrophones at 20, 30, 40 and 50 m respectively. Limiting the tilt of either the VLA or light bulb breaking apparatus limits this geometric error to be less than 1.5 ms for all ranges. Overestimates in path length (and therefore acoustic spreading losses) might explain measured bottom losses that are less than 0. Better measurements of ice thickness and bathymetry in the area would help to eliminate some of the uncertainty in ray path, while current measurements, or a tilt sensor on both the VLA and the bulb breaking apparatus would reduce the uncertainty in geometry. An increase in the sampling rate would increase the number of data points in the FFT and improve the estimate of energy within the light bulb signal. A higher sampling rate may also allow for the development of an algorithm to estimate the energy of two overlapping signals.
The shape of the bottom loss curve and the frequency dependence of the bottom loss indicate a thin layer of low density material overlays a denser sediment layer. This result agrees with core samples and seismic measurements from the region. The number of variables in the model make an inversion algorithm beyond the scope of this report although an inversion algorithm should be able to extract the likely layer thickness.
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5. References
1. Bucca, P.J. A compendium of environmental data collected during the Iceshelf-99 exercise. Technical Note 32, Naval Oceanographic and Atmospheric Laboratory, Stennis Space Center, Mississippi.
2. Clay, C.S, and Medwin, H. Acoustical Oceanography: Principles and applications. John Wiley & Sons, Toronto. 1977.
3. Dosso, S.E, Brooke, G.H. Measurements of seismo-acoustic ocean-bottom properties in the high Arctic. J. Acoustic. Soc. Am. 98(3), 1657-1666 (1995)
4. Hamilton, E.L. Geoacoustic modeling of the sea floor. J. Acoust. Soc. Am. 68(5), 1313-1340. 1980
5. Heard, G.J., McDonald M, Chapman, N.R., Jaschke, L. Underwater light bulb implosions: A useful acoustic source. IEEE Oceans 97 conference proc. Vol III. October 6-9 Halifax, pp 755-762.
6. Hunter, J.A., and Pullan, S.E. A vertical array method for shallow seismic refraction surveying of the sea floor. Geophysics, 55, 92-96 (1990)
7. Mayer, L.A., and Marsters, J. Measurement of geophysical properties of arctic sediment cores. Contractors report series 89-19, Defence Research Establishment Pacific, Victoria. 1989
8. Nelson, J.B., Harwick, D., Bower, M., Marcotte, D. MacPherson, M., Forsythe, D., Macnab, R. and Teskey, D. Preliminary analysis of data from the Lincoln Sea aeromagnetic survey 1989-1990. Geological Survey of Canada Current Research, Ottawa.
9. Todoeschuck, J.P. Seismic experiments in the Lincoln Sea during IceShelf 88. Contractors report 90-25, Defence Research Establishment Pacific, Victoria 1990.
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Iceshelf 2002 bottom loss analysis
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A short-range, shallow-water bottom loss experiment was conducted in the South Lincoln Sea,under an irregular rough ice canopy. The experiment was carried out using broadband signalsgenerated by imploding light bulbs at depths of 15, 30 and 50m and distances ranging from 15to 150 m from a vertical hydrophone array. Previous experiments in the vicinity of this site haveprovided limited seabed information. Preliminary modeling of the acoustic propagation at thislocation using a ray-based model is used to determine the bottom loss as a function of grazingangle for frequencies between 10 and 1000 Hz. This report will review the measurements, anddiscuss the challenges associated with the modeling, in view of the experimental uncertaintiesassociated with this complicated environment.
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