This article has been published in published been This has article or collective redistirbution other or collective or means this reposting, of machine, is by photocopy article only anypermitted of with portion the approval The O of B r e a k i n g Wav e s Oceanography

Shallow Water ’06 , Volume 4, a quarterly 20, Number The O journal of A Joint Acoustic Propagation/ Nonlinear Internal Wave Physics Experiment ceanography S B y D a j u n Ta n g, James N. M oum, James F. Ly n c h , P h i l a B B o t, R o s s C h a p m a n , P e t e r H . D a h l , T i m o t h y F. D u d a , g L e n G awa r k i e w i c z , S c o t t g L e nn , J o h n A . G o ff, ociety. C H a n s G r a b e r , J o h n K e m p, a N d r e w M a ff e i , J o n at h a n D . N a s h , a n d A r t h u r N e w h a ll opyright 2007 by The O ceanography S ceanography S S i n c e t h e e n d of the Cold War, acoustics systems and Navy opera- to be large and critical, but precious little the US Navy has had an increasing inter- tions need the “local ocean weather” as experimental data combining acoustics ociety. A ociety. S

est in continental shelves and slopes as well as the “ocean climate” as part of with a well-characterized coastal ocean Permission is reserved. ll rights granted to in teaching copy this and research. for use article R operational areas. To work in such areas the routine forecast, but the former is is to be found. Another challenge is to end all correspondence to: [email protected] Th e O or requires a good understanding of ocean not yet available. try to extrapolate the routinely measured acoustics, coastal physical oceanography, In ocean acoustics, the Navy wishes “single bounce” acoustic paths (those and, in the modern era, autonomous to operate both at low frequencies with one surface or bottom interaction) underwater vehicle (AUV) operations. (100–1000 Hz) and mid frequencies to predict how multiple-bounce sonar Each area presents challenges for both (1000–10,000 Hz), which poses ques- systems would perform. the scientist and the Navy. In physi- tions on a variety of spatial and temporal In using autonomous vehicles, rapidly cal oceanography, a complex interplay scales. For low-frequency acoustics, it has and efficiently sampling the oceanogra-

among winds, rivers, tides, and local become obvious that fully three-dimen- phy, particularly the temperature field, ceanography S bathymetry drives a nonstationary, shelf- sional (spatial) oceanography is neces- so as to optimize the placement and break front and the nonlinear inter- sary for propagation prediction. A bit performance of acoustics systems is a nal wave (NLIW) field. These strongly more surprisingly, it appears that fully major technical hurdle to be cleared. The ociety, P affect acoustic systems but are not three-dimensional (spatial) acoustics Navy is increasingly interested in using 1931, R O Box adequately understood. A key oceano- codes might be necessary as well, a big AUVs and gliders for such work, as they graphic challenge is to model the fully divergence from the two-dimensional can operate persistently over large areas, ockville, MD 20849-1931, USA ockville, epublication, systemmatic reproduction, four-dimensional ocean from the large- slice between source and receiver that comparatively inexpensively. scale circulation down to fine scales, has been adequate for ocean acoustics to To pursue these questions, the which include NLIW packets, internal date. At medium frequency, the effects of Office of Naval Research (ONR) spon- tides, jets, and density fronts. Both Navy NLIWs on sonar systems are predicted sored a large, multidisciplinary, multi- .

156 Oceanography Vol. 20, No. 4 Figure 1. A graphic overview of the SW06 experiment. Moored instru- mentation is deployed in lines in the along-shelf and across-shelf directions to observe shelf-break front and internal wave pack- ets. A fully three-dimensional array at the intersec- tion of the “T” is designed to study the wave-front length scales of nonlinear internal waves. A fleet of six gliders monitors mesoscale oceanography in the region. An “L”-shaped array of hydrophones consists of a horizontal and a vertical line array, and monitors sound transmissions throughout the experiment, while moored and shipboard sources transmit sig- nals. Ships carry out oceanographic, geologic, and acoustic research while networked to each other and to laboratories ashore to share information. Planes and satellites overhead image internal waves and other ocean processes. The bathymetry in the figure is an artist’s rendition showing generally correct but not exact features.

institution, multinational experiment off real-time data communication from a (see Figure 1). The geometry and place- the coast of New Jersey in summer 2006, number of shipboard experiments, and ment of the T were arranged to provide designated “Shallow Water ’06” (SW06). dozens of principal investigators tending good along- and across-shelf views of This large-scale experiment had three to their parts of this coordinated effort. the shelf-break front, the local eddy named components corresponding to Two of the logistical components of field, and the nonlinear internal wave the three major research and technology this effort merit special attention due to field, three oceanographic quantities of thrusts: in acoustics, LEAR (for Littoral their sheer size and novelty: (1) moor- primary interest. A small (10 x 10 km2) Environmental Acoustics Research); in ing layout, construction, deployment, moored oceanography array was placed physical oceanography, NLIWI (the Non and recovery, and (2) real-time data and at the crossing of the T to look at fully Linear Internal Waves Initiative); and in experiment-status communications via three-dimensional (spatially) nonlinear vehicles, AWACS (for Acoustic Wide Area the SW06 Web site. internal wave structure. Coverage for Surveillance). The T geometry was very efficient for Moorings notifying fishermen and other mariners EXPERIMENTAL SETUP The core measurement suite of the SW06 where our equipment was located—we AND LOGISTICS experiment was an array of 62 acoustics simply had to specify the endpoints and Over its two-month duration (mid- and physical oceanography moorings crossing of the T. As a result, no gear July to mid-September), the SW06 (57 long term, 5 short term) that were was lost to fishing activity. Amazingly, experiment entrained enormous deployed in a “T” geometry, with the we recovered every mooring that we resources: seven ships, 62 moorings, stem of the T stretching 30 km along- deployed, with about 95% data return, aircraft overflights and satellite cover- shelf at the 80-m isobath, and the top of despite having one lithium battery pack age, a fleet of ten oceanographic gliders, the T stretching 50 km across-shelf from explode and one mooring drift out to sea data-assimilating numerical modeling, the 500-m isobath to the 60-m isobath after Hurricane Ernesto.

Oceanography December 2007 157 Web Site deployment; ship, glider and aircraft a linear wave equation, and (2) nonlin- New challenges in the area of oceano- tracks; daily reports; weather informa- ear internal waves that have short length graphic experiment logistics, real-time tion; CODAR (Coastal Ocean Dynamics scales (hundreds of meters), are charac- field communication, and data fusion Applications Radar) and satellite imag- terized by large particle velocities, and and exchange led to the creation of Web- ery; and ocean-model results. These Web follow nonlinear wave equations such as based software tools to enable coordi- tools allowed us to collect and organize the Korteweg-deVries equation. These nated, real-time collaboration between this information into a usable form, nonlinear waves have a large acoustical researchers at sea and ashore. SW06 and also provided a searchable, time- effect, and we now know that they can be researchers could monitor the location based archive of the entire experiment. found all over the world’s ocean shelves. of several ships, dozens of moorings, The development of these tools was an Indeed, they are nearly as common as surface waves, given stratified waters. Internal-wave activity on the New sW06 researchers could monitor the Jersey slope is particularly well docu- location of several ships, dozens of mented through both satellite synthetic aperture radar (SAR) imagery (Jackson moorings, assorted underwater vehicles, and Apel, 2004) and previous in situ and other platforms in near real time. experiments (Apel et al., 2006). SAR imagery clearly reveals the surface sig- natures of wave packets (Figure 2) that assorted underwater vehicles, and other important contribution to the experi- appear to originate near the shelf break platforms in near real time. Information ment, providing the means for using and propagate shoreward. But this pic- gathered off the Internet was merged research vessels on scientific expeditions ture is not a totally simple one. Irregular with information collected during oper- as parts of a real-time ocean observatory. bathymetry near the shelf break, such ations at sea and inserted into a basic as canyons, can produce circular wave framework to provide additional sup- PHYSICAL OCEANOGRAPHY fronts that combine with the planar ones port. A primarily wireless network com- (NLIWI) from the shelf break and with each other. prised of satellite, shipboard, and global One of the principal foci of the experi- Along the New Jersey coast, the ambi- Internet links was used to synchronize ment was internal waves. These come ent stratification is also complicated Web sites on five ships and multiple in two general types: (1) linear internal by the existence of a shelf-break front shore-based servers so that all partici- waves that include long-wavelength (tens (a salinity-density front/current com- pants in the experiment could contrib- of kilometers) entities such as internal bination that meanders near the shelf ute and monitor platform locations/ tides and near-inertial waves, and follow edge), the frequent presence of warm-

Dajun Tang is Senior Engineer, Applied Physics Laboratory, University of Washington, Seattle, WA, USA. James N. Moum is Professor, Oregon State University, Corvallis, OR, USA. James F. Lynch ([email protected]) is Senior Scientist, Woods Hole Oceanographic Institution, Woods Hole, MA, USA. Phil Abbot is President, OASIS, Inc., Lexington, MA, USA. Ross Chapman is Professor, University of Victoria, Victoria, British Columbia, Canada. Peter H. Dahl is Principal Engineer, Applied Physics Laboratory, University of Washington, Seattle, WA, USA. Timothy F. Duda is Associate Scientist, Woods Hole Oceanographic Institution, Woods Hole, MA, USA. Glen Gawarkiewicz is Senior Scientist, Woods Hole Oceanographic Institution, Woods Hole, MA, USA. Scott Glenn is Professor, Rutgers University, New Brunswick, NJ, USA. John A. Goff is Senior Research Scientist, University of Texas at Austin, Austin, TX, USA. Hans Graber is Professor, University of Miami, Miami, FL, USA. John Kemp is Senior Engineering Assistant II, Woods Hole Oceanographic Institution, Woods Hole, MA, USA. Andrew Maffei is Senior Information Systems Specialist, Woods Hole Oceanographic Institution, Woods Hole, MA, USA. Jonathan D. Nash is Associate Professor, Oregon State University, Corvallis, OR, USA. Arthur Newhall is Information Systems Specialist, Woods Hole Oceanographic Institution, Woods Hole, MA, USA.

158 Oceanography Vol. 20, No. 4 mesoscales to NLIW scales, represent- ing an ambitious drive to cover yet more of the full-ocean spectrum with data- assimilating models. From an observational perspec- tive, the primary objectives were (1) to understand and describe the two- and three-dimensional structures of NLIWs in sufficient detail to aid in improving wave models beyond small-amplitude wave theories that are inadequate but still in use, (2) to define the energy evo- lution and transport, and (3) to identify wave-generation sites, times, and mecha- nisms. These are all necessary for predic- tion of wave generation and propagation over the continental shelf. In addition, important efforts include a quantita- tive physical assessment of surface radar signatures in terms of small-scale sea- surface variability.

Background Oceanographic Conditions and Nonlinear Internal Waves in SW06 Figure 2. Radarsat observation from August 13, 2006. Part of the across-shelf mooring array is shown in the figure to provide scale. The structure of the While NLIWs appear like clockwork in nonlinear internal wave packets, including some pronounced curvature of some parts of the world’s ocean (like the wave fronts, is clearly visible. Understanding the correlation of these sur- the South China Sea; Ramp et al., 2004), face images with subsurface observations is one of the objectives of SW06. prediction of the nonlinear internal wave climate over continental shelves is noto- riously difficult. During SW06, NLIW packets were observed at all phases of the core eddies shed from the Gulf Stream, defining both the space/time variability barotropic tide. This variability in the strong freshwater influences, near-iner- of the sound-speed field and the propa- timing of waves is not unlike previous tial wave activity, and the frequent pas- gation of individual nonlinear internal continental shelf observations (Colosi sage of tropical storms. The complex- wave trains through the acoustics array. et al., 2001; Moum et al., 2007). More ity that all this variability induces into Numerous local shipboard measure- importantly, and entirely counter to our the internal wave field is important to ments were made to investigate the expectations, larger and more frequent acoustics. As a matter of physical ocean- evolving wave structure and energetics waves appeared during neap tides than ography/acoustics synergy, the suite of and to support short-range acoustics during spring tides (which peaked on moored instruments was designed not experiments. A considerable modeling 11 August). One of the more interesting only to focus on fundamental aspects of effort combining oceanography, acous- challenges posed by our data will be to internal-wave physics but also to sup- tics, and Navy needs augments these reconcile this curious observation with port the acoustics measurements by observations. These models encompass our initial expectations.

Oceanography December 2007 159 Observations Intensive moored observations were designed to capture both mesoscale oceanography and the high-frequency wave climate. Water-column velocity, temperature, and salinity were measured with ~10-m resolution from ten dedi- cated oceanography moorings. Five bot- tom landers were deployed at 70-, 80-, 85-, 110-, and 125-m water depths to measure near-bottom density, pressure, and turbulence as well as full-water-col- umn velocity structure at 5-s intervals. Two Air-Sea Interaction Spar (ASIS) buoys were deployed at 70- and 80-m water depths to measure atmospheric conditions, to quantify the surface grav- ity wave field, and to measure near- surface currents and density. Most instruments were sampled at 30-s inter- vals to resolve high-frequency waves. Multiple ships were used to chase wave packets across the continental shelf, hopefully from their birth sites to their death sites. Shipboard experiments to track evolving wave groups, including slaloming through the three-dimensional moored array cluster, were conducted from R/Vs Endeavor and Oceanus. These observations included acoustic Doppler current profilers sampled at high spa- Figure 3. Detailed acoustic backscatter, velocity (components shown are in the direction of wave propagation, u, and vertical, w), and turbulence (ε) observations of a nonlinear internal wave tial and temporal resolution (to map packet tracked by R/V Oceanus while propagating onshore over the New Jersey shelf during the NLIW velocity field in detail) and SW06. Two isopycnals are plotted over the u image. The two columns show two stages of wave- in situ measurements of temperature, packet evolution. During the 5-h separation between these observations, the wave propagated 17 km (wave speed ≅ 0.8 m s-1), or almost 100 wavelengths. The wave-packet evolution is sig- salinity, and small-scale turbulence. nificant; it changes from a well-defined set of radar wave fronts and an internal velocity/density Highly detailed depictions of wave structure that is clearly representative of solitary-like waves to a less-ordered structure marked by extreme turbulence at the base of its velocity core. This in turn is associated with what appear groups obtained (Figure 3) from these to be small-scale Kelvin-Helmholtz billows (as seen in the acoustic backscatter). These observa- data show the evolving structures of tions were made using shipboard acoustics and the Chameleon turbulence profiler.Figure cour- individual waves and how their relative tesy Emily Shroyer, Oregon State University positions within wave groups change as they propagate onshore. Shipboard wave tracking also provided clear observations of so-called “varicose” (mode-2) waves in a geophysical fluid (that is, waves that

160 Oceanography Vol. 20, No. 4 both elevate isopycnals above a density swept acoustics system of four transduc- the modelers is to represent the sound- interface and depress them below) and ers to remotely detect the spectrum of speed field from the mesoscale to the the change in sign from depression waves backscattering generated by small-scale small-scale fluctuations caused by the to elevation waves as they shoal. sound speed microstructure created by nonlinear waves. Shipboard observations also included turbulence instabilities within waves. X-band radar measurements of the sea This measurement augments profiling GEOLOGY (LEAR) surface (Figure 3). These records pro- observations of turbulence by providing The seabed forms the bottom bound- vided depictions of the wave fronts over a continuous sequence between profiles. ary condition for shallow-water acoustic 3-nm ranges from the ships, extend- It also helps to identify the detailed phys- and oceanographic experiments, such as ing our understanding of the shapes of ical structure of the waves. SW06. While understanding the acoustic the wave fronts beyond their points of High-resolution pressure sensors were properties of the seabed is of obvious intersection with the ships. Wave-front deployed on three bottom landers along importance to the acoustic experiments, shapes were tracked even further by the array. Results show that the seafloor and the bathymetry and roughness is aircraft radar flights and satellite SAR pressure signal of the NLIWs observed by important to the oceanography, we also observations. One of the challenges of other means during SW06 is measurable seek a geologic understanding of the NLIWI will be to obtain a first-order (Moum and Nash, in press). This rela- sediments and the stratigraphic geom- understanding of the structure of the tively simple measurement suggests new etries in which they were deposited. This evolving wave fronts. ways to design NLIW detection antennae. type of information provides context To monitor the critical mesoscale vari- for understanding the results and for ability, a suite of 10 gliders was deployed Modeling applying what we learned to other areas. with the goal of continually flying six of Modeling of currents and stratifica- The stratigraphic geometries can help these through the moored array, a goal tion (including sound speed) from the guide extrapolation of bottom inverse that was largely achieved. These observa- synoptic to the wave scale is ongoing. results from one location to other parts tions were augmented by Scanfish sur- At large scales, the Regional Ocean of the survey region. veys extending across the shelf and over Modeling System assimilated data from One of the big attractions of the the continental slope to provide a broad all platforms to provide real-time maps SW06 area was the fact that, in addition perspective of the variability of the shelf- of the relevant fields. At intermediate to having interesting physical oceanogra- phy, the seabed had been examined and characterized by previous experiments, The seabed forms the bottom boundary thus saving a large amount of effort and money. Any further geological work condition for shallow-water acoustic and and stratigraphic surveying would be oceanographic experiments, such as SW06. answering more detailed questions about the bottom, in accord with SW06’s status as a “higher-order effects” type of experi- break front and other mesocale varia- scales, the nonhydrostatic MIT general ment, not an initial survey. tions in currents, temperature, and salin- circulation model was nested within the Much of the previous geological work ity. Author Abbot and co-investigators larger model to provide clues as to how in this area was sponsored by ONR, are preparing a WHOI technical report the observed NLIWs are generated. At in large part as a response to a system on these surveys. the scale of the waves, a novel model of buried, remnant river channels just Two new types of observations were based on optimized mesh-point place- below the seafloor that were suspected to exploited to investigate the NLIWs. ment provides detailed simulations of be the sources of “geo-clutter”(false tar- One of these was deployed from a ship small-scale tide-topography interaction get backscatter returns due to geology) in and used a broadband (120–600 kHz) and wave generation. The challenge to Navy sonar systems. Such buried chan-

Oceanography December 2007 161 deposited as sea level was falling in the lead-up to the last glacial maximum. In some locations, most notably the cen- tral site of the SW06 experiment, the top of the layered outer shelf wedge is very erose, cut by grooves oriented ~ east- west, and then filled with sediments that are acoustically transparent (Fulthorpe and Austin, 2004). This surface may be evidence of some sort of catastrophic flood event, or perhaps the grooves were created by icebergs grounding along an ancient seabed. New core samples should help us to resolve the origin of this boundary. At some time during the last glacial Figure 4. Schematic representation of stratigraphic elements along the strike-and-dip lines maximum, while sea level was much (Figure 5) of the SW06 experiment. The strike of a stratum is a line representing the intersection of that feature with the horizontal; the dip is the angle below the horizontal of a tilted stratum. lower and almost the entire continental The star represents the approximate location of the “cross-hairs” of the SW06 experiment. shelf was exposed, dendritic networks of river channels were carved into the mid-shelf wedge (the “C” horizon in Figure 4) and, where the wedge thinned, through R as well. When sea level subse- nels are quite common on continental overall geologic history of the SW06 quently rose following the melting of the shelves, and so the New Jersey shelf pro- region has grown immeasurably ice-age glaciers, the channels were filled vided a natural laboratory for looking (e.g., Duncan et al., 2000; Goff et al., by a sequence of estuarine sediments: a at this effect. The interested reader can 2005; Gulick et al., 2005; Nordfjord fluvial lag of sand and gravel at the base, find the history of this fascinating work et al., 2006), major gaps still remain. followed by muddy sediments deposited in the following references: initial map- The “R” horizon is the most prominent where the river met the tide, followed ping of fluvial channels (Davies et al., and widespread feature in the shallow by beach barrier sands (Nordfjord et al., 1992); multibeam surveys and bathym- subsurface (e.g., Milliman et al., 1990; 2006). These sediments, ~ 12,000 years etry (Goff et al., 1999); geophysical and Gulick et al., 2005; Figure 4). We know old in the SW06 region, form recogniz- geological sample data (e.g., Goff et al., from sampling and age dating that it is able layers in the chirp seismic record of 2004; Nordfjord et al., 2005, 2006); and approximately 40,000 years old, but the the channel-fill sediments. Continued long-coring efforts (Nordfjord et al., reasons for its existence are still the sub- sea-level rise caused the shoreline to pass 2006). As a postscript, it was found that ject of much speculation. The new SW06 over the buried channels, creating a wide- the “geo-clutter,” though appreciable, chirp data and future long coring should spread erosion surface referred to as the was often swamped by “bio-clutter” (i.e., provide critical new morphologic and “T” horizon. Large sand bodies formed reverberation returns from schools of stratigraphic constraints on the origin in the nearshore environment on top of fish on the shelf and often near the shelf of R. Lying on top of R is the outer-shelf T, and these were shaped into large bed- break [Makris et al., 2006]). A large ONR wedge, composed of alternating layers of forms (~ 1–5 m high, 1–4 km wide, and experiment to study this type of clutter sand and mud that are imaged as numer- 2–10 km long) called sand ridges. These was initiated as a result. ous layered reflections in chirp seismic features are so large that they persist to Though our understanding of the data. These sediments were most likely the present day and can be easily mapped

162 Oceanography Vol. 20, No. 4 in the multibeam bathymetry data. But Low-Frequency Propagation which allowed us to fill in the azimuthal the seabed does not become quiescent In doing the acoustics transmissions, propagation direction spectrum. with increasing water depth. Evidence we used both moored and shipboard There are a number of open questions in the multibeam and chirp data sug- sources and receivers. The primary about low-frequency, shallow-water gests widespread regions of recent and moored receiver site for propagation propagation. One of the most interesting modern erosion, which has removed the studies was at the along/across-shelf of these concerns the azimuthal depen- sand sheet in many places, exposing both intersection of the T-shaped mooring, dence of shallow-water sound propaga- the T and the R horizons at the seafloor. as that is where we have the best oceano- tion, specifically its strong dependence The result is a seabed with highly vari- graphic information. At that site, a long- upon the angle between the acoustic able sedimentary and physical properties, term horizontal/vertical line array with path and the propagation direction of which will be of great importance to the 48 hydrophone channels was deployed, the nonlinear internal wave field (when SW06 acoustics experiment results. sampling sound from 50–4500 Hz con- present). Sound can travel readily in the tinuously over a six-week period. This acoustic duct that is created between ACOUSTICS (LEAR) array was augmented by other receiv- individual soliton packet waves, in a Though acoustics was the largest over- ers, (mostly) near the same site, as well manner that keeps the acoustic modes all component of SW06, it makes sense as five long-term “single hydrophone separate and distinct. This leads to to discuss it after the oceanography receiver units” distributed along the higher-intensity regions of the acoustic and geology, as these set the context across-shelf line. The moored sources field, as seen in the top panel of Figure 5. for acoustic propagation and scatter- were arranged on the along- and across- On the other hand, sound that travels ing. The SW06 experiment had both shelf lines as well, as these are where we perpendicular to the propagation direc- low-frequency (100–1000-Hz) and had the best environmental support, tion of the nonlinear internal waves suf- medium-frequency (1000–10,000-Hz) and, also, these were “limiting cases” for fers intense coupling between the modes, components, and examined issues in the azimuthal variations of the acous- which can lead to the severe attenuation forward propagation, scattering, and tic path. These moored sources were of sound (depending upon where one inverse theory. augmented by some shipboard sources, places the source relative to the sur-

Figure 5. The top panel shows, in a top view, a computer model of ducting of sound between nonlinear internal wave crests (denoted by white dashed lines), a known and experimen- tally observed phenomenon. The bottom panel shows a top view of the ducting and horizontal dispersion of sound between two curved nonlinear internal wave crests, an effect that should be observable and provable using the SW06 data set. Color gradation denotes (relative) intensity in dB.

Oceanography December 2007 163 face mixed layer and thermocline.) At and water column. This influence can be MORAY1 receiving array from a source angles intermediate to these extremes, in the form of sound scattering, refrac- deployed from R/V Knorr, within the a combination of these effects occurs, tion, and attenuation. Uncertainties in frequency range 1–20 kHz. A second and we presently have a rather limited any or all of these categories are com- measurement transmitted both mid- and ability to predict the acoustic field for mon, so knowing the various mecha- low-frequency sound (0.05–4 kHz) at a this case due to both oceanographic nisms governing acoustic interaction range of 200 m around a circular path and acoustic complexity. with the environment is essential to encompassing the MORAY1 location. Another interesting question about improving sonar performance in shallow A new sediment acoustics measure- acoustic propagation in shallow water water. To gain quantitative understand- ment system (SAMS) was developed to is how much energy is reflected out ing of how sound interacts with these measure in situ sediment sound speed to of plane by internal waves and coastal environmental factors, it is essential a depth of 1.7 m with 0.1 m depth incre- fronts when the acoustic path is at low to measure all relevant environmental ments. This is accomplished by using a (grazing-angle) incidence to these ocean parameters at sufficient spatial and tem- vibro-core fitted with a hydrophone at features. The interference of a straight poral resolutions, which can be quite fine its tip to penetrate into the bottom in acoustic track (that does not interact for mid-frequency acoustics. controlled depth-steps while 10 acoustic with these ocean features) and a reflected The mid-frequency efforts of SW06/ sources located above the bottom trans- path can cause very large, fully three- LEAR were concentrated on receiving mit sound in the band of 2–35 kHz. This dimensional acoustic fluctuations, as arrays moored in the central area, where source-and-receive arrangement provides have been seen in numerous computer the water depth is about 80 m. Five many ray paths crisscrossing the sedi- simulations. Verification (or refutation) major measurements were made there: ment volume, so sediment sound speed of such effects is an interesting experi- (1) short-range acoustics interaction as a function of depth is directly imaged. mental question, and data from SW06 with the bottom and surface, (2) in situ Bottom backscatter in the frequency should shed light on this. Yet another measurement of sediment sound speed band of 3–10 kHz was measured by question of great interest to LF acoustics to a depth of 1.7 m, (3) bottom backscat- both a parametric source and by an is how the three-dimensional irregularity ter, (4) short-range propagation through omnidirectional source combined with of fronts and the curvature of nonlinear known internal waves, and (5) long- a 32-element, vertical-line hydrophone internal waves (which are often taken as range propagation to 10 km along the array. Supporting this effort, sediment two-dimensional, along-shelf symmet- ric entities to first order) affect acoustic propagation. There is an example of curvature effects in the bottom panel of A major objective in SW06 was to estimate Figure 5. These curvature effects can lead the regional geoacoustic environment using to both horizontal ducting and horizon- tal dispersion of sound rays (or “light the best present-day inversion techniques. pipe” and “rainbow” effects in optical parlance), and, in terms of Navy systems, false target bearings, a major concern. same track with multiple source depths, roughnesses were measured using a frequency bands, and different times. sediment conductivity probe that can Medium-Frequency Propagation To study bottom and surface reflec- measure one-dimensional roughness Many practical sonar systems operate in tion coefficients and their anisotropy, to a resolution of 10 mm in the hori- the mid-frequency band between 1 and short-range bottom/surface interaction zontal direction and 1 mm in the verti- 10 kHz. In shallow-water regions, the measurements were first made at ranges cal. In addition, a laser scanner was also performance of such systems is strongly from 50 m to 1000 m from multiple deployed to provide two-dimensional influenced by the bottom, sea surface, directions with respect to the moored roughness measurements to a resolu-

164 Oceanography Vol. 20, No. 4 tion of 1 mm in both the horizontal tom, especially at sites where there was dealing with a large component of water- and vertical directions. extensive geological “ground truth” of column fluctuation noise in the data. The effect of nonlinear internal wave the ocean bottom. The technical chal- However, it is completely unrealistic to trains on mid-frequency sound propa- lenge here is to perfect acoustic survey ask inverse methods to present an exact gation was measured with the goal of techniques for bottom properties, which, realization of the ocean between the making deterministic assessment of the despite much work over decades, are source and receiver. Recently developed impact by taking “snap shots” of the still being refined and improved. Of methods minimize the effects of water- internal waves along the acoustic path. equally strong interest to marine sedi- column fluctuations on such inverses, This was accomplished by a transmission from a source deployed from R/V Knorr to the MORAY1 receiving array stationed 1000 m away. A CTD chain with 50 ele- two of the most active and promising ments was towed at 6 kt in the vicinity of technologies in oceanography are the source and receiver by a second ship, powered AUVs and AUV gliders. R/V Endeavor. Both acoustics and CTD- chain measurements were made in the presence and absence of internal waves, so the impact of internal waves on mid- ment acousticians is the opportunity to based on knowing only the simple statis- frequency sound propagation can be measure the sediment sound speed dis- tics of water-column fluctuations. As the quantitatively evaluated persion (real and imaginary parts) over a SW06 experiment provides both high- Finally, mid-frequency, long-range broad frequency band. quality measurements of ocean fluctua- propagation to a 10-km range along the The most intensively studied site was tions and independent data on ocean- same track on different days with dif- the central area at 80-m water depth. bottom structure and properties, this ferent bandwidths was studied. This As mentioned, this site was first instru- method can be tested with some rigor. research focused on the temporal vari- mented with a mid-frequency hydro- Many teams investigated the bot- ability of propagation loss, and also phone array to measure acoustic reflec- tom-inversion issue using different tech- bottom parameter inversion and mode tivity from the seabed at frequencies from niques over a wide range of frequencies stripping due to surface scattering. 1–20 kHz. On a second visit to the site, in this experiment. The combination of we studied sound reflectivity and acoustic all the inversion results offers an oppor- Low- and Medium-Frequency propagation at lower frequencies, from tunity, through a synthesis process, to Bottom Inverses 4 kHz down to 50 Hz. The combined obtain a consistent bottom model of Underwater acousticians often use “geo- set of mid- and low–frequency data and the area that can be used to evaluate acoustic models” (local sediment layers accompanying measurements on geo- future sonar models. described by their density, compressional acoustic properties will provide a unique and shear wave speeds, and attenua- opportunity to both compare techniques AWACS tions) to characterize the bottom for and study dispersion properties. Two of the most active and promising making predictions of the sound levels Another of the current research chal- technologies in oceanography are pow- in the sea. A major objective in SW06 lenges in doing bottom geoacoustic ered AUVs and AUV gliders. As part was to estimate the regional geoacoustic property inversions, particularly at low of SW06, a component of the AWACS environment using the best present-day frequencies where there are large path (Acoustic Wide Aperture Coverage for inversion techniques. Several groups lengths between source and receiver, is Surveillance) project was conducted to collaborated in experiments that used to eliminate water column fluctuation utilize both powered and glider AUVs. sound sources over a broad band of effects from the inverse for the bottom. The objectives were to: (1) conduct frequencies to characterize the bot- Use of inverse methods often requires oceanographic surveys of an “interest-

Oceanography December 2007 165 Figure 6. (a) The top panel shows tracks of Oasis mobile acoustic sources (large red and blue circles) emitting sounds to sonobuoy receivers (darker red and blue arc-segments within the circles) in the vicinity of the shelf-break front, in order to study the effects of frontal and shelf oceanography on sound propa- gation. (b) The bottom panel displays average acoustic propagation loss at 900 Hz and clearly shows: (1) near azimuthal isotropy over the shelf region, and (2) anisotropy (more loss) where the acoustic paths cross the shelf-break front, in the angular sector from (roughly) 90 to 180 degrees. Prediction of the posi- tion of the shelf-break front was one of the oceanographic challenges of AWACS.

ing area” of ocean (e.g., near the shelf- veys from the ship for oceanography and bytes of data collected. We have made break front, or in a packet of solitons), Oasis mobile acoustic sources and sono- movies of acoustic arrivals at the arrays (2) create an oceanographic map of that buoy receivers for the acoustics. With this that graphically show their stability in area based on the data, (3) pick a geom- hurricane-induced “Plan B,” we were still oceanically calm periods and their gyra- etry most favorable for acoustic propa- able to perform the “adaptive acoustic tions during active periods. In addi- gation/detection based on that oceano- sampling” part of the project. This suc- tion, motion pictures of the nonlinear graphic map, and (4) use moored and ceeded well, with especially interesting internal waves recorded by our “three- sonobuoy-based acoustic sources and results coming from trying to predict the dimensional” mooring array are just receivers to demonstrate that the geome- shelf-break front and its acoustic effects, coming into being. This graphic imag- try so chosen was indeed an optimal one. as illustrated in Figures 6a and 6b. In ery, and much more analysis (some In pursuing this plan, small-boat these figures, we show the anisotropy in not quite so flashy), are providing the operations were needed for deploying acoustic propagation loss versus azimuth grist for a number of scientific publi- and recovering the small REMUS 100 for propagation in the region of the cations. We have already lined up spe- AUVs and Webb gliders that were to be shelf-break front. The anisotropy effects cial sessions in a number of technical utilized. Unfortunately, the fringes of due to fronts and internal waves were of society meetings and a special issue in hurricanes Ernesto and Florence prohib- particular interest in SW06. the Journal of the Acoustical Society of ited small-boat operations. (But fortu- America. We hope that this “prelude” nately for future programs, this incon- Postscript in Oceanography will stimulate readers’ venient small-boat technique has since In the year that has passed since the interest in the further publications that been superseded by fully “off the deck” SW06 experiment, great progress has will be coming out shortly from this operations!). Thus, we used Scanfish sur- been made in digesting the many tera- very successful experiment.

166 Oceanography Vol. 20, No. 4 Acknowledgements Alexander. 2005. Recent and modern marine ero- sion on the New Jersey outer shelf. Marine Geology First, we would like to thank our 216:275–296. ONR sponsors, particularly Drs. Ellen Goff, J.A., B. Kraft, L.A. Mayer, S.G. Schock, C. K. Livingston, Theresa Paluszkiewicz, and Sommerfield, H.C. Olson, S.P.S. Gulick, and S. Nordfjord. 2004. Seabed characterization on the Tom Curtin for their support of the New Jersey middle and outer shelf: Correlability SW06 experiment, and their help in and spatial variability of seafloor sediment proper- ties. Marine Geology 209:147–172. coordinating its efforts. Next, we would Goff, J.A., D.J.P. Swift, C.S. Duncan, L.A. Mayer, and J. like to thank the many co-PIs whose Hughes-Clarke. 1999. High-resolution swath sonar names do not appear on this paper investigation of sand ridge, dune and ribbon mor- phology in the offshore environment of the New (only direct contributors appear), but Jersey margin. Marine Geology 161:307–337. whose efforts and research in SW06 are Gulick, S.P.S., J.A. Goff, J.A. Austin Jr., C.R. Alexander Jr., S. Nordfjord, and C.S. Fulthorpe. 2005. Basal the topic of this paper. Third, we would inflection-controlled shelf-edge wedges off New like to thank the numerous technicians, Jersey track sea-level fall. Geology 33:429–432. Jackson, C.R., and J.R. Apel. 2004. An atlas of internal engineers, programmers, office person- solitary-like waves and their properties. Global nel, students, and others whose efforts Ocean Associates. 2nd ed. Available online at: http:// underpinned this large effort. Finally, the www.internalwaveatlas.com. Makris, N.C., P. Ratilal, D.T. Symonds, S. Jagannathan, captains and crews of the research ships S. Lee, and R.W. Nero. 2006. Fish population and that participated in SW06 get our hearti- behavior revealed by instantaneous continental shelf-scale imaging. Science 311:660–663. est praise and thanks. Overall, SW06 was Milliman, J.D., Z. Jiezao, L. Anchun, and J.I. Ewing. executed by a very talented, hardwork- 1990. Late Quaternary sedimentation on the outer ing, and collegial group, and they made and middle New Jersey continental shelf: Result of two local deglaciations? Journal of Geology both the experience, and the results, 98:966–976. fantastic. We all look forward to the next Moum, J.N., J.M. Klymak, J.D. Nash, A. Perlin, and W.D. Smyth. 2007. Energy transport by nonlinear few years of continued work together on internal waves. Journal of Physical Oceanography. this vast, rich data set. Moum, J.N., and J.D. Nash. In press. Seafloor pressure measurements of nonlinear internal waves. Journal of Physical Oceanography 37:1,968–1,988. References Nordfjord, S., J.A. Goff, J.A. Austin Jr., and S.P.S. Apel, J.R., L.A. Ostrovsky, Y.A. Stepanyants, and J.F. Gulick. 2006. Seismic facies of incised valley-fills, Lynch. 2006. Internal solitons in the ocean. WHOI New Jersey continental shelf: Implications for Technical Report WHOI-2006-04. erosion and preservation processes acting during Colosi, J.A., R.C. Beardsley, G. Gawarkiewicz, J.F. late Pleistocene/Holocene transgression. Journal of Lynch, C.S. Chiu, and A. Scotti. 2001. Observations Sedimentary Research 76:1,284–1,303. of nonlinear internal waves on the outer New Nordfjord, S., J.A. Goff, J.A. Austin Jr., and C.K. England continental shelf during the summer Sommerfield. 2005. Seismic geomorphology shelfbreak PRIMER study. Journal of Geophysical of buried channel systems on New Jersey shelf: Research 106(C5):9,587–9,601. Assessing past environmental conditions. Marine Davies, T.A., J.A. Austin Jr., M.B. Lagoe, and J.D. Geology 214:339–364. Milliman. 1992. Late Quaternary sedimentation off Ramp, S.R., T.Y. Tang, T.F. Duda, J.F. Lynch, A.K. New Jersey: New results using 3-D seismic profiles Liu, C-S. Chiu, F.L. Bahr, H.-R. Kim, and Y.-J. and cores. Marine Geology 108:323–343. Yang. 2004. Internal solitons in the northeastern Duncan, C.S., J.A. Goff, and J.A. Austin Jr. 2000. South China Sea, Part I: Sources and deep water Tracking the last sea-level cycle: Seafloor mor- propagation. IEEE Journal of Ocean Engineering phology and shallow stratigraphy of the latest 28(4):1,157–1,181. Quaternary New Jersey middle continental shelf. Marine Geology 170:395–421. Fulthorpe, C.S., and J.A. Austin Jr. 2004. Shallowly buried, enigmatic seismic stratigraphy on the New Jersey outer shelf: Evidence for latest Pleistocene catastrophic erosion? Geology 32:1,013–1,016. Goff, J.A., J.A. Austin Jr., S. Gulick, S. Nordfjord, B. Christensen, C. Sommerfield, H. Olson, and C.

Oceanography December 2007 167