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Acoustic Sensing for Ocean Research

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Acoustic Sensing for Ocean Research PAPER Acoustic Sensing for Ocean Research AUTHORS ABSTRACT Bruce M. Howe Ocean observatories have the potential to examine the physical, chemical, biological, Applied Physics Laboratory, University of and geological parameters and processes of the ocean at time and space scales previ- Washington ously unexplored. Acoustics provides an efficient and cost-effective means by which these James H. Miller parameters and processes can be measured and information can be communicated. Inte- Department of Ocean Engineering, grated acoustics systems providing navigation and communications and conducting acous- University of Rhode Island tic measurements in support of science applications are, in concept, analogous to the Global Positioning System, but rely on acoustics because the ocean is opaque to electro- magnetic waves and transparent to sound. A series of nested systems is envisioned, from small- to regional- to basin-scale. A small number of acoustic sources sending coded, low power signals can service unlimited numbers of inexpensive receivers. Drifting and fixed receivers can be tracked accurately while collecting ocean circulation and heat con- tent data (both point and integral data), as well as ambient sound data about wind, rain, marine mammals, seismic T-phases, and anthropogenic activity. The sources can also transmit control data from users to remote instruments, and if paired with receivers en- able two-way acoustic communications links. Acoustic instrumentation that shares the acoustic spectrum completes the concept of integrated acoustics systems. The ocean observations presently in the planning and implementation stages will require these inte- grated acoustics systems. INTRODUCTION as NEPTUNE (NEPTUNE Partners, ics, geodesy, hot vents, gas hydrates). Tech- ecent advances in oceanography have 2000), and enhanced coastal observatories nical questions about the navigation and prompted investigation of integrated acous- (Jahnke et al., 2004). A unifying theme for communications are discussed with thoughts Rtics systems for ocean observatories (IASOO, the OOI infrastructure investment is the about implementation, followed with con- 2002) that include navigation, communi- provision of seafloor junction boxes pro- cluding remarks. cations, and acoustical oceanography. Such viding power and communications. The an integrated system may be considered a expectation is that the OOI request for combination of Underwater GPS (UGPS), major research equipment and facilities 2. Science topics acoustic communications, and acoustic sen- centers will be included in the 2006 U.S. sors that support science. The purpose of federal budget. This investment (equivalent 2.1 Ocean Circulation this paper is to investigate the evolving con- to several large ships) will, with planned The use of neutrally buoyant and profil- cepts, primarily within the context of sci- increases, lead to a concomitant large sci- ing floats has been a cornerstone of ocean- ence scenarios, and to describe briefly some ence investment. ography for the last half-century. In the late technical aspects. We examine first some science scenarios 1950s Swallow floats misbehaved by mov- A major motivating factor is the recently (not meant to be exclusive) and the integra- ing at “high” speed (10 cm s-1) in seemingly created National Science Foundation tion of these ideas into the infrastructure of random directions rather than slowly in a (NSF) ORION (Ocean Research Interac- planned research oriented ocean observatory straight line (baffling the crew on the sail- tive Observatory Networks) program and and operationally oriented observing system boat trying to track them acoustically), thus the embedded Ocean Observatories Initia- efforts. We explore applications of acoustics awakening oceanography to the presence of tive (OOI) to further the sustained study in the areas of ocean circulation (currents, the mesoscale “weather.” A detailed descrip- of the ocean (Clark, 2001). The three in- heat content, turbulence and mixing, wind tion of the history of acoustically tracked frastructure elements of the latter consist and rain), the living ocean/ocean biosphere floats can be found at http:// of a coarse global array of buoys (DEOS, (fisheries, marine mammals, nekton), and www.po.gso.uri.edu/rafos/general/history/ 2001), a regional cabled observatory such seafloor processes (earthquakes, plate tecton- index.html. 144 Marine Technology Society Journal Since then it has become abundantly clear temperature and salinity data and to obtain chored sound sources provide an acoustic that the ocean circulation needs to be mea- a satellite navigation fix. During the time navigation system whereby precisely timed sured on all scales, from global and basin submerged the float is untracked. Accord- acoustic signals (an 80-second long 3-Hz scales that are climatically relevant down to ing to Davis and Zenk (2001): wide FM sweep at 260 Hz) spread out radi- the small scales that are important to deter- ally from each of the sources. Given the ar- mining elusive mixing processes. There are Some studies place a high premium rival times at a receiver equipped with a syn- complementary observing technologies that on using floats to represent fluid-parcel chronous clock, the receiver’s position can are relevant—satellites and radars for the sur- trajectories requiring the uninterrupted be determined to an accuracy of a few kilo- face (not considered here), and Lagrangian current following that can be achieved meters. Since the mid-1980s floats have been platforms and fixed Eulerian sensors for in only with acoustic tracking. The pen- deployed in many different projects world- situ point measurements, both of which can alty for autonomous [profiling] opera- wide to study ocean currents. be spanned by acoustic tomography. tion is a long time interval between The floats, also known as RAFOS floats known positions, which precludes re- (SOFAR spelled backwards), can be deployed Fixed small scale measurements The solving eddies unless cycling is rapid, to drift at any depth although greater acous- acoustic Doppler current profiler (ADCP) and periodic surfacing that interrupts tic ranges are possible if both the sound is now a ubiquitous oceanographic instru- the quasi-Lagrangian trajectory. For sources and the floats are not too far off the ment. It can measure profiles of velocity observations of subsurface velocity, it is sound channel axis. The floats record the ar- (more precisely the velocity of scatterers fol- likely that both the continuously rival times in their microprocessor memory. lowing the water) at ranges up to about 1 tracked neutrally buoyant RAFOS At the end of each float’s mission underwa- km. Because there are no moving parts, the floats and autonomous floats will be ter, it surfaces and telemeters all data back to instrumentation is robust and reliable. More needed. For acoustic floats a major shore. Typical missions last from months to recently the acoustic travel time current limitation to economical sampling can several years. Special float designs have been meter (ACM) has been used on scales of 0.1 be overcome by widespread deployment developed whereby a float surfaces briefly to to 1 m. The acoustic scintillation current of high-energy sound sources. It is, for telemeter its data before returning to depth meter (ASCM) has been developed to pro- example, entirely feasible today to in- to continue its underwater mission. To make duce average measurements of cross-beam stall enough sound sources that a float the floats mimic fluid motion accurately, they velocity averaged along the path, typically could be continuously tracked anywhere can be configured to drift with the waters about 1 km. These various acoustic current in the tropical or North Atlantic. Since both horizontally and vertically; these are measurement devices have largely replaced sound sources, like radio stations, can known as isopycnal floats. mechanical current meters. serve different users, the presently rather In regions of high shear, profiling floats Inverted echosounders (IESs) measure simply structured network of moored can give erroneous estimates of velocity if the round-trip travel time from the seafloor sound sources will require greater in- they are based solely on surface fixes. Simi- to the ocean surface. This is a measure of ternational coordination in the future. larly, if trying to measure abyssal currents, the integrated, averaged sound speed (~tem- The benefits of an organized RAFOS estimates can be severely compromised if the perature) along this path. When IESs are network will be linked with the respon- float comes to the surface. In these cases it is combined with bottom pressure and a ref- sibility of contributing parties to main- advantageous to remain submerged. The erence absolute velocity (such as a point- tain such arrays of sound sources over same is true for other mobile instrument ACM or an electrometer that gives an extended period of time on a basin- platforms such as gliders, autonomous un- barotropic velocity), much can be learned wide scale. Miniaturization of receiver dersea vehicles (AUVs), and bottom rovers. about the low mode ocean velocity and den- electronics and production in great With UGPS (using low amplitude, long sity structure (Meinen et al., 2002). These numbers could result in significant de- duration, coded, high bandwidth signals, sensors provide fundamental information cline in float prices. This, and the de- and new low power clocks), the precision about
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