OF ICE FLOES
6.4. SUBSURFACE FLOATS TRANSMITTING TO MOORED
displacement combined to form an ellipse. Elaborate de- ployment of Swallow floats as outlined gave us our first glimpse of the details of deep-water motions and thus contributed significantly to our understanding of several major oceanic subsurface current systems.
While Swallow was engaged in this pioneering devel- opment at the National Institute of Oceanography (later renamed the Institute of Oceanographic Sciences) in the United Kingdom, it seems that the neutrally buoyant float concept had also developed quite independently and simultaneously on the other side of the Atlantic (Gould, 2005).Stommel (1955)had called for direct measurement of deep currents and had suggested that it might be done using subsurface neutrally buoyant floats. However, his idea was that they should be tracked through the Sound Fixing and Ranging (SOFAR) channel by the floats creating regular explosions!
According to Gould (2005), at some juncture before 1966, the method of float tracking changed from hydro- phones mounted on a ship’s hull to two pairs of hydro- phones, each pair towed on a cable deployed from the ship’s quarter. The “square” of hydrophones separated by about 100 m fore and aft and by the breadth of the ship could be towed at speeds of 3e4 knots (limited by ship noise and hydrodynamic noise) and could determine at what stage a float was abeam of the hydrophone array. This method significantly reduced float-tracking time.
A limitation of Swallow’s method is that one ship can follow only a very small number of floats at one time. If several floats are released, e.g., at different depths, it is quite likely that they will perverse and drift off in different directions and the ship will not be able to keep track of them for long. Nevertheless, Swallow floats were really the first oceanographic devices to give us reliable information on the speed and direction of deep currents, and some of the results obtained in this new area have been unexpected.
Although several techniques have been proposed for direct measurement of subsurface currents, the most powerful and widely used, perhaps, has been the Swallow float. Further advancements in technology permitted tracking of these neutrally buoyant floats without a ship.
Other than Swallow, only a very small number of researchers used Swallow floats before the late 1960s.Gould (2005)reckons that this was perhaps attributable to a degree of ”mystique” about float tracking engendered by Swallow’s attention to detail in float preparation and the difficulties of float tracking using low-energy acoustic transmitters. This early limited use of floats contrasts with the larger number of laboratories that were measuring currents using moored instruments at that time. However, because of technological limitations of that era, such as power consumption and data storage capacity, moored current meters were also difficult to use for any period longer than weeks. Consequently, most users were confined to the shelf seas.
6.4. SUBSURFACE FLOATS
buoyant subsurface floats that are outfitted with acoustic pingers and released into the oceanic SOFAR channels.
The SOFAR channel, which is an acoustic wave guide, owes its existence to the fact that the speed of sound rea- ches a minimum value at a certain depth in the ocean. This is because the speed of sound is a function of pressure and temperature (and salinity, to a lesser extent). The speed of sound below the warm surface waters of the ocean decreases with increasing depth due to decreasing temperature. Below the main thermocline, where the thermal gradients become small, the effect of hydrostatic pressure on the speed of sound becomes dominant and thus increases as the depth is further increased. In the western Sargasso Sea, the minimum sound speed is 1,4922 m/s and is located at 1,200200 meters depth. The accuracy with which a float can be located depends directly on the stability of this minimum.
The significance of the minimum velocity becomes clear when it is realized that sound rays that are radiated within certain angles from the horizontal from a source at the depth of the minimum velocity (the sound channel axis) will be refracted back toward this depth. This acoustic energy, trapped in the vertical, radiates horizontally with a geometric rate of attenuation proportional to 1/R, where R is the distance, instead of 1/R2,as in the case of a spherical radi- ator. This low rate of attenuation of acoustic signals propa- gating in the SOFAR channel permits acoustic signals to be detected at great distances from the source. In addition to the geometric attenuation, there is also the frequency-dependent attenuation due to scattering and absorption. Thus, the acoustic signal is attenuated much more rapidly at higher frequencies. For this reason, better reception and greater ranges are obtained by transmitting at as low a frequency as possible. The dual virtues of vertical trapping and low rate of attenuation of acoustic energy in the SOFAR channels permit hydrophones to track the SOFAR floats drifting at far- off distances from the hydrophones.
This application of the SOFAR channel was suggested byH. Stommel (1949, 1955). He suggested that the basic instrument required for this purpose is the oceanographic equivalent of the meteorological constant-altitude balloon:
an unmanned subsurface buoy or float, devised so as to float at a nearly constant depth or along an isopleth of temper- ature or density and equipped with a clockwork designed to drop and/or fire SOFAR charges at certain predetermined times, say once a week, for periods up to half a year. The trajectory of each buoy could be determined from the times of arrival of the explosive sound wave at three SOFAR stations. He predicted that if this method proves feasible, the way will then be clear for a major advance in physical oceanography.
Encouraged by this suggestion, Rossby and Webb (1970)initiated an observational program to study abyssal water motions by tracking subsurface floats that were
released in the SOFAR channel, and the first Swallow float was launched in the SOFAR channel in January 1968.
These floats are now known asSOFAR floats.
SOFAR floats are similar in concept to the original Swallow floats, which have been used with great success.
However, the technology for SOFAR floats is much more demanding in terms of reliability, economy of power, batteries with high energy density per unit weight, a low- frequency (preferably less than 500 Hz) acoustic pro- jector with low weight and high efficiency, and corrosion resistance. These special demands of SOFAR floats arise because they act as acoustic beacons for tremendous distances (more than 1,000 km), which must operate over long periods of time (over a year).
Usable acoustic ranges that can be achieved depend largely on ambient acoustic noise. Unfortunately, the ocean is not noiseless. Ships, waves, rainfall, earthquakesdall generate noise in various parts of the frequency spectrum.
This imposes a limit on what signals can be detected. Most noise is due to surface agitation, although ships can cause serious interference at frequencies below 400 Hz. Acc- ording to Rossby and Webb (1970), the sea noise to be expected near Bermuda, Eleuthera, and Puerto Rico in the Atlantic Ocean is largely predictable from knowledge of wind conditions.
It has been found that useful acoustic ranges in excess of 1,000 km are possible for tracking SOFAR floats. In this method, at known time intervals, each float transmits a low- frequency acoustic signal. From the time of arrival of this signal at several widely spaced hydrophones (at ranges of approximately 500 to 1,000 km), the positions of the floats can be determined to within a few kilometers.
The SOFAR floats have been tracked by the use of two receivers separated by a large distance. If the travel times of an acoustic pulse from the SOFAR float to these two receivers are known, then the distance between the float and each receiver can be computed from ana prioriknowledge of the mean sound velocity in that ocean basin. The position of the float will be one of the two intersection points of the two circles whose centers are the positions of the two receivers and whose radii are the two pulse-travel distances between the float and the two receivers (see Figure 6.6).
FIGURE 6.6 Method of tracking a SOFAR float with the aid of two acoustic receiversR1andR2.
184 Measuring Ocean Currents
The ambiguity (i.e., which of the two intersecting points on the two circles represents the true position of the float) can be resolved by knowing its previous location or the location where it was launched. If the pulse-travel time is to be determined precisely, the time of transmission of the acoustic pulse from the float must be known precisely. For this purpose, pulses are transmitted from the float at pre- determined times. This means that the clock in the float must be extremely precise. The crystal clocks are known to drift with time. To circumvent this difficulty, three receivers are usually used rather than two. In this case the float position can be determined even if the clock in the float gains or loses time slightly. This is because of the fact that when three receivers are used, the difference in time of arrival between two pairs of receivers gives two hyperbolae (on a sphere), the intersection of which is the location of the float (Rossby and Webb, 1970). Because the “difference” in time of arrival is used, any error due to gain or loss of time (which is common to both of the received signals) gets cancelled out. To get sufficient data for this purpose, many pulses (usually a set of 100) are transmitted 1 minute apart every few hours. Because the deep currents are usually small, this methodology of tracking is not expected to give rise to serious errors in the position determination of the float.
In the receiver electronics, some precautions are needed to keep the influence of noise level to a minimum.
This is because the acoustic signal traversing a long distance (1,000 km or more in the horizontal) picks up noise in various parts of the frequency spectrum (noise generated by ships, waves, rainfall, earthquakes, and so on). Any slight nonlinearity of the signal amplifiers in the receiver side can generate harmonics, and this will raise the effective noise level of the system. This problem can be minimized by filtering out the noise components in the received signal and amplifying only the frequency band of interest. Usually the received signals are detected in an approximately 1 Hz wide band-pass filter centered at the transmission frequency.
Rossby and Webb (1970) have described an observa- tional program designed to study abyssal water motions in the western Sargasso Sea by tracking Swallow floats in the SOFAR channel. The development of floats that can be tracked by sound transmissions through the SOFAR channel can be traced back to Stommel’s original SOFAR concept;
but Rossby and Webb (1970) used a low-frequency (500e600 Hz) sound source rather than Stommel’s primi- tive idea of repeated explosions. The prototype SOFAR float constructed byRossby and Webb (1970)is a tightly engi- neered Swallow float with an acoustic transducer that transmits CW pulses according to a precisely timed sched- ule. Their float consisted of a spherical housing (in two hemispheres) of adequate stiffness and resistance to hydro- static pressure (Figure 6.7). Each hemisphere of the float was spun from aluminum alloy 6061-T6 and had an internal
diameter of 0.93 m, a minimum wall thickness of 25 mm, and a hydrostatic collapse safety factor of two or more.All exposed external surfaces were either machined hard anod- ized aluminum alloy or nonconductive synthetic material.
The float enclosed the required power supply and electronic equipment and supported the electroacoustic transducers suspended about 1.5 m beneath it. The electroacoustic transducer was a hollow cylinder of 750 mm diameter, 325 mm length, and 50 mm wall thickness, which vibrated in the fundamental radial mode. The construction was a composite of active barium titanate and passive aluminum alloy in a barrel-stave configuration pre-stressed into compression by circumferential glass fibers in tension and insulated with polyurethane. The electrical connectors to the transducers were protected against fish bite using nylon tubes.
The computational method of determining the position of the float is to assume an initial position, the last one, say, and then to calculate the travel times. These are compared to the measured values, and the resulting errors are used to estimate the new position. The process is repeated until the error is negligible. The travel time is estimated from the mean velocity of sound and the range, which is calculated using the Andoyer-Lambert correction for the ellipticity of the Earth.
It may be noted that three major international campaigns—POLYGON, MODE, and POLYMODE—
were planned and executed with the hope of better under- standing the large-scale circulation features in the Atlantic.
POLYGON was led by Leonid Brekhovskikh from the Acoustics Institute, Russia, involving six research vessels and an extensive network of current meters. The current meters were deployed in a pattern resembling the shape of a cross, dubbed the “POLYGON”, spanning a region of 113113 nautical miles. This experiment provided some interesting results. It looked as though some large-scale eddy or wave disturbances were travelling across the POLYGON site from east to west. Their scales were close to those of the planetary baroclinic Rossby waves.
FIGURE 6.7 Prototype SOFAR float.(Source: Modified from figure 3 of Gould, 2005.)
Mid-Ocean Dynamics Experiment (MODE) was led by Henry Stommel from the United States. According to Walter Munk, the POLYGON experiment “ignited the mesoscale revolution and that MODE defined the new order” and that “oceanography has never been the same”
since. POLYMODE was a combination of the Russian experiment POLYGON and the U.S. led MODE. Russian ships came into Woods Hole during this collaboration.
Scripps as well as Woods Hole oceanographic institutions from the United States were involved in this campaign.
POLYMODE experiment, led by Andrei Monin, took place in 1978.
The U.S. Air force maintained an array of Missile Impact Location (MIL) hydrophones that located the positions of test missiles that dropped SOFAR charges at the end of their flights. These hydrophones could monitor floats in much of the northwest Atlantic. The first two SOFAR floats were deployed in the Sargasso Sea in 1968 and showed that reception of signals was possible at ranges of up to 1,000 km and that float positions could be determined with accuracy of the order of 3e5 km. The premature failure of both floats after one week and after two days, respectively (they were designed for a life of 9e12 months), was worrisome and attributed to biological attacks on the floats.
The availability of the MIL hydrophones was encour- aging to carry out a systematic exploration of the ocean mesoscale over a substantial part of an ocean basin and thus paved the way for planning the 1973 Mid-Ocean Dynamics Experiment (MODE), (MODE Group, 1978). In the MODE experiment, a combination of floats, moored current/tem- perature recorders, hydrographic surveys, and bottom pres- sure gauges was used in a nine-month collaborative U.S./
U.K. experiment. Because the abrupt failure of both spherical-shaped SOFAR floats after only a few days of operation was a cause for concern, Rossby et al. (1975) adopted an alternate design for use in the MODE experi- ments at Bermuda triangle. This design was similar to the neutral buoyancy float originally designed by Swallow. The SOFAR floats constructed for MODE (seeFigure 6.8) con- sisted of three cylinders of aluminum alloy.
The central cylinder was 5.2 m long and 30.5 cm in diameter, with hemispherical aluminum end closures, and contained the battery and electronic equipment. The two short cylinders shown in the figure are the low-frequency acoustic transducers, which operate freely flooded. The upper end was open and the acoustic driver, which was a bender plate, was fitted to the lower end. These transducer tubes were mounted on the main housing with four heavy stainless steel studs. All 20 SOFAR floats constructed for MODE were equipped with two acoustic signaling systems: a low-frequency system for long-range shore locations and a high-frequency subsystem for shipboard location and command recovery. A 7.3 kg cylinder of lead was connected to the lower end through an electronically
controlled release mechanism and was jettisoned for float recovery on command from the surface. The small, high- frequency (10 kHz) ceramic transducer was connected to the bottom end. All signals were derived from a tempera- ture-compensated quartz crystal oscillator. Electrical energy was furnished by a single battery of 73 kg, 30 volt nominal, and 5 kW hrs. Over 90 percent of the energy was used in the low-frequency signal. The completed float weighed 430 kg. Ballasting was carried out in an enclosure attached to the Woods Hole dock. The floats also had a 10-kHz short-range navigation system to allow their location identification and subsequent recovery by a ship.
For identification of all 20 floats from the same receiver stations, the method used was to have a combination of one of three carrier frequencies (approximately 267, 270, and 273 Hz) and seven different pulse-repetition rates ranging from 1,437 to 1,443 transmissions per day. These three differing carrier frequencies and seven pulse repetition rates provided 21 channels, which was more than sufficient to track 20 floats. Finally, 20 MODE SOFAR floats, each with a one-year design life, were deployed at a target depth of 1,500 m. The floats were tracked from four widely spaced hydrophone sites (Bermuda, Bahamas, Grand Turk, and Puerto Rico) at ranges of approximately 500 to 1,000 km, as indicated inFigure 6.9. After amplification, filtering,
FIGURE 6.8 Photograph of the modified SOFAR float used in the MODE program. (Source: Rossby et al., 1975. Reproduced with kind permission of Dr. Thomas Rossby.)
186 Measuring Ocean Currents
and addition of timing, the signals were recorded and used for analysis.
The navigational resolution for the trajectory data was typically500 m and the accuracy was 2 to 3 km. A good fraction of the error was attributed to the assumption of a “universal” speed of sound of 1,492 m/s throughout the Bermuda triangle, where the MODE experiments were conducted. Trajectories of individual floats, operated at 1,500 m depth in the Bermuda triangle during the MODE experiment, are shown in Figure 6.10. The experiment revealed some new information on the structure and vari- ability of the deep-ocean currents. Some floats remained stationary for a year, whereas others covered hundreds of kilometers. The experiments indicated that the Blake- Bahama Outer Ridge had considerable influence on the organization of the eddy field in the MODE area. Regions of sudden swirls and large horizontal shear were also observed.
The MODE marked a quantum leap in our ability to observe the state of the ocean. Although the experiment
was still primarily ship based, the SOFAR floats enabled day-to-day objective mapping of the ocean mesoscale over an area 400 km square and revealed the long-term propa- gation of these features (Freeland et al., 1975;Freeland and Gould, 1976). The floats were also entrained into the Gulf Stream rings and were thus able to reveal both their rotation rates and propagation (Cheney et al., 1976).
Since the MODE experiment in spring 1973, acousti- cally tracked SOFAR floats have been regularly used as Lagrangian drifters to study subsurface currents in the western North Atlantic. By emitting acoustic signals on a regular schedule, their positions have been determined as a function of time at ranges in excess of 2,000 km, depending on acoustic propagation considerations, source power level, and ambient noise conditions. Initially, the use of SOFAR floats was restricted to areas of the western North Atlantic within reach of shore-based SOFAR hydrophones at Bermuda, Eleuthera (Bahamas), Grand Turk Island, and Puerto Rico.
In an alternate design,Webb (1977)used a cylindrical float as part of the POLYMODE programs in a major study of mesoscale circulation in mid-ocean. These floats, al- though used with no protective coatings, were reported to have performed well even after four years of continuous service. The signals were received at deep hydrophones from
FIGURE 6.10 Trajectories of individual floats, operated at 1,500 m depth in the Bermuda triangle during the MODE experiment (September 28, 1972, to December 31, 1974).(Source:Rossby et al., 1975. Repro- duced with kind permission of Dr. Thomas Rossby.)
FIGURE 6.9 Physiographic diagram of the western North Atlantic (straddling the rough bottom to the east, the Hatteras Abyssal Plain to the west, and further to the west the Blake-Bahama Outer Ridge), where MODE-I experiment was conducted. The MODE-I region is indicated by three concentric rings of 100, 200, and 300 km radius. The float- monitoring sites are indicated by thick rings.(Source: Rossby et al., 1975. Reproduced with kind permission of Dr. Thomas Rossby.)