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Engineering Applications of Underwater

Acoustics in the Downloaded from http://onepetro.org/JPT/article-pdf/21/10/1277/2222841/spe-2317-pa.pdf by guest on 30 September 2021 J. B. Hersey, Ofice of Naval Research

Introduction As the mineral industries exploit deeper and deeper the state of the art, the most generally useful part of areas of the ocean, they will almost certainly rely naval research and development probably will be the more and more heavily on underwater acoustical in- methods that have been developed for measuring the struments. One can only guess at the :.Mruments that environment. In fact, in the continuing interaction will prove useful, but it may be helpful to describe with the oil indust~ particularly, several of the navy’s some past experiences in naval research and develop- methods and apparatus have already bsen adopted, ment and indicate the paths of research and develop- ment that seem to be needed for producing a suitable Deep WaterExploration and Drilling array of acoustical tools for industry. The offshore oil development in the Gulf of Mexico, off Southern California, and in the Persian Gulf long MilitaryUse of UnderwaterAcoustics ago heralded the exploitation of the mineral wealth Underwater has been employed in undersea of the ocean. Most of the resulting production has warfare since World War 1. It is used for detecting, been in less than 100 ft of . In the past 5 years, tracking and localizhg enemy targets, either sub- however, there has developed a powerful thrust into marines or surface ships; and some weapons are fired deeper water that has already carried the oil industxy by means of acoustic sensors, The use of rather to the edge of the and beyond, both than optical or devices is dictated by the rapid in geophysical exploration and in drilling. Even very attenuation of electromagnetic waves in water conservative prognostications indicate that oil will be compared with that of air. The variability of the produced in several thousand feet of water in 10 to ocean both in place and time puts constraints on 20 years. The corresponding development of solid systems that the navies of the world must know about minerals from the sea floor is now estimated to be far in some detail — at least where they expect to oper- slower, Nevertheless, the geology of the sea floor is ate. As a result, an elaborate program of environ- impetiectly understood, to say the least, and there mental research, focused on the needs of sonar, has may well be some surprises awaiting us in hard min- paralleled and paced the development of military erals. Whether for oilfield development, which is systems. upon us, or mining, which will develop more slowly, Military sonar systems themselves are not general- it is clear that much of the work must be don? under- ly useful to indust~ because of their specialized de- water. It is equally clear that the men of this under- sign. Although some elements of these systems may water world are having to learn new conditions and be of interest because their development has advanced ways of workin~, and thev must have new instru-

Underwater sound has provided means for measuring distances for nearly half a century. The first successful means — — is still the one most widely used. It is likely that continued research can make it a valuable oil industry tool in oflshore exploration and operations. —

OCTOBER, 1969 1277 ments and new tools. Perhaps the most needed is the quantity is mean horizontal speed, and for such meas- family of instruments that helps them see and meas- urements as depth soundings, it is the mean vertical ure their surroundings. speed. Sound speed is well known to vary both horizon- Measurementof Horizontal Distances tally and vertically but the variability with depth is Over short distances sight, either directly or by means almost always far greater than in the horizontal over of television, will be the most useful. But the longest comparable distances, so much so that it is reason- probable visual distance is about 100 ft, which can be able to suppose that there is no horizontal change reduced to a few inches in murky water, At greater except near a feature like the or a river distances only sound is effective. Hence, there must mouth where fresh and salt water may form sharp be an array of acoustical sensors for all distances. boundaries. Thus far in technical work at sea, sound has been most used for measuring distances and relative mo- Effects of Heat tion. Pictorial representations of underwater scenes Solar heating causes the temperature and hence the can be made, as will be explained, but even the clear- sound speed to be higher near the surface of the ocean est of them falls far short of the clarity common in than at greater depths. The interaction between the Downloaded from http://onepetro.org/JPT/article-pdf/21/10/1277/2222841/spe-2317-pa.pdf by guest on 30 September 2021 scenes of optical vision. Most of these acoustical air and the water, mostly the action of wind and images are a visual display of measurements of dis- evaporation, will create a layer of nearly constant tance. In fact, nearly all current sonar systems pro- temperature at the surface (Fig. 1). This layer, the vide at best an accurate measurement of travel time isothermal or mixed layer, is a convenient yardstick of a sound wave between two points in the water and for horizontal distance measurements up to 100 miles some more or less refined measurement of its direc- or more. If this layer is strictly isothermal, then sound tion of arrival at the receiver. Travel time is inter- speed increases uniformly with depth and sound paths preted as distance through the formula r = c t, where are arcs of circles cuwing upwards and propagating r is distance, c is sound speed, and t istravel time. outward by repeated surface reflections (Fig, 2). The Time intervals can be measured with great accuracy variation in sound speed is about 2 percent of the these days even in remote places in the ocean. This thickness of the layer. For example, in a layer 100 ft fact is of fundamental importance in the design of thick, the sound speed at its base is about 2 ft/sec sonar displays, which depend on time interval meas- greater than at the surface, or about 0.4 percent urements. They should always be designed so that of the sound speed. The mean horizontal speed can the time measurement camot be a iimit on accuracy. be estimated more accurately so that the accuracy The , however, is a variable in both (about 0.2 percent) of distance measurements in this space and time. The maximum range of sound speed layer is limited by our ability to measure sound speed. in the ocean is approximately 4,700 to 5,400 ft/sec Another possible limit is our abiity to identify phase or 1,433 to 1,646 m/see. Briefly, the higher the tem- in the surveying system. It develops that this is far perature, or the hydrostatic pressure, or the salinity from limiting. This method has long been a standard of the water, the greater the speed, Thus, distance one in science for measuring the separation between measurements requiring no better than 5 percent ac- ships or instruments at sea. Only the most accurate curacy are well enough made by assuming the speed radar is competitive in accuracy or convenience be- to be 5,000 ft/sec or 1,500 m/see as a rule of thumb, tween ships or radar transponder buoys. It is obvious For tenacious but essentially historic reasons, 4,800 that even accurate radar is a feeble competitor com- ft/sec is also used by many. pared with suspended instruments on free-floating For more refined measurements of distance, a host submersibles. This method made possible the highly of factors must be considered in interpreting travel accurate distance measurements in the concentrated times as distances, For many practical jobs their program, commenced in 1949 and still continuing, effect can be ignored if measurements are to be made over the same general paths in the ocean, and if the average sound speed can be measured independently 80” 5006 over approximately the same path. There are a num- .— ber of things to beware of such as the likelihood of hOXEO LAYER temporal change in sound speed during the measure- ments, and the requirement truly to employ the same path in order to be sure that that independent speed measurement is good enough. For an experienced practitioner, the results of using this method can be remarkably accurate — within 0,1 percent — in a wide range of situations. If this empirical calibration is impractical, high accuracy is still possible, as a general rule, in water T deeper than 100 fathoms (200 meters). In shallower water, the problem is more complex. For the moment, 3440 8W TEMPERATURE [*F I SOUNO VELOCITY IF TISECl confining ourselves to the deeper water, a distance measurement requires that the appropriate average Fig. l—Temperature structure and corresponding sound velocity structure of isothermal or mixed layer speed be known. For horizontal separations, this ‘overlying negative thermocllne.

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of seismic refraction investigations in the deep ocean. SEA SURFACE The shot instant from an explosive charge on the o

shooting ship was radioed to the receiving ship, there 100 to be recorded on the same oscillogram with the pulse . called the “direct arrival” traveling in this layer (Fig. H 200 & 3). The onset of this pulse easily can be identified to ~ about 1 or 2 millisec at distances up to 100 miles. g 300 n The isothermal layer is patchy, especially in sum- % mer or in windless areas of the tropics, Hence, it can- 400 not be counted on everywhere all the time. Neverthe- 500 icss, it is reliable enough and is world wide, so that a [ measurement system can be based upon it. For exam- ple, it is rare that the so-called direct wave of deep- Fig. 2—Sound ray paths in isothermal or mixed layer. sea refraction seismology cannot be used on profiles 20to 50 miles long. Downloaded from http://onepetro.org/JPT/article-pdf/21/10/1277/2222841/spe-2317-pa.pdf by guest on 30 September 2021 Other Applications of Horizontal Measurement PRECISE 1 IME .._ . . ..r -— —.. v —T , --- CIib?4NfL - * ‘“ There have been a host of other applications, A very SHOT INSTANT ANO BuBBLE PuLSE

simple and obvious one is a system for recovering a DIRECT RAOIO ._l&~ ,b—_——._——— . . ..-...... lost, but still active, . A sound pulse sent LINK FROM - If+ SHOOTING SHIP I out from a projector at the ship is received by the sonobuoy and radioed back to the ship where it is recorded on a graphic recorder simiiar to an echo IONSET OF THE sounder. The ship proceeds on courses required to OIRCCT ARRIvAL shorten range to the sonobuoy while continuing to Fig. 3-Shot instant (via radio) and direct acoustic arrival record sound pulses until it is in sight (Fig. 4). An (via isothermal layer). acoustic transponder will do the same for a buoy- suspended instrument or a free submersible in the isothermal layer. The industrial application of this UA”?lCAL EL APSLO TIME -9 technique is straightforward. The acoustics of the MILES 00 ,,, ,,,881! 1111111111111,111111111111111111111118111!1111111llltlll llllllllillltll I layer is adequately known, The naval information on :k INSTANT z ..~ BUOY occurrence of the surface layer and its approximate SIGNAL IS . RECOVERfO PROJECTEO sound speed generally can be made available for spe- ~~ ,.~’ \ SUOYSwsmo cial areas. A variety of sound sources, receivers, re- ,.~’ \\\+ corders and associated amplifiers have already been . .--— — ,,,0’ developed, Mechanical deck handling equipment has .\\\’ been made for similar acoustical devices. g \\\\\\’ ~ ?.0 .- ._. _— ,. ——.—— . . — ,\!\\’ ~ ,,,l!l~ The SOFAR Channei : The characteristic acoustical feature of water deeper 40— — ——— than 1,000 fathoms in the open ocean is the SOFAR Fig. 4-Successive sound pulses received from lost channel, The general features of it are familiar; near sonobuoy during search and recovery. the surface the sound speed is high and decreases as the temperature decreases with increasing depth. Be- low a few thousand feet the temperature becomes nearly constant and at greater depth the sound speed SOUNO VELOCITY FTfSEC 4880 5000 5040 increases with hydrostatic pressure at a rate approxi- I iJ I mating 0.018 see-l as shown in Fig. 5. The broad speed minimum is the SOFAR charnel. In it sound is bent continually toward the speed minimum, thus SOFAR CHANNEL curving and recurving without encountering surface AXIS or bottom (Fig. 6), ,----- This channel is found little modified from this form wherever there is water deeper than 1,500 fathoms or so. It has been used for years to recover experi- mental hardware in the U. S. space program. A num- ber of hydrophores located at various accessible ( places and at the minimum sound speed, called the SOFAR channel axis, receive sound from a small ex- plosive charge incorporated in the unit to be recov- ered. The charge is detonated approximately at the axis depth and the arrival time of its sound is noted SEA FLOOR at three or more of these hydrophores, The time dif- Fig. 5— ed va depth in representative ferences between these several observations are used &kAR channel.

OCTOBER, 1969 12’79 to locate the unit, This method in principle is not rough topography, we may be greeted by such a scene distance-limited, the sound being detectable anywhere as Fig, 10. The detailed interpretation of this scene there is a clear water path. Accuracy of the method has not been made, even though many of us admire is quite satisfactory for recovery and depends on how such records as a form of decoration, This series of well calibrated the ocean area is for such a method. examples makes it clear that while some limited inter- Measurements can be and have been made over paths pretation of echo soundings made in deep water from from hundreds to thousands of miles long. a surface ship may be possible, the method simply Perhaps I am missing an important powibi!ity, but does not provide an accurate measurement of vertical I doubt this measurement will prove necessary for distance from bottom to surface. The “narrow beam” mineral exploitation. I have described it mainly to echo sounders improve this situation, but have been illustrate the scope of application of acoustics in the shown clearly not to solve it, With either broad-beam ocean. or narrow-beam sounders, serious errors have been made in rugged terrain in surprisingly shallow depths. Measurementof Vertical Distances In one horror story, an error in excess of 1,000 ft Let us now examine measurements of position in the was identified from soundings with a narrow-beam vertical, i.e., in depth. The first, historically and in instrument in an area where a representative depth is Downloaded from http://onepetro.org/JPT/article-pdf/21/10/1277/2222841/spe-2317-pa.pdf by guest on 30 September 2021 importance, is measurement of the ocean depth, 6,000 feet. This could have led to a costly miscalcula- which is done mostly today by echo sormdmg. A tion, but the group doing the work was aware of the sound projector mounted in or near the keel of a ship problem. Ultimately, it was necessary to lower an emits a sound pulse that is reflected from the ocean echo sounder on a cable (as in Fig. 11) to within floor and then returns to the projector. Usually this about 50 ft of the bottom, The sounder received an same instrument converts the echo to an electrical echo both from the nearby ocean floor and from the pulse, which records on a graphic recorder. Experi- surface, The sum of these echoes is a sounding ac- ence resulting from some 40 years with echo-sound- curate to within the uncertainty imposed by the sur- ing has led to the use of directional transducers of face waves, In using such a method, the echo sounder the “search-light” or “piston” design, which com- must be located by a suitable means since it will not monIy emit a pulse of sine waves in a radiation pat- in general hang directly below the surface ship. This tern consisting of one strong main lobe and several work was done when the present generation of sub- weaker minor lobes. The depends on the mersibles were still on the drawing boards. water depth and the surveying craft, but varies from It seems to me that where high accuracy is neces- 7 to 60 KiloHertz (KHz). A deep-sea model popular sary, submersibles are the deep-water sounding boats in this country for many years employs 12 KHz. of the future. They require very good underwater More recently, the transducers used have been tiely navigation, which can be supplied by sound, and dmectional and must be mounted in some sort of utilize the same echo sounders that have long been stable vertical platform. The reason for this shift available for making highly accurate measurements. is that the broader beamed instrument detects the Questions of economy and efficiency of operation stronger reflecting facets of the sea floor before it have largely to do with submersible design, not with passes over them, or, simiiarly, when they lie off to sonar. A suitable general plan for making detailed the side of the track of the ship. On the sounding oceanic depth surveys would be, first, to make a de- recorder, they present a characteristic crescentic, or tailed sounding survey from a surface ship, wldch eyebrow pattern, familiar to oceanographers (Fig. 7). could be the mother ship of the submersible. Follow- These crescents may represent fish or other objects ing this survey, the reference equipment of under- in the water (Fig. 8) or the edge of an underwater water navigation would be implanted, and the detailed cliff, or a shallow trench with its upper edges and its survey could be carried out by one or more submersi- inner comer reflectors (Fig. 9), In an area of very bles. Plans of this sort no doubt are in the m~lng or

ECHO SOUNDER RECORD SURFACEDIRECTION OF MOVEMENT _-

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Fig. -Sound ray paths of SOFAR channel with Fig. 7—Formation of crescentlc echo sequence as echo source on axis. sounder approaches and then passes over a strong echo point. \ 12$0 JOURNAL OF PETROLEUM TECHNOLOGY Downloaded from http://onepetro.org/JPT/article-pdf/21/10/1277/2222841/spe-2317-pa.pdf by guest on 30 September 2021

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OCTOBER, 1969 1281 .

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1282 JOURNAL OF PETROLEUM TECHNOLOGY .

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Fig. n-Depth measurements in mountainous regions Fig. 13—Side.scanning sonar record of a ridge of by means of suspended echo sounder. slates surrounded by sand. Downloaded from http://onepetro.org/JPT/article-pdf/21/10/1277/2222841/spe-2317-pa.pdf by guest on 30 September 2021

perhaps have actually been carried out. the deep , and they hold promise for measur- Many years ago some of us sought methods of ing physical properties of the sediments. As examples, improving the interpretation of echo soundings. Echo attenuation of sound has been measured by an analy- resolution could be sharpened by employing the short- sis of sub-bottom echo amplitudes. Similar analysis est sound pulses permitted by our transducers. We of a very large sample of bottom echo data has shown were able to resolve echoes whose onsets were about a high correlation between echo amplitudes (suitably 0,3 X 10-s seconds apart, or about 1.5 ft in water corrected for transmission effects) and physical prop- over 4 miles deep. By this program, we learned that erties such as grain size and porosity of the local bot- high frequency sound, well above 10 KHz, will pene- tom sediment, These are the merest beginnings. trate some bottom sedments to portray the layering Another product of studies with high resohition of the sediments themselves and reveal their relation- techniques is the precise positioning of instruments sldp to their surroundings. For example, 12 KHz with respect to each other, the sea floor, or the sur- sound will penetrate well over 100 ft into the sands face. The variety of applications includes photogra- and clays of the deep ocean basins as in this record- phy of the sea floor and of free-swimming fish, the ing from the Nares Basin 200 miles north of Puerto detection of differential movement of instruments RICO(Fig. 12). In a few places in the Mediterranean drifting in turbulent water, and the monitoring of Sea, similar echoes that come in after the bottom bottom dredges. These applications show that changes echo is received have been shown to be due to con- of a few inches in separation of objects a few miles trasting layers of sand or volcanic ash and clay. In apart can be reliable measurements, and that short the past few years, several echo sounders of lower distances can be measured by means of instruments , such as 3 to 6 KHz, have shown greater remote from the observer. Whereas these applica- penetration and very useful resolution. Used in con- tions have existed for some time, a more recent ap- junction with them, these instruments enhance great- plication has been in solving the drill re-entry prob- ly the capability of the vaiious seismic profilers and lem in drilling, other seismic reflection apparatus familiar to the oil exploration indust~. The instruments in the high fre- AreasThat Need FurtherInvestigation quency end of this group have been very useful in As we require more installations on the sea floor, revealing the nature of shallow sediment structures in especially on slopes in water deeper than 100 fathoms,

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OCTOBER, 1969 1283 .

● b it is necessary to develop Iileans of assessing their provide an excellent means of extending the useful- foundations. Evidence is accumulating from seismic ness of these and other physical analyses. profiling that sh.umpingis a common activity of sedi- It is evident that sonar can be successfully applied ments of the ocean even on very gentle slopes, We to a wide variety of underwater chores. However, need to know much more than we now do about what there is one area in which we are woefully deficient. causes slumps, how often they occur and how best We have not developed the ability to extract and pre- to circumvent their effects. I suggest that geological sent the wonderfully detailed information contained investigations, analysis of physical properties, and the in what we call reverberation, Side-looking underwater equivalent of soil dynamics arc needed as present partial scenes by means of extremely rudi- we exploit the ocean depths. This is a research and mentary signal processing (Fig. 13). I believe that as development program of major proporticm. Failure to we invade the deep ocean, it would be wise to develop carry it out will be rewarded sooner or later at least the ability to examine quickly on several scales the by destruction of property and possibly by loss of landscape over which we work. By improving side- life as well. Iooking sonar, by using multiple-beam echo sound- Our knowledge of slumping is from direct observa- ers, and by developing acoustical holography, we are tion by divers, from bottom photographs, from hear- approaching this capability. Of course, our aim should ing underwater sounds that seem like major land- be to depict not only the ocean floor, but also the Downloaded from http://onepetro.org/JPT/article-pdf/21/10/1277/2222841/spe-2317-pa.pdf by guest on 30 September 2021 slides, and from evidence of truly prodigious slides structures that lie beneath, We are currently fasci- turning up in seismic profiler studies of continental nated by the data from seismic profilers, and the side slopes and rises. From these sources we are beginning scanners. We can and should expect far more from to know the scale in time and distance of the proper an extension of all these techniques. JPT questions to be asked. A comparatively modest effort has gone into the study of the shearing strength and bottom stability of Manuscript received Swt. 19, 1968. Paper (SPE 2317) wes pre sented at SPE 43rd Annual Fall Meatlng held in Houston, Tex., Sept. sediments. I suggest that acoustical measurements will 29.Ott, 2, 196S.