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Engineering Applications of Underwater Acoustics in the Ocean

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Engineering Applications of Underwater Acoustics in the Ocean t2317 f’il I w Engineering Applications of Underwater Acoustics in the Ocean 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 acoustics 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 water. 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 sound rather to the edge of the continental shelf and beyond, both than optical or radar devices is dictated by the rapid in geophysical exploration and in drilling. Even very attenuation of electromagnetic waves in sea 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 sonar 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 — echo sounding — 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 Gulf Stream 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 speed of sound, 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 THERMOCLINE 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. 1278 JOURNAL OF PETROLEUM TECHNOLOGY . 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 .
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