Download - Study of acoustical fluctuations and ocean movements over one deep-ocean skip distance

Study of acoustical fluctuations and ocean movements over one deep-ocean skip distance*

John C. Beckerie

Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543

Howard W. Broek '

Bell Telephone Laboratories, Incorporated, Whippany, New Jersey 07981

•. O. LaCasce Or.

Bowdoin College, Brunswick, Maine 04011 (Received 16 December 1974) •

This experiment studies the relationship between ocean movements and acoustical fluctuations southwest of Bermuda. Sound transmission over a single skip distance RSR path was maintained for a 33-h period. During the same time interval, ocean changes were monitored both at the source and near the second turning point of the sound-ray path. Some correlations were observed among some of these observations.

Subject Classification: 30.20; 28.60.

INTRODUCTION

Fluctuations in the ocean should produce and be related to fluctuations in an acoustical signal transmitted through the region. Acousticalwavefront variations have been attributed to internal waves•; however, simultaneous measurements of transmission characteristics

were not available. A joint effort between Woods Hole Oceanographic Institution and Bell Telephone Labora- tories to make simultaneous acoustical and oceanographic measurements recorded time series of amplitude fluctuations and oceanographic data to permit a search with possible correlation calculations.

The experimental configuration is shown schematically in Fig. 1. This figure shows the ray path from the source to the receiver with one turning point or surface reflection near the source and a second near the receiver.

Oceanographic data were collected by the source ship MISSION CAPISTRANO and by R. V. CttAIN, which was in the vicinity of the second turning point. The transmitted sound pulses, center frequency 400 Hz, were received on deep hydrophones off Argus Island. The geometry and objectives of this experiment are similar to the experiment reported by Chuprov in 1966. •'

I. THE OCEANOGRAPHIC AND ACOUSTICAL DATA

The electrically pulsed sound source was located at a depth of 384 m and a range of about 83 km from the receiving hydrophone (bottom mounted at depth 1460 m). During the 33 h of the experiment, the source ship shift- ed position back and forth by only + 0.5 km, as deter- mined by Loran C navigation. The acoustic pulse was short enough in duration to permit identification of the single sound path depicted in the figure. Some multipath transmission studies have been made by several investigators. 3-0 At the source ship the temperature near the source was monitored by a quartz thermometer starting well in advance of the acoustical measurements, and a sequence of bathythermographs (BTs) was made on an hourly schedule. These observations are shown in Fig. 2.

R. V. CHAIN was stationed near the second turning point (Fig. 1) to permit determination of the importance of fluctuations there. At the deep turning point for this ray path the sound-velocity fluctuations are known to be relatively small, while the fluctuations should be larger in the surface layer and in the upper part of the main thermocline.

FIG. 1. Experimental configuration of sound source, receiver, and sound velocimeter.

832 J. Acoust. Soc. Am., Vol. 57, No. 4, April 1975 Copyright ¸ 1975 by the Acoustical Society of America 832

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833 Beckerie, Broek, and La Casce: Acoustical fluctuations and ocean movements 833

lOOO

500

o

-500,

-1ooo

85 DEPTH OF 21øC ISOTHERM (70 ø

i i i i i i i i , i i I i

.

OF 18.9øC ISOTHERM (66 ø 2

_ (c) , ..... 140 ' • • • ' ' ' ' ' ' ' ' ' 80 240 400 560 720 880 1040 1200 1360 1520

17.44[

17'14•/(d ) 17.04 u (•d) • -80

TEMPERATURE

0 8'0 ' ' ' ' ' ' ' ' ' ' ' ' ,4o ,oo TIME IN MINUTES

FIG. 2. Environmental observations at the source ship. (N. B.: Depth scale increases upward.)

At CHAIN, the sound velocity as a function of depth down to 1500 m was measured just prior to and just after the acoustical experiment. The curves in Fig. 3 for 26 and 29 May were obtained from salinity, pressure, and temperature measurements by use of Bissett-Berman STD equipment, while the data for 2 June was taken by use of a sound velocimeter. The similarity of the general appearance of these profiles does not tell anything about the correlation between the fluctuations of the

medium and the fluctuations of the acoustical signal.

During the acoustical experiment the sound velocity at the second turning point was monitored by raising and lowering a velocimeter between the depths of 50 and 650 m. One complete cycle took about 13 min. In the data of Figs. 4(b) and 4(c), points for the sound velocity at depths of 220 and 560 m are plotted. As expected, the sound-velocity variations were more pronounced near the knee of the sound-velocity profile at the top of the main thermocline (see Fig. 3). Also, Figure 4(a) shows the change in range with time. The data points have been connected by straight lines to indicate trends.

The receiving hydrophones were bottom mounted at depths below the main thermocline in the ocean. The amplitude of the received signal was recorded for a 33- h period starting at 0130 h, 1 June. The fluctuation in sound amplitude is plotted as a function of time in Figs.

5(a)-5(e) for five hydrophones, their order correspond- ing to increasing depth. The source was pulsed every 6 rain and the sampled values are the relative amplitudes of the received signal for the ray path shown in Fig. 1 (RSR). In order to emphasize the longer term changes in the sound amplitude, we averaged the amplitude of the second hydrophone [Fig. 5(b)] over 1-h periods to obtain Fig. 5(f).

II. AN INTERPRETATION OF THE DATA

In examining the oceanographic data, time variations in the layer from 50 to 600 m (Fig. 3) are of interest as an indication of internal waves. Temperature and sound- velocity measurements are possible ways of monitoring internal waves. H the water is isothermal, only measurements below the isothermal layer are useful. There, a change in depth of the isotherm or a variation in sound velocity is an indication of the motion of the water. Sound velocity at a given depth is primarily temperature dependent. Since internal waves produce currents in the surface layer, a variation in the range of the source ship might indicate these currents. However, other factors are involved here since the source ship had the assistance of another ship in maintaining position. Also, observati•)ns of fluctuations of temperature at a fixed depth may be contaminated by the to-and-fro motions of

J. Acoust. Soc. Am., Vol. 57, No. 4, April 1975


834 Beckerie, Broek, and La Casce' Acoustical fluctuations and ocean movements 834

VEL OC/TY (meters/second) 14! •0 1500 1510 1520 15:30

0 , , , , I , , , , I , , , , I , .... '....?r ' _

300 -

_

_ _

_ _

_ _

_

_

1200 •::' -- 2• MAY 1967 -

• ...... 2 JUNE 1967 _

_

1500

FIG. 3. Sound-velocity profiles at C//•I/N.

600

9OO

the ship in a region where a horizontal temperature gradient may exist.

In comparing the two curves in Figs. 2(b) and 2(e), there appears to be a general downward motion at about 600 and 1200 rain and an upward motion around 900 min. The horizontal motion of the ship [Fig. 2(a)] appears related to the vertical motion of the water column, Figs. 2(b) and 2 (e).

A power spectrum and autocorrelation function of the temperature-time series obtained from the quartz thermometer probe located at the source [Fig. 2(d)] were computed and plotted. The spectrum was calculated as follows: the best linear fit to the time series

was subtracted from each term in the series (i.e., it was demeaned and derrended). Then the autocorrelation function of the new series was computed and a cosine transform of the autocorrelation function was calculated.

Finally, the spectrum was smoothed by an operation called Hanning. 7 Relative spectrum level is shown in Fig. 6.

This spectrum is similar to the power spectrum for internal gravity waves; measurements of internal gravity waves reported by Voorhis ø indicated that the power spectrum varied at higher frequencies approximately as frequency to the -2 power. The straight-line fit to the spectrum in Fig. 6 indicates a falloff with frequency of approximately 20 dB per decade (inverse square). A slope of - 5/3 does not fit as well as this. A - 5/3

5O0

0

500

1000 t 1500 I •øøø I 2_500-

1525'20I• _•_ ff/•_ ACHA//V:SOUND VELOCITY AT 220meters

.•,••1522.20 t • • ,52,.7o[ (b) •1521 20 • ' ' • • ' ' ' ' ' ' ' ' ' • ' • • ' • ß

152_. 1 .P-Or

I/--x ̂ A //• ,, CH, ZlIN:SOUND VELOCITY AT 560 meters ,52o.7o' / /h t 151920 • • • • • • • • • • • • • • • • • •

80 240 400 560 720 880 1040 1200 1360 1520

T/ME/N M/NUTES

FIG. 4. Environmental observations at CtlAIN.



835 835 Beckerie, Broek, and La Casce: Acoustical fluctuations and ocean movements ,

.80

•.80 F

•o00 .......... .80

.oo_Cl, v .............

.•.80 F

t.[..00

.80

TIME IN MINU T•'$

FIG. 5. Relative amplitudes of received pulse signals as a function of time.

power law is obtained from Kolmorgorov 9 in a theory of turbulence.

At the second turning point, CHAIN had larger motions in maintaining its station than the source ship [Fig. 4(a)]. Consequently both the space,and time variations of the ocean state shown in Fig. 4 are inextricably interwoven. However, Fig. 4(c)would indicate that the sound-velocity fluctuations at 560 m are more pronounced and apparent- ly less sensitive to variations in space.

A correlation exists between the location of the source

ship [Fig. 2(a)] and the sound-velocity fluctuations at a depth of 560 m at the second turning point •ø [Fig. 4(c)]. To emphasize this visual correlation, the two curves have been superimposed in Fig. 7(a). The computed maximum correlation is about 0.5 and the maximum

occurs with a delay of about 4 h, that is, the sound-velocity fluctuations at CHAIN occur about 4 h after the range fluctuations of the source ship. The sound-ve-

locity fluctuations [Fig. 4(c)] are more peaked for the higher values of sound velocity. This should be expected in measurements at 560 m (Fig. 3) when a large- amplitude internal gravity wave causes the boundary of the nearly isothermal layer to descend into the region of the main thermocline.

Figure 7(b) indicates some visual correlation of the 21 øC isotherm depth variations and the sound-velocity fluctuations at CHAIN. There the delay is about 5 h. From these observations (Figs. 2, 4, and 7)we main- tain the conclusion that correlations in internal ocean

movements extend to a spatial separation of over 54 km, the east-west distance between the ships.

Comparisons of the acoustical fluctuations with the oceanographic data at the source and at the second turn-

ing point are shown in Fig. 8. For these figures we smoothed the acoustical pulse amplitude of Fig. 5(b) and then plotted it as a peak-to-peak variation.




102 I I

10'

• 10-'

104'

10-3 I .001

-2 SLOPE

SLOPE

.Ol

FRE•/E'NC Y (cycles/minute) FIG. 6. Spectrum of temperature at the source.

In Fig. 8(a) the acoustical fluctuations are compared wit. h the depth variation of the 21 øC isotherm at the MISSION CAPISTRANO. This visual comparison sug- gests that the broad-scale variations of the amplitude tend to increase when the isotherm depth deviates up or down from 77 m.

A possible explanation of these observations is that the motion of the source ship is partially controlled by the to-and-fro motions of the water, produced by an internal gravity wave. The 21 øC isotherm fluctuations at the source ship are an indication of such an internal wave. We estimated the internal gravity wave speed from a two-layer model of the ocean, with Ap/p the relative density change for a thin upper layer, thickness h, and the acceleration of gravity g. Accordingly,

[(•p/p)gh]•l•'-•[(2x10'•)(lOm/sec2)(800 m)]•/•'=4 m/sec.

An internal gravity wave propagating eastward at speed 4 m/sec would take about 4 h to reach the position of CHAIN which was east of MISSION CAPISTRANO by 54 kin. •0 An internal tidal wave, if present, should prop- agate eastward for this ocean region. u We also obtained an estimate of 0.04 m/sec for the ship speed in its to- and-fro motion due to an internal wave, and this is close to the value observed in Fig. 2(a). z0 Ray computations that use a typical sound-velocity profile indicate that, for an increase and decrease of the source range, a region of focusing on the wavefront can pass through the hydrophone position. Thus, the correlation observed between the 21 øC isotherm near the source and the

envelope of the acoustic pulse amplitude [Fig. 8(a)] is probably due to the motion of the source ship caused by the internal waves.

Figure 8(b) shows the envelope variations of the acous-

SHIP

_-,, MOVEMENT

\__

55

• 65

- SOURCE LOCATION

'--- _ \ ! \

\x•///,7•, - _ __/1 kk/• . I x -- ii-

/ - !

i i i 400 800 ! 200

(b) ,oo .oo ,oo I I I

1 2 3 4 5 6 7 8 9 10 11 12 13 !4 15 16 17 18 19 20 21 22 23 24 25 26 I I i i i I I I I I I I I I I I I I I I I ! I I I ! I ! F-DELAY INTERVAL--•

TIME IN HOURS

• SOUND VELOCITY AT C/Y4/N (560 meters) _• .... 21øC ISOTHERM FROM BT DATA AT - 1521.2 •

1520.2 "'! -,•

- -.4 i• - - -.2 0

0.0 •

+ø2 • - +ø4

'

- .1.0

FIG. 7. Comparison of oceanographic data at the source and near the second turning point. (a) Source ship movement and sound velocity at 560 m at C//A/N. (b) BT data at source and sound velocity at CHA/N.




.8O

.4o

-.40

-.80

,?

• SMOOTHED ACOUSTIC (ENVELOPE)

- PEAK-TO-PEAK PULSE AMPLITUDE FROM FIG. 5b

A -• --- 21 ø c ISOTHERM AT SOURCE A, (B INVERTED) A ___. [55

/ \ 65

75

.... Z'• 7'/Mœ (m/nute$?/\ ' /• / • I• .

85 'b

55

B

INVERTED SMOOTHED ACOUSTIC PULSE AMPLITUDE OF FIG. 5b _

.80 -

.40

-.40

--- SOUND-VELOCITY FLUCTUATIONS AT CHAIN (DEPTH 560m)

1522,20

1521.20

1520.20

lSl 9.20 ?IME (m/•u res]

FIG. 8. Comparison of the acoustic pulse amplitude and oceanographic data. (a) Acoustic variations and BT data at the source. (b) Acoustic variations and sound-velocity fluctuations at CHAIN.

tic pulse amplitudes, and the sound-velocity variations at 560 m with no lime lag. The correlation is not obvious. Computer calculation of the cross correlation indicates there is a maximum value of 0.4 for delays less than 4 h, and that the amplitude fluctuations lead the sound-velocity fluctuations by about 2 h.

The existence of a correlation reaching 0.4 for delays less than 4 h between the acoustical pulse amplitudes and the measurements at the second turning point is considered to be significant in view of the complicated additional effect from the internal wave motion at the

source. The fact that the correlation is not visually obvious may be due in part to incomplete oceanographic data and in part to the more complex geometry for the ray path at the second turning point as well as the motion of the source. Remember that the ray path enters and exits the upper layer in the vicinity of CHAIN, and that there should be acoustic amplitude variations cor- responding to variations in the depth of the knee of the thermocline in lzvo places near CHAIN. During the experiment we measured the oceanographic fluctuations at only one place somewhere between the entrance and exit to the upper layer. A future experiment could be designed to take into account more completely the oceanographic variations to seek an improvement of the correlation obtained.

III. CONCLUSIONS

In this experiment we are primarily interested in the relations between the fluctuations in the transmission

characteristics of the ocean and the fluctuations in the

acoustical signal transmitted through this region.

In summary, the data may be grouped as follows: (1) the ocean fluctuations at the source (Fig. 2), (2) the ocean fluctuations at the second turning point (Fig. 4), and (3) the acoustical signals received at the hydrophones (Fig. 5). These three groups of observations by them- selves show that in each instance the individual measure-

ments at each location are related.

When these three groups of measurements are ex- amined for correlations between the groups, interrela- tions are not immediately obvious. Visual comparison o of the oceanographic data near the two turning points (Fig. 7) are related to an internal wave. The comparison between the oceanographic data and the amplitude fluctuations of the acoustic pulse (Fig. 8) indicates some correlation. We do not readily verify in these measurements that the acoustic spectrum is double the frequency of the internal waves, as reported by Chuprov. •' How- ever, the acoustic fluctuations are more rapid than the internal wave fluctuations, and there is some suggestion in the visual correlations made that internal wave dis-




placements upward or downward can produce acoustic focusing (see Fig. 8).

ACKNOWLEDGMENTS

The authors want to acknowledge the help of Dr. Eli Katz in the planning phase of the work and during cruise 67 of CHAIN. A number of students also assisted during the cruise. This work was supported by the Office of Naval Research.

*Contribution No. 3477 of the Woods Hole Oceanographic In- stitution, Woods Hole, MA 02543.

1j. C. Beckerie, J. L. Wagar, and R. D. Worley, "Under- water acoustic wavefront variations and internal waves," J. A½oust. Soc. Am. 44, 295-296 (1968).

•'S. D. Chuprov, "On the observation of a sound signal in the presence of internal waves," Atm. Ocean. Phys. 2 (5), 551- 552 (1966).

3j. C. Steinberg, '•hase variations of sound in the Florida

Straits," J. Acoust. Soc. Am. 41, 1617 (A) (1967). 4Ross E. Williams, and Henry F. Battestin, "Coherent recom-

bination of acoustic multipath signals propagated in the deep ocean," J. Acoust. Soc. Am. 50, 1433 (1971).

5j. Clark, and M. Kronengold, "Long-period fluctuations of CW signals in deep and shallow water," J. Acoust. Soc. Am. 56, 1071-1083 (1974).

6R. P. Porter, R. C. Spindel, and R. J. Jaffee, økcoustic- internal wave interaction at long ranges in the ocean," J. Acoust. Soc. Am. 56, 1426-1436 (1.974).

?R. B. Blackman, and J. W. Tukey, The measurement of power spectra (Dover, New York, 1958), p. 14.

8A. D. Voorhis, "Measurements of vertical motion and the partition of energy in the New England slope water," Deep Sea Res. 15, 599-608 (1968).

SA. N. Kolmogorov, "The local structure of turbulence in in- compressible viscous fluid for very large Reynolds numbers," Doklady. Akad. Nauk SSSR. 30, 301 (1941).

10j. C. Beckerie, '*Spatial correlation of ocean movements and sound-velocity fluctuations," J. Acoust. Soc. Am. 45, 1050- 1051 (1969).

llA. Derant, Ebb andfiow (Univ. Michigan P., Ann Arbor, MI, 1958), p. 121.



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