volume scattering-strength profiles in the northeast pacific ocean

Received 1 April 1969 13.6, 13.10; 11.7

Volume Scattering-Strength Profiles in the Northeast Pacific Ocean

J. A. Scl•rG•Zl• ANt> R. G. TuI•N•Zl•

Defence Research Establishment Pacific, Victoria, B.C., Canada

A technique is described that permits the determination of the acoustic scattering strength of the ocean volume in terms of depth, frequency, and time. A pod of charges is lowered together with a hydrophone to various depths in the ocean and, by observing the amount of scattered energy produced by detonating units of the explosive charge pod, a profile of good resolution of the scattering strength versus depths obtained. The broad-band acoustic characteristics of the charges permit the spectral characteristics of the scattered returns to be determined. Observations made over an extended time period yield the time dependence of scattering. Measurements made in the northeast Pacific Ocean are given.

INTRODUCTION

Acoustic scattering layers in the deep oceans of the world have, for the most part, been examined from the ocean surface. The earliest work involved the use of

echo sounders 1.2 and more recently explosive charges detonated near the surface yielded information relating to the frequency characteristics of the layers? øIn this paper, a technique is described in which an explosive charge is detonated close to a receiving hydrophone and

• W. E. Batzler and E. C. Westerfield, "Sonar Studies of the Deep Scattering Layer in the North Pacific," Navy Electron. Lab. Rep. 334, San Deigo, Calif. (1953).

•' NDRC Summary Tech. Rep., "Physics of Sound in the Sea," Div. 6, Vol. 8, (1946).

a j. B. Hersey, R. H. Backus, and J. Hellwig, "Sound-Scattering Spectra of Deep Scattering Layers in the Western North Atlantic Ocean," Deep-Seas Res. 8, 196-210 (1961).

4 j. R. Marshall and R. P. Chapman, "Reverberation from a Deep Scattering Layer Measured with Explosive Sound Sources," J. Acoust. Soc. Amer. 36, 164-167 (1964).

5 R. P. Chapman and J. R. Marshall, "Reverberation from Deep Scattering Layers in the Western North Atlantic," J. Acoust. Soc. Amer. 40, 405-411 (1964).

0 B. A. Gold and P. Van Schuyler, "Time Variability of Volume Scattering in a Small Oceanic Area," J. Acoust. Soc. Amer. 40, 1317-1321 (1966).

7 G. B. Farquhar, "Preliminary Report on Scattering Layer Measurements in the Western North Atlantic," Naval Oceanog. Office Informal Manuscript IM No. 66-3, Washington, D.C. (1966).

s W. I. Aron, F. J. Bourbeau, and R. E. Pieper, "Acoustical and Biological Studies of the Deep Scattering Layer in the Eastern North Pacific," General Motors Corp., AC Electron. Defence Res. Labs. TR 67-13, Santa Barbara, Calif. (1967).

9 j. A. Scrimger and R. G. Turner, "Preliminary Scattering Strength Measurements in the N. E. Pacific Ocean: Integrated Strengths," Defence Res. Estab. Pacific Rep. 69-2, Victoria, B.C. (1969).

the close-in acoustic returns observed. By using a pod of charges lowered together with a hydrophone to various depths, and by observing the amount of scattered energy produced by detonating units of the explosive-charge pod, a profile of the scattering strength versus depth is obtained with good depth resolution. By observing the scattered return within 25-50 msec after the instant of detonation, a depth resolution in the range 60-125 ft is obtained. This technique is similar to one used by Anderson, 1ø who used a sparker-hydrophone combination at various depths in the ocean to obtain wide-band returns. The volume of water insonified

varies from about 90X 103 yd 3 at 200 ft depth to 18X 10 a yd 3 at 2000 ft depth, assuming that the acoustic pulse length is represented by one bubble-pulse interval (i.e., 5 msec at 200 ft and 1 msec at 2000 ft). This is smaller than that insonified by conventional echo sounders, but not quite so small as the volume (5X 103 yd 3) of water insonified by a dual-frequency horizontally directive pulsed system recently used by Anderson. TM The great advantage of explosive charges as sources lies, of course, in their ability to generate a broad band of frequencies. This fact enables continuous spectra of the scattered returns observed at the various depths to be observed.

•0 V. C. Anderson, "Wide Band Sound Scattering of the Deep Scattering Layer," Scripps Inst. Oceanog., Marine Physical Lab., S.I.O. Ref. 53-36 (1953).

11 V. C. Anderson, "Spatial and Spectral Dependence of Acoustic Reverberation," J. Acoust. Soc. Amer. 42, 1080-1088 (1967).

13 V. C. Anderson, "Frequency Dependence of Reverberation in the Ocean," J. Acoust. Soc. Amer. 41, 1467-1474 (1967).

The Journal of the Acoustical Society of America 771

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10:15:42

SCRIMGER AND TURNER

SITE

LOCATION--• o

135 o 130'

I I I I

D.R.E.P.

125 ø

F•G. 1. Site of measurements off the west coast of Vancouver Island.

Scattering-strength profiles in three octave bands between 640 Hz and 10.24 kHz were obtained in March

1968 at the location shown in Fig. 1, 48ø22'N 126ø42'W

in the northeast Pacific Ocean--a position about 80 nautical miles off the entrance to the Straits of Juan de Fuca. Water depth was 1000 f. The profiles were ob-

HYDROPHONE

PRE- AMP. &

SHOT BOX

DE TONATORS

Fro. 2. The expanding shock wave generated by detonation of one of the explosive charges in the pod, pictured several milliseconds after detonation. Scattered returns that reach the listening hydrophone are indicated by the arrows. The insert shows the hydrophone and pod configuration.

772 Volume 46 Number 3 (Part 2) 1969


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VO[,UME SCATTERING IN TIlE NORTHEAST PACIFIC OCEAN

20 dB.-T-'

SIGNAL

BLOCKIN•

Fro. 3. Rectified and smoothed pressure level • versus time traces of the scattered returns in the

5120- to 10240-Hz band observed at different NO•Sœ-• depths during daylight. The vertical bars indicate a peaking in received level corresponding to energy • reflected from a well-defined layer above or below the hydrophone-pod combination. At the 1355-ft depth, the hydrophone is in the layer. At the 730-ft level, the reflected energy from the layer and surface are arriving simultaneously. Above 730 ft, there is a poorly defined peak corresponding to reflection from a near-surface layer.

220'

'•'• •• 220'

345'

480'

550'

625'

I 680'

730'

812'

915'

I 1015'

I

1140'

1355'

SHOT DEPTH

tained using electrically detonated Seismocaps (Cana- dian Industries Ltd.--TNT equivalent 0.00146 lb) located close to a listening hydrophone. The spectral- energy flux density of these charges was obtained as a function of depth for this experiment and will be re- ported separately. A pod of 12 caps and a hydrophone were lowered to various depths. After detonation of a cap at each selected depth, the close-in scattered returns were observed, permitting depth dependence of the scattering to be determined. Spectra of the scattered energy were obtained for most shots.

Profiles were obtained during the daylight hours between 1500 and 1615 hours local time and after sunset

in the period 1850 to 2100 hours local time. Scattering layers were observed near the surface and in the region of 1200 ft. The near-surface scattering layer showed a marked increase in scattering strength after sunset.

I. METHOD

Figure 2 illustrates the technique of observing the acoustic scattering in situ. The volume scattering

equation developed by Urick la may be used to obtain scattering strength explicitly from this type of experiment. It is necessary, however, to consider the effect of pulse length, if scattering observations are to be made close to the charge-hydrophone pair. Consequently, a more general form of the scattering equation, in which pulse length is accounted for, is developed in Appendix A. However, it is evident, a priori, that the level of received sound intensity observed a short time (say less than 50 msec) after the receipt of the shock wave from the charge will be closely related to the scattering- strength value associated with the depth at which the monostatic charge and receiver are situated. The signal from the hydrophone is recorded broad-band up to 10 kHz at a number of gains on an FM tape recorder, and, in addition, is filtered into octave bands, logged, rectified, and then displayed on a multichannel strip-chart recorder. There is a difficulty with this latter method owing to the slow recovery of the system after the

za R. J. Urick, "Generalized Form of the Sonar Equations," J. Acoust. Soc. Amer. 34, 547-550 (1962).



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SCRIMGER AND TURNER

SIGNAL

BLOC..K I NG-,,•

SHOT DEPTH

250'

250'

500'

690'

NOISE .--• 655'

I

• 750'

• 880'

I '""' • ,,50' 1300'

Fro. 4. Rectified and smoothed pressure level versus time traces of the scattered returns in the

5120- to 10 240-Hz band observed at different depths during nighttime. The vertical bars indicate a peaking in received level corresponding to energy reflected from a well-defined layer above or below the hydrophone-pod combination. At the 1150-ft depth, the hydrophone is in the layer.

initial overload due to the arrival of the shock wave.

This difficulty was largely overcome by removing the input to the logarithmic system during the period of reception of the shock wave. A typical set of rectified and smoothed traces of the pressure-time record, in which the period of switch o.17 can be seen, is shown in Figs. 3 and 4. The logarithmic amplifying and recording gear has already been described in a previous report. TM However, for this experiment, a switch used to block the shock wave was incorporated between the post- amplifier and the octave-band filters.

II. RESULTS

Scattering-strength profiles are shown in the three octave bands between 640 Hz and 10.24 kHz for day and night conditions in Figs. 5-7. Some difficulty was encountered in reducing the pressure-time curves, such as shown in Figs. 3 and 4, owing to the presence of a

•4 D. W. Brown, "A Logarithmic System for Monitoring High Dynamic Range Signals from the Acoustic Backscatter of Under- water Explosions," Defence Res. Estab. Pacific Rept. 64-3, Victoria, B.C. (1964).

switching transient at the time of switch on of the block- ing switch. This transient, which can be seen in Figs. 3 and 4, was evident in all traces except for those sho•ing high levels of scattered sound pressure. The ambiguity associated with the switching-on transient was removed by assuming that the medium in the vicinity of the source (i.e., within 100 ft) was homogeneous and, therefore, that the decay in pressure level due to scattering close to the receiver wo'ald show a 20 logt dependence on time. Accordingly, a template was made up showing a 20 logt decay, as predicted in Appendix A. This template was used to find a match on a portion of the pressure level versus time trace close to the time of switch on.

Only that portion of the trace that matched the 20 logt template was used to determine My(z). When scattering levels were high, a match with the template was obtained over extended time intervals (100 msec). In these cases, values of pressure level were read at various times, e.g., 50, 75, and 100 msec, and the scattering strength calculated for each set of measurements. For a perfect 20 logt decay, these values of scattering strength would be the same, which was found to be the ca, se.



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VOLUME SCATTERING IN THE NORTHEAST PACIFIC OCEAN

I0 log M z I0 log M z

-,oo •8 -80 -80 -,oo •8 ~80

su. FACE t ' ] t I ---•---] I I SU.FACE [ i---------i------+- + -. +

500 --

t+ i e 5120 o - 10240 Hz.

•ooo • +•----+ PRE- SUNSET

O-- -- --O POST- SUNSET e//

Fro. 5. Scattering-strength versus depth profiles in the 5.12- to 10.24-kHz band.

-60

/

/• e 2560--5120 Hz.

I000 -- • ..... SUNSET • 0 .... • POST-- SUNSET

/ 1500 FT. --

Fro. 6. Scattering-strength versus depth profiles in the 2.56- to 5.12-kHz band.

Spectra of the energy scattered from various depths were obtained using digital-analysis techniques on the broad-band recordings and are shown in Figs. 8 and 9 for the post- and presunset periods, respectively. The portion of the signal produced by the scattered returns chosen for spectral analysis was 12 msec in length and was centered •,50 msec after the instant of detonation. The $0-msec time was chosen in order to take into

account the inaccuracies due to finite pulse length as described in the Appendix; the 12-msec signal length was long enough to provide an adequate sample for analysis (sampling rate was 40 000 samples/sec and the analysis bandwidth and spacing of spectral estimates were 400 and 200 Hz, respectively) but not so long as to exaggerate the corresponding acoustic-shell thickness depicted in Fig. 2. The shell dimensions were thus 125 ft in radius, 30 ft thickqa sampling volume of about 220X l0 s yd. *

III. DISCUSSION

The technique employed clearly shows the presence and movement of the scattering layers, as can be seen at once from the pressure versus time curves of Figs. 3 and 4. The deep scattering layer at about 1200 ft produces in classic form the predicted 20 log! decay in scattered returns received within the layer. In Fig. 3, the small amount of scattering at shallow depths contrasts sharply with the higher levels seen in Fig. 4 in the same (200-800 ft) depth range. A hump, visible in most of the traces of Figs. 3 and 4 (shown by the vertical bars), is attributable to reflection from the scattering layer when the receiving hydrophone and

pod are not situated in the layer. In Fig. 4, the pod and hydrophone were placed both above and below the layer, while in Fig. 3 only traces corresponding to the equipment located above the layer were obtained, owing to the ship's drift, which limited the depth to which the cable could be lowered. However, there are indications in Fig. 3 that correspond to the daytime condition of reflection from a near-surface layer.

-i

SURFACE -

I000 l I

1500 FT. •

I0 log M z

0 d8. -90 -70

80-- 2560 Hz.

+•--+ PRE-SUNSET

• O--- -- ---(9 POST-SUNSET

Fro. 7. Scattering strength versus depth profiles in the 1.28- to 2.56-kHz band.



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SCRIMGER AND TURNER

N

io dB.

POST- SUNSEeker_ •ø"X, 31

670'

1350';•,• V v BAR INDICATES -93 dB.

I I I I I 2 io

FREQUENCY K Hz.

Fro. 8. Frequency spectra of the energy scattered at various depths at night. Each specimen has been given an arbitrary vertical displacement from its neighbor for the sake of clarity, and the heavy horizontal bars on the left side of each spectrum indicate a scattering-strength value of --93 dB. The magnitude of a 10-dB increment in scattering strength is shown in the bottom left corner. The dotted lines connecting the spectra between 790 and 1350 ft indicate those line components that exhibit a S•JRE)I dependence.

The absolute values of scattering strength presented as depth profiles obtained for the three octave bands between 1.28 and 10.24 kHz (Figs. 5-7) show the change in form corresponding to daytime and nighttime

I I I • • 2 I0

FREQUENCY KHz.

Fro. 9. Frequency spectra of the energy scattered at various depths during daylight. Each specimen has been given an arbitrary vertical displacement from its neighbor for the sake of clarity, and the heavy horizontal bars on the left side of each spectrum indicate a scattering-strength value of --93 dB. The magnitude of a 10-dB increment in scattering strength is shown in the bottom left corner.

conditions. An increase in scattering strength of about 5 dB is seen in passing from each octave band to the next highest. Two prominent layers are visible--one at 1200 ft, the other at 300 ft; the deeper layer remains essentially unchanged over a diurnal period; the shallow layer shows a marked increase in scattering strength and thickness after sunset. Changes over the sunset in the lowest frequency band, 1.28-2.56 kHz, are different. Here the surface layer is little affected, while the deep

Q

1400 - -

__

1200--

__

I000 - -

8O0

RADIUS--0.2

+ -I.-

m

7 8

FREQUENCY ( KHz )

Fro. 10. The pressure dependence of the ii'n½ components in the spectra of Fig. 8 between the 790- and 1350-ft depths. The two curves depict a (PR•.ssua•.)• dependence for swim bladders of 0.29-cm and 0.26-cm radius. The values for the line components from Fig. 8 are indicated by



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1200-ft layer appears to show a decrease in scattering strength at night.

The scattering-strength spectra shown in Fig. 8 and 9 are consistent with the profile data. These spectra are of course best viewed in three-dimensional form, with the scattering strength plotted as a surface against frequency and depth. The most notable feature of the spectra is the strong line at 4.8 kHz. This shows up particularly in the presunset spectra of Fig. 9, where the broad-band level of scattered returns is much less

than that experienced at night. It should be mentioned at this point that the spectra of noise samples taken just prior to each shot do not show any trace of this line. However, since the frequency of this component appears to be independent of depth, it is difficult to account for the line in terms of current hypotheses, which attribute scattering to gas bubbles that are the swim bladders of fish. Such scatterers exhibit a resonant

frequency of scattering that depends on (PRESSUR•)• for a vertically migrating organism that maintains neutral buoyancy by keeping the volume of its swim

bladder constant; or on (PRESSUre.) 5• for an organism that maintains its swim bladder at constant pressure. Additional data will be sought in order to examine this phenomenon more closely. In addition, the contribution to acoustic scattering by siphonophores, which some investigators 15.16 consider significant, requires further study. Line components that show a (•ssu•.)l dependence show up in the deep layer of Fig. 8 (i.e., the spectra taken at depths of 790, 915, 1060, 1190, and 1350 ft). These components occupy the 5.6- to 7.5- and 6.4- to 8.3-kHz frequency range. Their fit to a (•R•s- su•)« dependence is shown in Fig. 10. More data is needed in order to comment fully on these line components-suffice it to say that they correspond to swim bladders of 0.29 and 0.26 cm radius.

•5 G. V. Pickwell, E.G. Barham, and J. W. Wilton, "Carbon Monoxide Production by a Bathypelagic Siphonophore," Science 144, 860-862 (1964).

16 ½½ ' G. V. Pickwell, S•phonophores and Their Released Bubbles as Acoustic Targets within the Deep Scattering Layer," Proc. U.S. Navy Symp. Military Oceanog., 4th, Washington, D.C., 1, 383-395 (1967).

Appendix A

In this Appendix, an expression is derived relating the observed scattered returns to the scattering strength of the medium. Also, the errors incurred by using this expression for short-range returns are examined.

Assume a shock wave is generated at t-0 at origin R=0 of Fig. A-! (a), and that the shock wave spreads spherically in a uniform scattering medium to distance R in time t. In Fig. A-! (b), let r be the time measured within the pulse of total length r0 as seen by a frequency- band-limited receiving hydrophone located at the origin, and let I(r) be the time-dependent intensity within the pulse for the frequency band of interest.

Consider an element of the shock pulse between r and (r+dr) (where 0< r< r0) as shown in Fig. A-1 (a). A hydrophone located at R=0 of Fig. A-l(b) will receive backscattered energy from a spherical shell due to this element of the pulse at time t given by

t= 2R/C-- r, (A1)

where C is the sound velocity in the water. The radius R of the shell is given by

R=«C(t+r), (A2)

and the thickness of the shell dR is given by

dR=«Cdr. (A3)

The shell volume dV is

(a)

dV= 4•'R2dR, (A4)

= (•'/2)C3(t--r)2dr. (A5)

Now define I(t) as the intensity at time t of the backscattered sound received by the hydrophone and dI(t)

R-•c(t-'C)

I

d R ,, ,•- c d '['-

(b)

f(T)

•_T'o

T •

Fro. A-! (a). The spherical shell from which the acoustic-pulse element of length dr is scattered at time t; (b) the time dependent intensity in the acoustic pulse.

The Journal of the• Acoustical Society of America 777


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SCRIMGER AND TURNER

O.

400-

_

800 -

_

1200 -

_

1600 -

_

2O00 -

MAX ERROR (dB) i.o 2.0 30

I I -

50 msec

Fro, A-2. The maximum error due to finite pulse length to be expected in the scattering-strength values derived from intensity measurements taken at three different times after initiation of the

pulse plotted as a function of the depth of detonation of the explosive source.

as that contribution from the pulse element dr. Also define M, as the fraction of energy backscattered by a unit cube of the volume such that M, satisfies the condition

I,./I,=M,,dF/r •, (A6)

where I, is the incident plane-wave intensity on the unit volume dV, and I• is the intensity of backscattered energy received at a point at distance r from dV. Then the intensity of sound backscattered from the volume element dV is

MvdVI(r)/R 2. (A7)

The instantaneous intensity dI(t) due to the volume element dV observed at the receiving hydrophone is given by

dI (t) = M,dVI (r)/R 4, (A8)

= W],

and the time-dependent total scattered intensity r(t) is

(t) = o - The integral is solved using a Taylor's series expansion for the integral and the Lagrangian form of the re-

mainder, in which case we obtain

8,rMv ;r0 I (t) = C-•--J o I (r)dr-½ - 16.z-M, ;•o O(r/t)I(r)dr Ct 2 J o I-•-o(•-/t)-I •'

8,rMvE 16,rM• ;•o O(r/t)I (r)dr + , Jo

where 0<0<1, and E is the total energy due to the charge in the frequency band of interest observed at unit distance.

The first term in the above expression can, of course, be obtained by making the proper substitutions in the equation given by Urick. 13 The second term may be considered a correction that grows in importance as the pulse length becomes an appreciable fraction of the time of observation, which is the condition that prevails when scattering observations are being made close to the charge-hydrophone combination. An error analysis is presented later, which defines the conditions under which this term may be omitted, since its inclusion would require a knowledge of the pulse shape. Hence, the first term alone is used to relate the scattering strength M, to the observed scattered intensity.

Using the relationship

I(t)=p2/pc, (A12)



10:15:42


where pc is the acoustic impedance of the medium, the scattering strength can be related to the received sound- pressure amplitude, i.e.,

p•'= Oc(8rrM,,E/CF) ; (A13)

if p is measured cgs units, then pc is in similar units, C is the sound velocity measured in yards/sec, E is the total energy in cgs units in the bandwidth of interest at 1 yd. Hence,

p•'=8•rM,,E(152 000)/1616F dyn/cm •'. (A14)

Since E is often expressed as the energy flux at 100 yd from the source, it is convenient to make the conversion in the above expression. Hence,

p•=8rrM•,E•oo104(152 000)/1616F dyn/cm 'ø (A15)

or, taking logs,

20 logp= 10 logM•+ 10 logEz00-- 20 logt+ 73.7. (A16)

Error Analysis

The maximum error e to be expected when using the expression given above is given by the term for the

remainder in the Taylor series expansion, viz.,

16•rM•

CF O O(,-/t)z - , (A17)

16•rM,, ;"ø OrI (r)dr e_• -- . (A18) do

If 0 and r are maximized, i.e., 0-1 and r= to, we have

16•rMvroE •_< . (A19)

Ct•(1 --to/t) •

The maximum error expressed as a percentage e% in using the expression given in Eq. A16 is, therefore,

27'0 X100%. (A20)

Figure A-2 shows the maximum errors as a function of depth that might be expected from scattered returns observed at 25, 40, and 50 msec after initiation of the pulse. The pulse length r0 is taken as the bubble-pulse interval at the depth of detonation.



10:15:42

volume scattering-strength profiles in the northeast pacific ocean

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