Television-echo sounder observations of midwater sound scatterers

RICHARD H. BACKUS and HAROLD BARNES*

(Received 30 January 1957)

Abstract--Simultaneous observations with echo sounders and underwater television have been made both in and out of deep scattering layers. All the evidence indicates that, with the type of sonic gear used, micro plankton was not responsible for either the observed deep scattering layer or for detectable echo sequences on the short range equipment. It was possible to " see " for the first time the sound scatterers responsible for the typical echo sequences from single animals and to confirm the deductions of JOHNSON et aL (1956) that individual small fishes give rise to the sort of echo sequences that have been seen in deep scattering layers. The type of echo record and its relation to the number of fish scattering the sound was observed.

INTRODUCTION

INOIV~UAL animals both in and out of deep scattering layers have recently been resolved by the use of short range suspended echo sounders (JorlNSON et al., 1956). Furthermore, using a camera mounted alongside the echo sounder, photographs were taken when strong echo sequences were being recorded in art attempt to identify the origin of the sound scattering. In these experiments images of fishes on the photo- graphs were always aecompained by strong echo sequences ; on the other hand strong echo sequences were not invariably accompanied by images of fishes and correlations with other types of organisms could not be established. Some of these uncertainties are without doubt due to the fact that the fields of view of the camera and echo sounder may not be identical ; this problem may be largely overcome if the approximate acoustic field is viewed directly and continuously. Such viewing is obtained if the camera is replaced by an underwater television unit. It has already been shown that plankton and fishes may be viewed and sometimes counted and identified by underwater television and some success has been reported in identifying the origin of traces on conventional echo sounder records (BARNES, 1955).

APPARATUS AND PROCEDURE

The television equipment has been described (BARNES, 1953). In some of the experiments a loose fitting canvas cover was fastened to the framework. A body of water at any level is therefore to some extent trapped so that movement of water past the window is reduced and the plankters are more easily recognized. The echo soundings were made with either an Edo UQN-1B (12 kc) transducer operating at the surface or a suspended echo sounder consisting of transmitting and receiving UQN-1B transducers operating at 34 kc. The Woods Hole Oceangraphic Institution precision graphic recorder (KNOTT and HERSEY, 1956) was used with both systems. The suspended echo sounder transducers were mounted on the gantry of the television camera and aimed at a point about seven feet away on the camera's optical axis.

*I wish to acknowledge both the receipt of a Research Fellowship from the Woods Hole Oceano- graphic Institution and leave of absence granted by the Council of the Scottish Marine Biological Association.

116

Television-echo sounder observations of midwater sound scatterers 117

The usual brass target ball (JOHNSON et al., 1956) was hung below the camera also on the optical axis but at a distance of about nine feet, The depth of the equipment was determined either by echo-sounding from the surface or by a wire measuring device on the supporting cable.

EXPERIMENTS USING THE SURFACE ECHO SOUNDER

A strongly developed deep scattering layer as well as a zone of weaker scattering near the surface was observed on November 8th, 1956 at 39 ° 46'N, 70 ° 5YW in water of 920 fathoms (Fig. 1). The dominant layer was between 90-190 feet with its axis (level of strongest scattering) at 140 feet. Beginning at 00.10 hr the television camera with the canvas cover and two lights near the window was lowered by stages into the layer ; so rigged the observations relate primarily to plankton. At each depth notes were made from the screen picture and a series of counts taken by the method already described (BARNES, 1955). Between 40 and 230 feet many small copepods were seen (about the size of adult Calanus finmarchicus) and subjective estimates indicated little change i'n population density with depth. A number of euphausids were present at 120 feet. It was evident from the echo sounder record that with the lights continuously in use the intensity of sound scattering was reduced in the immediate vicinity of the camera. The lowering was therefore repeated (by which time the layer extended from 115 to 200 feet, axis 170 feet) but turning the lights on at each successive depth only for sufficient time to make a count. Each set of counts (70-100) took about five minutes during which time there was no evidence of any change in population density. The results are given in Table 1.

Table 1. Mean number of plankters per count, 8 November, 1956

04.20-05.00 hr 13.50-14.40 hr

Depth (ft)

40 75

115 150 190 230

No.

1'2 0'96 0.79 0'58 0-58 0"59

Depth (ft)

40 80

120 160 195 230

No.

1.50 1.16 1.03 0.77 0.52

The results show that of the animals counted the greatest population density was at 40 feet ; there is no evidence that they were concentrated at the level of the scattering layer (115-200 ft) seen on the echo sounder unless the lights are considered to invalidate the results (see below for discussion).

The experiment was repeated during daylight on the same day at a position within five miles of the original observations ; by this time the deep scattering layer had migrated downwards and now lay between 960 and 1,680 ft with axes at 1,140 and 1,440 ft. The pictures appeared similar to those of the earlier experiment. The counts are shown in Table 1.

It is evident from this table that the population density and its relative distribution with depth was substantially the same for these small plankters during the night when the camera was within the deep scattering layer as during the day when the latter was well below the camera. This indicates that these small organisms were not

118 RaCnAaD H. BACKUS and HAROLD BARNES

responsible for the migrating deep scattering layer observed on the echo sounder. Although it was difficult to ascertain whether or not the lights affected the near

surface scatterers it was evident from the echo sounder record that, as alread3~ pointed out, the intensity of scattered sound was reduced- presumably by repulsion or dispersion of the scatterers- as the lights entered the deep scattering layer. When the camera was well within the layer, switching the lights on and off gave a rapid and marked change in the echo sounder record (Fig. 2). That the small plankters counted on the screen remained constant under such conditions indicates even more strongly that they were not responsible for the observed scattering layer. Furthermore the speed at which the sound scatterers moved away when the lights were turned on suggests that larger, strong-swimming animals were responsible. This behaviour resembles closely that of certain fishes, (RICHARDSON, 1952).

EXPERIMENTS WITH THE SUSPENDED ECHO SOUNDER

These observations were made some 25 miles to the south of those described above, at 39 ° 22'N, 70 ° 52'W on November 8th, 1956 in water of 1,350 fathoms; a less well developed scattering layer was present between surface and 250 ft with its axis at 120 ft. The surface echo sounder was used only to check the position of the suspended echo sounder-television equipment with respect to the scattering layer, the echo sounder records being taken from the suspended unit. The television camera was rigged for full forward viewing with the whole complement of lights (2 kW) and without a canvas cover. Notes from observations of the television picture screen were entered on the echo sounder record and for part of the time a remote marker switch was operated by the television viewer to locate precisely the screen observations on the echo sounder record.

The observations extended from 20.30 to 24.00 hr during which time fishes and small plankters (mainly copepods) were seen. The last were present throughout the whole period and they never gave rise to detectable echo sequences. In contrast there was a high correlation between the presence of fish in the picture and a strong echo sequence on the echo sounder record. Although the correlation was high it was not complete. Occasionally fish were seen on the screen when either no echo or very faint echoes were recorded. As indicated by subsequent experience this was pro- bably the result of imperfect alignment between transducers and television camera. On the other hand, the fishes were not specifically identified and therefore it is possible that some had no swimbladder and consequently low target strength. In contrast, echo sequences were occasionally obtained when fish wero not recorded as present on the picture. This too may have been the result of misalignment but in this case the possibility that fish or other animals may have passed unobserved on the screen must be taken into account.

Most of the individual observations were of short duration, the fish image on the screen and the accompanying echo sequence usually lasting a few seconds. Of particular significance and interest was a set of observations in which a number of fish 3in.-7in. in length), probably in this case attracted by the light, swam in and out of the field of view for about forty minutes.

The echo sounder used can resolve individual targets only ~vhen their ranges differ by more thanits ping length of about 0.2 milliseconds (about I ft). In these experiments one, two and three fish were individually resolved (Figs. 3-5). When a compact

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Fig. 4. As Fig. 3 but with fish in view on screen. Two echo sequences are discernible although much of one is obscured by the echo sequence from the target ball. A third echo sequence is discernible in the early part of the record at a range beyond that of the

television camera.

Fig. 3. Suspended echo sounder record made when a single fish was in view on

television picture screen.

Fig. 5. As Figs. 3 and4. 3 fish were viewed by the television and 3 echo sequences are discernible except when two or all fish are at same range.

Fig. 6. As Figs. 3! 4 and 5 with 6-8 fishes present m a compact group. Individual fish are not resolved and a more complex echo sequence was produced.

Television-echo sounder observations of midwater sound scatterers 119

group of 6-8 fish were present a single complex echo sequence was registered (Fig. 6) with continuous echo series whose length was greater than that of the ping. In such a case ; i.e., with several scatterers at the same or nearly the same range interference patterns (alternating maxima and minima in echo intensity resulting from phase differences) such as were seen by JOHNSON et al. (1956) might be expected. The absence of such interference patterns is probably to be explained by the fact that our recordings are overloaded.

The observations directly demonstrate that fishes give the type of echo sequence obtained by JoHNsoN et al. (1956) using the short range suspended echo sounder in deep scattering layers ; small plankters at the population density present on these occasions produced no detectable echo sequences. We have no results as yet on other animals, e.g. squid, which have been proposed as the source of sound scattering in deep scattering layers.

DISCUSSION

These preliminary experiments indicate that television is useful in identifying the types and sizes of animals which produce strong echo sequences. Correlations between animal and echo sequence may be made. The establishment of such correlations is independent of any reaction of the animals to light so long as they are not completely repelled from the field of view. Observations may be made at any depth with either natural or artificial light and a picture built up of an organism-echo sequence correla- tion.

In spite of the reactions of the animals comprising deep scattering layers to light it is believed that further work on these lines would be valuable. The lights may be switched on only when observations are made when it is unlikely that all the scatterers will be completely repelled.

Acknowledgements--We are indebted to the officers and crew of R/VBear, members of the underwater sound group at The Woods Hole Oceanographic Institution, particularly Mr. CHARLES S. Im,rIS, and Mr. A. G. RANDALL of The Marine Station, Millport, Scotland for their assistance. This work was done in part under contract NObsr-72521 with the Bureau of Ships, U.S. Department of the Navy.

Woods Hole Oceanographic Institution. Contribution No. 894 from the Woods Hole Oceanographic Institution

REFERENCES BARNES H. (1953) Underwater television and research in marine biology, bottom topography

and geology. Pt. 1, A description of the equipment and its use on board. Dtsch. Hydro. Zeit. 6, 123-133.

BARNES H. (1955) Underwater television and research in marine biology, bottom topography and geology. Pt. 2, Experience with the equipment. Dtsch. Hydro. Zeit. 8, 213-236.

JOHNSON H. R., BACKUS R. H., HERSEY J. B., and OWEN D. M. (1956) Suspended echo- sounder and camera studies of midwater sound scatterers. Deep-Sea Res. 3, 266-272.

KNOTT S. T. and HERSEY J. B. (1956) Interpretation of high-resolution echo-sounding techniques and their use in bathymetry, marine geophysics and biology. Deep-Sea Res. 4, 36-44.

RICHARDSON I. D. (1952) Some reactions of pelagic fish to light as recorded by echo- sounding. Min. Agric. and Fish. Fish. Invest. Ser. 2, 18 (1), 1-20.