the echolocation ability of the beluga (delphinapterus leucas) to detect targets in clutter

The echolocation ability of the beluga (Delphinapterus leucas) to detect targets in clutter

Charles W. Turl

Naval Ocean System Center, Hawaii Laboratory, Kailua, Oahu, Hawaii 96733

Debra J. Skaar

SEA CO Incorporated, 2835 Nimitz Boulevard, San Diego, California 92106

Whitlow W. Au

Naval Ocean System Center, Hawaii Laboratory, Kailua, Oahu, Hawaii 96733

( Received 16 July 1990; accepted for publication 21 September 1990)

A beluga (Delphinapterus leucas) was trained to detect different length cylinders in front of a clutter screen at five separation distances. The clutter screen consisted of 300, 5.1-cm-diam cork spheres located behind the targets. The clutter screen was used in two positions: perpendicular to the body axis of the beluga (90 ø grazing angle) and rotated 22 ø (68 ø grazing angle). Five hollow stainless-steel cylinders (3.2-cm-diameter and 0.38-cm wall thickness) of different lengths ( 14, 10, 7, 5, and 3 cm) were used as targets. Detection data were collected on the beluga's performance as a function of the separation between targets and the clutter screen. The beluga's performance was above 80% correct detection for the 14- and 10-cm cylinders as the separation distance decreased from 10.1 to 5.1 cm for both grazing angles. For all targets except the 3-cm cylinder, the beluga's detection was higher at 0-cm separation than at 2.5-cm separation. The beluga's detection performance was higher at 68 ø than at 90 ø grazing angle. The target strength of the cylinders and the clutter screen was measured in peak-to-peak amplitude and the energy of the incident and reflected echoes. For 0-cm target-to-clutter screen separation at 90 ø grazing angle, the echo-to-reverberation ratio at 50% correct detection was -- 1.0 dB, based on the peak-to-peak measurements, and - 5.1 dB based on the energy measurement. The results of this experiment suggest that a beluga can detect targets in 3.6 to 5.4 dB more reverberation than previously reported for a bottlenose dolphin ( Tursiops truncatus).

PACS numbers: 43.80.Jz, 43.80. Lb, 43.80. Nd

INTRODUCTION

The target detection sensitivity of any sonar is limited by interfering noise and reverberation. Turl et al. (1987) compared the target detection capabilities of a beluga and a bottlenose dolphin (Tursiops truncatus) in the same masking noise experiment. Masking noise with a flat spectrum from 30-160 kHz was projected from a spherical transducer located in front of the animal hoop station in line with the target. Detection performance was determined with a 7.62-cm- diam sphere at 16.5 and 40 m, and a 22.86-cm-diam sphere at 80 m. Target detection performance was determined as a function of masking noise level at each target range. At the 75 % correct response threshold, the echo-to-noise ratio was approximately 1.0 dB for the beluga and approximately 10 dB for the bottlenose dolphin. Au and Penner ( 1981 ) previously reported the echo-to-noise ratio at 75% correct response threshold for two different bottlenose dolphins was 7.4 dB and 12.8 dB. The difference in the echo-to-noise ratio

estimate at threshold for the beluga and bottlenose dolphin may include differences of echolocation emission strategies and signal-processing capabilities (Turl et al., 1987).

Reverberation is different from masking noise since reverberation is caused by the sonar itself, and is composed of

echoes scattered back to a sonar from objects and inhomo- geneities in the water and on its boundaries. The spectral characteristics and intensity of reverberation are similar to and proportional to the projected signal; therefore, target detection cannot be improved by increasing the intensity of the projected signal. In a reverberation-limited environment, target detection depends on the sonar system to dis- criminate between the target echoes and false targets and clutter that contribute to the reverberation.

The ability of a bottlenose dolphin to detect different length cylinders in front of a clutter screen was reported by Au and Turl (1983). Backscatter measurements of the tar-

gets and clutter screen were made with simulated dolphin signals to determine the echo-to-reverberation ratio (E/R ) based on the energy of the incident and reflected signals (E/RE ) and on the peak-to-peak values (E/Rp_p ). The dolphin's detection performance when the targets were located within the plane of the clutter screen indicated that the 50% correct detection threshold corresponded to E/RE value of 0.25 dB and E/Rp_e value of 2.6 dB.

In this study, a beluga was trained to detect different size targets in front of a clutter screen. Backscatter measurements of the clutter screen and targets were made so that the E/R could be determined.

896 J. Acoust. Soc. Am. 89 (2), February 1991 896

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I. METHODS AND MATERIALS

The experimental animal was a female beluga (D1637F). She had been the subject in other echolocation experiments and had received training in various experimental procedures. The experiment was conducted in San Diego Bay, California. The test enclosure was a 10 X 10-m floating pen with nylon net around the perimeter and bottom (Fig. 1 ). An instrumentation hut opposite the animal station provided shelter for the experimenter and electronic equipment. A 30-cm-diam hoop was submerged with its center at a depth of 1 m. An acoustic screen, that could be raised and lowered, was located in front of the hoop. When the screen was lowered, the beluga in the hoop could ensonify the target. The clutter screen and targets were on a 6 X 6-m floating pen, 5 m in front of the animal station.

The clutter screen consisted of 300 5.1-cm-diam cork

spheres with their centers spaced 15.2 cm apart. The screen was 8 m from the hoop station. The center of the Spheres was tied to braided nylon lines (20 spheres/line). The lines were tied to a 7.62-cm PVC pipe that was 30 cm above the water. A galvanized pipe, 36 cm below the last row of cork spheres, provided weight to keep the clutter screen vertical in the water.

Five target separation distances (AR) of 0, 2.5, 5.1, 7, and 10 cm in front of the clutter screen were used. At 0-cm

separation, the targets were located within the plane of the clutter screen. The target was lowered so that the center was 1 m below the surface at the same depth of the center of the animal hoop. The experimenter could lower and raise each target from inside the instrumentation hut.

A trial started with the animal in front of the experimenter. The acoustic screen was in the raised position and the targets out of the water. The beluga swam across the pen and inserted its head into the hoop station. A target was then either lowered into the water or all targets left out of the water. After the acoustic screen was lowered, the beluga echolocated for as long as it wanted. The animal swam back to the experimenter and responded by touching either a paddle on the right, indicating a target present, or a paddle on

FIG. 1. Experimental and floating pen configuration in San Diego Bay, Cal- ifornia. To show the position of targets, portions of the clutter screen have been removed in the figure.

the left, indicating a target absent. The animal was rewarded with fish for correct responses.

Data were collected in a morning and an afternoon session. A session consisted of 50 trials divided into five blocks

of ten trials. Equal numbers of target-present and target- absent trials were distributed randomly in a block based on modified Gellerman tables (Gellerman, 1933). Each target was assigned a separation distance before the first session of the day. The separation distance for each ten-trial block was randomized each day in such a manner that, after 5 days, each target had been tested 20 times at five separation distances. The clutter screen was used in two positions: perpendicular to the beluga ( 90 ø grazing angle) and rotated 22 ø ( 68 ø grazing angle). Each target was tested a minimum of 100 trials at both grazing angles.

Each echolocation click produced by the beluga was digitized using a Macintosh Plus computer equipped with an analog-to-digital converter (ADC). An Edo-Western 6166 hydrophone was located 2 m from the station hoop, 1 m below the surface. To prevent the system from triggering on snapping shrimp, the signal from the hydrophone was band- pass filtered between 20 and 200 kHz. The experimenter started data collection of each trial by turning on the ADC. The trial number, target separation, and the animal's response were cataloged with each click train and stored in random-access-memory (RAM) along with the number of clicks, interclick intervals in ms, and the peak-to-peak source level for each click. At the end of a session, these data were stored on a diskette. During each block of ten trials, the waveform from each click from one target-present and one target-absent trial was digitized at 500-kHz sampling rate ( 128 points per click) and stored on diskette. The peak frequency, bandwidth, and source level for each digitized pulse were measured on an Analogic Data 6000 spectrum analyz-

Targets were hollow stainless-steel cylinders (3.2-cm diameter and 0.38-cm wall thickness), 14.0, 10.0, 7.0, 5.0, and 3.0 cm long. By using cylinders with the same wall thickness and diameter, but of different lengths, the target strength could be varied without changing the echo structure. Backscatter measurements of the clutter screen and

targets were made using the procedures given in Au and Snyder (1980). The backscatter measurements of the clutter screen were made with the transducer located so that its 3-

dB beamwidth covered approximately the same area of the clutter screen that the beluga's beam (Au et al., 1987 ) would cover at a distance of 8 m. Since the cork spheres could move +_ 2 cm in the horizontal direction, each echo return would

be slightly different from the preceding one. The target strength of the clutter screen was averaged over 20 signals from the transducer.

The echoes of five consecutive signals with the 10-cm target placed 10.1 cm in front of the clutter screen are shown in Fig. 2. The echoes from the target look similar, while the echoes from the clutter screen exhibited some variations.

The clutter screen echoes consisted of reflections from many scattering centers all moving slightly relative to each other causing the echoes to vary from ping to ping. Values of the target strength based on energy (TSE) and on maximum

897 J. A½oust. So½. Am., Vol. 89, No. 2, February 1991 Turl et al.' Echolocation ability of the beluga in clutter 897


0 1.0 ms i ii I TARGET CLUTTER SCREEN ECHO

ECHO

FIG. 2. Echoes from the 10-cm-long target and the clutter screen, with a separation distance of 10.1 cm between the target and screen, for five consecutive pings.

peak-to-peak amplitudes (TSv_v) are shown in Table I for the clutter screen and the five targets. TSe was determined for the clutter screen by integrating the energy in a 1-ms window.

II. RESULTS

A. Performance

Figure 3 shows the results of the beluga's correct-detection performance as a function of the separation distance between the targets and the clutter screen ( AR ) at 90 ø [Fig. 3 (a) ] and 68 ø [ Fig. 3 (b) ] grazing angle. Only the target- present trials were used to generate the correct-detection curves. At both grazing angles the beluga's performance was greater than 80% correct detection for the 14- and 10-cm cylinders. Target detection accuracy decreased for all targets

TABLE I. Results of the target strength measurements of the clutter screen and cylinder targets. The target strength and echo-to-reverberation ratio based on the energies in the incident and echo signals are designated as TSe (dB) and E/Re (dB), respectively. The target strength and echo-to- reverberation ratio based on the maximum peak-to-peak amplitudes of the incident and echo signals are designated TSp_p (dB) and E/R p_p (dB).

Length TSv_v TSe E/Rp_p E/Re Target (cm) (dB) (dB) (dB) (dB)

Screen

90 ø '" - 25.8 q- 1.7 - !9.9 q- 1.0 68 ..... 27.8 ñ 1.3 - 20.3 q-0.6

Targets 1 14.0 -- 17.6 q- .2 -- 16.8 q- .2 d- 8.2 2 10.0 -- 19.3 q- .2 -- 18.3 q- .2 d- 6.3 3 7.0 -- 22.0 q- .2 -- 21.1 q- .2 d- 3.8 3 5.0 -- 25.1 ñ .3 -- 23.9 __ .5 d- 0.7 5 3.0 -- 29.3 ñ .3 -- 27.6 ñ .9 -- 3.6

d.3.1 d- 1.5 -- 1.2

-- 3.0

--7.7

IOO

80

= )

u. 40 •

• 5

20-- ß :3

GRAZING ANGLE -- 90'

I I I I 0 +2.5 +5.1 +7.6 +!0.1

lb)

!oo •

80

GRAZING ANGLE = 68'

20 i

I I I I 0 +2.5 +5.1 +7.6 +!0.1

a,q (ctl)

FIG. 3. Beluga's performance for target present trials as a function of AR for the five targets: (a) grazing angle = 90 ø, (b) grazing angle = 68 ø.

as AR decreased except at AR = 2.5 cm. For all targets, except the 3-cm cylinder, detection accuracy was lower at AR = 2.5 cm than at AR = 0. At 68 ø grazing angle, the beluga's performance was higher than with the clutter screen at 90 ø grazing angle.

The results for AR = 0 [Fig. 3 (a) ] are replotted in Fig. 4 as a function of E/R based on both energy and peak-to- peak amplitude. In this case the targets were within the plane of the clutter screen and the echoes from the target and clutter screen arrived at the animal simultaneously. The dashed lines indicate that the 50% correct detection threshold cor-

898 J. Acoust. Soc. Am., Vol. 89, No. 2, February 1991 Turl eta/.' Echolocation ability of the beluga in clutter 898


IOO

== 8o

I ,,"

I• ' ,I I ! a

-I0 -5 0 5 i0

E/R (dB)

300

200

IOO

ol

(a)

GRAZING ANGLE = 90"

I I I ;

+2.5 +5.1 +7.6 +10.1

nR (CM)

FIG. 4. Beluga's target detection performance as a function of the echo-to- reverberation ratio for a zero separation distance at 90 ø grazing angle. The open figures indicate echo-to-reverberation ratio based on energy and the closed figures indicate echo-to-reverberation ratio based on peak-to-peak amplitude. Also shown are the E/R values for bottlenose dolphin (after Au and Turl, 1983).

responded to an E/RE of -- 5.1 dB and an E/Rv_v of -- 1.0 dB. The E/R values for Tursiops (Au and Turl, 1983 ) are included in Fig. 4. The target detection sensitivity of both Delphinapterus and Tursiops in the clutter of reverberation is proportional to the E/R parameter in dB.

B. Echolocation signals

The average number of clicks per trial as a function of the separation distance for the five targets is shown in Fig. 5 at (a) 90 ø and (b) 68 ø grazing angle. The curves in Fig. 5 are for the correct detection trials. The number of clicks per trial generally increased as AR decreased from 10.1 to 2.5 cm. The beluga generally emitted fewer clicks per trial as the size of the target increased from 3 to 14 cm. The beluga emitted more clicks per trial for detecting the 10-, 7-, and 5-cm target at AR = 2.5 cm than at AR = 0 cm. With the clutter screen positioned at the 68 ø grazing angle, the beluga emitted fewer clicks per trial for all targets than with the clutter screen positioned at 90 ø .

Interclick intervals (ICI) of beluga click trains were greater than the calculated two-way travel time (TWT) for a signal to travel to the target and return. Average ICI for the beluga click trains were between 16 and 21 ms or approximately 6 to 11 ms greater than the TWT.

The peak frequencies of the echolocation clicks were similar (85 to 105 kHz) for all cylinders at the five separation distances; however, the bandwidth and amplitude were different at AR = 2.5 cm than at the other separation distances. For cylinders within the plane (AR = 0 cm) of the clutter screen and AR = 5.1, 7.6, and 10.1 cm, the predomi- nant bandwidth was between 30 and 35 kHz and the pre- dominant amplitude was from 200 to 205 dB re: 1/•Pa. At AR = 2.5 cm, 73% of the echolocation signals had bandwidths greater than 30 kHz compared with 65 % at AR = 0

(b)

300 TARGET (CM)

< 0 •4

.. I-I•o

=' 200, ß 3

• !o

0 +2.5 +5.1 +7.6 +10.1

AR (11'1)

FIG. 5. The average number of clicks per trial as a function of AR for targets at (a) 90 ø and (b) 68 ø grazing angle.

cm. Seventy percent of the signals hhd source levels of 210 dB or more for AR = 2.5 cm compared with 56% for AR = 0 cm. Figure 6 compares data for all cylinders com- bined for AR = 0 and 2.5 cm. The peak frequencies [Fig. 6 (a) ] are similar, whereas, the bandwidths [ Fig. 6 (b) ] are wider and amplitudes [Fig. 6(c) ] are higher at AR = 2.5 cm than AR = 0 cm.

To illustrate the difference in signal bandwidth, spectral-time plots for a trial at (a) AR = 0 and (b) AR = 2.5 cm for the 10-cm cylinder are shown in Fig. 7. The average waveform and spectrum for each click train are shown above the spectral-time display. The average peak frequencies are similar, whereas, the bandwidth is consistently wider (50 kHz) at AR = 2.5 than for the same cylinder at AR = 0.

DISCUSSION

The beluga's performance decreased as the separation distance between the clutter screen and targets decreased from 10.1 to 2.5 cm, for both grazing angles. Except for the 3-cm target at the 90 ø grazing angle and the 3- and 14-cm targets at 68 ø grazing angle, the beluga's accuracy was higher at AR = 0 than at AR = 2.5 cm. This behavior was unusual

899 J. Acoust. Soc. Am., Vol. 89, No. 2, February 1991 Turl eta/.' Echolocation ability of the beluga in clutter 899


(a) 50

• ,40

,.,_ :30

Z 20

eL I0

0 ß ß

70 75 80 85 90 95 100105110115

FREQUENCY (kHz)

,40-

• 5o

..=, IO

(b)

25 :30 :35 40 45 50 55 60

FREQUENCY ( kHz}

(c)

• '40 t u :30

• 2o

• IO

o

200 205 210 215 220 225

SOURCE LEVEL (rib re I pPa)

i •._•• SI = 216 • 256 It5

0 !00 200 FREQLIENCY (kHz)

ß • (kHz) see

(b)

L.•• •L = 218 dB ! fp=o9 kitz

! . 0 !00 200

FI•QUENCY (kHz)

!

o

F11EQUI•ICY (kHz)

FIG. 7. Spectral-time plots ofbeluga signals for a detection ofa 10-cm cylinder at (a) AR = 0 and (b) AR = 2.5 cm. The parameters shown in the average waveform and spectra are' SL = dB re: 1/tPa,f•, = peak frequency, and BW = 3-dB b'•ndwidth.

FIG. 6. Histograms of the digitized echolocation signals: (a) peak frequency, (b) 3-dB bandwidth, and (c) source level for AR = 0 and 2.5 cm.

since it seemed reasonable that performance would decrease continuously to AR = 0. Au and Turl ( 1983 ) reported that a bottlenose dolphin's detection of targets in clutter decreased linearly with the E/R and the dolphin's accuracy decreased monotonically as AR decreased to 0 cm.

Au (1980) suggested that Tursiops does not change the spectral characteristics of its echolocation signals in response to a specific target or echolocation task in Kaneohe Bay, but that the bottlenose dolphin increases the signal intensity and peak frequency of echolocation signals in order to maintain a high signal-to-noise ratio. It has been reported that Tursiops increases the number of clicks per trial as the target detection task becomes more difficult (Penner and Kadane, 1980; Au and Penner, 1981; Au et al., 1982; Au and Turl, 1983 ). Turl and Penner (1989) reported that a beluga emitted click trains composed of packets. In this experiment,

no packets were recorded. This difference may be due to either the difference in the task or the difference in target ranges.

At AR = 2.5 cm the beluga emitted more clicks (Fig. 5), with wider bandwidths [ Fig. 6(b) ] and higher amplitudes [ Fig. 6 (c) ] than when the target and clutter screen were co-planar. It seems the beluga may have changed its click emission strategy at AR = 2.5 cm in order to extract the target echo from the clutter. At AR = 2.5 cm some of the highlight features of a target may have been masked by the clutter screen, but the highlight structure of the target was not affected by the movement of the clutter when a target was co-planar (AR = 0) with the clutter screen.

The beluga's performance was higher at 68 ø grazing angle than when the grazing angle was 90 ø . The higher performance may have been due to the lower target strength of the clutter screen at 68 ø . At 68 ø grazing angle, with the target within the plane of the clutter screen, the echo of the clutter screen arrived at the animal before the target echo. At 90 ø grazing angle, the echoes from the target and clutter screen arrived at the animal simultaneously. The beluga's performance was not affected by early arrival of the clutter screen echo.

900 J. Acoust. Soc. Am., Vol. 89, No. 2, February 1991 Turl et al.' Echolocation ability of the beluga in clutter 900


The results of this experiment suggest that the beluga can detect targets in 3.6 to 5.3 dB more reverberation than Tursiops. The E/R based on energy represents the lowest estimate of the E/R since the entire clutter screen echo oc-

curred within the 1 ms integration time used. The E/R based on the peak-to-peak amplitude represents the highest estimate ofE/R. An estimate ofE/R at detection threshold for the beluga should be between these two extremes. We do not know how belugas process echoes in reverberation and the beluga's integration time is not known.

IV. CONCLUSIONS

The beluga can detect targets in higher reverberation than the bottlenose dolphin. When the target was co-planar with the clutter screen, target detection varied linearly with the calculated E/R, but the beluga's performance did not vary continuously as the target separation distance decreased to 0 cm. At AR = 2.5 cm, the beluga emitted more clicks per trial, higher intensity and wider bandwidth echolocation signals than at AR = 0 cm. We suggest that at AR = 2.5 cm, the target's highlights were masked by the clutter echo and the beluga changed its click emission strategy to extract the target from the clutter. The beluga's higher detection at 68 ø grazing angle is consistent with the lower backscatter measurements.

Acoustic backscatter levels from Arctic sea ice are

stronger than observed levels in the open ocean (Brown and Milne, 1967; Milne, 1964) and the backscatter levels tend to increase with frequency and grazing angle (Urick, 1967). Belugas' are commonly found in and around marginal ice zone (Finley and Davis, 1984), have been reported in pack ice, cracks, and polynyas (Kleinenberg et al., 1969; Turl, 1987), and occur in large numbers in very shallow and tur- bid estuaries (Fraker, 1977 ). Except for the narwhal (Mon- odon monoceros) and bowhead whale (Balaena mysticetus), few species of cetaceans are found in such unpredictable areas. The beluga's ability to detect low-amplitude echoes in high reverberation, along with a directional and narrow transmitted beam should provide for good target discrimination and navigation in such highly reverberant conditions. The beluga's bioacoustic abilities are not fully known, but the available information suggests that the beluga's echolocation system is uniquely adapted to function in the Arctic environment.

ACKNOWLEDGMENTS

We would like to thank Dr. Waldo K. Lyon, Dr. Alan Beal, Capt. Merrill Dorman, and Capt. Jack Sapol (retired) of the Arctic Submarine Laboratory (ASL) who supported this work. We greatly appreciate the support of Sarah Hop- kins of ASL. Bob Floyd, Brian Matsuyama, and Tom Pas- tore of the Hawaii Laboratory designed and built the signal data system. We extend our thanks to Bill Friedl who kindly reviewed the manuscript.

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Turl, C. W., Penner, R. H., and Au, W. W. L. (1987). "Comparison of the target detection capabilities of the beluga and bottlenose dolphin," J. Acoust. Soc. Am. 82, 1387-1391.

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Turl, C. W., and Penner, R. H. (1989). "Difference in echolocation click patterns of the beluga (Delphinapterus leucas) and the bottlenose dolphin ( Tursiops truncatus)," J. Acoust. Soc. Am. 86, 497-502.

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the echolocation ability of the beluga (delphinapterus leucas) to detect targets in clutter

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