Investigation of acoustic attributes of methane man-made bubbles ...

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Investigation of acoustic attributes of methane man-made bubbles ...

Investigation of acoustic attributes of methane man-made bubbles and their

further use for distinguishing from the fish

Jaroslava Frouzova a , Michal Tuser a , Petr Stanovsky b

a Biological Centre of the Academy of the Sciences of the Czech Republic, Hydrobiological

Institute, Na Sadkach 7, 37005 Ceske Budejovice, Czech Republic

b Institute of Chemical Process Fundamentals of the ASCR, Rozvojová 135

165 02 Praha 6, Czech Republic

Jaroslava Frouzova , Biological Centre of the Academy of the Sciences of the Czech Republic,

Hydrobiological Institute, Na Sadkach 7, 37005 Ceske Budejovice, Czech Republic, fax number

: +420 385 310 248 , e-mail address : jarkafrouzova@gmail.com

During hydroacoustical observations of fish in reservoirs and lakes of temperate zone, an

indispensable amount of methane bubbles rising from the bottom sediments toward the water

surface was registered. These bubbles essentially interfere with acoustical detection of fish,

thereby biasing an estimate of the fish abundance and biomass The experiment with methane

bubbles controlled production was made with using of Simrad EK60 echosounder working with

38. 120 and 400 kHz frequency from the depth of 6m. Six sizes (from 1 to 8 mm in diameter) of

methane bubbles were observed both vertically and horizontally. As an acoustic attributes were

observed target strength (TS) and its changes, speed and direction of moving and echo length.

Target strength of bubbles copy target strength of fish with the similar variability, so this does

not look like good parameter for distinguishing, moreover the TS in horizontal mode of

observation is stronger significantly than in vertical mode. Similar are also results with the echo

length. Direction and speed of moving of bubbles look more promising. As an interesting side

result seems to be observed changes of TS of particular bubbles in dependence on their density

(on the interval among bubbles) in the sound beam.

Keywords : Bubbles, methane, acoustics, fish

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1st International Conference and Exhibition on Underwater Acoustics

1. Introduction

During vertical beaming acoustic surveys in lakes or reservoirs of temperate zone, an

indispensable quantity of gas bubbles rising from bottom sediments toward the water surface can

be observed [1,2]. These gas bubbles as strong scatters, however, interfere with acoustic

detection of fish, thereby biasing an estimate of fish abundance and biomass [3]. Fortunately at

vertical deployment with a low survey speed, rising bubbles can be recognized for their vertical

movements, which are not frequent in fish. This could be utilized to quantify the density of gas

ebullition and consequently subtracted from the total densities of observed objects [4,5]. This

solution is possible to use only in water bodies with an adequate depth for applying vertical

beaming acoustics and when a surveying boat is sufficiently low for tracking rising bubbles. In

cases of shallower waters or surface-living fish communities [6,7], horizontal beaming acoustics

is a more appropriate method and distinguishing bubbles based on vertical movement cannot be

applied.

In this study, we used various sizes of artificial methane bubbles to investigate their acoustic

characteristics for distinct acoustic frequencies in both vertical and horizontal mode of the

observation with common type of echosounders.

2. Methods and material

The study of acoustic response and characteristics of gas bubbles was performed in the Římov

reservoir. The experimental site was situated in roofed-over boat dock with standing water of 6

m depth. A bubble production device was fixed at a special aluminum frame and placed right

below the dock. Prior to the field experiment, the production device was tested in a special tank

in laboratory using the high speed and high resolution camera to achieve the most accurate

measurement of bubble volume and shape. The size spectrum of bubbles was predicted on the

base of acoustic size composition obtained from common acoustic surveys of the Czech

reservoirs (Table 1). The artificial bubbles were consisted of methane using a set of nozzles with

pressure drop elements coupled with a set of solenoid valves [8]. The valves were controlled via

National Instruments CompactDAQ card by means of LabVIEW software. The pressure of

methane in supply line was controlled by a couple of pressure reduction valves with an electronic

pressure transducer (BD Sensors).

Also in the field, the size and behavior of rising bubbles were observed optically with

cameras SplashCam Delta Vision HD B/W, Ocean systems Inc.

All acoustic measurements were performed with Simrad EK60 split-beam echosounders

operating with frequencies of 38, 120 and 400 kHz. For vertical deployment, all transducers were

circular ones with nominal angles of 12° (38 kHz) and 7° (120 and 400 kHz) and mounted on a

special frame equipped with buoys. For horizontal beaming acoustics, an elliptical transducer

ES120-4 (4.3° and 9.2° nominal angles) was utilized and attached to a vertical pole, which

enables to alter the depth of horizontally aiming transducer (1 – 4 m). The acoustic equipment

was calibrated with tungsten-carbide standard targets [9-11]. The operating power was adjusted

to 100 W with 0.05 ms pulse rate. The pulse length was set to 256 ms for 38 kHz frequency, and

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1st International Conference and Exhibition on Underwater Acoustics

128 ms for both 120 and 400 kHz frequencies. The echosounder’s single echo detector threshold

was fixed conclusively to accept echoes with a minimum value of -80 dB.

All collected acoustic data were processed with Sonar5 Pro software (Balk and Lindem,

University of Oslo). Only bubbles recorded in the central beam were included into analyses. In

the case of elliptical transducer, collected data were restricted in magnitude of -1 and +1 degree

on the vertical axis.

3. Results and discussion

Values of acoustic target strength TS for all measured sizes of bubbles observed vertically

and horizontally are depicted in Figures 1 and 2. Target strength of measured bubbles did not

differ from TS of a fish, ranging from -60 to -40 dB. Generally, we can say that signals for the

smallest sized bubbles (S1) correspond to 4.5-cm long fish larvae[12], whereas signals for the

largest bubbles (S6) represent 10-cm long fish in both vertical and horizontal mode of the

observation [13]. Also, echo length cannot be applied to distinguish bubble from fish target.

When compared vertical and horizontal observation at 120 kHz, TS values for smaller sizes of

bubbles (S1 and S2) are weaker in vertical observation than horizontal one, ranging up to 14 dB

dependently on the depth. In contrast with other sizes, the difference is opposite, for S6 bubbles

the difference makes 16 dB. So consideration of spherical shape of rising bubbles and possible

subtraction this bubble size distribution obtained during vertical observation in horizontal

records [4] seems to be quite misleading.

TS of bubbles observed vertically did not change their TS size with their ascent. Similarly, TS

of bubbles in horizontal records failed to change with the depth of observation, except for the

smallest S1 bubbles, which were increasing TS. This S1 size is also interesting from the view of

unpredictably strong TS, which acts like the strongest target at all, and exceed the largest size of

bubbles about 5 dB in a depth of 1 m below the surface. This might be attributed to the fact that

bubbles of that size in unclean water (i.e. without other treatment as distillation or cleaning with

carbon filter) rise rectilinearly, and in this particular case bubble-bubble spacing were not large

enough to exclude multiple scattering. The influence of bubble separation on acoustic response

in the case of S1 bubble is ongoing.

A speed of bubble rising at 120 kHz failed to vary with the size and it is about 24 or 25 cm/s.

In the case of S5 bubble, its speed is visibly slower than other sizes, i.e. 21 cm/s. A speed of fish

is dependent on the species, its exhibiting behavior and body length. In our so far unpublished

cage study is well visible that a speed of 16-cm long bream (Abramis brama) during the day was

higher up to 29 cm/s than during the night when fish was relaxing and its speed was from 2 cm to

20 cm/s. A combination of speed and direction of movement can be a good indicator to

distinguish the echo of bubble from that of fish.

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1st International Conference and Exhibition on Underwater Acoustics

-35

-40

-45

TS (dB)

-50

-55

-60

-65

120 kHz

400 khz

38 kHz

-70

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16

Volume of bubbles (ml)

Fig. 1: Acoustic sizes of bubbles observed vertically at 38, 120 and 400 kHz

TS (dB0

-65

-60

-55

-50

s1

s2

s3

s4

s5

s6

-45

-40

0 1 2 3 4 5

Depth (m)

Fig. 2: Acoustical sizes of bubbles observed horizontally at 120 kHz in different depths

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1st International Conference and Exhibition on Underwater Acoustics

D

(mm) SD V SD

S1 1.23 0.05 0.00097 0.00001

S2 2.80 0.05 0.0115 0.00068

S3 3.17 0.1 0.0167 0.00156

S4 4.16 0.08 0.0378 0.00226

S5 6.75 0.12 0.1608 0.00864

S6 8.73 0.46 0.3484 0.05184

Table 1 : Diameter (D) and volume (V) of investigated bubbles

4. Acknowledgements

This study was supported from project no. P504/12/1186 of Czech Science Foundation.

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