SIDE-SCAN ANGULAR SETUP INFLUENCE IN THE DETECTION OF POSIDONIA OCEANICA FIELDS

SIDE-SCAN ANGULAR SETUP INFLUENCE IN THE DETECTION OF

POSIDONIA OCEANICA FIELDS

PACS: 43.30-g 43.30-Pc 43.30-Vh 43.30-Ma

M. N. Sletten, V. Espinosa, N. Sanchez-Carnero*, J. Freire*

Instituto de Investigación para la Gestión integrada de Zonas Costeras Universidad Politécnica de Valencia

C/. Paranimf 1

46730 Grau de Gandia, Spain

* Grupo de Recursos Marinos y Pesquerías Facultade de Ciencias

Universidade da Coruña C/.Alejandro de la Sota 1 15008 A Coruña, Spain

ABSTRACT

Low density biomass, as Posidonia Oceanica prairies, has shown to be a challenging backscattering target. As Posidonia is a crucial part of the seabed ecosystem, evaluation of seabed areas with its presence is becoming an important issue, and it has turned a strong motivation for our study. We analyse alternative side-scan transducer setups, comparing existing single beam echosounder and side-scan methods. In this context we are investigating the use of a vertical mounted side-scan transducer, where the downward bound of the main lobe is near the vertical with a width of 50˚ towards the horizontal. The initial attempts shows to be promising, gaining spatial resolution and increase in effective detection cross section, compared with traditional narrow single beam echo sounders and grazing side-scan sonar.

INTRODUCTION

The lateral wide angle of the side-scan sonar insonifies a large seabed area only from a single ping. Spatial information is conserved as the acoustic ping, at grazing angle, is time-sampled at sub milliseconds intervals. Despite of this valuable features, the side-scan sonar has some shortcomings as topographical information is absent, the backscattering echo depends on angle of incident and the targets' acoustical impedance. This is not the case for echo sounders operating at nadir angle, with elliptic spatial resolution. The use of vertical echosounders within biomass studies is widely extended, because of its ability, among others, to position targets in the water column.

PRINCIPLE OF OPERATION

Figure 1: Diagrams of the first bottom return and backscatering contribution from Posidonia depending on the incidence angle.

By a wide angle lobe close to the vertical direction we are able to have a constant integration cross section for all water depths (see Figure 1), much less sensitive to the boat movements (like rolling) than the narrow single beam echosounder. After correcting for the angle dependent resolution, this approach gives a more concise signal signature for biomass detection, with its measure of biomass density. Plotting the azimuth angle γ gives an idea of this feature, what is described by the simple equation.

γ=cos−1 1− 2 hp

ct

.

Here <hp> is the mean hight of the Posidonia and ct is the metric two way distance to first bottom backscatter.

Detectability and noise estimates

Detection of signal signatures is always dependent on the definition of the complementary out come. In this fashion noise denotes everything else than valid detection of the desired object. As a first approach, noise is defined by using diver classified transects. Since the objective is the detection of biomass in the lower part of the water column there is no need of take into account the sub bottom part of the ping. In the water column the noise is due to reverberation from surface, bottom or volume backscattering, where the sources are plankton, air bubbles or similar.

Below are three plots from the same transect recorded separately in consecutive takes in a campaign made in the natural park of Cabo de Gata (Almeria, Spain) [3]; therefore the coordinates of geographical positioning may vary between two measurements, introducing a certain error margin with respect to the ground-truthing. The operational frequency is 200kHz on all three setups.

As a main reference of standard bottom classification technique, a normal single beam (SB) recording is shown in figure 3. The equipment is a Simrad EA400 echosounder with a 200 kHz transducer, with a 7° circular main lobe. The detectability of different types of submerged vegetation has been demonstrated [1] and also positive results for Posidonia have been claimed by different authors [2]. Nevertheless, the conclusions about the feasibility of characterization with single beam echosounder point to the fact that the navigations conditions affect severely the results and no correlation with ground-truthing is positive [2, 3]. See Table 1 for comparison; we can observe that the vertical structures in the echogram does no follow the observations of the divers, and the artifacts in bottom can be directly attributed to the ship rolling.

Figure 4 is a dB representation of the raw intensity data from a vertically mounted side-scan transducer that is also operated by the Simrad EA400. “Vertically” means in this case that the center of the transversal beam is directed 25 degrees away from vertical direction, in opposition to the standard mounting of this device (“horizontal)”, with 25 degrees from horizontal. The transducer is positioned to the left, then the water column follows with biomass, and bottom backscatter as the continuous red line. The total dB-range is approximately 10 dB, nearly the same as the SB, but more detections are made of sea grass. Noise artifacts are clearly visible in the first parts the data, this is mainly due to water surface reflections. A strong vertical line is visible plot and is reflections from the surface from a side lobe.

Figure 5 shows the same transect with a horizontally mounted transducer. The noise level is about the same as the vertical mounted one, but the backscattered intensity from the Posidonia is also lower than 38 dB.

Figure 5: Horizontal side-scan, the intensity scale is in dB re 1 intensity unit. Rescaled

values from -14x10^3 as relative minimum

Table 1: Diver classification of the transect

Start Length Sediment Seagrass Height [cm]

0 15 sand Posidonia and Cymodocea 150

15 95 sand Dense posidonia 80

110 5 sand Absence

-115 5 sand Posidonia 75

120 10 sand Absence

-130 30 sand Posidonia 70

160 65 sand Absence

-225 10 sand Posidonia 75

235 20 sand Cymodocea

-255 65 sand Dense posidonia 56

320 5 sand Absence

-325 10 sand Posidonia 56

-CONCLUSIONS

The wide transversal angular aperture of side-scan sonar when the beam is directed close to vertical direction allows to detect and classify Posidonia Oceanica fields, preserving the bathymetric and substrate information in the bottom echo envelope. The use of this tool is encouraged versus the application of ordinary vertical single beam echosounders or more grazing angles of the side scan sonars.

REFERENCES

[1] J. Burczynski et al., Use of acoustics for detecting aquatic vegetation, ICES CM 2001/B:04

[2] C. Noel – C. Viala – B. Zerr, Acoustic characterization of underwater vegetations

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