Hydrological and Physical Characterization of Cinque Terre Marine Protected Area (Ligurian Sea) and Evaluation of Current Velocity and Direction by

AUV Navigation Tracks

M. Locritani1,2, O. Chiappini 3, R. Chiarabini 3, F. Bruni4, V. Ciccarelli4, L. Natale5, R. De Paulis6

1 Istituto Nazionale di Geofisica e Vulcanologia – Roma, Italy

2 Università di Genova – Genova, Italy 3 NATO Undersea Research Centre, La Spezia, Italy

4 Tecnomare S.p.A., 5 Parco Nazionale delle Cinque Terre, La Spezia, Italy

6 ENI exploration and production division, GEOLAB Dept., Milano, Italy

Abstract: The integrated study, concerning the experimental observations, was performed during the oceanographic cruises on-board the two NURC research vessels, in the period between 2008 and 2010 in the Marine Protected Area of Cinque Terre (North-West Mediterranean Sea, Italy). The aim of the research is to describe the environmental features using both hydrological parameters and AUV navigation tracks, acquired respectively by a multi-parametric platform (MEDUSA) and AUV MUSCLE. The innovative contribution of this work is the possibility to evaluate the bottom stream without direct current measurements, but using the navigation data recorded by AUV. This work is a complementary analysis to environmental support of the ENI S.p.A. Exploration and Production Division project.

Keywords: Hydrology, AUV, Cinque Terre, Ligurian Sea.

Abbreviations: Marine Protected Area (MPA), Ligurian Provençal Current (LPC), Module for Enviromental Deep-Under-Sea Analysis (MEDUSA), Autonomous Underwater Vehicle (AUV), Minehunting UUV for Shallow water Covert Littoral Expeditions (MUSCLE), Conductivity, Temperature, and Depth (CTD), Interferometric Synthetic Aperture Sonar (InSAS), NATO Undersea Research Centre (NURC), Istituto Nazionale di Geofisica e Vulcanologia (INGV), Doppler Velocity Log (DVL), Global Positioning System (GPS).

I. INTRODUCTION

The Marine Protected Area (MPA) of Cinque Terre is a very nice touristic place located in the northernmost basin of the western Mediterranean Sea, Ligurian Sea (Fig.1). The MPA is characterized by high and rocky coast that quickly deepen in the sea. In detail the continental shelf follows the coast and it is very close with the steep slope cut by canyon connected to the higher depth (more than 2000 m in the centre of the basin). The coastal morphology and the bathymetry could influence the current flow and the environmental features of the marine areas. In fact a lot of authors [1], [2] and [3] indicate the existence of a current flowing approximately along the isobaths and following the narrow shelf. This mainly cyclonic circulation (North-westward) is called Ligurian Provençal Current (LPC) and it is the result of two distinct northern streams: one (warmer) coming from the Tyrrhenian Sea through the Corsica Channel and the other (colder) flowing along the western Corsica coast [4]. The confluence of these currents generates a frontal system that separates the cold and salty central water from the coastal one [5], [6]. The MPA is located in this region of confluence. The LPC appears seasonally influenced, becoming more intense in the winter period and less in summer. Moreover in latter season both currents flowing northward, while in summer the Tyrrhenian Current reduces its intensity. The tide contribute is relatively unimportant in the Ligurian Sea [7]. The environmental protection becomes more important if the marine system has a very high value as in the specific site (the MPA), and the new technology could give a significant contribution to study this peculiar marine area. In detail this paper focus on the study of the hydrological parameters to describe the environmental features in the MPA of Cinque Terre. We use three kinds of instruments: a multi-parametric platform named MEDUSA, an Autonomous Underwater Vehicle (AUV) called

978-1-4244-4333-8/10/$25.00 ©2010 IEEE

MUSCLE (Minehunting UUV for Shallow water Covert Littoral Expeditions) and a CTD (Conductivity, Temperature, and Depth). Recently, a lot of AUV have been integrated with different kind of system to measure current (REMUS – Remote Environmental Monitoring Units [8]) or to calculate this (Underwater Gliders [9]). The innovative contribution of this work is to use the MUSCLE data, that usually doesn’t give the current measures, to deduce the intensity and direction of the bottom current.

Fig. 1 Map of the Ligurian Sea. The locations of the MEDUSA casts (43 red stars) and the CTD casts (2 blue stars) performed during the cruse on-board of the Alliance

(periods 19th-23rd June 2009) are shown. In the green circle is evidenced the area of the AUV mission (for detail see Fig. 2), where 12 casts are made.

II. DATA AND METHODS

From 2008 to 2010, an observational study has been conducted as a complementary analysis to support the ENI S.p.A. Exploration and Production Division project called InSAS (Interferometric Synthetic Aperture Sonar), whose aim is to monitor the sea bed surface in the MPA of Cinque Terre, Ligurian Sea. Two cruises have been made on-board the two NURC research vessels: the first one in autumn 2008 (December 18th) on-board the Leonardo R.V., the second one on summer 2009 (June 19th-24th) on-board the Alliance R.V. and another one is scheduled before the end of 2010. During the last one an additional measure of current velocity and direction with a conventional instrument (current-meter) could be due to evaluate the hypothesis explain in this study. The area of interest (12.5 Km2 wide) is 1 Km far away from the high and rocky coast of the Cinque Terre MPA. In detail the measures have been performed in the area opposite of the cape named Capo Monte Nero, where the bottom is almost flat (from 40 to 50 m of depth). The available instruments were: i) the INGV (Istituto Nazionale di Geofisica e Vulcanologia) multi-parametric platform named MEDUSA (Module for Enviromental Deep-Under-Sea Analysis) which was designed by Tecnomare S.p.A., recording physical and hydrological parameters of the water column; ii) the NURC (NATO Undersea

Research Centre) CTD probe, Sea-Bird SBE37; iii) the MUSCLE Autonomous Underwater Vehicle-mounted high-frequency imaging sonar, developed by Thales Underwater Systems according to high level specification provided by NURC [10], [11].

A. Hydrological data

The multi-parametric platform mounted the following sensors: CTD (SBE19-Plus), trasmissometer (BBRTD-347) and Oxigen

sensor (OPTODE 3830). The sampling frequency was 4 Hz for the first two parameters and 1 Hz for the Oxigen, but during the processing it was bring back on 1 Hz for whole sensors. The analyzed data were: temperature (°C), depth (m), salinity (psu), oxigen (mg/l) and turbidity (NTU). The NURC probe measured only temperature (°C), depth (m) and salinity (psu) with a sampling frequency of 1 Hz.

During the first cruise was made a test of MEDUSA consists of 9 casts located in the AUV mission area (see the sub-section 2.2 and Fig. 1). In the second cruise, on 22nd June 2009, 43 casts were performed (Fig. 1) by MEDUSA along the perpendicular and parallel transepts. The casts, distributed in 12.5 Km2, was spaced about 1 Km along-shore and 500 m cross-shore. In the opposite to Capo Monte Nero (the area interested by AUV missions) the MEDUSA sampling was closer, about 70 m (Fig. 1). To evaluate the temporal variability of the hydrological data, NURC probe made 23 casts with about 30 minutes of time lag. The 22 casts were performed from 19th to 24th June from 7:00 to 19:00 (UTM), in the coordinate: 44,0885 N - 9,7012 E, and only one of these was made simultaneously of MEDUSA cast in the coordinate: 44,0885 N - 9,7126 E (see Fig. 1).

B. AUV data

MUSCLE is a COTS Bluefin21 platform. It is about 3 m long, it mounts 270-330 kHz wideband InSAS optimized for shallow

water. The AUV navigation system is divided in two types: aided inertial navigation system and dead reckoning combined with acoustic long baseline navigation [12]. This system give an accurate underwater positioning system. Moreover the DVL (Doppler Velocity Log) acquired the speed of the AUV respect to the bottom. The MUSCLE can be pre-programmed for adaptive mission planning. During the two cruises performed for InSAS project a specific mission has been scheduled for the AUV.

We consider only the data relatively to the second cruise because in this period have been collected more hydrological data respect to the other cruise and in the same time period. In detail AUV mission was included in about 2.5 Km2, opposite to Capo Monte Nero, and was made by nine legs: five parallel (South-eastward (135°N) and North-westward (315°N)) and four perpendicular (South-westward (225°N) and North-eastward (45°N)) to the coast (Fig. 2, see the positions: A,B,C,D).

Each mission is 2 hours long. The AUV started from the surface in point A and deepen at 29 m of depth when arrived in point B. In this position the real mission started. At the end of the first leg (C) the MUSCLE turned and repeated the same trajectory (parallel to the coast) four times. Then the AUV come back to the surface, controlled the GPS (Global Positioning System) and in

the case of need corrected its position. The last trajectories are composed of four perpendicular legs starting from C and arriving in D at the fixed depth of 29 m (as in the previous case). The MUSCLE, during the mission, maintained the constant propulsion made by 225 fixed turns of the propeller, with an ideal speed of 2.5 knots (about 1.29 m/s). The mission was replied for five days from 20th to 24th June 2009, but we show only the data acquired in 22st June 2009.

To evaluate the velocity and direction of the current field we analyzed the AUV navigation tracks and the relative speed with respect to the bottom, comparing it respectively with the scheduled trajectories and the ideal speed.

Fig. 2 AUV mission. The AUV reaches the first trajectory, parallel to the coast, (B-C) five times, and the second one, perpendicular to the coast, (E-F) four times. At the fixed depth

of 29 m.

III. RESULTS A. Hydrological conditions

The hydrological data recorded by MEDUSA in December 2008 in the AUV mission area (see sub-section 2.2 and the Fig. 1),

performed during the cruise on-board the Leonardo R.V., show an homogeneous water masses characterized by about 15°C of temperature and 37.5 psu of salinity, almost constant in space and in depth. This result is probably due to the winter season and to the dimension of the considered area. While, the data acquired during the second cruise, on 22sd June 2009 on-board the Alliance R.V., suggests the presence of the stratification, with the thermocline at about 25 m of depth (Fig. 3). Moreover the extension of the area (12.5 Km2, see Fig. 1) allows to analyzed the horizontal and vertical distribution of the temperature, salinity, oxygen and turbidity. At surface, upper to the thermocline, the gradient of temperature (24°C - 22.5°C) and salinity (37.9 psu - 37.6 psu) weakly decreases from South-East toward North-West (Fig. 4). Then temperature decreases homogeneously toward the bottom, while salinity has a maximum value (37.9 psu) offshore in the centre of the area between 20 - 25 m of depth (Fig. 3 and Fig. 4). The oxygen (not shown) has an opposite horizontal and vertical gradient with respect to the temperature. The turbidity (not shown) shows a decreasing toward offshore and an increasing from the surface to the bottom. The minimum value of turbidity is measured in correspondence to the saltier water. This suggests the presence of an intrusion of saltier and clearer masses of water in the center of the area probably coming from offshore.

The analysis of the time series of hydrological data, acquired along the whole column of water by CTD casts from 19th to 24th June, in coordinate 44,0885 N - 9,7012 E, not reveals any significant variation along the second cruise period and during each days (from 7:00 to 19:00 UTM): temperature and salinity remain almost constant (Fig. 5). The cast made simultaneously by NURC CTD and MEDUSA hasn’t indicated any significant difference both in temperature and salinity along whole column of water, therefore the values are comparable.

Fig. 3 Hydrological section. The figure shows the last transept (toward offshore) performed on 22nd June 2009. The first panel describes vertical distribution of the temperature (°C) and the second panel of the salinity (psu).

Fig. 4 Hydrological map. The figure shows the surface horizontal distribution of temperature (first panel) expresses in °C, and salinity (second

panel) expresses in psu. The black points represent the stations where the MEDUSA cast were made on 22nd June 2009, during the cruse on-board of the Alliance R.V.

Fig. 5 Hydrological time series. The figure reports the trend of temperature (°C) and salinity (psu) acquired from 7:00 to 19:00 UTC on 23rd June 2009. The casts have been made by the NURC CTD in the eastern

blue point, in AUV area, shown in Fig. 1.

B. Current fields

A few studies, e.g. [13], speak about the guidance and tracking control of the AUV under current interaction. In detail we know that when the magnitude of current velocity reaches nearly the same order of vehicle, quite amount of drift is induced on the vehicle motion due to the interaction with current flow. When the vehicle transfers between two points in the current region, it is quite natural there happen considerable differences in elapsed time (or speed) and in tracing a different trajectory [13]. We utilized these differences in time and in the trajectories to evaluate the current velocity and direction. In particular, in this specific project, the scheduled MUSCLE missions included the perpendicular and parallel trajectories (Fig. 2) respect to the coast. We observe a change of the vehicle speed during the parallel transepts but not significantly in the perpendicular ones (Fig. 6). The Tab. 1 indicates the mean speed respect to the bottom, measured by the AUV during the nine

legs. We considered only the trajectories included in the A, B, C, D positions (Fig. 2). We can see a progressive general decreases of the mean speed value from the first leg to the last one. This reduction is probably due to run down of the AUV battery. Analyzing the data acquired along the parallel leg (1st,2nd,3rd,4th,5th) the speed changes in different way respect to the direction of the AUV motion: it increases of about 0.25 m/s for the AUV course of 135°N, and decreases of about 0.25 m/s when the MUSCLE went toward the direction of 315°N. In the other legs (the perpendicular ones) the AUV speed doesn’t change significantly with direction: it is always close to the ideal speed (1.29 m/s). Fig. 6 shows the paths direction and the time series of AUV speed during leg 1 and 2 respectively. This suggests the presence of a South-westward current approximately parallel to the coast at the vehicle navigation depth (29 m). The general current (LPC) flowing in the opposite direction, but for the peculiar bathymetry (canyon) of this zone it could be a formation of a countercurrent near the coast.

Tab. 1 The AUV mission of 22nd June 2009. The table indicated the mean direction (course in °N) of the trajectories and the mean AUV speed (m/s) calculated for each leg.

Leg 1 2 3 4 5 6 7 8 9

AUV course 135°N 315°N 135°N 315°N 135°N 225°N 45°N 225°N 45°N

Mean speed (m/s) 1.58 1.06 1.56 1.04 1.35 1.28 1.23 1.25 1.23

9.726 9.728 9.73 9.732 9.734

44.085

44.09

44.095

lon

lat

0 1000 2000 3000 4000 5000 6000 7000

11.5

2

records

spee

d [m

/s]

1° leg mean speed: 1.58 m/s2° leg mean speed: 1.06 m/s

1° leg: NW - SE2° leg: SE - NW

Fig. 6 AUV navigation tracks and the respectively speeds. The first leg (leg 1) is represented by the red arrows and the second one (leg 2) by the blue arrows. The first panel shows the trajectories obtained by the plot of the

coordinates acquired by the acoustic positioning of the AUV and the directions of the speed vectors, while lower panel shows the AUV speed time series and the mean values, reported in legend.

IV. DISCUSSIONS

This work concerning the analysis of the experimental observation of a marine integrated survey performed in a very nice touristic place (MPA of Cinque Terre, Italy) by a multi-parametric platform (MEDUSA) and by AUV (MUSCLE) in the period between 2008 and 2010. The MEDUSA data allow us to describe the hydrological parameters distribution in the MPA, determining the temporal and spatial variability of the column of water, while the analysis of the acoustic positioning data and of the relative speed of AUV give some information about the bottom current field.

The acquired data show that in winter the column of water results almost homogeneous, while in summer the stratification is evident (thermocline is at 25 m of depth). The time series (from 19th to 24th June 2009) indicated the stability of the parameters in this period. The paper focuses on 22sd June, when a hydrographic cruse on-board of the Alliance, and a MUSCLE mission was made almost simultaneously.

Hydrological data not evidenced a marked dynamic of the masses of water, indicating a decreases of the horizontal and vertical gradient of temperature toward North-West (Fig. 4), and toward the bottom, respectively (Fig. 3). This suggests the prevalence of warm shallow water probably fed by the Tyrrhenian current. At the same time the maps (Fig. 3 and Fig. 4) suggest the presence of a salty and clear water that intrudes toward the coast in the centre of the area between 20 and 25 m of depth in correspondence to the canyon, that quickly deepened toward 200 m (Fig. 1). This feature could be associated to the current, evaluated by the AUV, that suggests the presence of a South-eastward current (about 0.25 m/s) parallel to the coast at 29 m of depth.

In conclusion, this integrated study lets us to characterize the Cinque Terre MPA in the period of measurements. In detail the study allows to affirm that, in this specific coastal zone, the general circulation prevails and the hydrological data don’t change significant, except for the presence of an offshore intrusion probably due to the interaction of the general current with the bathymetry. This result could be correlated with the counter-current estimated with AUV navigation data. This local circulation is the result to the peculiarity of this small coastal area, located near the region of confluence of the Tyrrhenian and Corsica Current and close to the deep canyon that cut the continental slope.

In the near future (before the end of 2010) we should realize the in situ current conventional measurements to confirm these results.

ACKNOLWLEDGMENTS

The authors are grateful to the staff of the NURC Research Vessels (Leonardo and Alliance) for they instrumental support to

the cruises. Then a special acknowledgement to Dr. C. Carmisciano for the management of the INGV project and to Dr. G. P. Gasparini, Dr. P. Povero and Dr. M. Rixen, my PhD tutors, is due.

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