Deep seawater inherent optical properties in the Southern Ionian Sea

doi:10.1016/j.astropartphys.2006.08.006

Astroparticle Physics

Volume 27, Issue 1, February 2007, Pages 1-9

https://doi.org/10.1016/j.astropartphys.2006.08.006 Get rights and content

Abstract

The NEMO (NEutrino Mediterranean Observatory) Collaboration has been carrying out since 1998 an evaluation programme of deep sea sites suitable for the construction of the future Mediterranean km³ Čerenkov neutrino telescope. We investigated the seawater optical and oceanographic properties of several deep sea marine areas close to the Italian Coast. Inherent optical properties (light absorption and attenuation coefficients) have been measured as a function of depth using an experimental apparatus equipped with standard oceanographic probes and the commercial transmissometer AC9 manufactured by WETLabs. This paper reports on the visible light absorption and attenuation coefficients measured in deep seawater of a marine region located in the Southern Ionian Sea, 60–100 km SE of Capo Passero (Sicily). Data show that blue light absorption coefficient is about 0.015 m⁻¹ (corresponding to an absorption length of 67 m) close to the one of optically pure water and it does not show seasonal variation.

Introduction

The construction of km³-scale high energy neutrino telescopes will complement and extend the field of high energy astrophysics allowing the identification of the highest energy cosmic ray sources. The search for astronomical sources of high energy cosmic rays is possible with particles that reach undeflected the detectors. The observational horizon of high energy cosmic gamma rays and nuclei from ground based detectors is limited to few tens of Mpc by the interaction with cosmic matter and radiation: this should imply the well known GZK cutoff [1], [2] in the energy distribution of ultra high energy extragalactic cosmic rays. On the contrary, the low cross section of weak interaction allows neutrinos to reach the Earth undeflected from the farthermost regions of the Universe. Active Galactic Nuclei [3], Galactic Supernova Remnants [4], Microquasars [5] and Gamma Ray Bursters [6] are some of the most promising candidate of high energy muon–neutrino sources. On the basis of high energy neutrino fluxes, calculated using astrophysical models, neutrino detectors with an effective area of ≃1 km² will be able to collect, in one year, a statistically significant number of events from point-like astrophysical neutrino sources.

Underwater Čerenkov telescopes detect high energy neutrinos indirectly, tracking the Čerenkov light wavefront radiated, in seawater or in ice, by charged leptons originated in Charged Current neutrino interactions. Seawater, therefore, acts as a neutrino target and as a Čerenkov radiator. An undersea location at a depth of more than 3000 m provides an effective shielding for atmospheric muons background and allows the construction of such detectors, usually referred as Neutrino Telescopes [7]. Two smaller scale neutrino detectors, AMANDA and BAIKAL, have already collected and reported candidate neutrino events [8], [9]. AMANDA is located in the South Pole icecap [10] at a depth between 1400 and 2400 m. The present size is relatively small, about 25,000 m² for TeV muons, compared to IceCube [11], the future km³ detector now under construction. BAIKAL NT-200, the pioneer underwater detector, is deployed in the Siberian Lake Baikal at about 1000 m depth and has a detection area of few thousand 10⁴ m² for TeV muons [12].

In the Northern Hemisphere, the Mediterranean Sea offers several areas with depths greater than 3000 m; few are close to scientific and logistic infrastructures and offer optimal conditions to install an underwater km³ neutrino telescope. The future IceCube and the Mediterranean km³ will complement each other providing a global 4π observation of the sky. The long light absorption length of the Antarctic ice is expected to allow good energy resolution, the long light effective scattering length of the Mediterranean seawater should also allow excellent angular resolution. Three collaborations, NESTOR [13], ANTARES [14] and NEMO [15], are presently active in the Mediterranean Sea. NESTOR proposes the installation of a Čerenkov detector, with a tower-shaped geometry, moored a few nautical miles off the south-west tip of the Peloponnese (Greece), at about 4000 m depth. ANTARES is building a detector in the vicinity of Toulon (France) at ≃2450 m depth to possibly detect astrophysical neutrinos and to demonstrate the feasibility of a km³-scale underwater neutrino telescope.

The NEMO Collaboration is active in the design and tests for the Mediterranean km³ neutrino telescope. After a long period of R&D activity, at present the collaboration is ready to install a prototype station (NEMO phase 1) at 2000 m depth, 25 km offshore the town of Catania, in Sicily (Test Site in Fig. 1). Since 1998 we have performed more than 25 oceanographic campaigns in the Central Mediterranean Sea in order to characterize and eventually seek an optimal submarine site for the installation of the Mediterranean km³ [16]. Three areas close to the Italian Coast have been compared, on the basis of two requirements: depth > 3000 m and distance from shore <100 km. Two of these sites are trenches located in the Southern Tyrrhenian Sea close to the Alicudi and Ustica Islands (at depth ≃3500 m). Measurements of deep seawater optical properties were performed by the NEMO Collaboration in these sites and results were published [17]. The third site is a submarine plateau, whose average depth is ≃3500 m, located at a distance of 40–100 km South East of Capo Passero, Sicily (see Fig. 1). In this paper, we report on deep seawater optical properties (absorption and attenuation coefficients) measured in the Capo Passero marine region during a period extending from December 1999 to July 2003. The results refer to two sites located ≃60 km (36°30′N, 15°50′E) and ≃80 km (36°25′N, 16°00′E) offshore Capo Passero, hereafter indicated respectively as KM3 and KM4. The programme of characterization of deep seawater in Capo Passero site, carried out by the NEMO Collaboration, includes also long term measurements of optical background (due to bioluminescence and ⁴⁰K radioactive decays), water temperature and salinity, deep sea currents, sedimentation rate and bio-fouling. The results of this work are presented elsewhere [18] and will be published soon.

Section snippets

Effect of water optical properties on underwater neutrino telescopes

The propagation of light in water is quantified, for a given wavelength λ, by the water inherent optical properties (IOP): the absorption a(λ), scattering b(λ) and attenuation c(λ) = a(λ) + b(λ) coefficients. The light propagation in water can be described by the laws: $\begin{matrix} I_{a} (x, λ) = I_{0} (λ) e^{- x \cdot a (λ)} \\ I_{b} (x, λ) = I_{0} (λ) e^{- x \cdot b (λ)} \\ I_{c} (x, λ) = I_{0} (λ) e^{- x \cdot c (λ)} \end{matrix}$ where x is the optical path traversed by the light and I₀(λ) is the source intensity. A complete description of light scattering in water would require the knowledge of

Measurements of water optical properties with the AC9

We carried out light attenuation and absorption measurements in deep seawater using an experimental setup based on the AC9. This device performs attenuation and absorption measurements, independently, using two different light paths and spanning the light spectrum over nine different wavelengths (412, 440, 488, 510, 532, 555, 650, 676, 715 nm) [26], [27], [28]. The setup designed for deep seawater measurements consists of an AC9, powered by a submersible battery pack, connected to an Idronaut Ocean

Comparison of deep sea sites in the Central Mediterranean Sea

The first measurements of IOP in Capo Passero were carried out in December 1999, in the KM3 and KM4 sites. A comparison among the vertical profiles of salinity, temperature, a(440 nm) and c(440 nm) as a function of depth, recorded in the two sites is shown in Fig. 4. Between 1250 m and 3250 m depth, the water column in the site KM3 shows variations of the attenuation coefficients as a function of depth. We attribute this variation of c(λ) to extra sources of light scattering, due to particulate

Long term study of optical properties at the Capo Passero site

In order to verify the occurrence of seasonal variations of deep seawater IOPs in KM4, we are continuously monitoring this site using the experimental setup described above. The data collected during oceanographic campaigns of December 1999, March 2002, May 2002, August 2002 and July 2003 are reported here. In Fig. 6 the profiles of water temperature, salinity, a(440 nm) and c(440 nm), as a function of depth, are shown. The whole collected data sample consists of: 2 deployments in December 1999

Conclusions

The NEMO Collaboration measured, as a function of depth, the salinity, temperature and inherent optical properties in several abyssal sites of the central Mediterranean Sea using an experimental apparatus consisting of an AC9 transmissometer and a standard CTD probe. In order to compare the water transparency to Čerenkov light of different sites we have averaged the measured values of c(λ) and a(λ) in a range of about 400 m, at the depths which are suitable for the deployment of a km³ neutrino

Acknowledgement

This work has been been conducted in collaboration with: Department of Physical Oceanography INOGS (Trieste), Istituto Sperimentale Talassografico CNR (Messina) and Istituto di Oceanografia Fisica CNR (La Spezia). We thank Captains E. Gentile, V. Lubrano, A. Patané, the officers and the crew of the R/V Alliance, Thetis and Urania for their outstanding experience shown during the sea campaigns.

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^✠: Deceased.

¹: On leave of absence Dipartimento Interateneo di Fisica Università di Bari, Via E. Orabona 4, 70126 Bari, Italy.

²: Present address: Istituto Centrale per la Ricerca Scientifica e Tecnologica Applicata al Mare, via Casalotti 300, 00166 Roma, Italy.

³: Present address: DAPNIA/SPP Bt 141 CEN Saclay, 91191 Gif-sur-Yvette, France.

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Deep seawater inherent optical properties in the Southern Ionian Sea

Abstract

Introduction

Section snippets

Effect of water optical properties on underwater neutrino telescopes

Measurements of water optical properties with the AC9

Comparison of deep sea sites in the Central Mediterranean Sea

Long term study of optical properties at the Capo Passero site

Conclusions

Acknowledgement

Phys. Rep.

Astron. Astrophys.

Astropart. Phys.

Nucl. Phys.

Nucl. Instrum. Methods Phys. Res., Sect. A

Appl. Opt.

Astropart. Phys.

Ann. Rev. Nucl. Part. Sci.

Astrophys. J.

Phys. Rev. Lett.

Phys. Rev. Lett.