The acoustic signature of shallow hydrothermal brine of Panarea: source mechanism recognition and behaviour changes over mid-term observations

Passive acoustic may represents a sustainable and safe method for long-term investigation of hydrothermal vents, as direct measures can be challenging due to the extreme environmental conditions (high temperature and acidic fluids) as demonstrated in the recent past [Heinicke, et al. 2009, Longo et al. 2021]. Here we present preliminary results of short-term monitoring of the submarine hot hydrothermal spring located in the hydrothermal field within the islets about 2 nautical miles  E of Panarea island, in the NE sector of Aeolian arc (Aeolian Island, Italy). The so-called “Black Point” spring shows peculiar characteristics with respect to to the surrounding field; it is featured  by extremely high temperatures close to 140 °C, very low pH (with values spanning between 2.8-3.5) and dark/brown fluid emissions from the vent [Italiano and Nuccio, 1991, Müller 2011].

The main scope of the present work is to describe for the first time the spectral signature of the shallow hydrothermal brine using non invasive passive acoustic methods and to provide a useful and long-life tool to track the flow rate evolution along the time. High resolution acoustic records were collected using a dedicated battery powered smart hydrophone installed in the proximity of the thermal emission in different periods, providing a panoramic view of the investigated phenomena. The application of various methods of spectral analysis and metadata extrapolation permitted the identification of different energetic frequency peaks and narrow tones, diverging from the ambient background noise, depicting distinct features in terms of bandwidth and energy levels. The bandwidth extension, comprising both infrasonic and audible bands, suggest the coexistence of different source  mechanisms, as previously highlighted in different deep hydrothermal sites [Little et al. 1990, Crone TJ et al. 2006, Smith and Barclay 2021]. The application  of customised thresholding algorithms allowed the identification of the acoustic source related to bubble nucleation processes induced by the turbulent flowing fluid throughout uneven conduits. Afterwards, the analysis of  the PSDs of each record emphasised the temporal evolution of both the power spectral levels and the frequency peaks during the observing period.
Preliminary results show persistent and  almost constant contributions in well defined frequency range. Frequency shifts testify a behaviour change of the hydrothermal vent turbulence due to natural forces. Furthermore, vibrational induced signals and the presence of narrow tonal components, due to the conversion of seismic energy into acoustic waves along the solid-liquid interface, were identified in the range [50 - 150] Hz, underlying the complexity of mechanisms and the hidden information that can be extracted from the hydrothermal area. In addition, coupling short-time passive acoustic with multidisciplinary data coming from   the pre-existing fixed underwater infrastructure, it was possible to observe the evolution of the hydrothermal field activity over time. The ongoing research demonstrates how a deep understanding  of the acoustic sources could shed light over the behaviour of the hydrothermal reservoir, acting as a powerful proxy to identify fluid flux change induced by magmatic contribution over long-term deployments.