Listening to Hydrothermal Vents
Brendan Smith – Brendan.Smith@dal.ca
Dr. David Barclay – David.Barclay@dal.ca
Department of Oceanography
Life Sciences Centre, 1355 Oxford St.
PO Box 15000
B3H 4R2, Canada
Popular version of paper ‘2pAO6 – Passive acoustic monitoring of hydrothermal vents at the endeavour hydrothermal vent field’
Presented Tuesday morning, November 30, 2021
181st ASA Meeting
Long-term monitoring of hydrothermal vents is challenging due to their high temperature and caustic fluid properties. Passive acoustics provides a sustained vent monitoring method from a safe distance. Long-term acoustic records and hydrophone arrays may be used to investigate the sound producing mechanisms of hydrothermal vents. The initial results from an analysis of 6-months of single hydrophone acoustic data collected at the Main Endeavour Hydrothermal Vent field in the North-East Pacific, and a short-term array deployment at the same location demonstrate features of the vent’s signature.
The monitoring hydrophone, operated by Ocean Networks Canada (ONC) is within 10 meters of a black smoker hydrothermal vent. During a servicing cruise in the fall of 2021, ONC deployed the Deep Acoustic Lander (DAL), an autonomous acoustic recorder carrying a four-channel hydrophone array, shown in Fig. 1. The difference in received signals across the array can be exploited to identify hydrothermal vent generated noise and separate it from possible interferences, such as flow noise, wind generated wave noise, and ship noise.
The Deep Acoustic Lander being deployed by Ocean Networks Canada using an ROV near a black smoker hydrothermal vent [Credit: Ocean Networks Canada]”
Despite the vigorous, high-temperature flow seen from black smoker chimneys, they do not produce loud acoustic signals relative to the ocean’s background noise. However, several acoustic source mechanisms have been proposed to generate both tonal and broadband sounds (Lighthill, 1952; Little, 1988; Crone et al., 2006).
Fig. 2 compares audio spectra and vertical coherence from the DAL hydrophone array deployed at an initial standoff distance of 200 m, then subsequently repositioned to within 3 m from the vent outlet. Increased broadband infrasonic (1 – 10 Hz) and low frequency (100 – 200 Hz) energy is observed when the sensor is positioned near the vent, and tonal components at 4, 5, 7, 8, and 9 Hz are observed in the spectra. A reduction in coherence in the infrasonic band indicates flow noise while the coherent tonals may be generated by the vibrating vent structure.
Caption: “Figure 2: (a) Acoustic power spectra, (b) real and (c) imaginary vertical coherence <3m (solid) and >200m (dashed) from vent”
The outflow rate and temperature of hydrothermal vent fluid can modulate due to tidal variations in overburden pressure, causing a correlated variation in sound level (Barreyre & Sohn, 2016; Xu & Di Iorio, 2012; Larson et al., 2007; Crone & Wilcock, 2005; Crone et al., 2006). Tidal-period variations in sound level over 6 months of audio data were observed by carrying out a spectral analysis of power spectral density levels, shown in Fig. 3. Variations in sound level with the diurnal and semidiurnal tidal components are seen at infrasonic (1 – 10 Hz) and low (100 – 400 Hz) frequencies. The semidiurnal variability below 10 Hz is attributed to flow noise (Fig. 2) due to either tidal currents or vent plume entrainment. Variability between 100-400 Hz, above the flow noise regime, is generated by vent plume outflow and mixing.
Caption: “Figure 3: Periodic variability of power spectral density”
Combining the long-term records with data recorded on the Deep Acoustic Lander’s hydrophone array will allow the relationships between physical forcing and hydrothermal vent sound generation mechanisms and acoustic signatures to be further determined.
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