M³: Towards Long-Term Acoustic Monitoring of Gas Emissions using
Underwater Cabled Observatory Technology
Abstract
Natural methane gas release from the seafloor is a widespread phenomenon
that occurs at cold seeps along most continental margins. Since their
discovery in the early 1980s, seeps have been the focus of intensive
research, partly aimed to refine the global carbon budget. However,
deep-sea research is challenging and expensive and, to date, few works
have successfully monitored the variability of methane gas release over
long time periods (> 1 yr). Long-term monitoring is
necessary to study the mechanisms that control seabed gas release. The
M³ project, funded by the German Ministry of Education and Research,
aims to study the temporal and spatial variability of gas emissions at
the Southern Hydrate Ridge (SHR) by acoustically monitoring and
quantifying gas effluxes over several years. Located 850 m deep on the
Cascadia accretionary prism offshore Oregon, the SHR is one of the most
studied seep sites and persistent but variable gas release has been
observed for more than 20 years. Since 2015, the Ocean Observatories
Initiative’s (OOI) Cabled Array observatory, provides power supply and
two-way communication to the SHR, making it an ideal site for continuous
long-term monitoring work. In this work, we present how we will take
advantage of the OOI infrastructure and deploy several instruments on
the seabed for at least 1.5 year. A multi-beam “overview” sonar
mounted on a rotor will identify every gas bubble stream located within
200 m from the sonar location. A scanning “quantification” sonar will
be used to estimate the amount of gas that is released from discrete gas
streams. A camera system and a CTD probe will help process and analyze
the hydro-acoustic data. All instruments will be powered and controlled
from land through the OOI infrastructure. We present the instrument
design, the operation protocol, as well as the data processing steps and
expected results.