Context
STEMM-CCS (Strategies for Environmental Monitoring of Marine Carbon Capture and Storage) was an EU funded project with the goal of demonstrating the effectiveness of different monitoring of marine CCS facilities. That is the objective is to detect CO2 bubbles which, in the unlikely event of a leak occurring, would indicate the release of CO2 in gaseous form.
The focus of STEMM-CCS was a controlled release of CO2 in the North Sea in a location where CO2 storage might be considered. The chosen site was in 120 m of water. A remotely operated directional drilling rig was used to insert a curved pipe into the sediment, so it was 3 m below undisturbed sediment, then a hose was connected and the gas turned on using an ROV. Gas then worked its way through the sediment and into the water column. Gas is released continuously throughout the experiment, the rate of release is increased at five points. See the video https://www.youtube.com/watch?v=XmmITi4fxwI for an overview along with the paper [1]. The results of the project are summarised in [2]. The cruise report [3] contains detailed technical information about all aspects of the experiment, including the passive acoustic data collection (see section 7 of [3]).
Challenge Goals
The sounds of the bubbles are relatively quiet and, beside the typical ambient oceanic noise, there are several additional noise sources. On the seafloor there are other experiments, some of which use pumps and acoustic sensors and produce higher frequency noise, dominating above 20 kHz. In the lowest frequency band, below 100 Hz, the sounds of a distant seismic survey can be detected. In addition the research vessel (James Cook) is on station some distance from the experiment (ca 1 km) and there are sounds from the engines and propulsors that occur at times. The passive data is dominated by those sounds when the James Cook is over the site, as it is when the ISIS ROV is deployed. The longest period when these competing noise sources are least problematic is during
Background Material
Data was collected using a 5-channel recorder (RS-Orca ACE https://rsaqua.co.uk/product/orca/), from hydrophones positioned on a plane. The sample rate was 96 kHz. Recordings were made on a duty cycle of 5 mins on 5 mins off. The resulting data are stored in a 5-channel wav file which are named according to the convention: yyyy-mm-ddThh-mm-ss.wav.
The acoustic system is designed to listen to the sounds of bubbles leaving the sediment. The centre frequency of the sound from each bubble is characteristic of the bubble’s volume. Thus, by detecting the bubble sounds and analysing the frequency content we can estimate the amount of gas leaking from the sediment. The use of an array of 5 hydrophones offers the chance to enhance the signal to noise ratio and to estimate the location of the leaks. See [4] for a description of the processing applied to date.
Useful references
[1] Flohr, A. et al. “Towards improved monitoring of offshore carbon storage: A real-world field experiment detecting a controlled sub-seafloor CO2 releaseæ, International Journal of Greenhouse Gas Control, 106, 2021, https://doi.org/10.1016/j.ijggc.2020.103237.
[2] Connelly, D. et al. “Assuring the integrity of offshore carbon dioxide storageæ, Renewable and Sustainable Energy Reviews, 166, 2022, https://doi.org/10.1016/j.rser.2022.112670.
[3] “JC180 Cruise report- Strategies for the Environmental Monitoring of Marine Carbon Capture and Storage, STEMM-CCSæ https://www.bodc.ac.uk/resources/inventories/cruise_inventory/reports/jc180.pdf
[4] Li, J. et al. “Acoustic and optical determination of bubble size distributions – Quantification of seabed gas emissionsæ, International Journal of Greenhouse Gas Control, 108, 2021, https://doi.org/10.1016/j.ijggc.2021.103313.