Variability in acoustic backscatter and fish school abundance at a shallow water CCS site

Abstract

Carbon capture and storage (CCS) is one approach used to reduce carbon dioxide (CO2) emissions to the atmosphere. Subsea CO2 storage sites must have low risk of leakage and have minimal impact on the marine environment. Single beam echosounders (SBES) are a good technology for detecting CO2 leakage in the water column for application in a marine Measurement, Monitoring and Verification (MM&V) framework. Characterisation of active acoustic data is needed to determine whether levels of water column backscatter enable CO2 gas leaks to be detected, and to understand acoustic false positives caused by naturally occurring acoustic signals (e.g., fish schools). We used acoustic data collected by SBES fitted to two mobile and two fixed sensor platforms over four years to characterise the site of the proposed CarbonNet CCS Project subsurface CO2 storage reservoir. We observed diel and seasonal variability in water column backscatter and fish school abundance. Water column backscatter was highest at night and in Austral summer and autumn periods but was at a level that would allow small CO2 gas leaks to be detected. Fish abundance was highest during the day and in Austral summer and autumn. To reduce the likelihood of acoustic false positives, surveys designed to detect CO2 gas leaks using active acoustic technologies could prioritise data collection at night and during the months where fish school abundance is lowest. Understanding these sources of natural variability can guide survey strategies and response actions.