Advanced sonar methods for detecting and monitoring marine gas seeps

Detecting gas seeps at the seafloor at an early stage is of importance in order to prevent environmental as well as economical consequences, and to gain a better understanding of the amount of greenhouse gases seeping into the oceans and potentially reaching the atmosphere. It is well known that gas bubbles in water are visible in sonar images. However, limited efforts have so far been directed towards optimizing acoustic methods for reliable and potentially automatic seep detection. Current acoustic methods rely heavily on an operator's ability to scrutinize the data and to recognize gas seeps because of their characteristic appearance. This approach is not well suited for systematic monitoring where consistent and reliable detection of all potential seeps is vital. In this project, we use the characteristic properties (acoustic, spatial, and temporal) of gas bubbles in water in order to develop optimized methods for gas seep detection. We have proposed two new methods for (semi)-automatic gas seep detection. One method is based on an interferometric sidescan sonar, the other method is a modification of existing synthetic aperture sonar (SAS) data processing targeted at reliable detection of bubbles in the water column. We have also studied broadband sonar data in order to gain a better understanding of how the frequency dependent behavior of bubbles in water can be utilized for more reliable detection and classification of bubble seeps. In addition, we have developed a tool for enhancing seep visibility in deep water with poor SNR conditions using a broadband split-beam echo sounder.

With the increasing challenges related to CO2 emissions, extensive research is being directed toward CO2 storage in subsea reservoirs. There are several operational storage sites including the Utsira Formation above the Sleipner field, Norway. This has cr eated a need for reliable monitoring methods to ensure that the injected CO2 stays in the reservoir as intended. The main monitoring strategy today is time-lapse seismics, where subsurface layers down to the reservoir are imaged at intervals of one year o r more. While seismic studies can reveal information about the long-term movement of the gas, these studies are time-consuming and costly and do not offer detailed information about potential gas seeps at the seafloor. The acoustic properties of gas mak e sonar a highly suitable method for detection and localization of gas seeps at the seafloor. Existing sonar methods can detect gas seeps, but are not well suited for large-scale monitoring. The main objective for this project is to develop new reliable a nd cost-effective sonar methods for monitoring gas seepage into the oceans, based on specific properties of the gas. Expertise in acoustics, signal processing, and geosciences will be combined in an effort to reach our objectives. These methods will compl ement but not replace seismic studies. Three main research tasks are defined - Investigation of acoustic and spatio-temporal properties of gas seeps - Adaptive sonar processing targeted toward gas seep detection - Synthetic aperture sonar (SAS) process ing for gas seep detection The project will have access to unique sonar data from the Sleipner field, and from several natural gas seeps. This data was acquired using an autonomous underwater vehicle carrying a sidescan sonar with SAS capabilities. The results of these studies should be of interest nationally and internationally for improved monitoring of existing and future CO2 storage sites, as well as for environmental monitoring of the seafloor.