Abstract
Multibeam echosounder systems (MBES) can record backscatter strengths of gas plumes in the water column (WC) images that may be an indicator of possible occurrence of gas at certain depths. Manual or automatic detection is generally adopted in finding gas plumes, but frequently results in low efficiency and high false detection rates because of WC images that are polluted by noise. To improve the efficiency and reliability of the detection, a comprehensive detection method is proposed in this paper. In the proposed method, the characteristics of WC background noise are first analyzed and given. Then, the mean standard deviation threshold segmentations are respectively used for the denoising of time-angle and depth-angle images, an intersection operation is performed for the two segmented images to further weaken noise in the WC data, and the gas plumes in the WC data are detected from the intersection image by the morphological constraint. The proposed method was tested by conducting shallow-water and deepwater experiments. In these experiments, the detections were conducted automatically and higher correct detection rates than the traditional methods were achieved. The performance of the proposed method is analyzed and discussed.
Highlights
New generation multibeam echosounder systems (MBES), such as the Kongsberg EM710/EM302 [1,2,3]and Teledyne Reson 7125/8125 [4,5] can collect water column (WC) data
Gas plumes are formed by gas leakage [19,20,21]
Whether methane gas bubbles reach the sea surface depends on the water depth and initial bubble size
Summary
New generation multibeam echosounder systems (MBES), such as the Kongsberg EM710/EM302 [1,2,3]and Teledyne Reson 7125/8125 [4,5] can collect water column (WC) data. WC data include the full acoustic information from the MBES transducer to the seafloor [1] and provide an effective way to find underwater objects such as fish schools [4,6,7,8], wrecks [9,10,11], eelgrass [12], gas plumes [13,14], and internal ocean waves [15,16]. The search for methane hydrate by detecting gas plumes from WC data is a new and low-cost alternative method to the traditional seismic exploration [2,18]. Gas leakage may be modulated by temperature and pressure. The driving force of leakage is the production of methane at depth and resulting overpressure, possibly linked to fluid migration, and the buoyancy of the gas. Methane gas bubbles usually dissolve completely during their rise. Whether methane gas bubbles reach the sea surface depends on the water depth and initial bubble size. Many bubbles dissolve after rising a few dozens of meters [22]
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