*Article* **Optical Flow-Based Detection of Gas Leaks from Pipelines Using Multibeam Water Column Images**

**Chao Xu 1,2,3, Mingxing Wu 2, Tian Zhou 1,2,3,\*, Jianghui Li 4, Weidong Du 1,2,3, Wanyuan Zhang <sup>2</sup> and Paul R. White <sup>4</sup>**


Received: 11 November 2019; Accepted: 30 December 2019; Published: 1 January 2020

**Abstract:** In recent years, most multibeam echo sounders (MBESs) have been able to collect water column image (WCI) data while performing seabed topography measurements, providing effective data sources for gas-leakage detection. However, there can be systematic (e.g., sidelobe interference) or natural disturbances in the images, which may introduce challenges for automatic detection of gas leaks. In this paper, we design two data-processing schemes to estimate motion velocities based on the Farneback optical flow principle according to types of WCIs, including time-angle and depth-across track images. Moreover, by combining the estimated motion velocities with the amplitudes of the image pixels, several decision thresholds are used to eliminate interferences, such as the seabed, non-gas backscatters in the water column, etc. To verify the effectiveness of the proposed method, we simulated the scenarios of pipeline leakage in a pool and the Songhua Lake, Jilin Province, China, and used a HT300 PA MBES (it was developed by Harbin Engineering University and its operating frequency is 300 kHz) to collect acoustic data in static and dynamic conditions. The results show that the proposed method can automatically detect underwater leaking gases, and both data-processing schemes have similar detection performance.

**Keywords:** multibeam echo sounder; water column image; gas emissions; automatic detection; optical flow
