Quantification of methane bubbles in shallow freshwaters using horizontal hydroacoustical observations

Abstract

Methane represents an important greenhouse gas, and its ebullition is a significant way of releasing gas from bottom sediments of shallow fresh waters to the atmosphere. Estimation of ebullition is complicated because of high spatiotemporal variability; however, a hydroacoustical survey represents an effective method for quantifying it. Commonly used vertical beaming in deep waters can be quite limited in very shallow waters. This study was thus aimed to investigate the possibility of using a horizontally oriented sonar beam for gas bubble quantification. Artificially prepared methane bubbles of various sizes, ranging from 2.5 to 905 × 10−3 mL (1.7–12 mm of equivalent spherical diameter), were released from a depth of 6 m in a freshwater reservoir. The acoustic target strength (TS) of these bubbles was observed using both the vertical and horizontal beams of a 120 kHz frequency split‐beam sonar. TS obtained in both the vertical and horizontal modes increase with growing bubble size. However, for identical bubble size, the vertical observation gives stronger TS than the horizontal one. Further, TS distribution around mean value is wider with an increase in bubble size, and this distribution is greater in case of the horizontal mode of observation than vertical. It was observed that during bubble rise TS changes for both the vertical and horizontal mode of observation lie within the range of standard deviation of TS measurement; hence, depth is not relevant for TS regression models used in depth up to 6 m.

Seasonal and Spatial Dynamics of Gas Ebullition in a Temperate Water‐Storage Reservoir

“…More than 13 thousand bubbles were acoustically detected with sizes from 1.2 to 43 mm in equivalent diameter according to Frouzova et al (). Bubble TS distribution possessed a slight bimodal character (Figure a) with a smaller peak at −60.5 dB bin (corresponds to 2 mm in equivalent diameter) and the highest peak at −54.5 dB bin (corresponds to 3.8 mm in equivalent diameter).…”

“…With respect to the mean acoustic target size (TS), the bubble traces were divided into one‐decibel bins ranging from −65 to −35 dB. Each midpoint of i TS bin was converted into adequate gas volume V i using the formula from Frouzova et al (): …”

A New Look at Seafloor Venting: Natural Gas Hydrate Derivatives

“…Characterizing emissions is improving our understanding of the transport processes and allowing estimates and quantification of fluxes (Wilson, Leifer, and Maillard 2015, Leifer and Judd 2015, Leifer 2015, von Deimling et al 2015, Vielstädte et al 2015, Weber et al 2014, Wiggins et al 2015, Nauw, Linke, and Leifer 2015, Jerram, Weber, and Beaudoin 2015, Frouzova, Tušer, and Stanovsky 2015 that will provide more detail to the CH 4 budgets involved. Furthermore, following the Deep Water Horizon disaster in the Gulf of Mexico detailed mapping of the region near the wellhead revealed multiple sources of natural hydrocarbons (Smith et al 2010).…”