Near-Bottom Acoustical Detection of Bubble Streams Emanating From Natural Seafloor Seeps in the Gulf of Mexico

Abstract

Abstract A method is described for acoustical detection of bubble streams in the water column while conducting swath bathymetry surveys at 20 m altitude with a deep-diving ROV. Such bubble streams emanate from natural seafloor seeps. The same acoustical tools are then used to measure the flow rates of the detected bubble streams. The goals of this work are to quantify the volume flux emanating from natural seeps found on salt domes and mud volcanoes in the Gulf of Mexico. A combination of acoustical and optical tools was used for this purpose. The sonar used for this work was a forward-looking multibeam echo-sounder operating at 500 kHz. In detection mode, the sonar axis was tilted about 30º above horizontal to avoid ensonifying the bottom. In this mode, small bubble streams (individual bubbles rising in a column about 0.2 m in diameter) were detectable at ranges in excess of 50 m. Once a bubble stream was detected, the ROV would be navigated towards it until visual contact was established with video cameras installed aboard the ROV. Then, the sonar head was tilted upward and acoustic data were collected in imaging modes and in Doppler modes to track the ascent of bubbles in the water column. The main results of this work are bubble flow rates about 0.2 m/s, and bubble streams of a much smaller extent than anticipated from independent detections with a shipboard echo-sounder. Introduction The Mississippi Canyon protraction area (NH16-10) (BOEM, 2000) on the outer continental shelf of the northeastern Gulf of Mexico is an active oil exploration and extraction area associated with numerous salt domes and mud volcanoes. Natural hydrocarbon seeps found on these domes contribute oil and gas to the marine environment. It is estimated that 95% of the total offshore oil input into the Gulf of Mexico is due to natural seeps (OSB, 2003). However, the episodic release rates of these seeps make them difficult to detect. Shipboard acoustical methods have successfully detected such seeps when they include bubble streams emanating from the seafloor (e.g. Greinert et al., 2006; NOAA, 2011). When investigating known seeps with near-bottom video survey tools, the seeps are difficult to find because bubble streams are small and the field of view of the cameras is limited to a few meters. The objectives of the work presented here were to use acoustical methods to detect and measure the flow rates of natural seafloor seeps during near-bottom surveys at three sites in the northeastern Gulf of Mexico: Biloxi Dome, Dauphin Dome and Mars Mud Volcano.