Exploring the capabilities of an 18-kHz split-beam scientific echosounder for water column mapping and seafloor positioning of methane seeps in the northern Gulf of Mexico

Underwater methane seeps support diverse biological communities on the seafloor and, in cases of bubble survival to the surface, contribute to the quantity of atmospheric methane. The National Oceanic and Atmospheric Administration (NOAA) ship Okeanos Explorer completed two research cruises for seep mapping and characterization in the northern Gulf of Mexico during August and September of 2011 and April of 2012. A 30-kHz Kongsberg EM 302 multibeam echosounder (MBES) and an 18-kHz Simrad EK60 split-beam scientific echosounder were employed to detect and observe seeps during multiple transects over areas of known seep activity at depths of approximately 1500 m. This presentation includes analyses of EK60 data from both research cruises with emphasis on seep mapping in the water column and seep source positioning on the seafloor using EM 302 MBES observations of seeps as benchmarks. Uncertainty associated with interferometric principles employed by the EK60 and limits to midwater positioning capability imposed by its beam pattern are discussed. The importance of sound speed measurement at the transducer face and the effects of refraction correction are estimated by comparison of isovelocity and constant-velocity layer models using sound speed profiles collected during the research cruises.