Multibeam Echosounder System Optimization for Water Column Mapping of Undersea Gas Seeps

The ability to image water column scatterers has broadened the utility of multibeam echosounders and these systems, which were initially designed for hydrographic purposes, are now being used more often for physical, chemical and biological oceanographic studies of water column phenomena. It must be remembered, however, that multibeam echosounders were designed for bathymetric measurements and many features of modern systems reflect this: (1) frequency encoded, multi-sector systems are able to stabilize imaging geometry to ensure near constant sounding spacing and density on the seafloor, (2) dual swath systems allow for increased sounding density and faster survey speeds, (3) FM pulses allow for improved range performance and (4) transmit focusing improves angular resolution in the nearfield. These improvements to multibeam systems augment their abilities from the hydrographic point of view, however, their impact on those who wish to quantitatively study water column scatterers is poorly understood. In this work, we discuss field trials in which a Kongsberg Maritime EM302 multibeam echosounder was optimized for water column mapping of natural seabed gas seeps through repeat observations of a known set of seeps in the Gulf of Mexico. A systematic mapping campaign immediately followed the trials with hundreds of seeps detected in area measuring ~85 km x 130 km. Of particular interest to the hydrographic community are the efforts undertaken to accurately ray trace and georeference water column scatterers as the algorithms employed can be applied to water column imaging in support of least-depth detection over wrecks and other hazards to navigation