BROADBAND ACOUSTIC MEASUREMENTS OF A CONTROLLED SEEP WITH MULTIPLE GASES FOR VERIFICATION OF FLUX ESTIMATES THROUGH BUBBLE DISSOLUTION AND TARGET STRENGTH MODELS

Abstract

To verify existing models for conversion of acoustic target strength to estimates of the total volume of methane gas released from the seafloor through the water column, a synthetic seep system was designed and fabricated. This system creates individual bubbles of the sizes most commonly found in gaseous methane seeps, \u3c1 to 5mm radii, which can be released at any interval at depths up to 200m. The synthetic seep system was deployed off the coast of New Hampshire adjacent to the Isles of Shoals to a depth of 55m. Acoustic backscatter from 16-24kHz was collected by steaming over the synthetic seep multiple times with a suite of broadband splitbeam sonar systems. Each iteration ensonified a predetermined and calibrated bubble size created by the system at depth. These data represent a direct field measurement which was used to test models describing bubble evolution and acoustic scattering during the ascent through the water column for bubbles of different sizes and gasses. Validating these models directly tests the ability of broadband sonar systems to estimate the transport of gas from the seabed to the ocean and atmosphere. Acoustic data from 2.35mm radii argon bubbles, and 2.45mm radii nitrogen bubbles are consistent with bubble evolution and target strength models which validates spherical approximations made in both the mass transfer and acoustic scattering assumptions. Based on the similar uncertainty values for these bubble sizes, these data add confidence to the claims of previous studies which use similar methods for methane flux approximations