To verify existing models for conversion of acoustic target strength to estimates for 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 a specific sizes most commonly found in gaseous methane seeps, 1 to 5 mm radii, which can be released at any interval and at any water depth. The synthetic seep system was deployed off the coast of New Hampshire in an approximate depth of 50 m. Acoustic backscatter from 10 to 100 kHz was collected by steaming over the synthetic seep multiple times, each with a predetermined and calibrated bubble size created by the system at depth. These data represent a direct field measurement which tests models describing bubble size evolution during ascent through the water column, as well as models for acoustic scattering from bubbles of different sizes. Validating these models directly tests the ability of broadband sonar systems to acoustically monitor the transport of gas from the seabed to the atmosphere.