Seismo-acoustic signatures of shallow free gas in the Bornholm Basin, Baltic Sea

• We investigate the seismo-acoustic expressions of shallow gas.
• We compare seismo-acoustic data with different frequencies.
• In acoustic data shallow gas appears as point scatterers and acoustic blanking.
• In seismic data the gassy layer appears as a reflector with reversed polarity.
• In seismic data shallow gas can modify the wavelet shape of the seafloor reflection.

Research on gas-charged sediments is generally based on identification and mapping with hydroacoustic methods, and seismic sections are typically verbally described. The terminology, however, does often not properly account for different instrumentation, as the seismic response of gas-charged sediments is frequency-dependent, and seismic appearance may vary with the instrumentation used. We therefore combine and compare the expressions of shallow gas in high frequency Parasound data and lower frequency multichannel seismic data for the Bornholm Basin, Baltic Sea, to generalize the descriptional approach and to indicate distinct imaging differences. Gas bubbles strongly scatter the acoustic energy of a high frequency signal, shallow gas appears therefore in Parasound data as a cloud of point scatterers and acoustic blanking. The seismic waves undergo reflection from the gassy sediment, but with sufficient transmission, and the top of the gassy sediment appears as a reversed polarity reflector. In seismic data, free gas can modify the seafloor reflection in different ways depending on the proximity of the gas front to the seafloor. High reflectivity at this interface causes reverberation of the water bottom reflector. Gas signatures also reveal relative measures about the amount and concentration of gas bubbles, distribution of bubble sizes, and the depth distribution of gas.