Utilizing a variety of multichannel seismic and multi-frequency acoustic instruments, this thesis focuses on the investigation of the frequency dependence in seismo-acoustic imaging of shallow free gas and the development of methods to characterize gas occurrences and estimate the content in Baltic Sea sediments. A data set collected over the gassy area of the Bornholm Basin, SW part of the Baltic Sea, was processed and analysed. The identification of seismo-acoustic gas signatures in the Bornholm Basin revealed dependence on the measurement frequency as well as on the resonance of gas bubbles in the sediment. Interval velocities obtained from seismic data were shown to be used for the assessment of free gas concentration in situ in shallow marine sediments. With the multi-frequency acoustic approach, measurement of the compressional wave attenuation allows quantification of free gas content in shallow marine environments as long as the measurement frequency reveals the resonance frequency peak
