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Abstract

In marine seismic acquisition, the bandwidth of the recorded signal is limited, on both the low and high end, and it is usually accepted that by towing the source deeper the low-frequency content in the seismic data increase. However, Mayne and Quay found in 1971 that the low frequency response (between 0 and 10Hz) increased when the air-gun is towed at shallow depths. Recently, Landrø and Amundsen confirmed these results, they suggested that for frequencies between 0.25 and 0.8 Hz the signal response is more or less 10 dB stronger when the source is fired at 3m in comparison with one source fired at 7.5m. They also suggested that these ultra-low frequencies between (0.1 and 1 Hz) are generated by the rising of the bubble. In this thesis, this ultra-low frequencies generated by a large seismic bubble were studied and discussed, using computational simulations and field data.An empirical relation for the pressure on the far-field was found and is suggested that the signal output in the ultra-low frequencies can be enhanced by increasing the volume of the air-gun and towing the gun at shallower depths. This approach could be relevant and eventually may lead to changes in the air-gun design with the focus in enhancing the ultra-low frequency output, which can lead to higher-resolution seismic images, better signal penetration, and better suitability for seismic inversion.