Marine Air-Gun Arrays And Ghost Cavitation

The major objective of seismic sources is to generate a broadband signal that results in a high signal to noise ratio for seismic exploration. Despite the desire to obtain a broadband signal, we do not normally include ultra-high frequencies (above 5 kHz) in this objective. Hence, we want to avoid generating strong high-frequency signals since such signals might disturb marine mammals. A wellknown mechanism for creation of underwater high frequency signals is cavitation. For marine air-gun arrays it has been suggested (Landrø et al., 2011) that the collective acoustic signal caused by free surface reflections (referred to as “ghost cavitation”) from many air-guns being fired simultaneously might lead to cavity creation. This hypothesis has been strengthen by further studies (Landrø et al., 2013, Landrø et al., 2016, Khodabandeloo et al., 2017 and Khodabandeloo and Landrø, 2017). It is also well documented that single air-guns emit frequencies above 5 kHz, and there has been significant development aiming to attenuate the high-frequency signals generated by single air-guns (Coste et al., 2014). It has also been suggested to exploit the high frequency signals generated by air-gun arrays for leakage detection above a hydrocarbon reservoir or a CO2 storage site (Landrø et al., 2017). In this paper, we will give a short overview of ghost cavitation, and discuss both acoustic and photographic evidence for the phenomenon, and briefly discuss how we can attenuate the high frequency signal by simple means.