Calibration of Seismic Sources during a Test Cruise with the new RV SONNE

Abstract:

During autumn 2014, several test cruises of the brand new German research vessel SONNE were carried out before the first official scientific cruise started in December. In September 2014, BGR conducted a seismic test cruise in the British North Sea. RV SONNE is a multipurpose research vessel and was also designed for the mobile BGR 3D seismic equipment, which was tested successfully during the cruise. We spend two days for calibration of the following seismic sources of BGR:

• G-gun array (50 l @ 150 bar)
• G-gun array (50 l @ 207 bar)
• single GI-gun (3.4 l @ 150 bar)

For this experiment two hydrophones (TC4042 from Reson Teledyne) sampling up to 48 kHz were fixed below a drifting buoy at 20 m and 60 m water depth – the sea bottom was at 80 m depth. The vessel with the seismic sources sailed several up to 7 km long profiles around the buoy in order to cover many different azimuths and distances.

We aimed to measure sound pressure level (SPL) and sound exposure level (SEL) under the conditions of the shallow North Sea. Total reflections and refracted waves dominate the recorded wave field, enhance the noise level and partly screen the direct wave in contrast to ‘true’ deep water calibration based solely on the direct wave.

Presented are SPL and RMS power results in time domain, the decay with distance along profiles, and the somehow complicated 2D sound radiation pattern modulated by topography. The shading effect of the vessel’s hull is significant. In frequency domain we consider 1/3 octave levels and estimate the amount of energy in frequency ranges not used for reflection seismic processing. Results are presented in comparison of the three different sources listed above.

We compare the measured SPL decay with distance during this experiment with deep water modeling of seismic sources (Gundalf software) and with published results from calibrations with other marine seismic sources under different conditions: E.g. Breitzke et al. (2008, 2010) with RV Polarstern, Tolstoy et al. (2004) with RV Ewing and Tolstoy et al. (2009) with RV Langseth, and Crone et al. (2014) with RV Langseth.