R/V Marcus G. Langseth seismic source: Modeling and calibration

Abstract

This paper presents analyses of calibration data of the R/V Marcus G. Langseth seismic sources that were collected in 2007–2008. The analysis includes measurements of the levels and azimuthal directivity of the Langseth two‐string source, which is used for 3‐D seismic surveys and compares these data with multichannel seismic hydrophone array data in shallow water. The analysis also shows the important contribution of seafloor and subseafloor reflected and refracted arrivals and the associated effects of water depth and seafloor slope. As expected and predicted by modeling, azimuthal directivity depends greatly on the “footprint” dimensions of the seismic array. In shallow water, where the acoustic field is dominated by near‐vertically traveling reflected and refracted waves, data recorded using a towed MCS array are a useful proxy for single hydrophone calibration data. In deeper water, the easily modeled direct arrivals are exceeded in amplitude by seafloor reflected and subseafloor refracted energy at offsets which depend upon the water depth, limiting the applicability of a priori modeling that does not include these interactions. In addition, in a seafloor sloping environment, amplitudes depend greatly on whether the receivers are upslope or downslope from the source.