Estimation of the Horizontal Directivity Pattern of a Seismic Source Based on Spatial Measurements on the Shelf

Abstract

Based on spatial acoustic measurements carried out on the relatively shallow northeastern shelf off Sakhalin Island during seismic surveys, estimates of the horizontal directivity pattern of the seismic vessel’s source array were constructed at distances of 1.2, 2.4, and 4.6 km in three frequency bands: 10–350, 10–70, and 70–350 Hz. The estimation methodology measures the energy of seismoacoustic pulse signals propagating in the water column along a series of acoustic paths with nearly identical seabed profiles. In the 70–350 Hz band, signal energy is primarily carried by water-column normal modes, and the resulting directivity pattern exhibits clear directionality closely matching the theoretical pattern calculated for deep-water conditions. In the low-frequency band (10–70 Hz), the directivity pattern shows no dependence on the azimuthal angle. Both experimental and theoretical analyses demonstrate that estimation of the directivity pattern in the 10–70 Hz band may be inaccurate when there are spatial heterogeneities in the seabed, specifically variations in the compressional-wave speed . These findings are directly relevant to assessing the acoustic impact of seismic surveys in coastal shelf regions.