Reflected acoustic energy from geological layers during seismic reflection surveys

Abstract

Acoustic propagation is significantly impacted by seabed characteristics, which play a large role in propagation modeling. Shallow seabed characteristics comprise a notable area of research due to their impacts on bottom loss, but deep seabed characteristics are often ignored. At low frequencies (several hundred Hertz, particularly below 100 Hz) and at ranges less than that corresponding to the seafloor critical angle, these deep layer characteristics have non-negligible effects. Those effects are explored here using a subset of data from a marine seismic reflection survey, MGL2104, in an environment with a nearly constant ∼2.6 km bathymetry. The source is a 5700 in.3 airgun array and reflections are measured by a 1200 channel, ∼15 km streamer, with both arrays at 12 m depth. The results show that in one-third–octave bands below 100 Hz, a significant fraction of the reflected energy (sometimes >50%) at certain ranges in the water column is attributable to sub-seabed layers, and the seafloor reflections only become the dominant source at ranges where the reflection path approaches a critical angle. The analysis also considers the effects of layer depths on reflected energy, demonstrating that increased depth does not necessarily correlate with decreased energy reflected in the water column.