Identifying mid-water targets using the higher frequencies emitted by seismic sources of opportunity

Seismic sources are routinely employed by the oil and gas industry to identify hydrocarbon reserves beneath the seabed, and by researchers to image the sub-seabed for geophysics and to identify geo-hazards such as tsunami-generating areas. For mitigation purposes, it is paramount to identify animals in the water column, but they can be missed by surface observations (if they are diving or in bad weather) or by Passive Acoustic Monitoring (if they remain silent). For operational reasons, it is also important to know about any other sizeable objects below the water surface. Seismic sources emit high-amplitude broadband sounds, typically below 300 Hz, directed toward the seabed. They can also radiate energy up to 20 kHz into the water column, and it can be used as a “source of opportunity”. We use these higher frequencies (between 500 Hz to 20 kHz) to investigate their potential in identifying a variety of mid-water targets, with data from surveys conducted in challenging environments (two in shallow waters, 7–25 m deep, one in deep water, >1,500 m deep) with seismic sources up to 4,500 cubic inches in volume. The shallow-water surveys used a fixed source and freely drifting buoys, whereas the deep-water survey used a towed source with passive acoustic monitoring (PAM) vessel closely follow the seismic vessel to record data. The time spreads of individual shots recorded and the SNR at frequencies between 20 Hz – 20 kHz were compared between the surveys. Based on target strengths of potential targets at different ranges, and on benchmarked models of acoustic propagation, 2-D plots of measured vs. expected levels can be used to detect “hidden” targets of different sizes (from 0.5 to 20 m). The analyses suggest that, at 500 Hz, it is possible to confidently detect mid-water targets within the exclusion zone, and potentially going much further, to as deep as 2 km and as far as 2 km from the source. This has important implications for real-time mitigation and protection of marine mammals, which can be detected even if they are submerged and silent.