Summary

We propose two ideas to reduce ocean noise generated by seismic surveys. One is a new low-pressure source; an evolution of conventional airguns with some significant mutations. We estimate that low-pressure sources will reduce high frequency noise and strengthen weak low frequency signal, providing significant reductions in ocean noise from seismic surveys, improved sub-salt and subbasalt imaging, and better blocky earth models. The other idea is to use more receivers and less shots per area in wide-azimuth long-offset surveys. In essence to shoot less and make each shot count more. We propose the utilization of swarms of motorized unmanned surface vessels towing streamers. Such swarms will provide the wide azimuths and the far offsets that are required to image deep targets under complex overburden with less shooting per area compared to conventional wide azimuth solutions that use additional sources rather than additional receivers.

Introduction

One obvious idea to reduce ocean noise is to use seismic sources that generate less unusable high frequency energy. The airguns that we are using today are very inefficient; only a few percent of the energy that they release generate acoustic waves at useful frequencies. A lot of energy is wasted to heat, cavitation causes too much of the remaining energy to generate acoustic waves at frequencies that are too high to be useful to us. Waves over 220 Hz are taken out by the anti-aliasing high cut of our 2-millisecond analog to digital converters. We can use 1 millisecond and sub millisecond sampling, but we don’t, except in sitesurveys that anyway need only small source to image shallow hazards. There is no sense in using higher sampling rates because depending on the target depth and overburden’s attenuation and complexity, waves over 25- 150Hz are attenuated and scattered. Although we do not know the actual impacts of high frequency energy on marine life, it is generally believed that by reducing them as much as possible is a step in the right direction.

The seismic industry is attempting to address the issue of potential environmental impact from seismic sources. Marine Vibroseis may be an excellent long-term solution; no cavitation, just turbulence, and controlled sweeps that generate very little energy at useless high frequency harmonics. Marine vibroseis has been in development and testing for a long time (Dragoset, 1988; and many more publications) but it still remains a future solution.

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