How Can We Emit Enough Energy From a Marine Seismic Vibrator?

Abstract

Environmentally, marine seismic vibrators offer a safer way to emit acoustic energy into the earth. Even though they might produce a similar sound exposure level (SEL) to a corresponding airgun array, the SEL injury threshold for marine mammals is significantly higher for continuous sounds than it is for impulsive sounds. It is this reduced animal sensitivity, rather than the vibrator’s lower peak pressure, that gives the vibrator its environmental advantage over airguns for marine mammals. Geophysically, there has always been concern about the marine vibrator’s ability to generate enough energy, especially at the low frequencies. For all pulsating acoustic sources, there is an inherent reduction of output at low frequencies. It is indeed difficult to generate enough acoustic energy from a marine seismic vibrator array if the energy requirement is to match the spectrum of an airgun array. But airgun arrays typically produce an excess of energy. By configuring the sweep so as to emit only as much energy as is actually needed to produce the required image signal-to-noise ratio we can make marine vibrators viable.