3.4 High Frequency Signals from Airguns


Only a few studies have been published which describe measurements of airgun signals in the kilohertz (kHz) frequency range. In this section we discuss models for the generation of these high frequencies from airgun arrays.


If you want to find the secrets of the universe, think in terms of energy, frequency and vibration. Nikola Tesla (1856–1943)


In 2003 scientists conducted a broad-band (0–80 kHz) study of various gun arrays at the Heggernes Acoustic Range near Bergen, Norway, an institution operated by the Norwegian, Danish, Dutch and German navies for noise measurements of military and civil vessels. Te spatial dimensions and volumes of the source configurations were small, on average 10–20 dB lower than arrays used by the exploration industry. Additionally, the noise generated by the seismic vessel itself was recorded.


3.4.1 Ship-generated Noise


Most of the emitted source energy had frequencies below 150 Hz, far in excess of the low-frequency noise generated by the source vessel. Spectral source levels (measured in decibels) were highest below 100 Hz, dropping off continuously with range and frequency so that at 1 kHz they were approximately 40 dB, and at 80 kHz approximately 60 dB lower than the peak level. Above 1 kHz, spectral levels agreed almost


Figure 3.20: Photograph of an airgun fired underwater. Notice the four bubbles emerging from the ports of the gun.


completely with the noise generated by the vessel, meaning that if low-level, high-frequency (>1 kHz) spectral components were emitted, they were masked by ship- generated noise. Tis is of particular importance for marine mammals with pronounced high-frequency hearing sensitivity like toothed or beaked whales (see Section 3.9). Te results indicate that any high frequency signals created by these relatively small airgun arrays are so weak that they would not disturb these marine mammals significantly more than normal ship traffic, as shown in Figure 3.21.


3.4.2 Airgun High Frequency Signals


What causes high frequency signals from an airgun? Te first and most obvious cause is that the rapid movement


of the air escaping through the airgun ports (Figure 3.20) creates cavities in the water close to the gun. Tis effect is the same as cavities created on propellers, or those associated with turbulent flow in water, as shown on Figure 3.22. Since we believe that these cavities are created close to the source, it is reasonable to assume that the amount and strength of the cavities are dependent on the design of the airgun. Hence, there might be differences between high frequency noise from airguns produced by different manufacturers. It is also evident that the triggering mechanism, the so-called solenoid, which is an electrical coil


Figure 3.21: Broadband-smoothed amplitude spectra from 38.2 litre gun array at 550m range, received by a hydrophone at 35m depth, compared to noise generated by the seismic vessel. Below 1 kHz the amplitude spectrum of the airgun signal differs significantly from the vessel’s noise spectrum due to the low-frequency energy emitted by the airgun, while above 1 kHz the amplitude spectrum of the airgun signal coincides with the vessel’s noise almost completely. This indicates that the slow spectral level decay is mainly caused by ship-generated noise.


150 140 130 120 110 100 90 80 70 60 50


10 Airgun array


Vessel Noise


100


1000 Frequency (Hz) 107


10000


100000


Amplitude spectrum (dB re 1µ Pa/Hz)


Modified from Breitzke et al, 2008


Langhammer 1994


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