Reducing high-frequency ghost cavitation signals from marine air-gun arrays

Abstract

Ghost cavitation, which is a term describing that cavitation bubbles are generated acoustically, has been hypothesized to occur when the ghost reflected signals from many individual air guns beneath the sea surface produce a pressure that is close to zero in the water above the source array. Ghost cavitation is typically observed some milliseconds after the ghost reflection, and it may last for 5-15 ms, depending on the configuration of the source array. The cavitation process subsequently generates a weak high-frequency signal. To investigate this potential signal model and mechanism, we have performed a dedicated source experiment. We found that the distance between the source strings in a source array is a major factor that influences the amount and strength of the high-frequency signal. By increasing the separation distance from 6.5 to 8 m, we have observed a significant decrease in the high-frequency signal. Further, the amount of ghost cavitation can be reduced by increasing the distance between the guns. Also single sub-arrays may create ghost cavitation sound, of course weaker in signal strength compared with full arrays, in agreement with the model. Conventional air-gun modeling can be used to predict where ghost cavitation can occur. Therefore, in principle, a workflow could be developed to quantify grossly if and how much high-frequency signals could be generated by this mechanism, given the source array configuration, and further change the configuration to reduce to a very minimum the high-frequency signals, if deemed necessary. For an airgun array consisting of two subarrays separated by 6 m and fired at 9 m depth, we found that the high-frequency signals emitted between 1 and 10 kHz were of similar strength to the noise from conventional cargo ships, depending on their size and the vessels' speed.

Characterizing the acoustic properties of the cavity cloud generated close to an air-gun array as a time-dependent effective medium

“…This phenomenon was first hypothesized by recording high frequency (> 10 kHz) signals in a seismic air-gun array field measurement (Landrø et al, 2011). It was further analysed (Landrø et al, 2013) and investigated based on a dedicated field experiment (Landrø et al, 2016). The ghost cavitation phenomenon in an air-gun array was numerically modelled and could successfully predict the recorded highfrequency ghost cavitation signals and the associated low-frequency component (Khodabandeloo and Landrø, 2017a).…”

Detecting gas leakage using high-frequency signals generated by air-gun arrays

“…Coste et al (2014) showed that the high-frequency signal generated by a single air gun can be reduced by approximately 10 dB for the frequency range between 0.5 to 3 kHz by implementing a new air gun design. The environmental aspects related to the highfrequency signal generated by air gun arrays have been discussed by several authors, see for example Coste et al (2014) and Landrø et al (2016).…”

“…When we apply a 5-10 kHz bandpass filter (Figure 3d), we observe that the high-frequency signal occurs at a much later time: around 94.5 ms for the near-offset trace and 97.5 ms for the far-offset trace. In this case, we attribute the major part of the signal to be caused by the ghost cavitation effect (Landrø et al, , 2013(Landrø et al, and 2016. The time shift of 3 ms between the near-and far-offset trace indicates that there is a shift between the onset of the source signature for the near and far offset and the maximum peak amplitude.…”

“…The time shift of 3 ms between the near-and far-offset trace indicates that there is a shift between the onset of the source signature for the near and far offset and the maximum peak amplitude. It might also suggest that the ghost cavity cloud is elongated, as shown in Figure 7 in Landrø et al (2016). To investigate this time shift further needs more investigation and accurate modeling of both the air gun array and the ghost cavitation process.…”

“…The more guns and the closer they are, the more the ghost cavitation effect will contribute to the high-frequency signals. A dedicated experiment was performed in 2011 where this hypothesis was further tested by for instance increasing the distance between two subarrays in a seismic array (Landrø et al, 2016). They found that the strength of the high-frequency signals was reduced by increasing the distance between subarrays from 6.5 to 8 m.…”