1 0.8 0.6 0.4 0.2 0 40 1 0.8 0.6 0.4 0.2 0 42 44 46 48 50 Time (ms) b 52 54 56 58 3.4.5 How to Reduce the Ghost Cavity Signal 0 2 4 6 8 10 Time (ms)
Figure 3.30: Measured (a) and modelled (b) ghost cavitation signal for various high pass filters, showing a good correlation. The red curve in (b) represents modelled signal assuming a sea surface reflection coefficient of -0.2, and the black curve corresponds to -0.6.
12 14 16 18 20
A dedicated experiment (Landrø et al., 2016) tested if the ghost cavity signal could be reduced by, for instance, increasing the distance between the sources in a source array. Figure 3.31 shows that this is indeed possible: by increasing the distance between two subarrays in a conventional marine airgun source array, we found that the high frequency signal (above 130 kHz) is significantly reduced by increasing the separation distance from 6.5 to 8m. In the frequency domain, it is clearly shown that
0.15
6.5m 8.0m Infinite (only port array)
0.1 0.7 ms 0.05
the difference between 6.5 and 8m subarray distance is increasing with frequency, as shown in Figure 3.32. Tis means that the simplest way to reduce the ghost cavitation signals that occur for large compact airgun arrays is to increase the distance between the subarrays. Furthermore, this field experiment showed that the ghost cavity signal decreases with increasing source depth. It was also found that an airgun array consisting of two subarrays gives high frequency signals between 1 and 10 kHz that are similar to conventional cargo ships.
we find that this simple model gives a reasonably good similarity between measured and modelled ghost cavity signals, as shown in Figure 3.30.
a
7 ms and reaches a maximum after 3.5 ms. Using the famous Rayleigh formula which relates the collapse time (T) of a cavity to its radius (R):
T = 0.915R ph
0
38
40
42 Time (ms)
Figure 3.31: Amplitude spectra of two subarrays separated by 6.5m (black), 8m (red) and one subarray only (blue). Notice that the slope is steeper for the single array.
150 140 130 120 110 100 90 80 70 60
44
46
48
6.5m separation 8.0m separation
Infinite separation (only one subarray)
100 101 Frequency (kHz)
Figure 3.32: Absolute value and smoothing of the ghost cavity signals for various distances between two subarrays in the source array. Black and red curves correspond to 6.5 and 8m subarray separation distance, respectively. A 130 kHz high pass filter has been applied to the data prior to absolute value and smoothing. The blue curve shows the same type or response for the case where only the port array is fired. The grey and light red boxes show where the signal has been saturated.
110 102
SPL (dB; re 1 µPa, 1 Hz, 1m)
Amplitude
Amplitude
Amplitude
Landrø et al., Geophysics, 2016
Landrø et al., Geophysics, 2016
Landrø et al., Geophysics, 2016
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