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Reducing the volume of a marine seismic source array can cause a significant reduction in the effect upon and interaction with marine fauna. Small sources are favourable in environmentally sensitive areas; however, the conventional understanding is that larger sources are required for improved signal-to-noise ratio and deep penetration. Results from a number of reduced source-strength tests suggest that this is only partly true; at the lower end of the seismic bandwidth, we can benefit from more power (we are governed by non-shot-generated noise), but for the majority of the seismic bandwidth, we can indeed accept less power (we are governed by shot-generated noise). We look at the system as a whole and use a 4C towed streamer that is towed deep. This amplifies the low frequencies and compensates for the lack of low-frequency signal from the smaller source array. This allows us to acquire data with a small source array (typically three or four times smaller than normal) while maintaining the data quality of a conventional towed-streamer system using a larger source array. The high-frequency limit of the conventional pressure only streamer is also overcome by the inherent notch filling of the 4C measurements.

We address the low-frequency deficiency of marine airgun source arrays. The output well below the lowest bubble resonance frequency is determined by the total quantity of air released, not how the air is distributed between guns in the array. However, the output at and above the lowest bubble frequency depends on the details of the array. Increasing the low-frequency output requires a bigger bubble. This will shift the bubble resonance frequency downwards. We design the large bubble using a ‘hypercluster’ of conventional airguns, which are placed so the released air behaves as one large bubble oscillating with the period associated with the combined air quantity. The airguns are placed close enough for the released air to frequency-lock, but not necessarily so close that the bubbles coalesce. Of course, we could generate the same resonance frequency by using a single enormous airgun, but we do not want to do that because this would severely limit the mid-frequency output, and it would also pose significant operational challenges. Frequency-locking enables array designers to utilize more of the total high-pressure air capacity for generating low-frequency sound.

We discuss how to design a marine source array that emits the same pulse shape in all directions. We do this by analysing the relative contribution of the three main components of the farfield source spectrum: the ghost response, the aperture smoothing function, and the monopole spectra of the individual source elements. Counter to intuition, reducing the lateral array dimensions, such that it approximates a point aperture, will not result in an isotropic source; it will create a dipole because of the sea-surface reflection. For single depth arrays, the lateral geometry must be tailored such that the aperture smoothing function offsets the angular variation of the ghost response, in particular at frequencies close to the first ghost notch. Multi-depth arrays can be made insensitive to the lateral geometry by combining the source depths, such that the first ghost notch is far beyond the highest frequency of interest. This results in smaller spectral variation with take-off angle than that of single depth arrays. There is no advantage in using more than two layers.