Fundamental Principles of Isotropic Marine Source Design

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.