Vibroseis, Nonlinearity, and Harmonics
The ear is, however, endowed with a property which the eye does not possess, namely that of creating waves of entirely new frequencies out of the disturbances which fall on it.— James Jeans (1877–1946)
If we were to survey the universe of seismic sources in use today for production-seismic data in the petroleum industry, it would reveal only a few serious contenders (Liner, 2008). Since the beginning of commercial seismology in about 1930, many seismic sources have been developed, tested, and tossed into the Darwinian struggle for market survival as a reliable commercial source. At present, only three sources account for the vast majority of data acquisition.
In marine seismic applications, the air gun is ubiquitous. Several dispersive phe-nomena arise in the use of air guns and air-gun arrays, including ghosting and radiation patterns. Recall that we are using the term dispersion in a generalized sense meaning frequency-dependent phenomena, not just seismic-velocity variation with frequency.
The ghost is an interesting example of dispersion wherein the physical source interacts with the ocean surface to form a plus-minus dipole whose time signal is a strong function of frequency and propagation angle (Krail and Shin, 1990). For a given source depth, the radiated field can have one or several interference notches at frequen-cies that vary with angular direction away from the source. These show up in the mea-sured seismic data as spectral nulls called ghost notches. To further complicate the picture, ghosting occurs on both the source and receiver sides of acquisition.