Seismic Source Decomposition

Abstract We exploit the differences that exist between the radiation fields of a point source and an array to design a marine seismic source with desired power spectral and directional characteristics, whose far-field time signature is known precisely from measurements. The desired power spectral characteristics are created by firing a series of point source units sequentially, such that their time signatures do not overlap. The effective power spectrum of the whole series of signatures can be made equal to the sum of the power spectra of the individual signatures, and can therefore be designed to suit the desired application by an appropriate choice of source units. The key to the subsequent processing of the recorded data is to measure the pressure wave generated by each point source unit with a hydrophone placed closeby, but in the linear radiation field. The position of this hydrophone relative to the source unit must be known accurately in three dimensions. The depths of the source units and their relative spatial positions at the instants of firing must also be known. From these measurements the far-field signature of the sequence in any azimuth can be deduced, and the impulse response of the earth can therefore be recovered from the recorded reflection data. The desired directional characteristics can be created by arranging the source unit separations such that each source unit reaches the desired position at the prescribed firing instant. The deconvolution of the derived sequence of signatures simultaneously performs the source array simulation. Introduction In the seismic experiment the measurable response x(t) is regarded as being composed of the impulse response of the earth g(t), convolved with the far-field signature of the seismic source s(t), plus some noise n(t). Thus: (1) x ( t ) = s ( t ) + g ( t ) + n ( t ) where the asterisk * denotes convolution. The object of initial data processing of the measurements is to recover the earth impulse response g(t) from the measurable quantity x(t) with as high a degree of fidelity as possible. In order to do this there are three basic requirements which must be met :The signal-to-noise ratio must be large.The frequency bandwidth of the generated far-field signature s(t) must be broad.The shape of the far-field signature s(t) must be known. This means that both the amplitude and phase as a function of frequency must be known. All these three requirements are met if the seismic source produces a high-energy, broad bandwidth signature of known shape. In attempting to meet the first two requirements at sea, with seismic source arrays, it is usually the case that the third requirement is not met. In this approach we aim to meet all three. Marine Seismic Source Arrays The marine seismic sources currently in use are of two types: point sources and arrays. Point sources are those which have dimensions which are small compared with the wavelengths of seismic radiation which they generate.