A Powerful Marine Seismic Source Seismic Tuned Array of Steam Jets

ABSTRACT The sudden ejection of steam into the water generates an acoustic wave which corresponds to the expansion peak of the signature. As the bubble expands its internal pressure decreases and the equilibrium position is overshot due to kinetic energy. Under the hydrostatic pressure, the bubble pressure falls and practically reaches the saturation pressure of the cold water. From this time, corresponding to the minimum pressure of the signature, the bubble starts its implosion according to Lord Raleigh's law. The implosion ends in a shock of the water against itself. The well-known qualities of a punctual steam seismic source are an exceptionally sharp impulse signature and no bubble effect. The basic system consists of 1 to 4 identical subarrays towed behind the seismic vessel. Each subarray comprises 4 guns emitting 4 different bubble periods and synchronized on the implosion peak (fig. I). Firing these 4 guns together results in a powerful and well shaped signal (fig. 2) with good spectral characteristics (fig. 3). Thus tuned, Starjet preserves the advantages of a punctual source and is characterized by an excellent repetitivity of the emitted signal, a clean pulse (no tail), good ratio between implosion and first peak, a 15 bar-meter peak-to-peak amplitude for each subarray. This seismic source has to be used when penetration and definition are required in seismic surveys. After 5 years of research and development Starjet's industrial use began in June 1982 with a series of tests conducted in the North-Sea and the Mediterranean. Seismic results are discussed in relation with the pulse characteristics and wavelet processing. INTRODUCTION Implosive sources are becoming ever more widely used because of their superiority in high frequencies in comparison with conventional airguns. Nevertheless there are three drawbacks with the general use of such sources.firstly, they generate a double impulse. The first when the bubble is formed under waterdischarge of the steam (explosion) in the case of steam sources; discharge of water in the case of water sources, with electric current used to cause the sudden generation of steam in the case of sparkers. The second impulse is of course the implosion itself. This double impulse in the time domain expresses itself by major oscillations of the frequency spectrum due to phase differences between the components of each of the two impulses. The resulting signal is generally difficult to deconvolve.secondly, the spectrum toward the low frequencies is limited by the maximum bubble period these sources can produce, since the minimum frequency emitted with a sufficient level is in fact the inverse of the maximum bubble period.thirdly, synchronizing a subarray of implosive guns is difficult in rough seas due to the fact that the time implosions occur depends on the local hydrostatic pressure at the instant of the shot according to the well-known Rayleigh's formula. The Starjet system presented here has been designed to overcome these three drawbacks.