Seismic Energy Sources (1969)

PREFACE This paper is a condensation of a talk presented at the 38th Annual Meeting of the Society of Exploration Geophysicists held in Denver, Colorado, September 29-0ctober 3, 1968. The complete paper is published in handbook form, available from United Geophysical Corporation, Pasadena, California. INTRODUCTION For many years dynamite and other high explosives have provided the principal energy source for seismic prospecting systems. Probably no other type of source provides such a compact package of concentrated energy, and no other source provides such a simple means for rapid and almost instantaneous release of energy. However, for various economic, technological, and political reasons, the last few years have seen a rapid increase in the use of alternative type of energy sources, particularly in marine seismic survey operations. The present-day geophysicist may choose from a proliferating spectrum of sources including dynamite, black powder, gun powder, ammonium nitrate, PRlMACORD*, AQUAFLEX, AQUASEIS, FLEXOTIR*, electric sparkers, SSP*, WASSP*, SONO-PROBE*, PINGERS*, BOOMERS*, gas exploders, DUSS*, DINOSEIS*, GASSP*, AQUAPULSE*, air guns, PAR*, PNEUMATIC ACOUSTIC ENERGY SOURCE*, SEISMOJET*, AIRDOX*, CARDOX*, HYDRO-SEIN*, etc. In addition to these impulsive type sources, there is the sustained, vibratory, underwater chirper system, VIBROSEIS*. Discussion in this paper will be limited to impulsive type sources. The essential characteristic of any underwater impulsive source is that it impart a high initial outward acceleration to the surrounding water. The energy stored in the source device must be converted suddenly into potential energy of compression and kinetic energy of outward motion in the water medium. All the impulsive devices mentioned above meet this requirement in varying degrees. However it is not surprising that there are wide variations in the quantity of energy delivered to the water. The energy range from the strongest to weakest source mentioned is over one hundred thousand to one. Accordingly, one purpose of this paper is to supply the geophysicist with a convenient basis for estimating the approximate energy delivery of various type sources. A further requirement of seismic energy source systems is that the effective energy delivery be sufficient to provide a satisfactory reflection signal/noise ratio at the greatest reflection times of interest In the case of weaker sources it is necessary to resort to multiple source arrays, "vertical" stacking, and common reflection point stacking. The reflection signal/noise ratio must be built up to required levels. It is a second purpose of this paper to describe how seismic effectiveness increases with an increase in the size of arrays and the number of stacks. A substantial portion of the total energy released to the water by any seismic source device is not radiated immediately in the form of a seismic wave, but is retained and stored temporarily as kinetic flow energy in the water. At a predictable later time the outward flow of water is reversed, and inward flow converges to produce a secondary radiated seismic pulse. This is called the first "bubble pulse," and the phenomenon is called bubble oscillation. It will be shown that the oscillation period, or time to the first bubble pulse, provides a convenient means for estimating the energy of a seismic source device.