A NEW SEISMIC SOURCE: VAPORCHOC

Abstract

A very important drawback of the quasi totality of impulse marine seismic sources is the secondary emission of bubble pulses following the useful impulse.

The intensity of these parasitic pulses can be of the same order of magnitude as that of the main pulse in the seismic frequency band. This phenomenon is due to the physical nature of the system in which energy is deposited, i.e. the source itself, the surrounding medium and their inter-reaction.

The Vaporchoc illustrates one way to modify the system so as to prevent, at one moment, the accumulation of potential energy in an elastic medium, so far getting a total transfer of the energy to acoustic form.

To reach this goal, a mass of superheated steam is released in water through a special valve; this steam creates a growing bubble, while giving during this phase a very small part of its energy into an acoustic pulse.

A few moments later, steam is condensed due to both adiabatic expansion and thermal exchanges. The bubble becoming empty begins to collapse; near the end of the collapse phase, the kinetic energy of water flowing toward the centre of the bubble is converted into an intense acoustic pulse, due to the non linearity of the mechanical system. No further evolution of the system is possible as there remains no compressed gas acting like a spring to recycle the system.

There is a major difference in the emitting processes of conventional explosives or compressed air systems and of Vaporchoc.

In compressed air or explosives systems, the main acoustic emission takes place within the first pulse, the presence of air or gases acting as a spring gives successive bubble pulses.

In Vaporchoc, on the contrary, there is a small acoustic radiation due to the first pulse, as the steam is released smoothly; the main acoustic emission takes place within the second pulse and, as steam is condensed before this second pulse, no gas can act like a spring and there are no other pulses.

The conversion into acoustic energy is made near the centre of the bubble and the apparatus is not submitted to the very high pressure which takes place in the emitting zone.

This new source can be operated at the highest repetition rate compatible with record length allowing high order common depth point methods.