ABSTRACT

The Simplon water gun is a new pneumatic seismic source. It creates an acoustic pulse by implosion. Its acoustic efficiency is higher than other pneumatic sources, it is free of bubble pulses, and its power spectrum ranges from a few cycles to several ldlocycles. Firing is very accurate and signals from several guns can be added. Signal amplitude can be varied through operating pressure or chamber volume change. The principle of operation is described.. Technical data, signal characteristics, and marine seismic reflection profiles illustrate the suitability of the water gun as a seismic energy source.

INTRODUCTION

The Centre Oceanologique de Bretagne has developed a new seismic source to replace preceding dynamite equipment for use during its geophysical cruises. Main objectives considered were not only signal and energy characteristics, but also versatility, cost, safety, volume, and weight. Development was a-cooperative project between COB and SODERA, a Toulon-based corporation. It led to the water gun described in this paper.

PRINCIPLE OF OPERATION

The water gun is basically an axial flow air gun coupled to a piston and cylinder assembly. The piston moves freely in the cylinder. Shooting sequence is illustrated in Fig. 1 and can be summarized as follows.

First, the air gun is brought to pressure. The piston is maintained against the air gun outlet by hydrostatic pressure and the cylinder is filled with water.

Secondly, the gun is fired. Compressed air propels the piston at high velocity (100 to 200 m/sec). The piston in turn expels the water contained in the cylinder. This water plug displaces the surrounding water at great velocity.

Thirdly, the piston is rapidly decelerated and the moving water mass is separated from the piston and a cavity is formed by inertia.

Fourthly, the cavity implodes and creates an acoustic signal. The air is let out of the gun through a hose and the piston is pushed back against the air gun outlet by hydrostatic pressure. The gun is then ready for another shot.

CHARACTERISTICS

In the water gun, energy transfer is purely mechanical and highly efficient. For the cavity to exist and in order to create an adequate signal, it, is necessary to control the imparted velocity and deceleration of both the piston and the propelled water mass. Piston velocity can be varied through action upon pressure and volume of the air chamber. Piston deceleration function can be changed by different piston and cylinder design.

The water gun is simple and requires no other controls than the ones of the air gun. The cylinder and piston form a lagged and reliable assembly. Signal characteristics can be modulated by varying operation parameters such as the pressure. Signal can also beamed by modification in design such as air chamber or cylinder volumes.

At this date, the water gun is available in two sizes with characteristics given in the following table.

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