Precision Air-Gun Contrallers and Monitor Systems

ABSTRACT Large air-gun arrays are popular seismic energy sources. Optimum performance occurs when all guns in the array are individually programmed to fire in a fixed time sequence. A precision control and monitor system has been developed for arrays up to 30 guns. The geophysicist can program the relative fire time of all guns and monitor individual gun performance. INTRODUCTION HISTORY Offshore seismic exploration began in earnest about 30 years ago. Dynamite and other chemical explosives were the first energy sources used and dominated the field until the late fifties. Then, economics, safety, environmental and political considerations forced the industry to look to other sources of energy. Among the sources used or tried, have been air-guns, VIBROSEIS*, DINOSEIS*, AQUAPULSE*, steam power, cavitation, sparkers, and implosion devices. The most popular non-explosive source today is the air-gun, available in various forms from different manufacturers. BASIC OPERATION The air-gun is a device for translating compressed air into seismic energy for use in offshore or marsh operations. A typical airgun is the Bolt PAR* air-gun shown in Figure 1. The air-gun receives air from an external source and, upon electronic command, releases the air explosively into the water to create an acoustic excitation. Basically, the gun consists of an upper cavity and lower chamber with a shuttle and dual piston arrangement to control the air release. In operation, high-pressure air is applied to the upper cavity. An equalizing vent through the shuttle fills the lower chamber. The upper piston is slightly larger than the lower piston so that the mechanical advantage holds down the shuttle, sealing the lower chamber until a firing command is received. When an electronic fire command is received, a solenoid in the upper cavity vents a small volume of air to the lower Bide of the upper piston, unbalancing the holding forces. When no longer restrained, the shuttle moves rapidly upward due to the pressure on the lower piston. As the shuttle moves upward, the lower piston moves past the ports releasing the air from the chamber into the surrounding medium. The shuttle continues to travel upward until stopped by the cushion of trapped air in the upper cavity. Then the shuttle springs back downward to seal the chamber again. The airgun is then ready to begin another cycle. USAGE Because the air-gun releases energy in proportion to air pressure and chamber volume, it might seem that desired impulse characteristics could be obtained simply by varying the pressure and size of the gun. In practice, however, the characteristics of the air-gun impulse in relation to its surroundings are too complex to permit such simple solutions. In fact, many elements other than simple source characteristics enter into the process of gun selection. Typical of these are gun to water-surface distance and gun to water-bottom distance. 1 In normal operation, the pulse from a single air-gun, regardless of size, is generally less than desirable.