STAGARAY System Improves Primary PUlse/Bubble Ratio in Marine Exploration.
Authors: Kologinczak John 
Published: October 2008

The STAGARAY? marine seismic energy source is a seven element, tuned air gun array designed with the objectives of obtaining maximum power over a broad spectrum of frequencies in the seismic band and maximum attenuation of subsequent pressure pulses caused by bubble oscillations. Various approaches were considered to achieve these objectives. The ultimate configuration was influenced by the desire for operating reliability and practical application of a theoretical approach. The small number of guns used in the final configuration requires little maintenance and the source fired at 1,800 psi produces an acoustic pulse with a peak amplitude of 12 bar meters (12 atmospheres at one meter distance from the source). Bubble suppression is better than 12 to I measured in terms of primary pulse amplitude to subsequent bubble pulse amplitude.

TOWING ARRANGEMENT

As part of the towing arrangement (see figure I), each gun is suspended from a buoy to a depth of about 35 feet. The array length is 61 feet with sufficient distance between guns to prevent inter-bubble pressure distortion or bubble coalescence. For consistent control of gun depth, the nearest gun is towed 60 feet behind the vessel, and both the string of guns and the string of buoys are towed by separate cables. The array is deployed and retrieved by a system of winches and a trough attached to the deck.

TEST SETUP

To analyze the signature characteristics of the source output with no bottom reflection interference, it is necessary to make measurements in relatively deep water. The signatures shown were taken in 1,200 feet of water with a calibrated hydrophone towed 180 feet below the source. The hydrophone was attached to an anchor and the boat speed was adjusted until the hydrophone was directly beneath the energy source.

The hydrophone output was amplified by a calibrated, fixed-gain, flat-response amplifier and recorded digitally at a one millisecond sample rate. The measurement standard used was the actual pressure observed in the far field at 55 meters (180 feet), corrected to represent pressure at one meter from the source. This is a standard method of acoustical measurement. Pressure decay is proportional to the inverse of the distance squared.

PRESSURE CHARACTERISTICS

When the ports of an air gun are opened, the pressurized air contained in the gun chamber is released and pressure is relieved as the bubble grows. The two curves in figure 3 show bubble growth and pressure characteristics of an air gun pulse. The solid curve is bubble radius as a function of time, while the dashed curve is the associated pressure of that bubble. The primary pressure peak occurs at the instant the compressed-air is released. Subsequent pressure peaks (or bubble pulses) are generated during the oscillation period when the bubble is smallest, which is during maximum acceleration of the bubble. The oscillation rate is a function of volume and pressure. This plot is based on 120 cubic inches of air at 2,000 psi and the assumption that the pressurized air could be released instantaneously.