End-fired Vertical Marine Source Array
Authors: Kerekes Bibi Cagdas 
Published: May 2011

The paper is based upon an original idea that beaming the seismic source energy within the vertical angles of the exploration interest, source-generated interference can be reduced. The study emanates from previous on-shore work by Pan American Petroleum (S. T. Martner, et. al.) and Atlantic Richfield (Elias Z. Ata, et.al.)

This new seismic source design significantly improves signal-to-noise ratio by enhancing signal in the presence of source-generated interference such as water-bottom reverberations, scatter, surface ghosting and the like, that are otherwise difficult, if not impossible to remove completely in data processing.

The current use of air-gun sub-arrays are mainly designed to attenuate bubble oscillation and are ineffective in reducing source-generated interference resulting from geological complexities located above the exploration objectives. Although end-fire source arrays have been used in land seismic surveys the same was never designed nor used in marine seismic surveys due to the popularity and predominance of air-gun sub-arrays. This current proposal involves the use of individual air-guns in a near-vertical deployment whereby the topmost gun is fired first followed by guns located below with pertinent time delays that correspond to the sea-water velocity. Such end-fire source array will reinforce the downward propagating pressure wave by constructive interference while attenuating all other wave propagation by destructive interference. Such destructive interference is physical, i.e., superimposing rarefaction upon pressure, thereby eliminating further propagation of such pressure wave in water. Depending upon the exploration objectives a source array can be designed to beam source energy towards intended targets while avoiding beaming such energy in any other direction. (Copyright 2011, Offshore Technology Conference.)

Background Theory

Seismic source arrays in general can be looked upon as antennas that radiate seismic energy. The direction of such radiation depends upon the array characteristics, the velocity of medium within the array and the emitted frequency spectra of the source elements. The performance of symmetrically distributed arrays can be described by their response in the domain of attenuation in decibels and spatial frequency in wave-numbers. To illustrate the theory Figure-1 shows the response of a simple uniform array of 12 elements spaced at 10 meter interval. The main-lobe at small wave-numbers represents the useful seismic energy radiating towards intended seismic targets. The side-lobes towards the higher wave-numbers represent undesirable seismic energy radiating away from intended seismic targets. The aim in seismic array design is to attenuate side-lobes that are in close proximity to the main-lobe. Optimization can be achieved by increased number of elements in a centrally weighted arrangement to reduce such side-lobe interference. The response can be transformed into polar plots in the domain of angles from vertical and attenuation in decibels by setting the frequency and medium velocity constant. Figure-2 shows such transformation for 45 Hertz dominant frequency and sea-water velocity of 1525 m/s. Figure-3 shows the same array response in polar coordinates. Any given array for higher component frequencies would have narrower main-lobes thus array directivity enhances the high frequency components of the emitted source spectrum.

Development

Directivity of simultaneously fired source arrays is perpendicular to the deployed direction. To design a directionally efficient horizontally deployed source array for marine application would involve towing a " football-field size?? array of many air-gun sub-arrays fired simultaneously. Practicality and current industry practice limits the number of air-gun sub-arrays to around four per source unit towed in close vicinity. As a result beaming characteristics are uncared for.