Pneumatic Acoustic Energy Source*

Abstract

In recent years considerable work has been done to devise a satisfactory non‐dynamite seismic system that would replace dynamite in offshore areas. Prior to the advent of digital recording and processing, the non‐dynamite sources have generally not provided the depth of penetration or the resolution required for satisfactory seismic interpretation. More recent developments in non‐dynamite offshore marine sources include adaptation of the Vibroseis from a land unit to a marine unit, and adaptation of the Dinoseis unit from a land to a marine unit. The SUE (Seismic Underwater Explorer) system is a thermodynamic non‐dynamite source utilizing a mixture of propane and oxygen detonated in a special chamber approximately 15 feet below the water surface. This source gives penetration to more than 4 sec in areas typified by Gulf of Mexico type geology and shows deeper penetration than had previously been obtained by dynamite along the western United States in areas with 20 lb charge limitations. A pneumatic source, the airgun, has been in production use in the United States since June 1966. This non‐dynamite source provides an intriguing amount of versatility and can be expanded to provide additional energy as necessary to obtain the penetration desired. Tests using systems comprised of from eight to twenty‐three airguns show penetration in excess of 5 seconds in many areas. Power spectra comparisons both in amplitude and frequency content demonstrate that this is a controlled source generating a controlled seismic wavelet and a controlled frequency spectrum that can be tailored to fit requirements of particular areas. Sample sections obtained in the Gulf of Mexico and the Pacific Ocean offshore California show adequate penetration to 5.0 seconds reflection time. Quantitative measurements with the airguns demonstrate the effect of: Variation of the number of guns in the system; Shaping the frequency spectrum by using different sizes of airguns in the system; Effects on signal‐to‐noise ratios as a result of stacking several small energy sources together; Reproducibility of the initial pulse wavelet from shot to shot. The improvement in record quality as a result of advanced digital processing with non‐dynamite sources is comparable to that obtained with dynamite sources. Non‐dynamite sources make additional improvements possible where high source multiplicity is advantageous. Excellent dynamic correlations yield accurate velocity control as well as definitions of apparent velocities attributable to multiples and primary‐to‐multiple amplitude relationships. Non‐dynamite sources are being used more and more extensively in offshore exploration. The advent of digital recording and processing provides a means for improving depth of penetration and resolution of many non‐dynamite sources.

A Method for Calculating the Output Pressure Waveform from an Air Gun

“…Predicted graphs (Fig. 6) using this theory can be seen to be in very close agreement with results obtained experimentally by Giles (1968) using guns of entirely different design.…”

“…In commercial prospecting, arrays of guns of different sizes are very often used to produce a broad bandwidth, high-amplitude signal. (B. F. Giles 1968. ) Many of the arrangements in use today have been arrived at by trial-and-error methods.…”

“…This theory was first discussed by Gilmore (1952). His paper is an unpublished report, and so, rather than just quoting the final equations, thus giving no idea of how they were derived, a slightly abbreviated form of his argument is given in Section 2.…”

Modelling large scale airgun-bubble dynamics with highly non-spherical features

“…To satisfy the global demand for oil, offshore oil resources become of increasing relevance [1]. To find them airguns are widely used as a seismic source in geophysical exploration [2], as they are safe, cheap and environmentally friendly. An airgun contains a chamber filled with highly compressed air.…”

Measurement of Airgun Waveforms

“…The use of a waveform in synthetic seismic reflection profiling modelling will be presented in a future paper. Attempts have been made to measure airgun waveforms (Giles 1968;Ziolkowski 1970;Mayne & Quay 1971;Schulze-Gatterman 1972;Giles & Johnston 1973) but these are not considered to satisfy adequately all the requirements: known impulse response of the recording system, adequate depth of water so that bottom reflections do not interfere with primary arrivals, known geometry of gun and receiver, and measurement at a sufficient depth to measure the downgoing wave into the sea bed. These effects are considered separately.…”