3.1 Airguns for Non-Experts The airgun has long been the most popular marine seismic source.


Double, double, toil and trouble; airguns fire and ocean bubble. With apologies to Shakespeare’s three witches in Macbeth,


and thanks to our colleagues Bill Dragoset and Jan Langhammer.


A seismic source is defined as any device which releases energy into the earth in the form of seismic waves. Te major source type in marine exploration is the airgun array, which since the 1970s has been by far the most popular. Te airgun can be described as a chamber of compressed air that is released rapidly into the surrounding water to create an acoustic pulse. Te airgun is the most commonly used source because the pulses are predictable, repeatable and controllable, it uses compressed air which is cheap and readily available, and it has only a minor impact on marine life.


3.1.1 Size and Geometry


An airgun volume is measured in litres (l) or more commonly, by the conservative petroleum geophysicist, in cubic inches (in3


Typical volumes of individual airguns used by the exploration industry vary from 20 in3


(0.3 litres) to 800 in3 ). (13.1 litres), while


academic seismic refraction studies can use volumes up to 1,600 in3


(26.2 litres). An airgun array consists of 3–6 subarrays called


strings, each string containing 6–8 individual guns, so that the array usually involves between 18 and 48 guns, although in special cases as many as 100 guns an array can be used. Te airgun array volume is the sum of the volumes of each gun, and is typically in the range 3,000–8,000 in3


(49.2–131.6 litres). Te airguns hang in the sea beneath floats between 3m and


10m below the sea surface, generally at about 6m, except in refraction studies when a deeper deployment is needed. Te gun pressure most commonly used by the seismic industry is 2,000 psi (138 bar). During a survey the guns fire every 10–15 seconds. It is common to arrange several (2–4) airguns in a cluster,


with the guns so close together that they behave as a larger single gun. Te main purpose of clustering is to improve signal characteristics, since the bubble motion (see Section 3.1.2.1) is reduced by this configuration. Te energy sent out by airgun arrays is predominantly


directed vertically downwards. Te broad band of frequencies from the array form a pulse with peak-to-peak amplitude in the range 14–28 bar-m, corresponding to 243–249 dB re 1 μPa-m vertically downward. Te amplitude levels emitted horizontally tend to be 15–24 dB lower. Tese numbers are frequency dependent. By filtering out high frequencies there is less deviation between amplitude levels vertically and horizontally. Here, ‘dB re 1 μPa-m’ means decibel value peak-to-


peak relative to the reference pressure one micropascal at a reference distance of one metre. Confusing units? Read the box on definitions and for a guide of the physical principles of airguns and the basic sound measurement units. Our focus for the moment is on the vertically downward travelling ‘far- field’ signature of an airgun array as this signature provides a quantitative measure of the array’s performance.


94


Figure 3.1: Deployment of airgun subarray (or string). 3.1.2 Bubble Oscillations


When compressed air is suddenly released into the water an oscillating bubble forms. Tis process is described in Parkes and Hatton (1986): “Initially, the pressure inside the bubble greatly exceeds the


hydrostatic (external) pressure. Te air bubble then expands well beyond the point at which the internal and hydrostatic pressures are equal. When the expansion ceases, the internal bubble pressure is below the hydrostatic pressure, so that the bubble starts to collapse. Te collapse overshoots the equilibrium position and the cycle starts once again. Te bubble continues to oscillate, with a period typically in the range of tens to hundreds of milliseconds.” Te oscillation is stopped due to frictional forces, and the buoyancy of the bubble causes it to break the sea surface. If


PGS


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