Introduction to Air Guns and Air Gun Arrays

The effect of seismic operations onmarine mammals has been debatedvigorously for years. Some feel thatthese operations could harm the ani-mals. Others, based on anecdotal evi-dence of marine mammals swimming(or even playing) near active air-gunarrays, feel that harmful effects areunlikely. They claim that such evi-dence indicates, at the very least, thatair guns do not physically damage theanimals. Still others have relied onacoustic measurements to argue thatany potential effect on the marinemammal population within a seismicsurvey area is negligible. Because ofthe importance of the discussion, theseismic industry has conductednumerous tests and monitoring stud-ies to try to address this issue. To date,those studies have not identified anyharmful long-term effects due to theproximity of marine mammals to airguns. Why then, does the debate con-tinue?

The answer may lie in the shearcomplexity of the issues. Air-gundesign, underwater acoustics, animalbehavior, and marine mammal phys-iology are complex subjects and inter-actions between them are even morecomplicated. Although many debateparticipants are experts in one or moreof these fields, none is an expert in all.Thus, individuals can interpret thesame data in different ways and reporttheir interpretations using differentterms. Suppose two individuals withvarying backgrounds observed a dol-phin jumping near an active air-gunarray. One might see a dolphin leap-ing from the water to avoid the noisewhile the other may conclude thatsame dolphin is playing in the airbubbles.

Because of this communicationproblem and a lack of definitive sci-entific studies, no clear consensus hasbeen reached about how air guns affectmarine mammals. Thus, many orga-nizations have recommended mitiga-tion practices until a clear answer isfound. One common mitigation pro-cedure is air-gun ramp-up, in whichguns in an array are turned on indi-vidually over several minutes.Presumably, any animal that finds thesound annoying will leave before thesound becomes loud enough to do anyharm. Another mitigation measure isto employ trained observers to watch

for marine mammals near a seismicsource and turn the source off if theycome within a certain range of the guns(based on sound pressure levels).These measures have been adopted byseveral countries, but there is little evi-dence that they are effective. There is,however, a great deal of evidence thatthey are costly to the seismic industry.How then should we identify poten-tially harmful long-term effects of air-

gun arrays on marine mammals anddesign appropriate, sensible mitiga-tion procedures if such effects arefound?

The answer is improved commu-nication and cooperation among allconcerned. Many scientific studies arebeing planned to learn more about theeffects of sound on marine mammals,particularly on their hearing, andabout the effectiveness of mitigationprocedures. If those studies are to pro-vide useful data, the seismic industry,government agencies, environmentalscientists, and marine biologists needto share their expertise. Thus, this shortarticle is offered in a cooperative spiritto those not intimately familiar with airguns and how the seismic industryuses them.

Air guns and air-gun arrays. A typi-cal air gun is a relatively simplemechanical device that stores com-pressed air in a reservoir and releasesit rapidly through small ports when afiring command is received. The portsare opened and closed by either anexternal movable piece called a sleeveor an internal movable piece called ashuttle (Figure 1). When an air gunfires, part of the energy contained inthe escaping compressed air is con-verted to sound, thereby generating aseismic signal that travels into theearths subsurface. Air guns are about4-8 inches in diameter. Air reservoirsrange from 10 to 500 in3. The operat-

892 THE LEADING EDGE AUGUST 2000 AUGUST 2000 THE LEADING EDGE 0000

Introduction to air guns and air-gun arraysBILL DRAGOSET, Western Geophysical (a division of Baker Hughes), Houston, Texas, U.S.

Figure 1. Two typical internal-shut-tle air guns.

Figure 2. Signature of a single 40-in3 air gun as recorded by a hydrophone300 m below the gun.

ing air pressure is usually around 2000psi, and guns are generally deployed3-10 m below the water surface.

The characteristic sound producedwhen an air gun fires underwater iscalled its pressure signature (Figure2). The signature has three main com-ponents: (1) the so-called direct arrival,the sound produced when the airguns ports first open; (2) the sourceghost, the reflection of the direct arrivalfrom the water surface and which hasopposite polarity from that of the directarrival because the reflection coeffi-cient of the water-air boundary is neg-ative; and (3) the bubble pulsesproduced by the expansion-collapsecycle of the air bubble created in thewater when an air gun fires. Note thateach bubble pulse contains soundcoming directly from the bubble, fol-lowed by the sound from the bubblethat reflects from the water surface.

The important properties of an air-guns signature are characterized bytwo parameters, strength and bubbleperiod. Strength is simply the ampli-tude of the sound, measured in pres-sure units at the direct arrivals peak(peak strength) or from the directarrivals peak to the ghost arrivalspeak (peak-to-trough strength). Thebubble period is the time between con-secutive bubble pulses. The signaturein Figure 2 has peak strength of 1.5 bar-m (these units are explained below)and a 57-ms bubble period. Thestrength and bubble period of an airgun depend on size, depth in thewater, and initial firing pressure. Table1 summarizes the relationshipsbetween these five quantities.

For a couple of reasons, the soundfrom a single air gun is generally notan acceptable seismic source. First, thesignature of a single gun is too weakto produce a good signal-to-noise ratioat depth. Secondly, because of bubblepulses, the signature of a single air gunis far from being an ideal impulsiveseismic wavelet. Data shot with a sin-gle gun face difficult deconvolutionprocessing to remove bubble pulsereverberations. Both problems can beovercome using the tuned air-gunarray concept, in which many guns ofdifferent, carefully selected volumesare fired simultaneously. As shown inFigure 3, direct arrivals from individ-ual guns sum coherently below thearray, thereby producing a soundmuch louder than that from a singlegun. On the other hand, sounds frombubble pulses add up incoherently,thereby attenuating them relative tothe direct arrival. The amount of suchtuning is described by the primary-to-

bubble ratio (PBR), which is the peak-to-trough strength of the direct arrivaldivided by the peak-to-trough strengthof residual bubble pulses. The tuned-array signature at the bottom of Figure3 is much closer to the ideal seismicwavelet, a perfect impulse, than sig-natures from the individual guns.

As shown in Table 1, peak soundpressure by an air gun is proportionalto the cube root of the volume of com-

pressed air stored in the gun. This sim-ple relationship means that the mostefficient way to create sound using afixed compressed air capacity is to par-tition that air among an array of manysmall air guns rather than an array ofa few large guns. For example, two 50-in3 air guns fired together are almost60% more efficient than a single 100-in3 gun (2 (50

1/3 / 1001/3) = 1.59). To

make use of this efficiency, a typical


Figure 4. Plan view of typical air-gun array. Numbers below the gun sta-tions (green circles) are gun volumes (in3). The 155 3 notation indicatesthree guns, each with a volume of 155 in3, so close together that their airbubbles coalesce after the guns fire. Such so-called cluster guns producesound more efficiently than a single large gun. (Figure courtesy ofSchlumberger).

Gun volume (V)Firing pressure (P)Gun depth (D in m)

Strength (S)S V1/3

S Pcomplicated

Bubble period (T)T V1/3

T P1/3

T (10 + D)-5/6

Table 1. Dependence of an air-gun signature on gun parameters

Figure 3. Concept of tuned air-gun array. Blue signatures come from indi-vidual guns whose volumes, in cubic inches, are on the left. If those sixguns are placed in an array and fired simultaneously, they produce the redsignature at a hydrophone 300 m below the array. The arrays PBR is 8.6.

marine seismic source has a large num-ber of small guns rather than a fewlarger guns. Some air-gun arrays haveas many as 100 guns, but 25-50 is moreusual. A typical array is shown inFigure 4.

Signature measurements. A disad-vantage of having an array-like seis-mic source is that measuring theoutput is difficult. Sound pressure cre-ated at some distance by a single airgun is inversely proportional to thatdistance. Therefore, a detector at anydistance from the gun can measure itspressure output, provided that themeasured quantity is corrected for thedistance that the sound travels. For anarray, however, detector placement iscruciala position must be foundwhere the detector is equidistant ornearly equidistant from all elements inthe array. Only in this way is it possi-ble to correct measured pressure prop-erly for the distance that the soundhas traveled and to have sounds fromeach element arrive simultaneously atthe detector.

In seismic exploration, the pres-sure output of a marine source is mea-sured by an experiment called afar-field signature test. As shown inFigure 5, the detector is a hydrophonecentered under the array. The termfar-field refers to the depth of thehydrophone relative to array size andmeasurement bandwidth. In essence,far field means that the hydrophone isfar enough away so that sounds fromindividual guns reach it within onesampling interval. For an averagearray and a 1-ms sampling interval,far-field distance is about 300 m. Anadditional 300 m is required betweenthe hydrophone and the water bottomto prevent reflected sound from inter-fering with the measurement.

The SEG-approved unit for far-field strength of an array is the bar-m.A bar is a unit of pressure, equivalentto 14.5 psi (about 1 atmosphere) or 1011Pa (micro-Pascal). The bar-m unit isobtained by multiplying measuredpressure expressed in bars by the dis-tance between the sensor and thesound source. The advantage of thebar-m unit is that source strength ischaracterized by a single numberrather than by two numbers (the pres-sure and where it was measured). Inthe far field, peak-to-trough strengthin bar-m can be converted to actualpeak positive pressure at any distanceby taking half the peak-to-trough num-ber and dividing by that distance. Forexample, at 300 m a peak-to-trough 60bar-m source creates a peak positive

pressure of 0.1 bar [.5(60/300)]; at 1000m it creates a peak positive pressureof 0.03 bar [.5(60/1000)].

Maximum peak pressure of an air-gun array. What is the peak positivepressure at 10 m or 1 m below an arrayor at any distance to the side? Thoseand similar questions can be importantwhen trying to determine the impactof a marine source on the environment,especially in shallow water. In fact,interesting questions are What is themaximum peak pressure to which ananimal could be exposed near an air-gun array? and Where does thatpressure occur? They cannot beanswered simply by applying the dis-tance conversion from bar-m to bar toa far-field measurement. The simple

conversion fails because, at the loca-tions in question, the shape of the sig-nature changes, not just its overallscale.

Signature shape changes near anarray or to its side due to the arrayeffect. Consider, for example, whatwould happen if the hydrophone inFigure 5 were 300 m aft of the sourcerather than directly below it. DistancesAand B would no longer be equal andpeak pressures arriving at the phonefrom the corresponding guns wouldno longer coincide in time. The signa-tures direct arrival would then bedefocused or spread out, with loweramplitude due to the noncoincidentpeak arrival times. The same sort ofsignature shape distortion occurs atany location other than at a far-field


Table 2. Configuration of modeled air-gun arrayString #

111111222222333333

Element #

123456123456123456

Volume (in3)

150 + 15090 + 70

115805540

150 + 15090 + 70

115805540

150 + 15090 + 70

115805540

In-line coordinate (m)

0.04.37.7

10.412.915.00.04.37.7

10.412.915.00.04.37.7

10.412.915.0

Cross-linecoordinate (m)

-10.0-10.0-10.0-10.0-10.0-10.0

0.00.00.00.00.00.0

10.010.010.010.010.010.0

Figure 5. Equipment configuration for a far-field signature measurement.For a typical array, distance A = 300 m.

distance directly below an array.Consequently, maximum peak pres-sure produced by an air-gun array can-not be established simply by scalingfar-field measurements.

Nevertheless, establishing mini-mum peak pressure for an array isquite easy: Select the largest single gun.Measure (or calculate) its peak pres-sure in bar-m at a convenient location.Scale that to absolute pressure at 1 m.

With a single air gun, simple dis-tance conversion is valid because thereare no array effects that change thepeak pressure as location changes.With the guns currently used in theseismic industry, minimum peak pres-sure within an array is typicallybetween 2 and 3 bar. Maximum peakpressure created by an array is deter-mined by how two competing phe-nomenon interact. On one hand is thefocusing effect due to array geometry,and on the other is the inverse scalingof pressure with distance from asource. So, the question becomes: Canarray focusing effects more than com-pensate for the inverse scaling to pro-duce sound levels greater than 2-3 bar,and if so, what is the spatial extent ofsuch sound levels?

To answer these questions requiresspecial air-gun source measurementsor detailed modeling of an air-gunsource. For this paper, I have resortedto air-gun modeling because it is muchsimpler and less costly than field mea-surements. The main advantage ofmodeling is that it allows calculationof pressure produced by an array atany underwater location. Thus, onecan easily map the full 3-D extent ofan arrays pressure field, an exercisethat would take days of field mea-surements to accomplish.

The modeling software, used inthe seismic industry for more than 15years, uses coupled partial differentialequations to represent the interacting,oscillating air bubbles produced whenan array is fired. As shown in Figure6, modeled signatures agree quitewell with measured signatures. Themodeled signature in Figure 6 wascalculated as if all air guns firedsimultaneously. In an actual array,however, firing times are spread over1-2 ms due to mechanical variationsin their firing mechanisms. Because ofthis difference, the peak pressure ofthe modeled signature is somewhatlarger than that of the measured sig-nature.

The particular array modeled isone designed by Western Geophysical.It consists of 24 air guns arranged inthree identical strings. The strings are

deployed in a horizontal plane 6 mbelow the water surface. Each stringhas six source elements. The first twoelements of each string consist of twoair guns each, arranged in a cluster. (Acluster means guns so close togetherthat when fired they behave as a sin-gle larger gun.) The final four elementsof each string are single air guns. Allguns are sleeve guns with a com-pressed air reservoir at a pressure of2000 psi prior to firing. Total volumeof the array is 2250 in3, and far-fieldpeak strength is about 57 bar-m.

Table 2 shows gun volumes andsource element horizontal coordinates.Figure 7 shows a plan view of thearray. In Table 2 the terms in-line and

cross-line refer to the direction the shipsails and the perpendicular to the saildirection, respectively. Note that theorigin of the horizontal coordinate sys-tem coincides with the most forwardelement in the center string. Modeledpressure results are tabulated in thesame coordinate frame. Note also thatthe array is symmetric about the cen-ter string in the cross-line direction.This means that pressures need to bemodeled for only one half of the array(the starboard side was chosen), sincethey, too, will be symmetric.

The first modeling experimentcomputed peak positive sound pres-sures in the horizontal plane of thearray. This was accomplished by posi-


Table 3. Peak positive pressure (bar) in the horizontal plane of the array inFigure 7In-line (m)

-10.0-7.5-5.0-2.50.0

2.154.36.07.7

9.0510.4

11.6512.9

13.9515.017.520.022.525.0

= 2 m0.801.041.351.832.503.003.083.113.343.433.453.292.902.411.951.300.990.780.65

= 5 m0.791.151.491.942.823.794.254.004.204.754.263.983.313.032.451.631.120.870.66

= 8 m0.761.001.331.782.503.003.143.143.343.433.443.292.902.411.951.330.990.750.58

= 12 m0.480.610.831.392.482.983.033.093.303.393.413.252.862.381.920.970.580.420.35

= 15 m0.400.570.740.971.411.902.122.112.102.372.131.991.651.521.230.810.560.440.33

= 0 m0.941.211.532.05

3.87

4.65

5.45

5.30

4.71

1.501.140.910.75

Figure 6. Comparison of measured and modeled signatures for a small air-gun array.

Crossline

tioning strings of 19 imaginaryhydrophones at cross-line coordinates0, 2, 5, 8, 12, and 15 m. The hydrophonestrings, parallel to the in-line direction,had phones every 2.5 m from in-linecoordinates -10 to 25 m. Note that thepattern monitors pressures inside andoutside the array. Results are shown inTable 3. The pink shaded area is out-side the boundaries of the array. Theempty cells in the second column from

the left correspond to coordinateswhere the pressure could not be com-puted because an air gun was too close.The largest peak positive pressure forthis modeling experiment (yellow cell)occurs near the center of the middlegun string. A number of conclusionsare immediately apparent:

The largest pressures occur insidethe boundaries of the array

Outside the boundaries, pressuresdecay rapidly due to inverse scaling

The largest pressure occurs near thevery center of the array

The moderately higher pressures at5 m compared to 2 or 8 m is an indi-cation of a focusing effect

Focusing effects are relatively weak.At no point does pressure rise muchabove what one would expect byplacing a hydrophone midwaybetween two guns

The second experiment examinedadditional locations within the middlegun string. Results are in Table 4. Theyellow cell marks the maximum peakpositive pressure found for this arrayamong all of the modeling experi-ments. Note that the largest positivepeak pressure within the plane of thearray remains below 6 bar. The thirdexperiment examined pressures atnear-field locations below the hori-zontal plane of the array. Based on theresults in Tables 3 and 4, it is evidentthat maximum peak pressures occurin the center of the array. Therefore, thethird experiment was restricted to thevertical plane containing the middle

gun string. Results are in Table 5. The6-m results (at the central string) arecopied from Table 3 for comparison.Clearly, inverse scaling reduces thepressure more rapidly than arrayfocusing increases pressure.

The final experiment examinedpeak pressure below the 9.05-m posi-tion in the central gun string over anextended depth range. (9.05 m waschosen because it is between the twolocations in Table 4 with the highestpressures.) Table 6 shows the resultsalong with the 6-, 7-, and 8-m resultsfrom Table 5.

Once again, array focusing was notable to overcome the decrease in pres-sure due to inverse scaling.

Discussion.Afew caveats are in order.First, the modeling did not includeeffects of sound reflected upward fromthe water bottom. Generally, suchenergy is quite weak compared to thesound produced directly from anarray. However, if an array is within 1m of the water bottom, upwardreflected energy can contribute signif-icantly to peak pressure. Second, thearray had cross-line spacing betweenelements (10 m) that was significantlygreater than average in-line spacingbetween elements (3 m). If cross-linespacing had been much smaller, say 3m, the maximum peak positive pres-sure would be that due to contribu-tions from four air guns rather thanthat from just two air guns. In both sit-uations, peak pressure would behigher than reported here. Third, peakpressures created by air guns depend


Table 5. Peak pressures below thecentral gun stringIn-line (m)

-2.50.0

2.154.36.07.7

9.0510.4

11.6512.9

13.9515.017.5

Depth =7 m

2.194.613.514.263.774.694.274.854.314.503.713.541.58

Depth =8 m

2.182.843.393.663.553.623.733.593.202.822.482.261.63

Depth =6 m

2.05

3.87

4.65

5.45

5.30

4.71

1.50

Table 4. Peak positive pressures atadditional locations within thecentral gun string

In-line (m)1.01.52.83.35.36.78.79.4

11.411.9

Crossline = 0 m4.974.023.974.315.115.055.685.675.455.26

Table 6. Peak positive pressures in a vertical line below the centerof the array

Depth (m)6789

10121417202530

In-line = 9.05 m5.454.273.733.473.393.423.102.782.562.061.88

Figure 7. Plan view of the air-gun array used for the modeling study.Numbers next to each gun site are gun volumes (in3). Red circle marks theorigin of the coordinate frame.

on the bandwidth used for the mea-surement or calculation. The resultsreported here were computed for a 1-ms sampling interval (correspondingto a 500-Hz bandwidth) that encom-passes most energy produced by anarray. A broader band, however, maygive slightly larger pressure results.

The maximum peak positive pres-sure was 5.68 bar at the center of thehorizontal plane containing the array.This is about the pressure that shouldbe expected by placing a phone mid-way between the 115-in3 and 80-in3guns with all other guns turned off.The reason that the pressure peakshere rather than between the 300-in3and the 160-in3 guns is the differentgun separations. The two smaller gunsare only 2.7 m apart while the twolarger guns are 4.3 m apart. Similarmodeling was done for the Schlum-berger array in Figure 4. That arrayhad a maximum peak pressure ofabout 7 bar between the first two gunstations of the center string.

Array focusing effects are evidentin the modeled results. For example,Tables 3 and 6 show that pressuredecreases slower with vertical distancebelow the center of the array than withhorizontal distance to the side. That isexactly the behavior expected from anarray whose elements are in the hori-zontal plane. Nowhere within oraround the array was the focusingeffect strong enough to overcome thedecrease in pressure due to inversescaling and produce a pressure greaterthan that expected between two guns.

These results suggest a generalprocedure for determining maximumpeak positive pressure of arrays simi-lar to the one modeled: Measure (orcalculate) the pressure at variouspoints between closely spaced adja-cent guns in the plane of the array.Exact gun sizes and separations willdetermine which pair produces themaximum pressure. An upper limitcan easily be established simply bydoubling the peak pressure at 1 m fromthe largest gun in the array. Unless anarray contains guns significantly largerthan those in the modeled array, theupper limit of the maximum peak pos-itive pressure should be around 6 bars(measured in a 500-Hz bandwidth).

Concluding remarks. Modelingshows that peak maximum pressureproduced by an array is roughly a fac-tor of 10 less than what one wouldexpect by naively scaling a far-fieldmeasurement back to 1 m. While it iscomforting to know that a 100 bar-marray does not produce pressure of 100

bar in the water, that by itself does notlead to any conclusions regarding theimpact of air guns on marine animals.That impact likely depends on a num-ber of phenomena upon which Ihavent even touched in this intro-ductory paper. For example, eachspecies of mammals may be most sen-sitive to sound in a particular band-width. Because sound produced by anarray is not distributed evenly acrossthe frequency spectrum, some speciesmay be bothered more by air gunsthan others. In addition, the soundfrom an array that an animal at somedistance actually experiences dependson the interaction between the array,the near-surface geology, and the waterdepth. This is particularly true forarrays in shallow water, wherechanges in water-bottom conditionsor water depth can create dramaticchanges in the modal propagation ofsound in the water layer.

Because of their reliability andfavorable sound characteristics, nearlyall marine seismic surveys in the past20 years have used tuned air-gunarrays. During that time, the seismicindustry has learned much about airguns: the characteristics of the soundthey produce, how to measure thatsound reliably, how to design optimalarrays, and how to calculate the soundfrom an array. That accumulatedknowledge can make an importantcontribution to studies involving themarine environment and man-madenoises. I hope that if everyone con-cerned with the environment workstogether, it wont be overlooked.

Suggestions for further reading. Acomprehensive method for evaluatingthe design of air guns and air-gunarrays by Dragoset (TLE, 1984). Astan-dard quantitative calibration procedurefor marine sources by Fricke et al.(GEOPHYSICS, 1985). Air-gun array specs:A tutorial by Dragoset (TLE, 1990). TheMarine Seismic Source by Parkes andHatton (D. Reidel Publishing, 1986).Desired seismic characteristics of anair gun source by Larner et al.(GEOPHYSICS, 1982). Marine Mammalsand Noise by Richardson et al.(Academic Press, 1995). LE

Acknowledgments: I thank WesternGeophysical for permission to publish this paper,Schlumberger for providing one of the exam-ples, and Jack Caldwell, Doug Bremner, andLaurent Meister for contributions and sugges-tions.

Corresponding author: william.dragoset@ west-geo.com


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Introduction to Air Guns and Air Gun Arrays