Airgun Arrays

Marine Seismic Source Arrays – J. Diebold, L-DEO 1

Marine Seismic Source Arrays

J. Diebold, L-DEO January 2003

Peak Pressures in the Time Domain To create a marine seismic source pulse, a number of airguns are simultaneously triggered, or “fired.” Each one releases a bubble of compressed air, which expands at a regular, rapid rate, creating a pressure front - a sound wave - in the surrounding water. Compared to the surrounding water, this air bubble is quite easily resized, and it acts like a spring, overexpanding and recompressing repeatedly. Each re-expansion causes a secondary pressure front, called a bubble pulse. Here is an example, recorded during testing of R/V EWING’s airguns:

To reduce the unwanted “ringing” that results from this, these airguns (the “seismic source array”) each release different amounts of air, creating different sized bubbles, each ringing with a different periodicity, creating a “tuned” seismic signal:

Depending on its size, and a few other factors, including towing depth and compressed air pressure, each airgun’s bubble creates a different amount of pressure in the water:


In theory, the pressure produced by differently sized airguns is proportional to the radii of the bubbles they produce, or the cube root of the bubble’s volume. To double the pressure output by a single airgun requires another airgun eight times as big. On the other hand adding a second airgun of equal size will give the same result, with a more economic (1/4 as much) increase in total air volume. This is another reason that arrays with large numbers of small airguns are preferred over those with only a few large guns. An additional incentive is that for most air gun designs, there is an upward limit to the cube-root rule, clearly shown in this plot, for volumes above 420 cubic inches. The pressure and energy created any marine seismic source dies off rapidly with distance, according to simple mathematical rules. Two principal physical facts, however, conspire to complicate the result. Taking EWING’s largest seismic source array as an example, we can quickly show how the operating environment and operational requirements affect the acoustic “footprints” created by airgun arrays. If all EWING’s 20 airguns could be fired in exactly the same place, and in a world where the air and water had the same acoustic properties [impedances] this would be the result:


Acoustic pressure (sound level) would decrease regularly in all directions, as predicted by the simple rule of cylindrical spreading. Every time distance doubled, pressure would fall off by 6 dB, and at ten meters, pressure would be 20 dB less than that at one meter. It’s impossible to have all twenty airguns in exactly the same place, however, and they don’t work right if they are fired too close to each other, so the EWING’s array is spread out, to cover a horizontal area of 5 x 36 meters. This simple fact changes the way pressure is created in the surrounding ocean:

Directly beneath (and above), and not too close to the array, distances between a point of observation and each airgun are not very much different. Therefore, the pulses arrive at nearly the same time, and add together as shown in the “tuning” picture, above. Off to the side, however, the distances differ enough that each pulse arrives at a slightly different time. Individual peaks don’t line up, and the maximum sound level actually produced is much lower than it is the same distance directly below the array. The second physical fact affecting sound production is a very powerful and inescapable one – the sea surface, or air-water interface. Since sound propagation in air and water are so different, when sound waves in the water bounce off the sea surface, the reflections are negative images of the original, and almost as loud.


The EWING airgun array’s beam pattern is vertical, with an elliptical footprint, longer (fore-and-aft) than it is wide. In all non-vertical directions, pulses from individual airguns arrive out of phase, greatly diminishing the effective, or perceived, source level. This effect is further enhanced by the fact that the primary airgun signal is closely followed by its negative image, reflecting from the air-water interface. At near-horizontal angles, the arrival times for these two signals are identical, or nearly so, and the negative “ghost” signal image cancels or diminishes the signal further. In any given direction, the perceived signal falls off thereafter according to the cylindrical [1 / R] spreading rule – amplitude level falls off by 6 dB every time the distance from the source is doubled.

RMS dB and the safety radius The peak-to-peak levels charted above are easy to measure from airgun array signals, but historically, all of the research on acoustic avoidance behavior of marine mammals has quantified the sound levels in terms of RMS [root-mean-square] a measure which is entirely appropriate for many acoustic signals found in the marine environment [shipping noise, Navy sonar, etc.] Although it is less appropriate for impulsive airgun signals, the RMS measure has been used in most published studies anyway, so that meaningful comparisons could be made. The protocols used for the RMS calculation in most


published research are diagrammed here, applied to the signal predicted by our modeling for a point 4000 meters aft of EWING’s 20 airgun array, at a depth of 1200 meters:

This difference of 18 dB between the peak-to-peak and RMS dB levels for the same signal are typical for impulsive airgun signals [W.J. Richardson, pers. com., 2002.] Despite this apparent underestimation, published studies using this RMS determination still showed that whale avoidance behavior only occurred with airgun sounds at higher dB levels than for other, more constant noise types such as rig and shipping noise. Therefore, it is correct to use this measure of sound pressure levels [SPL] for calculating the safety radiuses to be observed for the various EWING seismic source arrays. Over the past few years, most IHA mitigation programs approved by NMFS have used an SPL of 180 dB RMS to define a safety radius. Any cetacean observed within this radius is defined to have been “taken” under the MMPA, and the IHA strictly limits the number of such “takes” that will be allowed during a particular activity. It is important to note that 180 dB “takes” do not involve any physical harm to cetaceans [and it is generally agreed that pinniped and fish species have even higher permissible levels.] Physical harm is not likely for SPLs below 220 dB, though it is generally agreed that exposures of 180 dB or above are to be avoided. Airgun arrays are rated according to several measures, primarily the peak-to-peak pressure levels, observed far from the array itself, but amplitude-corrected or back projected back to a standard reference point, one meter from the array’s center. Array output, measured thus in the “far field” and at a point within this primary downward


beam, will provide a value well below the rated value, due to geometrical spreading and distance from the array. If the proper correction for geometrical spreading is applied, the rated value will result. As the observation point is brought closer to the array itself, the physical distribution of the individual airguns in the array begins to have an effect, and the actual pressure measured will fall well below that predicted by applying the simple spreading law to the rated array output. In the “near field” the distances from a single observation point and the various array elements differs significantly enough that the contributions from those elements arrive at different times, out of phase. It is instructive to examine an up-close view of the RMS dB levels predicted by modeling within EWING’s largest standard source array. Here we show a vertical athwartships slice, directly through the fore-and-aft center of the 20-airgun array. Individual airguns are shown as circles, and all twenty of the array’s gun positions are projected onto the plot, though it actually intersects only six of them. Values were calculated on a 1m x 1m mesh, and the highest contour level shown is 222 dB [RMS.] Only the few largest guns and those located within the model plane show RMS levels so high. This is why physical harm by EWING’s airgun arrays to marine mammals is an extremely unlikely occurrence.

The very high pressure gradients above the array are due to the free surface or “Lloyd’s mirror” effect discussed previously. Directly below the array, its downward directivity begins to show in the form of largest separation of contour lines. Contouring the overall pressure field in terms of RMS signal power produces a now-familiar pattern:


Since the fore-and-aft extent of EWING’s array is smaller than the athwartships dimension, this directionality is less marked in front of and behind the array. The distances therefore to the 180 dB contours, or isopleths, is greater in the fore-and-aft than athwartships directions, and we use these worst case distances to determine safety radiuses:

Below, we chart the maximum horizontal extent of the isopleths, and for comparison include the levels of some more familiar sounds that have been carefully measured and presented in published literature.


Distribution of Energy in the Frequency Domain

When assessing seismic source arrays, either for their ability to penetrate the earth, or to determine their impact on marine life, peak pressures are poor measure by which to compare. It is far better to see how energy is produced as a function of frequency. Since lowest frequency sound does the best job of returning echoes from deep within the earth, marine airgun arrays, which put most of their power into low frequencies, have been chosen to form EWING’s seismic source. Here is a modified view of the EWING test signature presented at the beginning of this paper:

Here, the surface ghost has been [nearly] removed. On the left is the near field signal, and on the right, its spectrum. Maximum energy is found at around 12 cycles per second [Hz.] This frequency happens to correspond to the period of the bubble pulse, but the overall trend of highest energy at low frequencies, and a steady falloff with increasing frequency is quite clear. As we described above, considerable effort is made in “tuning” airgun arrays to reduce the effects of the bubble pulses.

Typical airgun array signature from “Airgun Arrays and Marine Mammals”, IAGC, 9/2002.


Frequencies between 3 and 1000 Hz were included in this recording of a seismic airgun array, with output levels that are fairly standard for exploration by the petroleum industry. The peak-to-peak amplitude is approximately 160 Bar-Meters, equal to 264dB. This value, however, is dependent not only on the intrinsic power of the source, but also upon the the range of frequencies included in its measurement, and conversion from the analog [observed and perceived] to the digital [suitable for calculations] domain. For this reason, meaningful results and understanding are better obtained in the spectral domain, where energy is displayed as a function of frequency.

In the spectral domain, maximum energy is around 214dB, at 60Hz. The average level falls off steadily thereafter. The regularly spaced notches and repeated arch shapes correlate to a towing depth of 6 meters. Increased falloff at frequencies above 875 Hz is due to analog filtering. The authors have marked the overall spectral trend for us. This trend is defined by two values: 210 dB at zero Hz; 152 dB at 1000 Hz. Each is much lower than the corresponding signature’s peak-to-peak 264 dB. The connection between the 264 dB peak-to-peak measurement and the much lower levels seen here can be made by integrating the spectrum – calculating the area underneath the curve. The higher the frequencies included, the greater the integration sum, and the greater the peaks, which contain virtually all of the small amount of high frequency energy in an Airgun signal. As we show next, most of the energy produced by a seismic airgun array is found in its signal’s low frequencies. The signal’s general “shape” at higher frequencies is very similar for all 2000 psi airgun arrays, with fine scale variations introduced by differences in tuning and towing depth. Since virtually all available seismic signal recordings include the low frequency components and vary only in the extent that higher frequencies are


included, the spectral domain is the best place to compare various seismic arrays. Here, for example is the spectrum of the 20-airgun array used aboard EWING’s cruise EW0210:

Peak intensity for this array in the frequency domain is approximately 221 dB, around 20 Hz, compared with the approx. 214 dB peak for the industry array example given earlier. Although the EWING spectrum is only accurate up to 250 Hz or so, the rate of falloff can be seen to be nearly identical, and we expect its relative amplitudes at high frequency to be about the same. The right hand plot above measures the EWING spectrum in a standard way – again, integrating the spectrum from low to high frequency, but now showing the cumulative result in terms of energy flux. The curve climbs steeply, but then flattens out markedly at about 80 Hz. It is clear that most of the energy created by the array travels at subaudible frequencies. The predominance of low frequencies in all seismic airgun array signals and the falloff of energy at higher frequencies can be quantified by examining additional examples.


This example (provided by the manufacturer, SSI) of the spectrum for a single “GI” airgun, intended for high resolution profiling, shows almost exactly the same rates of spectral falloff as the EWING and IAGC array signatures, supporting the claim that while amplitudes vary, spectral shapes, and particularly the rates of falloff at higher frequencies, do not.

Returning to the particularly clear and finely-sampled IAGC airgun array spectrum presented earlier, we can begin to quantify the trends in spectral falloff. There is clearly a steep initial falloff of energy across frequencies below 400 Hz. Thereafter, the falloff


rate lessens a bit. The sampling rate for the signature from which this spectrum was obtained was too small [probably 2kHz, or 500 µsec/sample] to represent frequencies above 800 Hertz. This limitation is typical of marine seismic data. Acousticians and mammal researchers typically use higher rates, often as high as 41kHz, so we next turn to a published example in that field.

In this example Goold & Fish (1998) have presented broadband spectra of a different industry airgun array, measured at distances between 750 and 8000 meters (four traces at the bottom). Due to the limitations of the hardware used to make the measurements, the spectra are not usable at low [less than 400 – 500 Hz] frequencies and the signal levels of the 8 km example fell below the usable threshold of their system above 10kHz. At the top, I have corrected the spectra for spreading loss and superimposed the results. Spectral falloff, as observed in the previous graph, continues though less steeply with increasing frequency, and can be divided into three zones; 800 Hz – 2 kHz, 2 – 6.5 kHz, and all frequencies about 6.5 kHz. Given a good, low-frequency spectrum of a particular seismic airgun array, these results allow us to predict its output over the remaining frequency range, up to 20kHz or higher. Of great importance is the fact that different mammalian species communicate at, and are sensitive to, quite different ranges of frequency. True assessment of the likelihood of behavioral disturbance by seismic sources can only be made by comparing the characteristics of source spectra with the acoustic sensitivity of particular species. A very useful spectral-domain summary of various sound levels is presented by Ketten (1998): Here acoustic levels produced by a variety of man-made systems are compared to the


audible thresholds and frequency ranges determined for the hearing of a variety of mammalian species. Although the “Airgun” is represented by a single, peak value, the measurements presented above indicate that this is a reasonable simplification, since above the 1 KHz range, airgun array energy falls below the levels produced by the propulsion of small boats carrying whale watchers and marine mammal researchers.

[from Ketten, 1998; NOAA Technical Memorandum NMFS-SWFSC-256] The seismic airgun signal is “broadband” in that its energy is spread over a broad range of frequencies, corresponding to a very short, impulsive time signal. This is especially apparent when comparing the seismic spectrum to that of the kind of sonar used for determining ocean depths and for detecting and tracking submarines. These sonars are designed to emit a signal which is nearly monochromatic – the energy is confined to a narrow range of frequencies, and usually a range which is quite different from that present in the seismic source spectrum. Despite the limitations imposed by the sampling interval and filtering in the example above, the airgun array spectrum has energy at frequencies above 1000 Hz, although the energy continues to diminish as frequency increases. The exact peak-to-peak values measured, therefore, depend a lot on the range of frequencies included. The few


measurements that have been made of seismic airgun array output (including the EWING’s) at high frequencies show that the energy continues to fall off as frequency continues into the range of typical sonars, such as those used by the Navy in the Bahamas in 2000 during an exercise that resulted in the stranding of a large number of whales:

The Navy’s report on the Bahamas incident indicates that the sonars involved were operating at 3.3 kHz, issuing two kinds of “pings” – constant frequency and “chirp”, sweeping between 3250 and 3350 Hz. Both kinds of pings were issued in sets of three, over a two-second period. Individual “pings” were as long as half a second, and for several (more than three) hours, one of the sonars was operating at levels above the nominal 235 dB [RMS] rating. Anecdotal information from ONR indicates that this level may have been 240 dB, RMS. A 3.3 kHz, 240 dB RMS half-second “ping” contains significantly more total energy than EWING’s 20 airgun seismic array, and the rise time, described in the Navy report as “one or two cycles” is quite rapid – half a millisecond or less. In contrast, the rise times for airgun signals are typically 2 – 4 milliseconds – significantly less impulsive, and thus less likely to cause injury. For comparison, I have superimposed, on a single plot and at the same scale, three curves; the IAGC (1002) published example reproduced in detail earlier, The combined, decreasing falloff curves for the IAGC spectrum and that matching the observations of Good & Fish (1998) and lastly, the spectrum of a synthetic sonar “ping” exactly matching the description given in the Navy’s report on the Bahamas strandings (2002). This “ping” is 0.5 seconds long, rises to full 235 dB RMS amplitude in two cycles, and sweeps a short band of frequencies centered on 3.3kHz. The gradual falloff plotted for this spectrum below 170 dB is largely due to limits in the numerical resolution of the spectral calculation. It is clear that at frequencies above 1.5 kHz, the industry airgun array signal, even back-corrected to a nominal 1 meter distance, has fallen to beow 160 dB, and at 3.3kHz, it is 60dB below the sonar operating at the officially stated 235 dB RMS level. For mammals who are sensitive to kHz-and-higher frequencies, airgun arrays, even large ones used by exploration industry 3D seismic vessels, and research ships such as R/V EWING, are highly unlikely to cause injury or distress.


Looking sideways All of the airgun array spectrums shown in the previous section are calculated for the far-field signal, as observed at some distance directly beneath the array, and then corrected for spreading losses. Most marine creatures, however, experience the associated wavefields at positions horizontally removed from the source array. Including this horizontal dimension introduces two essential modifications to the relatively simple signals considered up to this point. One is a spectral shaping, quite similar to that resulting from towing depth, but now carried to extremes. The second is the creation of high-energy water column multiples. This second effect is of great importance in the shallow waters of marginal seas. Although variations will be introduced by water column velocities, the exact bottom character and topography of any particular area, the primary features of multipathing and horizontal propagation are to be found in the modeling presented here, with its assumption of a homogeneous, horizontal seafloor. The changes in amplitude due to cancellation by free surface ghosting were briefly addressed at the end of the first section, and it is useful to examine here the associated changes in the frequency domain. For practical purposes, the ocean surface is a perfect reflector for upcoming sound waves in the water. Since the speed of sound is far less in air than it is in water, a negative, or reversed phase reflection is generated. If the upcoming wave has a peak strength of +1, the downgoing reflected wave has a strength of –1, and at a receiver somewhere beneath the sound source, the time delay between primary and reflected pulses will depend on distance of the source beneath the sea surface. The resulting effect is a “plus one – minus one” time domain operator which is convolved with the source’s near field signature to form the far field signature. The reason this relationship is so useful is that both the operator and the near field signature have characteristic frequency spectrums, which can be multiplied together to produce the same result as convolution in the time domain.

While changing the towing depth of a tuned seismic source will not change its near field signature greatly, the change in the ghosting operator can be very significant, though at the same time very predictable, as indicated by these simple relationships.


The subject of this paper is not seismic source tuning per se, but an example showing the same source towed at two different depths is instructive:

This shows why marine geophysicists seeking high temporal resolution (requiring a broadband source) tow sources and receivers more shallowly than researchers who are trying to image deep structure (best done with low frequencies). The most important observation, however, is that the shorter the time between primary and ghost signals, the less low frequency energy is preserved.

As the horizontal offset between a source and a receiver increases, the interval time between direct and surface-reflected arrivals decreases, with no change in tow depth. The resulting “ghosting” operator is illustrated above for a 60-meter deep receiver [or mammal] moving away from a 6-meter deep source. As offset increases, low frequency


energy is diminished dramatically. High frequency energy is not enhanced, but low frequencies are diminished. For the first time, we consider the interaction of seismic waves with the seafloor:


For reference, the multiples can be classified according to “N”, the number of surface and seafloor reflections experienced by a wave front between source and receiver. The downward angle of incidence is smallest for greatest N, and largest for N = 1. At small angles of incidence, much of a seismic wave’s energy penetrates the seafloor, and continues downward, doing its intended work of sampling the underlying geological structures. Depending on the velocity of sound at or close below the seafloor, waves with angles of incidence larger than some particular (“critical”) value will be totally reflected, instead. Since beyond this point no energy can propagate downwards, the wave is totally (or “postcritically”) reflected. These postcritical waves have much more energy than other reflected waves. Depending on water depth and seafloor velocity, postcritical reflection will occur for all waves first reflecting beyond some “critical” distance (XC in the figures above) from the source. When the seafloor is horizontal, the multiple arrivals will be received in order, from 1 to N, and it is the nature of the hyperbolic geometry involved that the time between arrivals will increase as the arrivals come in:

Although the model for these arrivals is simple, the arrivals are not. As just described, the single-bounce N = 1 arrival is the earliest, and time between each sunsequent arrival increases. Arrivals from 1 to 14 are postcritical, and much stronger than the later arrivals, which have lost most of their energy into the seafloor. Although N = 1 has the shortest path, it is most weakened by the interference between “direct” and surface-reflected arrivals described in the first section of this paper. The same interference also causes subsequent variations, seen as “beating” in the signature. This seismogram was created with a point source – array directionality, if included, would diminish “low N” arrivals even further. In the realm of marine mammal mitigation, two very important features are illustrated in this example. First, a simple, impulsive signal has given rise to a reverberating, longer signal. As a result, its RMS measure will be close to the peak value, not reduced by ~16dB as in the simple deep water models seen earlier. Second, there arises the only potential seen so far for the creation of high frequency energy, through the shortened


interval time between low-N arrivals. The next two graphs, as well as the signature on this page, show that although we need in every case to consider the possibility, in most instances, no additional danger will be created for marine mammals.

As source-receiver distance increases, so does the frequency with which low-order multiples arrive. The time interval between the successively arriving peaks may decrease to the point at which reverberatory trains in the kilohertz range are created. The offsets at which this occurs are dependent on water depth, but are in general quite high, and surface ghost interference will virtually eliminate low-order multiple energy.


Although reflection coefficients for the postcritically reflected multiples are unity, the corresponding individual arrivals still suffer the normal 1/R spreading loss. This is reflected in the plot above, which uses the same model and has the same organization as the previous graph. Kiloherz – plus energy is created only for multiples at offsets of 14 km and above, where all multiples have greatly diminished amplitude, even neglecting the strong interference and array directivity effects previously described.

Airgun Arrays

Documents

Transcript of Airgun Arrays