Airgun Array Research Articles

How can we reduce the environmental impact of marine seismic surveys?

Marine seismic sources emit acoustic energy in the form of the seismic wavefield used for the remote sensing of subsurface impedance contrasts in the earth. The environmental impact of seismic sources is typically measured in terms of impulsive acoustic pressure (the sound pressure level, SPL) and the accumulated acoustic energy (the sound exposure level, SEL). We use global examples of the following marine source concepts to quantify the relative SPL and SEL in each scenario: • Large arrays of air guns activated simultaneously with no significant overlap in emitted acoustic pressure, • Small arrays of air guns activated simultaneously or in rapid succession with overlap in emitted acoustic pressure, • Individual air guns activated continuously with overlap in emitted acoustic pressure, and • Towed marine vibrators operated continuously. Continuous sources clearly have the lowest SPL and SEL. Examples from various basin settings are shown where benefits in data quality and survey efficiency may also complement the lower environmental impact. Another surprising geophysical outcome is that continuous sources with low SPL do not have compromised signal penetration to deep target depths compared to traditional large arrays of air guns activated simultaneously. These outcomes are relevant to how future marine seismic surveys might be designed to meet stricter environmental controls as well as presenting various new opportunities for how the surveys could be acquired more efficiently and processed.

Marine Vibrator Concepts for Modern Seismic Challenges

Aside from historical issues of mechanical durability and efficiency, the design of marine vibrators (MVs) for towed streamer operations are confronted by several practical challenges to their different possible applications: 1. Alternatives to conventional air gun arrays for flexible and creative acquisition geometries, 2. Low power alternatives to air gun arrays for environmentally sensitive applications, and 3. High power ultra-low frequency sources specific to Full Waveform Inversion (FWI) optimization. One relevant consideration to MV operations is the volume of water that must be displaced per cycle to achieve a desired Sound Pressure Level (SPL); increasing exponentially as the frequency of interest decreases, and becoming significant at frequencies less than about 5 Hz. This is particularly relevant for FWI optimization as the frequencies of interest are in the range of 1-6 Hz. Another consideration is that ultra-low frequency output theoretically benefits from deeper towing enhanced by the well-known free-surface ghost effect, but in practice, deeper towing is confronted by an air spring effect that increases the force required per cycle to generate a desired SPL, and is due to the surrounding hydrostatic pressure at depth. Environmental motivations to develop low power vibrator concepts are driven by regulatory restrictions upon received SPL and Sound Exposure Level (SEL), and we demonstrate how low power MVs can be configured to yield low SPL and SEL metrics without compromising geophysical performance—in contrast to the use of air gun arrays with few elements. MVs enable independence from large compressors and may be more easily deployed in spatially distributed geometries than air gun arrays. We present examples of flextensional MV development verified by numerical modelling, tow tank testing, and field verification; collectively supporting the principles discussed herein.

Shot repetition: An alternative seismic blending code in marine acquisition

In blended seismic acquisition, or simultaneous source seismic acquisition, source encoding is essential at the acquisition stage to allow for separation of the blended sources at the processing stage. In land seismic surveys, the vibroseis sources may be encoded with near-orthogonal sweeps for blending. In marine seismic surveys, the sweep type of source encoding is difficult because the main source type in marine seismic exploration is the air-gun array, which has an impulsive character. Another issue in marine streamer seismic data acquisition is that the spatial source sampling is generally coarse. This hinders the deblending performance of algorithms based on the random time delay blending code that inherently requires a dense source sampling because they exploit the signal coherency in the common-receiver domain. We have developed an alternative source code called shot repetition that exploits the impulsive character of the marine seismic source in blending. This source code consists of repeated spikes of ones and can be realized physically by activating a broadband impulsive source more than once at (nearly) the same location. Optimization of the shot-repetition type of blending code was done to improve the deblending performance. As a result of using shot repetition, the deblending process can be carried out in individual shot gathers. Therefore, our method has no need for a regular dense source sampling: It can cope with irregular sparse source sampling; it can help with real-time data quality control. In addition, the use of shot repetition is beneficial for reducing the background noise in the deblended data. We determine the feasibility of our method on numerical examples.

Monitoring and mitigation of sound exposure from seismic surveys for a feeding whale population

Seismic surveys are an essential tool in the search for and periodic assessment of hydrocarbon deposits for oil and gas production. Airgun arrays, which are to date the overwhelmingly predominant sound source for these surveys, generate a high level of low-frequency pulses which can cause disturbance to marine life, particularly species with good low-frequency hearing such as baleen whales (Southall et al., 2007). Despite numerous studies (see Nowacek et al., 2007 for a review), substantial data gaps remain in terms of impacts of seismic surveys on cetacean physiology, behaviour and population dynamics. There is broad acceptance, however, of the need to mitigate the exposure of animals to noise from seismic surveys, especially for endangered populations or species (Nowacek et al., 2013). The northeastern shelf of Sakhalin Island, Russia, is an important feeding ground of a critically endangered population of grey whales (Eschrichtius robustus) which convene annually in the region near the mouth of Piltun lagoon. Sakhalin Energy Investment Company Ltd (Sakhalin Energy) operates two platforms in the Piltun-Astokhskoye licence area. In 2010 and 2015, Sakhalin Energy conducted repeat (4D) seismic surveys to characterize changes to the subsurface from hydrocarbon production at the two platforms, required for efficient positioning of new wells; the original 3D data had been acquired in 1997. In 2015 the surveyed region was approximately twice as large as in 2010, and another operator (Exxon Neftegas Limited or ENL) also conducted seismic surveys to the immediate north and south of the Piltun-Astokhskoye area on a co-ordinated schedule.

Characterizing the acoustic properties of the cavity cloud generated close to an air-gun array as a time-dependent effective medium

Characterizing the acoustic properties of the cavity cloud generated close to an air-gun array as a time-dependent effective medium

Do nearby seismic airguns reduce singing effort in humpback whales?

The high-level percussive sounds generated for seismic sea floor exploration have the potential to disrupt normal behaviors of whales. This study assesses if there is any reduction in the singing behaviour of migrating humpback whales in response to nearby airguns. Singing whales were acoustically tracked as they migrated along the coastline of south-eastern Queensland. A 20 cubic inch airgun or 140 cubic inch array of airguns was towed through the study area for 1 hour with the airguns firing every 11 sec (active treatments) or without the airguns operating (controls). Singing activity across each day was measured by counting the number of singing whales within the 10km-radius study area every 10 min, from 0700 to 1700, including during experiments. Singing activity during active periods and controls were compared with each other as well as with pseudo-randomly selected 1 hour periods when experiments were not underway (baseline). Changes in singing effort were also recorded by noting the number of instances of a singer stopping or starting during these periods. Preliminary analyses show no significant differences in singing activity, nor changes in singing effort, between active, control, or baseline periods.The high-level percussive sounds generated for seismic sea floor exploration have the potential to disrupt normal behaviors of whales. This study assesses if there is any reduction in the singing behaviour of migrating humpback whales in response to nearby airguns. Singing whales were acoustically tracked as they migrated along the coastline of south-eastern Queensland. A 20 cubic inch airgun or 140 cubic inch array of airguns was towed through the study area for 1 hour with the airguns firing every 11 sec (active treatments) or without the airguns operating (controls). Singing activity across each day was measured by counting the number of singing whales within the 10km-radius study area every 10 min, from 0700 to 1700, including during experiments. Singing activity during active periods and controls were compared with each other as well as with pseudo-randomly selected 1 hour periods when experiments were not underway (baseline). Changes in singing effort were also recorded by noting the number of insta...

Validation of airgun array modelled source signatures

Characterizing the source levels from seismic airgun arrays to increase confidence in model accuracy has been identified as a requirement by the scientific and regulatory community. To comply with regulatory requirements in Australia, a measurement program was conducted to validate the source signature predictions of JASCO’s Airgun Array Source Model. The validation program measured a range of commercial airgun arrays, including a 2380in3 array, and was conducted in 80m of water with array passing directly over the recorder on the seafloor. For the 2380in3 array, the maximum measured PK was 220.62 dBre1μPa. Due to the proximity of the measurements, interesting features such as ghost cavitation and airgun recharge were observed in the data. The measurement study results were used to validate the modelled far-field source levels through a comparison between the measured received sound-levels and predicted received sound-levels at a real receiver point in the far-field of the source. The predictions were made using a wavenumber integral model coupled to the airgun source model. The program measured received sound levels in the endfire, broadside and vertical directions, and as the results showed good agreement with the modelling results, provided a validation of the complete modelled source signatures for the array.

Assessment of safety zones for marine mammals at noise exposure from industrial activity in the Russian Arctic

E&P of hydrocarbon along with shipping are the main sources of manmade noise in the ocean. But very few information on the noise from industry activities in Arctic is currently available. Over the next decade we expect the increasing level of anthropogenic sound in the Russian Arctic due to planned large scale construction of Oil&Gas projects including the Shtokman project in the Barents Sea, one of the world's largest natural gas deposits. Noise from seismic surveying, pile driving and construction can damage hearing or disquiet marine mammals. To protect marine mammals from pulsed noise of seismic survey, pile driving and chronic continuous noise at different stages of the Shtokman project construction we estimated the sizes of safety zones with SPL thresholds of 180 dB (zone of injury) and 120 dB for continuous noise (behavior disturbance). Modeling of the transmission loss was done by Normal Mode and PDPE models based on the environmental characteristics in the Barents Sea and real spectra of noise sources—construction vessels and seismic airgun array. The results demonstrate changes in the Safety Zones footprints depending on the stage of construction, season, specific environmental conditions like ice cover interface or presence of gas saturated sedimentary layer.

Simulated cavitation noise from strong nonlinear coupling in a multi-bubble system

The mutual nonlinear coupling between cavitation bubbles in a bubble cloud has often been blamed for the complex nature of the cavitation noise. This noise is undesirable for two reasons. First, it affects the source air-gun signature which is crucial in later stages of seismic signal processing. Second, the generated noise may fall in the same frequency regime which is used by some types of whales for communication, and therefore interfere. The complete nonlinear theory of bubble interaction is used to solve the governing Keller-Miksis equation (KME) to simulate the experimentally observed cavitation noise. A qualitative comparison is made with the results from other scientific investigations and interesting inferences are drawn. It is believed that the findings reported here may have implications in the air-gun array design in seismic exploration industry since air-guns are the indirect source of cavitation noise. It may also have an impact on the study of communication systems in marine mammals.

Acoustically induced cavity cloud generated by air-gun arrays-Comparing video recordings and acoustic data to modeling.

For seismic air-gun arrays, ghost cavitation is assumed to be one of the main mechanisms for high-frequency signal generation. Ghost cavitation signals are weak for seismic frequencies (<300 Hz) and do not contribute to seismic reflection profiling. In the current experiment, the ghost cavity cloud is monitored by a high-speed video camera using 120 frames per second. This is, as far as the authors know, the first convincing photographic evidence of ghost-induced cavitation. In addition to video recording, acoustic signals were recorded with a sampling rate of 312.5 kHz using broadband hydrophones suspended 17 m below the array. The pressure drop around the source array is estimated using air-gun modeling followed by a phenomenological modeling of the growth and collapse of each vapor cavity. The cumulative effect of cavity collapses is modeled based on linear superposition of the acoustic signals generated by individual cavities. The simulated acoustic ghost cavitation signal and the corresponding cavity cloud show good agreement with the field data.

Seismic Signature of an Untuned Large‐Volume Airgun Array Fired in a Water Reservoir

Seismic Signature of an Untuned Large‐Volume Airgun Array Fired in a Water Reservoir

Propagation of Underwater Noise from an Offshore Seismic Survey in Australia to Antarctica: Measurements and Modelling

An offshore seismic survey was conducted over the western edge of the continental shelf in Bass Strait in 2006. Underwater noise from this survey was recorded on an autonomous sound recorder deployed in the Southern Ocean on the Antarctic continental slope. Sound emission and propagation models were verified by experimental measurements using parameters and position of the airgun array and characteristics of the underwater sound channel. A parabolic equation approximation method was used to calculate the sound field over the continental slope of Australia, and then, a normal mode model was employed to account for the transmission loss due to sound scattering by surface waves south of the polar front. The numerical predictions are consistent with the measurement results within a few dBs for the sound exposure and energy spectral density levels. It is also demonstrated by measurements and modelling that the best coupling of a near-surface sound source with the deep underwater sound channel takes place when the source is located over the continental slope at a sea depth of about half of the channel’s axis depth. The model can be used to predict masking effects of man-made underwater noise on the communication environment of marine mammals in Antarctica.

Experimental and numerical study of the effects of a wall on the coalescence and collapse of bubble pairs

Two-bubble interaction is the most fundamental problem in multi-bubbles dynamics, which is crucial in many practical applications involving air-gun arrays and underwater explosions. In this paper, we experimentally and numerically investigate coalescence, collapse, and rebound of non-buoyant bubble pairs below a rigid wall. Two oscillating vapor bubbles with similar size are generated simultaneously near a rigid wall in axisymmetric configuration using the underwater electric discharge method, and the physical process is captured by a high-speed camera. Numerical simulations are conducted based on potential flow theory coupled with the boundary integral method. Our numerical results show excellent agreement with the experimental data until the splashing of the jet impact sets in. With different ranges of γbw (the dimensionless distance between the rigid wall and the nearest bubble center), the interaction between the coalesced bubble and the rigid wall is divided into three types, i.e., “weak,” “intermediate,” and “strong.” As γbw decreases, the contact point of the two axial jets migrates toward the wall. In “strong interaction” cases, only an upward jet towards the upper rigid wall forms and a secondary jet with a larger width appears at the base of the first jet. The collapsing coalesced bubble in a toroidal form splits into many smaller bubbles due to the instabilities and presents as bubble clouds during the rebounding phase, which may lead to a weakened pressure wave because the focusing energy associated with the collapsing bubble is disintegrated.

The behavioural response of migrating humpback whales to a full seismic airgun array.

Despite concerns on the effects of noise from seismic survey airguns on marine organisms, there remains uncertainty as to the biological significance of any response. This study quantifies and interprets the response of migrating humpback whales (Megaptera novaeangliae) to a 3130 in3 (51.3l) commercial airgun array. We compare the behavioural responses to active trials (array operational; n = 34 whale groups), with responses to control trials (source vessel towing the array while silent; n = 33) and baseline studies of normal behaviour in the absence of the vessel (n = 85). No abnormal behaviours were recorded during the trials. However, in response to the active seismic array and the controls, the whales displayed changes in behaviour. Changes in respiration rate were of a similar magnitude to changes in baseline groups being joined by other animals suggesting any change group energetics was within their behavioural repertoire. However, the reduced progression southwards in response to the active treatments, for some cohorts, was below typical migratory speeds. This response was more likely to occur within 4 km from the array at received levels over 135 dB re 1 µPa2s.

Low-frequency acoustic signal created by rising air-gun bubble

In the seismic industry, there is increasing interest in generating and recording low frequencies, which leads to better data quality and can be important for full-waveform inversion. The air gun is a seismic source with a signal that consists of the (1) main impulse, (2) oscillating bubble, and (3) rising of this air bubble. However, there has been little investigation of the third characteristic. We have studied a low-frequency signal that could be created by the rising air bubble and find the contribution to the low-frequency content in seismic acquisition. We use a simple theory and modeling of rising spheres in water and compute the acoustic signal created by this effect. We conduct tank and field experiments with a submerged buoy that is released from different depths and record the acoustic signal with hydrophones along the rising path. The experiments simulate the signal from the rising bubble separated from the other two effects (1 and 2). Furthermore, we use data recorded below a single air gun fired at different depths to investigate if we can observe the proposed signal. We find that the rising bubble creates a low-frequency signal. Compared with the main impulse and the oscillating bubble effect of an air-gun signal, the contribution of the rising bubble is weak, on the order of 1/900 depending on the bubble size. By using large air-gun arrays tuned to create one big bubble, the contribution of the signal can be increased. The enhanced signal can be important for deep targets or basin exploration because the low-frequency signal is less attenuated.

High frequency ghost cavitation - a comparison of two seismic air-gun arrays using numerical modelling

Ghost cavitation is probably the mechanism behind the majority of high frequencies (above 5 kHz) generated by seismic air-gun arrays. Such high frequencies are less important in seismic reflection imaging. High frequency sound might impact marine fauna and particularly marine mammals. In this paper the array signatures and high frequency ghost cavitation signals for two different arrays are simulated using numerical modelling. It is observed that one array has slightly more (20%) energy within the seismic frequency band (1-100 Hz) but emits significantly more energy (150%) for frequencies above 5 kHz.

Determining the behavioural dose-response relationship of marine mammals to air gun noise and source proximity.

The effect of various anthropogenic sources of noise (e.g. sonar, seismic surveys) on the behaviour of marine mammals is sometimes quantified as a dose-response relationship, where the probability of an animal behaviourally 'responding' (e.g. avoiding the source) increases with 'dose' (or received level of noise). To do this, however, requires a definition of a 'significant' response (avoidance), which can be difficult to quantify. There is also the potential that the animal 'avoids' not only the source of noise but also the vessel operating the source, complicating the relationship. The proximity of the source is an important variable to consider in the response, yet difficult to account for given that received level and proximity are highly correlated. This study used the behavioural response of humpback whales to noise from two different air gun arrays (20 and 140 cubic inch air gun array) to determine whether a dose-response relationship existed. To do this, a measure of avoidance of the source was developed, and the magnitude (rather than probability) of this response was tested against dose. The proximity to the source, and the vessel itself, was included within the one-analysis model. Humpback whales were more likely to avoid the air gun arrays (but not the controls) within 3 km of the source at levels over 140 re. 1 µPa2s-1, meaning that both the proximity and the received level were important factors and the relationship between dose (received level) and response is not a simple one.

Numerical study on attenuation of bubble pulse through tuning the air-gun array with the particle swarm optimization method

A numerical method for improving the quality of the far-field signal generated by a marine seismic air-gun array is presented, in which particle swarm optimization is used in conjunction with the theory of the oscillating spherical bubble to tune the array. Two sets of objective functions, including the primary-to-bubble ratio and the variance of the normalized amplitude spectrum, are built to screen the array. With this method, attempts are made to improve the far-field signals generated by four typical arrays, including the planar array, spatial array, irregular array and delayed array. It is concluded that the far-field signal can be largely improved just through the simple adjustment of the chamber volume, the firing depth, the horizontal location or the firing time of each individual gun in the array. We believe that this method will be useful in the air-gun array design and helpful to find a desirable array from a population of air-gun array distributions for a brief time.

On firing an air gun very shallow

Recently, two of the authors asked the question “Is it optimal to tow air guns shallow to enhance low frequencies?” By combining recordings of signatures from sources deployed at depths from 3 to 20 m, they concluded that for an air gun array with sources at several depths, more low frequencies would be introduced into the source signature. We have found the significant and notable additional experimental result that firing a [Formula: see text] air gun at approximately 1.3 m in depth gives a broadband signature with low and high frequencies and with notable downgoing amplitude strength. The experiment was not dedicated to the 1.3 m gun depth test, and only one shot was fired at this depth. Nevertheless, we found that the air gun at approximately 1 m depth produces significantly low frequencies less than 2 Hz compared with the guns fired deeper. We suggest that very shallow firing of large volume guns is of interest for VSPs as well as for site surveys due to the high frequencies that are generated and the almost bubble free signature.

A modelling comparison between received sound levels produced by a marine Vibroseis array and those from an airgun array for some typical seismic survey scenarios

Marine Vibroseis (MV) may provide a marine seismic sound source that has less environmental impact than conventional airguns. Modelled sound levels from a realistic MV array and airgun array with similar downward energy at frequencies <100Hz were compared under three scenarios: shallow, deep, and slope. Changing the layout of the MV array's higher frequency sources reduced sound exposure levels (SELs) by 4dB. At 100m range this MV was 20dB lower in peak-to-peak sound pressure level vs. the airgun array, decreasing to 12dB lower at 5km, the maximum modelled range for peak levels. SELs were less clear-cut, but for both shallow and deep water, MV produced 8dB lower SELs than the airguns at 100km range because of MV's reduced bandwidth. Overall, MV produced lower broadband SELs, especially at long range, and lower peak pressure, especially at short range, than airguns.