Gun Array Research Articles

A new method of alternating shooting of two different seismic sources for deep geophysical surveys

In the latest geophysical survey crossing the Ninety East Ridge of the Indian Ocean, a new method was employed to perform proportional double seismic source excitation and synchronously receive signals from the sea surface and the seabed. The two seismic sources used for excitation were two sets of gun arrays with different energies and dominant frequencies, a G gun array and a Bolt gun array. The G gun array consisted of 3 G.II guns with a total capacity of 450 in3 and a dominant frequency of 20–100 Hz. The Bolt gun array consisted of 4 Bolt 1500LL air guns with a total capacity of 6000 in3 and a dominant frequency of 10–40 Hz. The seismic receiving system comprised a 480-channel seismic streamer towed from the sea surface and 21 ocean bottom seismometers (OBS). During offshore operations, the integrated navigation system produced equidistant trigger signals at an interval of 50 m. The trigger signals were distributed to the G gun array and Bolt gun array at a ratio of 3:1 after passing through a pulse signal proportional distributor. The two sets of gun arrays fired alternatingly at a given ratio. The receiving equipment on the sea surface and seabed simultaneously received the seismic signals excited by the two sets of gun arrays. After targeted data processing, in addition to the seismic profile generated by the conventional G gun seismic source, the deep seismic profile generated by the Bolt gun seismic source and the survey profile of the active-source OBS were obtained simultaneously. The penetration depths of the three sets of profiles reach 2 km, 6 km, and 30 km, respectively, greatly improving the efficiency of offshore deep-sea seismic surveys.

Acoustic field characteristics of seismic surveys in the shallow water environment of the Bohai Sea

AbstractAir gun arrays are designed to generate strong energy pulses of less than 100 Hz for offshore oil and gas exploration. In shallow seas, due to the directivity of air gun arrays, the complex propagation conditions and the limitations of acquisition methods, it is difficult to accurately evaluate the spatial time and frequency transmission characteristics of airguns. To alleviate the existing lack of knowledge about the transmission characteristics of air gun pulses in the shallow waters of the Bohai Sea, we analysed three‐dimensional ocean bottom cable seismic data collected during a seismic survey in the Bohai Sea, China, in June 2017, which were used to evaluate the reception level, frequency domain characteristics and spatial distribution characteristics within a 5.55 × 5.575 km area. The results showed that in shallow water, multipath propagation causes downfrequency chirplike sounds and an increased sound field. Within 0.3 km, the transmission model follows a cylindrical diffusion, and the sound loss rate decreases as the transmission distance increases until reaching 8 km. Signal capture by the sound channel results in a high transmission loss frequency band, whereas multimode interference weakens the transmission loss. The gun array directivity and the transmission channel are the two main factors affecting the acoustic field. This research provides an improved understanding of the acoustic field of airguns in the Bohai Sea, which can benefit the acoustic modelling of air gun arrays, the design of future seismic exploration projects and the conservation of marine life.

Seismic surveys near gray whale feeding areas off Sakhalin Island, Russia: assessing impact and mitigation effectiveness

In 2015, two oil and gas companies conducted seismic surveys along the northeast coast of Sakhalin Island, Russia, near western gray whale (Eschrichtius robustus) feeding areas. This population of whales was listed as Critically Endangered at the time of the operations described here but has been reclassified as Endangered since 2018. The number and duration of the 2015 seismic surveys surpassed the level of previous seismic survey activity in this area, elevating concerns regarding disturbance of feeding gray whales and the potential for auditory injury. Exxon Neftegas Limited (ENL) developed a mitigation approach to address these concerns and, more importantly, implemented a comprehensive data collection strategy to assess the effectiveness of this approach. The mitigation approach prioritized completion of the seismic surveys closest to the nearshore feeding area as early in the season as possible, when fewer gray whales would be present. This was accomplished by increasing operational efficiency through the use of multiple seismic vessels and by establishing zones with specific seasonal criteria determining when air gun shutdowns would be implemented. These zones and seasonal criteria were based on pre-season modeled acoustic footprints of the air gun array and on gray whale distribution data collected over the previous 10 years. Real-time acoustic and whale sighting data were instrumental in the implementation of air gun shutdowns. The mitigation effectiveness of these shutdowns was assessed through analyzing short-term behavioral responses and shifts in gray whale distribution due to sound exposure. The overall mitigation strategy of an early survey completion was assessed through bioenergetics models that predict how reduced foraging activity might affect gray whale reproduction and maternal survival. This assessment relied on a total of 17 shore-based and 5 vessel-based teams collecting behavior, distribution, photo-identification, prey, and acoustic data. This paper describes the mitigation approach, the implementation of mitigation measures using real-time acoustic and gray whale location data, and the strategy to assess impacts and mitigation effectiveness.

Real-time acoustic monitoring with telemetry to mitigate potential effects of seismic survey sounds on marine mammals: a case study offshore Sakhalin Island

Exxon Neftegas Ltd. (ENL) carried out three 4D seismic surveys during the summer of 2015. Seismic operations in two of these fields (Odoptu and Chayvo) ensonified the nearshore feeding area of Korean-Okhotsk (western) gray whales (Eschrichtius robustus), potentially disturbing feeding activities. Following model-based optimization of the source design to minimize its lateral acoustic footprint, pre-season modeling was used to compute the acoustic exposure along each survey line. Real-time acoustic data facilitated implementation of mitigation measures aimed to minimize disturbance of whales. Acoustic data originated from underwater recorders deployed on the seafloor. Two complementary approaches were used to transmit recorded sound data to a computer housed at the Central Post (CP), where decisions regarding mitigation shut downs were made. In the first approach, a limited bandwidth (2–2000 Hz) sampling of the data was transmitted via cable to a surface buoy, which relayed these data to a shore station up to 15 km away via digital VHF telemetry. At the shore station, acoustic impulses from the seismic surveys were processed to compute impulse characteristics in the form of estimates of sound exposure level and peak sound pressure level, as well as one-minute-average 1/3-octave power spectral density coefficients, which were then transmitted to the CP via the internet. In the second, the pulse characteristics were computed through algorithms running on an onboard processor in each recorder’s surface buoy and sent directly to the CP computer via an Iridium satellite uplink. Both methods of data transfer proved viable, but Iridium transmission achieved the goal without the need for any shore based relay stations and is therefore more operationally efficient than VHF transmission. At the CP, analysts used the real-time acoustic data to calibrate and adjust the output of pre-season acoustical model runs. The acoustic footprint for the active seismic source, advancing synchronously with the motion of the seismic vessel and changing as the sound propagation environment changed, was computed from the calibrated and adjusted model output and integrated through the software Pythagoras with locations of gray whales provided by shore-based observers. This enabled analysts to require air gun array shutdowns before whales were exposed to mean square sound pressure levels greater than the behavioral response threshold of 163 dB re 1 μPa2. The method described here provides a realistic means of mitigating the possible effects of air guns at a behavioral response level, whereas most seismic surveys rely on pre-established mitigation radii to manage the risk of injury to a whale.

Facial characteristics of air gun array wavelets in the time and frequency domain under real conditions

How to determine the wavelet of air gun source and how to improve the performance of air gun source have gradually become two important topics in the field of air gun. The existing performance evaluation system of air gun array can only evaluate the performance of air gun from a single point of view, and can not characterize the change of three-dimensional acoustic field produced by air gun array. We first use the Van der Waals air gun wavelet model to simulate the three-dimensional air gun spatial wave field under real conditions. The experimental and modeled data are in good agreement at different pressure amplitudes and frequencies. Then, based on the three-dimensional spatial acoustic field simulated by the model, we proposed a facial characteristics to evaluate the time domain and frequency domain characteristics of the three-dimensional acoustic field. In this air gun wavelet evaluation system, we extend the evaluation parameters such as zero peak, peak-to-peak amplitude, bubble period and main bubble ratio from single wavelet to acoustic field, from one dimension to three dimension. By introducing the instantaneous property of wavelet, we can observe the amplitude, phase and frequency of the far-field wavelet from different angles. The experimental results show that the proposed air gun wavelet evaluation system can evaluate the spatial wave field from any Angle with multiple parameters, and effectively guide the optimization of the structure of the air gun array.

Temporal Variability of Thermohaline Fine-Structure Associated With the Subtropical Front Off the Southeast Coast of New Zealand in High-Frequency Short-Streamer Multi-Channel Seismic Data

Seismic oceanography generally makes use of multi-channel seismic reflection data sourced by air gun arrays and long hydrophone streamers to image oceanographic water masses and processes—often piggybacking on surveys that target deeper geological features below the seafloor. However, due to the acquisition methods employed, shallow (upper 200 m or so) regions of the ocean can be poorly imaged with this technique, and resolution is often lower than desirable for imaging fine-structure within the water column. In 2012, we collected a set of higher-resolution seismic lines off the southeast coast of New Zealand, with a generator-injector airgun source and hydrophone streamer configuration designed to improve images of shallower water masses and their boundaries. The seismic lines were acquired with coincident expendable bathythermograph deployments which provides direct ties between physical oceanographic data and seismic data, allowing for definitive identification of the Subtropical Front and associated water masses in the subsurface. Repeat acquisition along the same transect shows significant temporal variability on the scale of hours, illustrating the highly dynamic nature of this important ocean boundary. Comparisons to conventional low-frequency seismic data in the same location show the value of high-resolution acquisition methods in imaging the near-surface of the ocean and allowing subsurface features to be linked to their expressions at the surface.

Abnormal interference of very low frequency sound field in shallow water

In a multi-channel seismic exploration experiment in the South Yellow Sea, a tuned air gun array with a shooting interval of 37.5m was used as broadband pulse source, and a horizontal line array with 336 elements is used to receive transmitted signals. The received single shot records in the range-time domain are transformed into an interference pattern in the range-frequency domain using Fourier transform. It is found that there is an abnormal interference pattern of the very low frequency (VLF) sound field in the stratified shallow water environment. It is a complex nonlinear structure, no longer regular striations in the range-frequency domain. The mechanism of this particular interference pattern is interpreted according to normal mode theory. In shallow water stratified leaky waveguide, VLF sound propagates as waterborne modes and bottom-trapped modes. These two kinds of normal mode interfere with each other and form the particular stable interference pattern. In the experimental environment, the group speeds for these two type normal modes change differently with frequency. And it causes slope of interference striations change from positive to negative with increase of frequency. The theoretical simulation results are in good agreement with the experimental data. It is shown that the structure of interference pattern is sensitive to bottom structure and geoacoustic parameters.

Facial Features of an Air Gun Array Wavelet in the Time-Frequency Domain Based on Marine Vertical Cables

Facial Features of an Air Gun Array Wavelet in the Time-Frequency Domain Based on Marine Vertical Cables

Quantitative Evaluation of Air Gun Leakage in Offshore Seismic Exploration

Air gun seismic source is widely used in marine seismic data acquisition, and air leakage is a common phenomenon. When there is an abnormal situation such as air gun leakage, and the quality control cannot be carried out according to the gun closing standard, the influence of the air leakage of a single air gun or multiple air guns on the entire gun array is considered from the aspects of near-field wavelet correlation and energy consistency. Finally, this paper has put forward the quantitative analysis and evaluation method for the whole gun array, which can provide quantitative evaluation reference for the excitation quality of air gun array.

A Modeling Comparison of the Potential Effects on Marine Mammals from Sounds Produced by Marine Vibroseis and Air Gun Seismic Sources

Concerns about the potential environmental impacts of geophysical surveys using air gun sources, coupled with advances in geophysical surveying technology and data processing, are driving research and development of commercially viable alternative technologies such as marine vibroseis (MV). MV systems produce controllable acoustic signals through volume displacement of water using a vibrating plate or shell. MV sources generally produce lower acoustic pressure and reduced bandwidth (spectral content) compared to air gun sources, but to be effective sources for geophysical surveys they typically produce longer duration signals with short inter-signal periods. Few studies have evaluated the potential effects of MV system use on marine fauna. In this desktop study, potential acoustic exposure of marine mammals was estimated for MV and air gun arrays by modeling the source signal, sound propagation, and animal movement in representative survey scenarios. In the scenarios, few marine mammals could be expected to be exposed to potentially injurious sound levels for either source type, but fewer were predicted for MV arrays than air gun arrays. The estimated number of marine mammals exposed to sound levels associated with behavioral disturbance depended on the selection of evaluation criteria. More behavioral disturbance was predicted for MV arrays compared to air gun arrays using a single threshold sound pressure level (SPL), while the opposite result was found when using frequency-weighted sound fields and a multiple-step, probabilistic, threshold function.

A profile study of OBS deep geological detect in theEast China Sea

The use of ocean bottom seismometer (OBS) is of great significance to explore the tectonic evolution of the East China Sea. To study the characteristics of the deep geological structure from the East China Sea shelf basin to the Ryukyu trench uplifting fold belt, the NW-SE active source deep seismic experiment (OBS-2015) was carried out in the East China Sea. By using the delayed fire source of multilayer gun array, wide-angle seismic reflection and refraction signals up to the Moho can be obtained in the continental shelf and Okinawa Trough, and various seismic phases such as Ps, Pg and PmP can be recognized. The data acquisition results of OBS deep geoscience seismic exploration in the East China Sea show that the effective seismic phase up to Moho surface can also be obtained by using the three-dimensional array source composed of medium and small capacity air guns, which provides a new way for the selection of excitation source for OBS deep exploration. After data preprocessing, identifying and picking up seismic phases, a 2D velocity structure profile was obtained by ray tracing and travel time forward and inversion modeling. Geological interpretation and comprehensive study show that the East China Sea shelf basin is a Mesozoic-Cenozoic sedimentary basin with the maximum superimposed Mesozoic-Cenozoic thickness of 13 km. The crust of the East China Sea shelf area is the extension of the continental crust to the sea. The Moho surface has small fluctuation and the buried depth is about 30 km. From the Diaoyu Island fold to the Ryukyu Island Arc, the Moho Surface fluctuates sharply, rising rapidly from west to East. The crustal thickness of the Okinawa Trough area is only about half the thickness of in the East China Sea shelf basin. It is a high velocity geological body with the thinnest central axis of approximately 13 km and rapid thickening to both sides (approximately 19 km). The high velocity (up to 7.1 km/s) abnormal body develops above the Moho surface in the Okinawa Trough area. The Moho surface in the eastern depression has uplifted significantly. This shows an obvious mirror image relationship with the extensional basin. The vertical velocity structure of the Okinawa Trough is quite different from that of the continental crust and is close to that of the oceanic crust. It is speculated that the thick high-speed body developed on the Moho surface is the manifestation of a large amount of upwelling of mantle derived materials during the back arc stretching process. Combined with the phenomena of low-speed abnormal magma chamber in the shallow part and the penetration of magmatic rocks into the sedimentary basement layer, it is believed that the crust in the southern Okinawa Trough along the survey line has ruptured and entered the initial stage of sea-floor spreading, and the initial oceanic crust has appeared in the southern Okinawa Trough and the crust has atypical oceanic crust characteristics. This experiment provides important basic data for studying the deep crustal structure and regional geological evolution of the East China Sea.

Assessing auditory masking for management of underwater anthropogenic noise.

Masking is often assessed by quantifying changes, due to increasing noise, to an animal's communication or listening range. While the methods used to measure communication or listening ranges are functionally similar if used for vocalizations, they differ in their approaches: communication range is focused on the sender's call, while the listening range is centered on the listener's ability to perceive any signal. How these two methods differ in their use and output is important for management recommendations. Therefore it was investigated how these two methods may alter the conclusions of masking assessments based on Atlantic cod calls in the presence of a commercial air gun array. The two methods diverged with increasing distance from the masking noise source with maximum effects lasting longer between air gun pulses in terms of communication range than listening range. Reductions in the cod's communication ranges were sensitive to fluctuations in the call's source level. That instability was not observed for the listening range. Overall, changes to the cod's communication range were more conservative but very sensitive to the call source level. A high level of confidence in the call is therefore required, while confidence in the receiver's audiogram and soundscape is required for the listening range method.

Ships and air guns reduce social interactions in humpback whales at greater ranges than other behavioral impacts

Understanding the interactions between human activity in the ocean and marine mammals is a fundamental step to developing responsible mitigation measures and informing policy. Here, the response of migrating humpback whales to vessels towing seismic air gun arrays (on or off) was quantified as a reduction in their likelihood of socially interacting (joining together). Groups were significantly less likely to participate in a joining interaction in the presence of a vessel, regardless of whether or not the air guns were active. This reduction was especially pronounced in groups within a social environment that favored joining, that is, when singing whales or other groups were nearby. Seismic survey mitigation practices are designed primarily to prevent damage to whales' hearing from close-by sources. Here, we found potentially detrimental behavioral changes at much greater ranges, and much lower received levels, than those used for current mitigation recommendations.

Effects of the Rough Sea Surface on the Signature of a Single Air Gun

The calculation of air gun source signatures gives insight into applications such as air gun array design, deghosting and the impact of sound on marine life. Single air gun source signatures were calculated from the numerical solution of a set of differential equations based on different branches of physics. Some characteristic parameters of air guns were obtained from the Svein Vaage broadband air gun data set (SVBAD) measurements to calibrate the model. The comparison between measured and modeled air gun signals helps to investigate the accuracy of air gun source models. The modeled air gun signatures compared well with measurements from the SVBAD for the case of a calm sea surface of sea state 2 or less. The source ghost signal modeled for a rough sea surface showed amplitude and phase changes, affecting the ghost notches, which may explain discrepancies between the SVBAD measurements and modeled air gun signals at frequencies above 150 Hz.

Guest Editorial Special Issue on Verification and Validation of Air Gun Source Signature and Sound Propagation Models

The papers in this special issue focuses on the verification and validation of models for predicting the sound field radiated by air guns and air gun arrays. This issue aims to address the demand for increasingly precise assessments of environmental impacts, requiring accurate characterization of the air gun array sound field, including the interface between source signature models and sound propagation models. To address this need, a modeling workshop was held in July 2016. Participants in the 2016 workshop were asked to predict, for specified scenarios, the time series of sound pressure and sound particle acceleration at different positions relative to the source, as well as the sourcewaveform. Air gun modeling experts worldwide, including the 2016 workshop participants, were invited to contribute to this special issue.

Low Frequency Broad Band Acoustic Propagation in Andaman Sea

During November 2017, an active source seismic survey was performed in Andaman sea to study the acoustic propagation characteristics. High power low frequency acoustic signals generated by 20-air gun array onboard ORV Samudra Ratnagar were recorded from INS Sagardhwani at four different depths within 8 km ranges in shallow and deep waters. Low frequency sound levels were estimated using root mean square and power spectral values. Amplitude levels were analysed with respect to arrival time variation with frequency and is presented.

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.

Water guns affect abundance and behavior of bigheaded carp and native fish differently

Water guns have shown the potential to repel nuisance aquatic organisms. This study examines the effects of exposure to a 1966.4 cm3 seismic water gun array (two guns) on the abundance and behavior of Bighead Carp Hypophthalmichthys nobilis, Silver Carp H. molitrix (collectively referred to as bigheaded carp) and native fishes (e.g., Smallmouth Buffalo Ictiobus bubalus). Water guns were deployed in a channel that connects the Illinois River to backwater quarry pits that contained a large transient population of bigheaded carp. To evaluate the effect of water guns, mobile side-looking split-beam hydroacoustic surveys were conducted before, during and between replicated water gun firing periods. Water guns did not affect abundance of bigheaded carp, but abundance of native fish detected during the firing treatment was 43 and 34% lower than the control and water guns off treatments, respectively. The proximity of bigheaded carp to the water gun array was similar between the water guns on and water guns off treatments. In contrast, the closest detected native fish were detected farther from the water guns during the water guns on treatment (mean ± SE, 32.38 ± 3.32 m) than during the water guns off treatment (15.04 ± 1.59 m). The water gun array had a greater impact on native fish species than on bigheaded carp. Caution should be taken to the extrapolation of these results to other fish species and to fish exposed to water guns in different environments (e.g., reduced shoreline interaction) or exposure to a larger array of water guns, or for use of water guns for purposes other than a barrier.

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.

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.