Marine Airgun Research Articles

Toward a Practical Appraisal for Waveform Tomography of Band- and Offset-Limited Marine Seismic Data

We present a generalized workflow to retrieve high-resolution P-wave velocity ( <formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex>$V_{ P}$</tex> </formula> ) models of complex Earth's subsurface structures from traditional marine near-vertical seismic reflection experiments. These records have typically offsets too short to map refraction phases and lack low-frequency information. The workflow is composed of three steps: 1) downward continuation (DC) of seismic records to the seafloor to recover diving wave information; 2) travel-time tomography (TTT) of first arrivals obtained from DC data, to retrieve a kinematically correct model; and 3) full-waveform inversion (FWI) of the original streamer dataset, starting with the model obtained with TTT and sequentially introducing higher wavenumber details into the model. We show that the TTT allows overcoming the issues associated with the nonlinearity intrinsic to FWI. We also disentangle envelope and phase from the waveform to choose the objective function most suitable for FWI. We assess the accuracy of initial models and predict the quality of the FWI results by quantifying the early arrival cycle skipping between original and simulated data. The efficiency of the workflow is tested with a challenging synthetic target model, containing vertical boundaries with strong velocity contrasts and velocity inversions embedded in a checkerboard-like pattern. We show that workflow steps 1) and 2) provide a TTT model that is not cycle skipped at the frequencies available in most marine seismic experiments and thus allow step 3) FWI to obtain high-resolution <formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex>$V_{ P}$</tex> </formula> models of the subsurface using band- and offset-limited field datasets, traditionally collected in marine airgun and streamer acquisitions.

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.

Acoustic generation of underwater cavities-Comparing modeled and measured acoustic signals generated by seismic air gun arrays.

Underwater vapor cavities can be generated by acoustic stimulation. When the acoustic signals from several air guns are reflected from the sea surface, the pressure drop at some locations is sufficient for cavity growth and subsequent collapse. In this paper the generation of multiple water vapor cavities and their collapses are numerically modeled and the results are validated by comparing with field data from a seismic air gun array test. In a first modeling attempt where cavity interaction is neglected, a correspondence between measured and modeled data is found. Then, this correspondence is improved by assuming that the acoustic signal generated by the other cavities changes the hydrostatic pressure surrounding each cavity. This modeling can be used to estimate the amount and strength of high frequency signals generated by typical marine air gun arrays, given that a calibration step is performed prior to the modeling.

Environmental source modelling to mitigate impact on marine life

Alex Goertz, Jens Fredrik Wisloff, Francis Drossaert and Jaafar Ali discuss how modelling the source output of marine airgun arrays can be used for planning marine mammal mitigation measures as part of marine seismic survey design and environmental permitting.

3-D imaging of Marlborough, New Zealand, subducted plate and strike-slip fault systems

SUMMARY We present a 3-D seismic velocity model of the northern South Island, New Zealand, using several combined passive and active-source data sets. This area is the transition from Hikurangi subduction in the North Island to oblique continental collision along the Alpine fault and the Southern Alps, and geodetically observed deformation is consistent with plate motion being primarily taken up on a series of rotating strike-slip faults. We carry out simultaneous inversion for Vp and Vp/Vs and hypocentres using earthquake data recorded by the permanent seismic network and three temporary seismic arrays, also using active-source marine airgun data recorded at onshore stations. We image the subducted Hikurangi slab as a continuous unbroken feature extending from the active subduction zone to the north. Beneath the Marlborough region, the 3-D hypocentres define the subducted slab as a smooth curved feature to over 250-km depth, whereas below the junction of the Awatere and Alpine faults, the seismicity abruptly ends at 100-km depth. The subducting Pacific plate has a thick crust, including greywacke upper crust, which slows subduction. The interaction of the subducted plate with the translating overlying plate forms a 50-km-long zone of apparent crustal thickening with Vp < 7.0 km s−1 observed to 40-km depth. The Australian Plate has a high-velocity block to 65-km depth, aligned with the Separation Point pluton and trending oblique to the subducted slab. This zone of thicker Australian lithosphere may also hinder subduction. The Marlborough faults trend about 25° to the strike of the underlying subducted slab, so that the relation of faults to the slab varies along the fault strikes, and the overlying plate ranges in thickness from 20 to 50 km along the faults. Thus, the fault interaction with the subducted slab varies, from translating material directly above the subducting plate, to bottoming into a thick ductile region. The lower crust has high Vp/Vs under the Marlborough faults (except for a region of Haast schist), which may indicate excess fluid released by the subducted plate.

Comparative velocity structure of active Hawaiian volcanoes from 3-D onshore–offshore seismic tomography

We present a 3-D P-wave velocity model of the combined subaerial and submarine portions of the southeastern part of the Island of Hawaii, based on first-arrival seismic tomography of marine airgun shots recorded by the onland seismic network. Our model shows that high-velocity materials (6.5–7.0 km/s) lie beneath Kilauea's summit, Koae fault zone, and the upper Southwest Rift Zone (SWRZ) and upper and middle East Rift Zone (ERZ), indicative of magma cumulates within the volcanic edifice. A separate high-velocity body of 6.5–6.9 km/s within Kilauea's lower ERZ and upper Puna Ridge suggests a distinct body of magma cumulates, possibly connected to the summit magma cumulates at depth. The two cumulate bodies within Kilauea's ERZ may have undergone separate ductile flow seaward, influencing the submarine morphology of Kilauea's south flank. Low velocities (5.0–6.3 km/s) seaward of Kilauea's Hilina fault zone, and along Mauna Loa's seaward facing Kao'iki fault zone, are attributed to thick piles of volcaniclastic sediments deposited on the submarine flanks. Loihi seamount shows high-velocity anomalies beneath the summit and along the rift zones, similar to the interpreted magma cumulates below Mauna Loa and Kilauea volcanoes, and a low-velocity anomaly beneath the oceanic crust, probably indicative of melt within the upper mantle. Around Kilauea's submarine flank, a high-velocity anomaly beneath the outer bench suggests the presence of an ancient seamount that may obstruct outward spreading of the flank. Mauna Loa's southeast flank is also marked by a large, anomalously high-velocity feature (7.0–7.4 km/s), interpreted to define an inactive, buried volcanic rift zone, which might provide a new explanation for the westward migration of Mauna Loa's current SWRZ and the growth of Kilauea's SWRZ.

Imaging a plate boundar using double-sided onshore-offshore seismic profiling

Seismic profiling across the ocean-land transition is a known challenge that requires a combination of data collection techniques. Seismic acquisition may require the use of swamp buggies, bay cables, ocean bottom hydrophones or seismometers, and/or marine air guns. The onshore-offshore seismic profiling technique, which uses moderate-to-widely spaced land instruments to record densely spaced marine air-gun sources, undershoots the coastline to provide detailed images of subsurface regions that cannot be seen in either marine or land profiles. Intermediate to wide-angle raypath geometries are common in this technique. Useful analysis methods include arrival-time velocity tomography or wide-angle prestack migration, neither of which is based on vertical-incidence raypaths.

Urban seismic experiments investigate Seattle Fault and Basin

In the past decade, Earth scientists have recognized the seismic hazards that crustal faults and sedimentary basins pose to Seattle, Washington (Figure 1). In 1998, the US. Geological Survey and its collaborators initiated a series of urban seismic studies of the upper crust to better map seismogenic structures and sedimentary basins in the Puget Lowland. These studies are called the Seismic Hazard Investigations of Puget Sound (SHIPS).In March 1998, we conducted our first SHIPS study, an investigation of the upper crustal structure of the Puget Lowland, using marine airgun sources and land recorders [Fisher et al., 1999].The study was nicknamed Wet SHIPS. In September 1999, we obtained a seismic refraction line to study the upper crustal structure in the Seattle area in a land‐based study nicknamed Dry SHIPS [Brocher et al., 2000] (Figure 1). In March 2000, we recorded the demolition of the Seattle Kingdome sports stadium using a dense array of seismic recorders for a detailed site response study; this study was nicknamed Kingdome SHIPS (Figure 1).

Crustal structure across the Pacific margin of Nicaragua:evidence for ophiolitic basement and a shallow mantle sliver

Seismic wide-angle measurements across the Pacific margin of Nicaragua were carried out using ocean bottom hydrophones and land stations recording marine airgun shots. The structure and the P-wave velocity of the subducting Cocos and overriding Caribbean Plates were determined by modelling wide-angle data and further constrained by coincident seismic reflection, borehole and gravity data. The oceanic crust of the Cocos Plate is 5.5 km thick, with a thin pelagic sediment cover. The plate boundary can be traced to 40 km depth and is generally similar to configurations derived earlier from the local seismic network. A major feature of the upper plate is an 80-km-wide high-velocity, high-density rock unit reaching from the front of the margin to about the middle of the shelf. This wedge-shaped unit is 15 km thick beneath the shelf edge and is composed of a 5-km-thick upper part with velocities increasing from 3.5 km s−1 near the trench to 5.2 km s−1 at the shelf break overlying a 10-km-thick lower part with velocities of 6.0 km s−1. Analysis of the gravity field requires densities of 2.6–2.7 g cm−3 for the upper part and 2.9 g cm−3 for the lower part of this unit. These velocities and densities suggest that the margin wedge is composed of ophiolitic rock similar to those sampled offshore Guatemala and exposed on the Nicoya Peninsula, Costa Rica. The velocity structure of this ophiolitic unit ends about 50 km offshore. Landward, the basement underneath the Sandino Basin is formed by older igneous rock drilled beneath upper Cretaceous sedimentary rock onshore Nicaragua. Beneath the ophiolitic basement we find a sliver with velocities typical of mantle material that begins at depths of 12–18 km and coincides with the down-dip limit of the seismogenic zone. Mantle densities are required for the sliver to match the gravity data. In a tectonic reconstruction, the suture of an oceanic plateau on the Farallon Plate against the Chortis Block in upper Cretaceous time is suggested. Suturing left the former trench and margin in deep water, consistent with the late Cretaceous to Palaeocene deep-water sediment of the Rivas Formation at the base of the Sandino Basin. Suture of the Farallon Plate and Chortis Block might have initiated the strike-slip movement along the Motagua–Polochic Fault System. The development of a new subduction zone in Eocene–Oligocene times left the ophiolitic basement and a sliver of oceanic mantle attached to the Chortis Block and shifted the volcanic arc about 70 km southwestwards, close to its present position.

An experimental comparison of three direct methods of marine source signature estimation

Three methods for estimation of the pressure wavefield generated by a marine airgun array are tested experimentally and compared. In the trial a variety of radiation angles and array configurations were used and some large synchronization errors were deliberately introduced. The source was equipped with near‐field hydrophones and a subsource ministreamer. A tethered far‐field hydrophone was used so that the three estimated far‐field signatures could be compared with an independent measurement.The knowledge of the source signature is important for on‐board source array QC, deconvolution, multiple attenuation, stratigraphic trap prediction, modelling and inversion, AVO analysis and reservoir monitoring.The methods perform very well and give estimates whose frequency‐domain spectra match the measured spectra to within a few dB and within a few tens of degrees of phase over the tested bandwidth of 3.5–110 Hz. The time‐domain error‐energy is typically only a few per cent of the signal energy for radiation angles within about 30° of the vertical. The third method proved to be sensitive to an experimental shortcoming leading to overloading of the ministreamer and meaningful comparison was not possible for some test configurations.

Migration of wide‐aperture onshore‐offshore seismic data, central California: Seismic images of late stage subduction

We conducted a high shot and receiver density, onshore‐offshore seismic experiment in 1986 across the central California transform margin as part of the Pacific Gas and Electric / EDGE Deep Crustal Geophysical Survey. The data set consists of land recordings of 406 marine air gun shots fired along a 70‐km track across the Santa Maria basin. The seismic vessel's track was collinear with 154 land seismic recorders deployed over 50 km across the Sur Obispo terrane and part of the Salinian block. The source‐receiver offsets range from 2.5 km to 120 km. The unique recording geometry, and high shot and receiver density, permit imaging the deeper crustal structure of the California transform margin but pose serious challenges for conventional seismic imaging methods. Consequently we developed a unified approach to wide‐angle imaging which incorporates a tomography technique for obtaining initial migration velocity models, and a prestack Kirchhoff depth migration with depth focusing of the main reflectors for velocity model refinement and imaging. This approach retains the velocity and dip information in the data and allows user control over the imaging process. We applied this processing to subsets of the onshore‐offshore data to obtain an upper crustal velocity model, and a 32‐km‐long, wide‐angle depth migrated image of the lower crust, in the coastal region under the Santa Maria basin, and the Sur‐Obispo terrane. The migration image shows a part of an underplated oceanic crustal layer, interpreted as the Monterey plate (Meltzer and Levander, 1991; Miller et al., 1992), which dips coastward at depths of 18 to 28 km. The plate appears to be kinked or imbricated beneath the coastline. The image correlates well with existing wide‐angle seismic interpretations based on ray tracing (Walter and Sharpless, 1987; Putzig, 1988; Howie et al., 1993). The migrated lower crustal image has more structural information than is apparent in conventional ray‐based interpretations along the same, crossing, and adjacent profiles. The crustal image demonstrates that automated imaging techniques applied to wide‐angle data can provide images of crustal structure comparable in fidelity to conventionally processed and migrated near‐vertical incidence data.

Two-dimensional modelling and interpretation of seismic wide-angle data from the Western Gulf of Bothnia

In late 1989, wide-angle reflection and refraction profiles were shot in the Baltic Sea, the Bothnian Sea and the Gulf of Bothnia in order to study the lithospheric structure of Archaean and Proterozoic domains. The present study concerns the crustal structure and velocity variations beneath the Gulf of Bothnia area. Data collected at six land stations, recording marine airgun shots fired along BABEL line 2 in the Gulf of Bothnia, have been interpreted. The results from traveltime inversion show an upper crustal thickness of 13–22 km with P- and S-wave velocities varying from 5.5 to 6.5 and 3.3 to 3.7 km/s, respectively. The middle crust has a thickness of 13–15 km, with P-wave velocity varying from 6.3 to 6.9 km/s and S-wave velocity from 3.6 to 3.9 km/s. The lower crustal layer with thickness ranging from 9 to 20 km shows a high P- and S-wave velocity of approximately 7.5 and 4.4 km/s, respectively. The P-and S-wave velocities for the upper mantle are about 8.0 and 4.5 km/s. Calculated Poisson's ratio ranges from 0.20 to 0.24 for the upper crust and 0.25 to 0.28 for the middle and lower crust, and 0.29-0.31 for the upper mantle, respectively. The Moho depth varies between 42 and 55 km along the profile. The Moho topography shows a smooth undulation underneath the central part of the Gulf of Bothnia. A steep offset of about 8 km occurs beneath the southernmost part of the profile. Northward-dipping reflectors have been recorded in the lower crust. These lower crustal reflections line up perfectly with the northward-dipping upper mantle reflectors displayed in the vertical reflection section from the same line.

Seismic images of a tectonic subdivision of the Grenville Orogen beneath lakes Ontario and Erie

New seismic data from marine air-gun and Vibroseis profiles in Lake Ontario and Lake Erie provide images of subhorizontal Phanerozoic sediments underlain by a remarkable series of easterly dipping reflections that extends from the crystalline basement to the lower crust. These reflections are interpreted as structural features of crustal-scale subdivisions within the Grenville Orogen. Broadly deformed, imbricated, and overlapping thrust sheets within the western Central Metasedimentary Belt are succeeded to the west by a complex zone of easterly dipping, apparent thrust faults that are interpreted as a southwest subsurface extension of the boundary zone between the Central Metasedimentary Belt and the Central Gneiss Belt. The interpreted Central Metasedimentary Belt boundary zone has a characteristic magnetic anomaly that provides a link from the adjacent ends of lakes Ontario and Erie to structures exposed 150 km to the north. Less reflective, west-dipping events are interpreted as structures within the eastern Central Gneiss Belt. The seismic interpretation augments current tectonic models that suggest the exposed ductile structures formed at depth as a result of crustal shortening along northwest-verging thrust faults. Relatively shallow reflections across the boundary region suggest local, Late Proterozoic extensional troughs containing post-Grenville sediments, preserved possibly as a result of pre-Paleozoic reactivation of basement structures.

Surface‐to‐borehole fault imaging in a buried horizontal waveguide—An experimental study in the southern North Sea

One fifth of the United Kingdom’s energy is derived from the gas fields of the southern North Sea. The area is divided into large fault blocks, which are laced with minor faults. Surface reflection seismic fault maps of the main producing horizon are distorted by salt rollers or pillows in the overlying high velocity Zechstein evaporites. We have developed an inexpensive method of confirming the presence of minor faults in the relatively flat base of the Zechstein around a well that would simplify production planning. Deep well sonic logs indicate that seismic velocity drops abruptly beneath the base Zechstein and then rises gradually with depth. This profile is characteristic of a whispering gallery, or a buried, variable‐index beam waveguide. If base Zechstein faults scatter energy efficiently into the waveguide, then the faults might be mapped by moving a seismic source over the surface and recording the mode converted arrivals at a geophone within the guide. To test this possibility, over 3000 shots were fired at 27-m intervals in thirteen lines spaced by ≈500 m from a 4436-cu in. (0.075-cu m) marine air gun array into a 3 km deep triaxial VSP geophone, stationed beneath the Rotliegendes. Secondary arrivals were found to move out across the raw field geophone gathers in the manner expected of down‐going compressional to both guided compressional and guided shear vertical mode conversions [P-P and P-SV] at the base of the Zechstein. Prestack migration yielded a map of mode conversion activity at the edges of known faults with throws of 30 + m over an area of [Formula: see text]. We conclude that there is an extensive seismic waveguide beneath the Zechstein in the southern North Sea. Faults scatter down‐going P waves efficiently into the guide; noise levels are low at the bottom of a borehole. Vertical reverberation follows a surface source as it moves away from the wellhead. Horizontal waveguiding reduces geometric spreading loss. These factors make it possible to map small reservoir faults with high‐resolution at long ranges from an exploration well.

Estimation of relative site effect, coda Q and scattering strength from the records obtained by a large airgun experiment

Abstract By using a large-capacity marine airgun, we carried out a seismic refraction experiment around the Bandai volcanic area, in the southern part of Tohoku, NE Japan. The analysis of the observed waveform data from the airgun shots shows that the coda wave amplitude is independent of hypocentral distance after a lapse time of about 9 s at any observation site with epicentral distance Q c −1 ∝ ƒ n ), where the power n is found to be about −0.8. In the earlier part of the lapse time, the coda wave energy concentrates around the seismic source, which indicates that the scattering effect is very strong in the study area. The shapes of waveform envelopes at 5, 10 and 20 Hz show that the forward scattering factor is stronger at higher frequency than that at lower frequency.

Introduction to Special Section on EDGE and Related Seismic Projects, Onshore‐Offshore California

The continental margins hold a special interest to Earth scientists because they mark the complex transition from oceanic to continental crust. This transition often involves a number of interacting processes, including subduction, accretion, and deformation of the oceanic lithosphere and growth and modification of the continental lithosphere, particularly the crust. The past decade has seen a great deal of progress in geophysical studies of continental margins, in large part due to the successful application of marine multi channel seismic (MCS) surveys that have provided unprecedented details on the deep structure.An opportunity arose in 1986 to augment, at a relatively modest cost, an MCS survey commissioned by the Pacific Gas and Electric Company (PG) across the central California margin. At the initiative of the Houston Area Research Center (HARC) and Rice University, funds were provided to the M CS contractor to record several additional MCS profries, and land seismographs were deployed to record the marine air gun shots and land shots prepared by the U.S. Geological Survey. Ocean bottom seismographs were deployed on two of the profiles by Oregon State University. This study constitutes the first of many profiles of opportunity completed or planned by EDGE, a National Science Foundation and industry funded consortium of institutions committed to geophysical studies of continental margins.

Recording marine airgun shots at offsets between 300 and 700 km

This paper demonstrates that ‐ under favorable conditions ‐ by using multichannel recording and subsequent stacking of adjacent records marine airgun shots have been detected at offset distances up to 700 km, the maximum offset at which we attempted to record data. Besides a powerful airgun array, a low noise environment at the recording site and the elimination of static shifts are the prerequisites to obtain refracted and reflected arrivals from the crust and upper mantle at such large offsets. Primary arrivals detected at offsets between 400 and 700 km image the upper mantle from 70 to about 120 km depth. Stacking of neighbouring shots and/or receivers successfully increases the signal‐to‐noise ratio, if the traces have been corrected for offset differences, which requires knowledge of the apparent phase velocities. The data presented here were collected in autumn 1989 during the BABEL Project on the Baltic Shield.

67B. Crustal Structure around Bandai Volcano Estimated from Fan-Shooting Seismic Observations Using Marine Airgun.(Abstracts of Papers Presented at the Spring Meeting of the Society)

67B. Crustal Structure around Bandai Volcano Estimated from Fan-Shooting Seismic Observations Using Marine Airgun.(Abstracts of Papers Presented at the Spring Meeting of the Society)

エアガンによる磐梯山周辺域の地殻構造探査

エアガンによる磐梯山周辺域の地殻構造探査

エアガン実験で観測されたコーダ波の振幅特性

We have carried out a seismic refraction experiment around Bandai volcanic area, the southern part of Tohoku, by using a large capacity marine airgun. Analysis of observed seismograms from the airgun shots shows that the coda wave amplitude at each observation site is independent of hypocentral distance. This indicates that the coda wave energy is homogeneously distributed in the medium beneath the refraction profile. The coda wave amplitude correlates well to the amplitude of the direct P wave from distant earthquakes and inversely correlates to the Bouguer anomaly. The relative site amplification of the coda wave is nearly the same as that of the direct P wave, and is considered to be related to the average density or the thickness of sedimentary layer just beneath the observation site. By using the coda wave amplitude at each observation site, we can evaluate the relative site amplification. Correction of the site amplification factor gives a smooth and less scattered distribution of the P wave amplitude from the airgun shots along the refraction profile.