Large Airgun Research Articles

Behavioral Response Study on Seismic Airgun and Vessel Exposures in Narwhals

One of the last pristine marine soundscapes, the Arctic, is exposed to increasing anthropogenic activities due to climate-induced decrease in sea ice coverage. In this study, we combined movement and behavioral data from animal-borne tags in a controlled sound exposure study to describe the reactions of narwhals,Monodon monoceros, to airgun pulses and ship noise. Sixteen narwhals were live captured and instrumented with satellite tags and Acousonde acoustic-behavioral recorders, and 11 of them were exposed to airgun pulses and vessel sounds. The sound exposure levels (SELs) of pulses from a small airgun (3.4 L) used in 2017 and a larger one (17.0 L) used in 2018 were measured using drifting recorders. The experiment was divided into trials with airgun and ship-noise exposure, intertrials with only ship-noise, and pre- and postexposure periods. Both trials and intertrials lasted ∼4 h on average per individual. Depending on the location of the whales, the number of separate exposures ranged between one and eight trials or intertrials. Received pulse SELs dropped below 130 dB re 1 μPa2s by 2.5 km for the small airgun and 4–9 km for the larger airgun, and background noise levels were reached at distances of ∼3 and 8–10.5 km, respectively, for the small and big airguns. Avoidance reactions of the whales could be detected at distances >5 km in 2017 and >11 km in 2018 when in line of sight of the seismic vessel. Meanwhile, a ∼30% increase in horizontal travel speed could be detected up to 2 h before the seismic vessel was in line of sight. Applying line of sight as the criterion for exposure thus excludes some potential pre-response effects, and our estimates of effects must therefore be considered conservative. The whales reacted by changing their swimming speed and direction at distances between 5 and 24 km depending on topographical surroundings where the exposure occurred. The propensity of the whales to move towards the shore increased with increasing exposure (i.e., shorter distance to vessels) and was highest with the large airgun used in 2018, where the whales moved towards the shore at distances of 10–15 km. No long-term effects of the response study could be detected.

Geodynamic nature of the Okhotsk Sea lithosphere. An overview of seismic constraints

A combined interpretation of the geological, seismological and active-source seismic data in the Okhotsk Sea (Sea of Okhotsk) region allows describing the structure and geodynamics of this transition zone between the continent and the ocean. The interpreted data on the crust and uppermost mantle structure of this region are based on the recent seismic profiles with detailed system of observation carried out during the last decades in the Okhotsk Sea. Large air guns and 119 ocean bottom stations were used to study the lithosphere structure of beneath the sea down to 70 km depth. Detailed P- and S-waves interpretation methods were applied for these profiles. The study shows the Okhotsk Sea crust to be of the continental type composed mainly of the felsic rocks. Only within the elongated basin along the Kuril island arc, the crust thickness is reduced and the velocities increased to the values typical for the oceanic crust. The new seismic data are also obtained on the upper mantle structure. The detailed mathematical modeling of the observed mantle waves revealed unusual type of the wide angle reflections recorded at the first arrivals which previously interpreted as refractions. As a result, two reflective boundaries M and M1 are revealed beneath the sea depression, with velocities of 7.9 and 8.0 km/s. Along the Sakhalin Island a deep mantle fault was traced down to the depth of 70 km. This deep fault is traced in the north-south direction far into the Pacific ocean and into the east Asia. The seismological data show the Okhotsk Sea to be surrounded by deep faults of global nature. In addition to the Kuril arc zone limiting the sea depression in the eastern part, the deep Magadan fault separates the depression from the continent. These data suggest the Okhotsk Sea region as a separate micro-plate of the continental type.

3D seismic datasets applied to mineral exploration: revisiting three Canadian case studies

The benefits of working with legacy seismic data are broad and valued in a wide range of geoscience applications and result from the comprehensive volume of industry and public seismic data acquired over several decades. Legacy seismic data exist in many parts of the world including areas that, due to stricter regulatory processes, environmental restrictions, demographic changes, or other factors would be challenging or even impossible to survey today. Examples are numerous but one includes 2D marine seismic data acquired in the 1980s with a large airgun array to provide a deep cross-section through the Appalachians beneath the now sen-sitive Gulf of St Lawrence, eastern Canada (Marillier et al., 1989). Sometimes benefits arise through the use of newly developed tech-niques to extract additional information from legacy seismic data. This idea is obviously not new and many examples can be found in the literature. One example is the use of extended Vibroseis cor-relation developed in 1980s (Okaya and Jarchow, 1989) applied to increase the depth range of marine Vibroseis seismic data acquired in 1971 in two of the Great Lakes of North America (Milkereit et al., 1992). Marine seismic data in the Great Lakes are sparse, but contain information about deep Grenvillian terranes that, in this case, could only be revealed using a technology developed more than a decade after data acquisition. Reflection seismology has been used for mineral exploration for more than 30 years (Reed, 1993). During that period, many case studies demonstrated the utility of the method for imaging deep lithological contacts and structures indirectly associated with mineralization, and for the direct imaging of deposits. Malehmir et al. (2012) provide a comprehensive overview of key accomplishments for various commodities and mining locations in Canada and around the world. In particular, many examples demonstrate the advantages of the 3D seismic method for providing high-resolution images of subsurface structures and deep-seated ore deposits in a variety of geological environ-ments. Several of these legacy 3D seismic data sets have been reprocessed with advanced and novel techniques to provide more reliable and detailed images of the subsurface. Here, we show three early examples of the application of 3D seismic data for mineral exploration in the Bathurst, Sudbury, and Val d’Or mining camps, Canada (Figure 1). Specifically, we demonstrate that reprocessing of 3D datasets from earlier applications of seismic reflection can refine the imaging of known ore deposits and can lead to the identification of new exploration targets. For the case studies presented here, most improvements resulted from processing and imaging strategies built from an understanding of the response of mineralized bodies on seismic data and improving specific steps that prevented their proper imaging on final volumes. The case studies are introduced according to their year of acquisition, starting from the oldest (i.e., the Trill 3D survey in the Sudbury mining camp) to the most recent (e.g., the Louvicourt 3D survey in the Val d’Or mining camp). All 3D seismic surveys are located in areas with limited access and difficult terrain and as a result, were acquired with explosives. Surface conditions varied for each survey area but included steep hills, extensive swamps, and thick glacial sediments. Geology also varied at the three sites. Host rocks are either nearly-transparent-to-seismic waves or are highly heterogeneous and generating seismic scattering potentially masking the response of the mineralized body. In all three areas, however, mineralized bodies scattered or reflected energy in a preferred direction, and were observed only on a limited number of prestack seismic traces. In some cases, such a sparse signature was buried in noise when stacking all traces and required particu-lar attention to be properly imaged.

Application of large capacity air gun in three-dimensional crustal structure exploration of the Pearl River estuary area

The air gun source has been widely used as an artificial source in seismic exploration of the sea area because of its simple operation, low risk, automatic continuous operation and almost no influence on the observation environment. In 2015, we carried out the 3-D crustal structural exploration project in the Pearl River estuary, 13 seismic sounding explorations were set up in the sea area, and activated air gun source for more than 12,000 times. Through records of ocean bottom seismograph (OBS) and fixed seismic stations, we found that seismic records of OBS can be clearly identified Pg, PmP, and other phases, the records of the fixed seismic stations can be clearly identified the air gun shot events. The results show that the energy of the air gun shot in this field exploration is quite good, and data can support the study of the three-dimensional structure of the crust in the sea by air gun shot records of seismometers.

Cetacean behavioral responses to noise exposure generated by seismic surveys: how to mitigate better?

<p>Cetaceans use sound in many contexts, such as in social interactions, as well as to forage and to react in dangerous situations. Little information exists to describe how they respond physically and behaviorally to intense and long-term noise levels. Effects on cetaceans from seismic survey activities need to be understood in order to determine detailed acoustic exposure guidelines and to apply appropriated mitigation measures. This study examines direct behavioral responses of cetaceans in the southern Mediterranean Sea during seismic surveys with large airgun arrays (volume up to 5200 ci) used in the TOMO-ETNA active seismic experiment of summer 2014. Wide Angle Seismic and Multi-Channel Seismic surveys had carried out with refraction and reflection seismic methods, producing about 25,800 air-gun shots. Visual monitoring undertaken in the 26 daylights of seismic exploration adopted the protocol of the Joint Nature Conservation Committee. Data recorded were analyzed to examine effects on cetaceans. Sighting rates, distance and orientation from the airguns were compared for different volume categories of the airgun arrays. Results show that cetaceans can be disturbed by seismic survey activities, especially during particularly events. Here we propose many integrated actions to further mitigate this exposure and implications for management.</p>

Deep seismic soundings on the 1-AP profile in the Barents Sea: Methods and results

Profile 1-AP with a length of 1300 km intersects the Barents Sea from The Kola Peninsula to Franz Josef Land. The combined Common Depth Point (CDP) and Deep Seismic Sounding (DSS) seismic studies were carried out on this profile. The DSS measurements were conducted with the standalone bottom seismic stations with an interval of 5–20 km between them. The stations recorded the signals generated by the large air guns with a step of 250 m. Based on these data, the detailed P-velocity section of the Earth’s crust and uppermost mantle have been constructed for the entire profile and the S-velocity section for its southern part. The use of a variety of methods for constructing the velocity sections enabled us to assess the capabilities of each method from the standpoint of the highest reliability and informativity of the models. The ray tracing method yielded the best results. The 1-PR profile crosses two large basins—the South Barents and North Barents ones, with the thickness of the sediments increasing from 8 to 10 km in the south to 12–15 km in the north. The Earth’s crust pertains to the continental type along the entire profile. Its thickness averages 32 to 36 km and only increases to 43 km at the boundary between the two basins. The distinct change in the wave field at this boundary suggests the presence of a large deep fault in this zone. The high-velocity blocks are revealed in the crust of the South Barents basin, whereas the North Barents crust is characterized by relatively low velocities.

Airgun array sound measurements in the northeastern Chukchi Sea

Underwater acoustic measurements were made during eight 2D/3D seismic surveys with large airgun arrays and nine shallow hazards surveys using small arrays (2–4 airguns) in the shallow (30–50 m) waters of the northeastern Chukchi Sea between 2006 and 2013. The acoustic measurements were made with calibrated seabed-deployed recorders to assess potential noise exposures to marine fauna. Data were acquired from directly under the airgun arrays to over 150 km distance. The seismic pulses contain signals above ambient noise between approximately 5 Hz and 15 kHz. Modal structure develops by about 1 km distance from the sources at frequencies between approximately 30 Hz and 600 Hz, but it is strongest from 100 Hz to 300 Hz. This ocean environment therefore acts as a frequency band-pass filter, selectively passing the intermediate sound frequencies to long distances. The modes are strongly dispersed, with low frequency components arriving up to 2 s later than the higher frequency sounds of the same mode. This causes received signals to have a down-swept spectral shape. We present sound level measurement results from all of the surveys and we discuss the frequency-dependent acoustic transmission loss in context with the modal propagation characteristics of this ocean environment.

An experimental study on the excitation of large volume airguns in a small volume body of water

A large volume airgun array is effective in generating seismic waves, which is extensively used in large volume bodies of water such as oceans, lakes and reservoirs. So far, the application of large volume airguns is subject to the distribution of large volume bodies of water. This paper reports an attempt to utilize large volume airguns in a small body of water as a seismic source for seismotectonic studies. We carried out a field experiment in Mapaoquan pond, Fangshan district, Beijing, during the period 25–30 May 2009. Bolt LL1500 airguns, each with volumes of 2000 in3, the largest commercial airguns available today, were used in this experiment. We tested the excitation of the airgun array with one or two guns. The airgun array was placed 7–11 m below the water's surface. The near- and far-field seismic motions induced by the airgun source were recorded by a 100 km long seismic profile composed of 16 portable seismometers and a 100 m long strong motion seismograph profile, respectively. The following conclusions can be drawn from this experiment. First, it is feasible to excite large volume airguns in a small volume body of water. Second, seismic signals from a single shot of one airgun can be recognized at the offset up to 15 km. Taking advantage of high source repeatability, we stacked records from 128 shots to enhance the signal-to-noise ratio, and direct P-waves can be easily identified at the offset ~50 km in stacked records. Third, no detectable damage to fish or near-field constructions was caused by the airgun shots. Those results suggest that large volume airguns excited in small bodies of water can be used as a routinely operated seismic source for mid-scale (tens of kilometres) subsurface explorations and monitoring under various running conditions.

Imaging igneous rocks on the North Atlantic rifted continental margin

SUMMARY Extruded basalt flows with thicknesses of several kilometres occur ubiquitously along the rifted continental margins of the northern North Atlantic. Their total volume exceeds 1 million km 3 , and may reach several million cubic kilometres. Intruded igneous rock comprising the complementary melt fraction to that extruded at the surface should exist in the lower crust with a volume at least as large as that of the extrusive basalts. To image the extrusive and intrusive igneous rocks on the continental margin near the Faroe Islands, in 2002 we shot a 375 km long deep-penetration seismic profile across the margin using 85 ocean bottom seismometers for wide-angle acquisition and shot a separate reflection profile using a 12 km streamer for optimal imaging. By using large airgun sources tuned to produce low frequency energy we were able to constrain the seismic velocity structure of the whole crust. We imaged not only layering and structure within the extrusive basalts, but also lower-crustal intrusions on the continent–ocean transition (COT). Combination of good velocity control on the lower-crust together with direct imaging of the extrusive basalts enables us to constrain the volumes of extrusive and intrusive igneous rocks produced near the Faroe Islands to be 340–420 and 560–780 km 3 per kilometre along strike, respectively. The COT marked both by high-velocity intrusions and lower-crustal layering is surprisingly narrow, with only 50 km separating unstretched continental crust from fully oceanic crust. In contrast, the extruded basalts flow up to 150 km landward at the paleo-surface.

Refraction and reflection Seismic profilings to investigate Eocene arc-KY0715 cruise-

We carried out a deep wide-angle seismic experiment using a large airgun array and 110 ocean bottom seismographs (OBSs) along the Ogasawara Ridge from 26 November 2007 to December 25 (KY07-15 cruise) using R/V Kaiyo of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). The Ogasawara Ridge is an extinct arc that was produced in Eocene period. In addition, this arc collided with the Ogasawara Plateau and this event is expected to have severely deformed the crust. The objectives of this cruise are to understand the typical characteristics of the Eocene arc and the crustal deformation by collision with the Ogasawara Plateau. An airgun-OBS seismic line with a length of approximately 594 km was set along the Ogasawara Ridge through the gentle southern root near the collision point by the Ogasawara Plateau. We shot a large airgun array with a total volume of 12,000 cu. in. and recorded the seismic signals on OBSs with four components and a 12-channel hydrophone streamer. Moreover, we investigated the northern elongation of the Eocene arc by reflection survey using a G-gun array with a total capacity of 600 cu. in. and a 16-channel hydrophone streamer. In this paper, we summarize the seismic experiments and introduce the OBS data and reflection data.

Wide-angle seismic profiling across the middle Izu-Ogasawara arc for understanding boundary structure between the mature and juvenile arcs - KY0609 cruise -

We carried out a deep wide-angle seismic experiment using a large airgun array and total 95 ocean bottom seismo- graphs (OBSs) in the middle Izu-Ogasawara arc area, which was conducted by R/V Kaiyo of Japan Agency for Marine-Earth Science and Technology (JAMSTEC) from August 3, 2006 to September 1, 2006 (UTC) named as KY06-09 cruise. The seis- mic line runs from the Izu-Ogasawara trench to the eastern foot of the Kinan Seamount Chain in the Shikoku basin through the northern tip of the Ogasawara trough, the volcanic front located off south of the Sofugan and the Miocene Nishi-shichito ridge. Most important objective of this cruise is to construct a velocity structure across this area and clarify commonalities and dif fer- ences by comparison of velocity structures obtained with previous studies in the whole of Izu-Ogasawara arc regions. The key structures are the continuity of the crustal layers of the Ogasawara ridge and trough, and the current active arc obtained by p re- vious KY05-02 and KY05-11 cruises. And the high velocity lower crust with P-wave velocity of over 7.0 km/s is also one of the key structures because it was discovered at the arc-backarc transition zones in the western Izu-Ogasawara arc and the Mariana arc. The seismic signals propagated from an airgun array with a total capacity of 12,000 cu.in. were recorded by not only OBSs but also a 12-channnel streamer towed from stern. In this paper, we introduce the seismic experiment, especially the schedule and examples of OBS and reflection data.

Wide-angle seismic profiling across an active rifted zone in the middle Izu-Ogasawara arc –KY0511 cruise–

We carried out a deep wide-angle seismic experiment using a large airgun array and total 110 ocean bottom seismo- graphs (OBSs) in the middle Izu-Ogasawara arc area, which was conducted by R/V Kaiyo of Japan Agency for Marine-Earth Science and Technology (JAMSTEC) from October 5 to November 3, 2005 (KY05-11 cruise). Objectives of this cruise are to know a velocity structure of the across the northern Izu arc, especially velocity variation between an old Eocene arc beneath t he forearc and a new active arc, and a relationship between the structural heterogeneity with the velocity variation and the activ e backarc rifting. These are important keys to clarify nature of the oceanic arc growth. An airgun-OBS seismic line was set from a trench slope break adjacent to the Izu-Ogasawara trench to the western Shikoku Basin through the forearc basin, the volcanic front, the Sumisu rift, the Miocene rear arc, the eastern Shikoku Basin and the Kinan seamount chain. We shot a large airgun array with total volume 12,000 cu. in. and recorded the seismic signals on OBSs with four components and a 24-channel hydrophone streamer. In this paper, we summarize information of the seismic experiments and introduce OBS data and reflec- tion data.

Wide-angle seismic profiling of oceanic island arc in the southern Izu-Ogasawara arc –KY0502 cruise–

We carried out a deep wide-angle seismic experiment using a large airgun array and 110 ocean-bottom seismo- graphs (OBSs) in the southern Izu-Ogasawara arc area. The experiment was conducted on the R/V Kaiyo of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) from 12 February to 11 March 2005 (KY05-02 cruise). Objectives of this cruise were to determine the velocity structure of the entire arc-backarc system and the difference of crustal structures formed in the Oligocene, Eocene and the present to elucidate crustal growth. An airgun-OBS seismic line was set from the Izu- Ogasawara Trench near Chichijima to the Shikoku Basin across the southern Izu-Ogasawara arc in the arc-backarc system. The OBSs and a 12-channel hydrophone streamer recorded seismic signals, which were shot using a large airgun array with total capacity of 12,000 cubic inches. In this paper, we summarize information of the seismic experiments and introduce OBS data and reflection data.

Risks posed to the Antarctic marine environment by acoustic instruments: a structured analysis

The risks posed by a range of acoustic scientific instruments were assessed by the construction of matrices of scale and likelihood. We recognized six levels of impact ranging from none or short term, minimal behavioural response (Level 1) to multiple injuries and fatalities and/or compromised populations (Level 6) and six levels of likelihood ranging from “Expected in almost all instances” (Level 1) to “cannot see how it could happen” (Level 6). Typical scientific instruments ranging from acoustic releases to large air gun arrays were assessed. To provide a perspective for the risks of scientific operations, other activities were also ranked. These included large chemical explosions, submarine detection sonars implicated in some mass strandings of cetaceans and normal Antarctic shipping activities. The conclusion reached was that most scientific instruments pose a similar or lower risk than normal shipping operations. High source-level equipment poses some risk to individual animals' hearing and so should be mitigated. Likewise, survey planning should be designed to avoid trapping animals in narrow, constricted sea ways. Long term, cumulative impacts are still difficult to detect in areas with greater anthropogenic noise than the Antarctic but we concluded that any possible long term impacts should be mitigated by maintaining the low levels of activity using high source-level equipment through data sharing and survey planning.

Demonstration of comet sample collection by penetrator

We describe laboratory tests to investigate and demonstrate the acquisition and encapsulation of a subsurface sample from a comet analogue using a coring penetrator. The penetrator imbeds itself in the target, coring out a sample during the impact itself. Mechanisms seal the sample in a canister and the canister is spring-ejected from the rear of the penetrator where it can be retrieved in free-flight by a mother spacecraft, which thus need not perform a landing. We describe the penetrator vehicle, sample preparation and testing technique using the large airgun at the University of Arizona, and the performance results which indicate the technique is an attractive option for comet nucleus sample return.

Advances in seismic imaging through basalts: a case study from the Faroe–Shetland Basin

New seismic reflection data have been used to image intra- and sub-basalt features beneath the Faroe–Shetland Basin in the North Atlantic, where the highly reflective top and base boundaries of flood basalts and their complex internal structure make successful seismic imaging difficult. This study demonstrates that appropriate acquisition and processing of marine seismic data from hydrophone streamers and ocean-bottom seismometers (OBS) has the potential to enhance significantly imaging of intra-basalt and sub-basalt seismic reflections. The intersection of a new seismic reflection profile recorded in 2002 with a seismic profile recorded in 1998 allows a direct comparison of advances in sub-basalt imaging over this period and an interpretation of geological structures and seismic velocities at the intersection. To achieve better resolution of sediments below basaltic layers, surface seismic reflection data using a broad-band, low-frequency source have been recorded. By using a source wavelet with a significantly enhanced frequency spectrum centred at 10 Hz, generated from a large (167 l) airgun array tuned to the first bubble pulse, a wavelet is produced that is capable of penetrating through thick basaltic sequences. Furthermore, recording the low-frequency reflections to large offsets along a 12 000 m long streamer with 3.125 m single sensor spacing allowed effective suppression of multiple reflections, enhanced the precision of velocity analyses and produced better migration results. Seismic data along the same profile were also recorded on a dense deployment of 85 four-component OBS at 2 km or 6 km station spacing. These complementary ocean-bottom data allowed the large-scale velocity variations within and beneath the basalt to be constrained by recording at large offsets the termination of the diving rays ( c . 18 km) propagating through the basalt. Integration of seismic reflection data and OBS recordings yields good seismic velocity estimates and, therefore, increased confidence in the interpretation of intra- and sub-basalt features and lithology. Four major stratigraphic units characterized by their seismic velocities and reflection characters were identified: (a) post-basalt sediments; (b) basalt sequence; (c) sub-basalt unit; and (d) the top of presumed Cretaceous basement. The top boundary of the basalt sequence is defined by a strong reflection with a steep velocity increase from 3.5 km s −1 to 4.5 km s −1 in the c . 200 m thick layer above it. The basalt sequence here is c . 2.5 km thick. Beneath the base of the basalt there is a velocity decrease from 5.8 km s −1 to 5.2 km s −1 . The sub-basalt unit is subdivided further into hyaloclastites mixed with sediments and sills, and sub-basalt sediments separated by another velocity inversion from 5.2 km s −1 to 4.5 km s −1 . The Cretaceous basement with velocities above 5.5 km s −1 lies underneath these sub-basalt sediments.

Testing low/very low frequency acoustic sources for basin-wide propagation in the Indian Ocean

Low/very low frequency acoustic signals were transmitted to distant receivers in the Indian Ocean. The aim was to test methods for characterizing the hydroacoustic capability of the International Monitoring System (IMS) that discriminates for nuclear tests in the region. Several acoustic sources were deployed between Seychelles and Fremantle, Australia, and the IMS receivers comprised a network of hydrophones off Diego Garcia and Australia. Two of the three acoustic sources tested produced basin-scale propagation of impulsive signals. Single glass spheres imploded within the sound channel produced a clear signal at frequencies above ∼40 Hz, at ranges of hundreds to a thousand kilometers. Five-sphere glass implosions were recorded at ranges up to 4400 km. Near-sea surface shots from a large airgun array were recorded in several cases at ranges of hundreds to thousands of kilometers, the frequency of the highest signal-to-noise ratio arrivals varied within the 5–100 Hz band. High background noise level was a key factor at IMS stations that did not detect the airgun signals in the 5–15 Hz band. In a few cases, details of bathymetric features that are not well represented in the digital elevation model contributed to unexpected variation in relative signal levels between IMS stations.

Must magmatic intrusion in the lower crust produce reflectivity?

The Færoe–Iceland Ridge (FIR) provides a laboratory in which to investigate the reflectivity and velocity structure of thick crust generated above a mantle plume in order to constrain models of underplating and the origins of lower-crustal layering in an environment dominated by young igneous processes. Over 600 km of common midpoint (cmp) data were collected along and across the FIR using a large airgun array with a 240-channel streamer. The interpretation of these data has been integrated with a velocity model of the crust and upper mantle along the FIR obtained from wide-angle seismic arrivals into ocean bottom and land seismometers. Due to the intermediate water depths and the presence of basalt near the water bottom, specialized processing steps were required for the cmp data. A wave equation-based multiple attenuation scheme was applied to the prestack data, which used a forward model of the multiple series to predict and attenuate multiple energy. Array simulations were applied in the shot and receiver domains in order to minimize spatial aliasing and reduce low apparent-velocity noise. Most of the sections over the central (oceanic) portion of the FIR show no pronounced reflectivity, although occasional Moho and/or lower-crustal reflections are observed. We believe that the poor reflectivity results largely from a lack of physical property contrasts rather than being an effect of acquisition or processing, although we also conclude that residual energy from strong multiple reflection remains in the final sections. Amplitude decay and reflection strength vary along the FIR, but there is good signal-to-noise ratio to travel times of at least 9 s (i.e., into the lower crust), implying that the reduced reflectivity beneath the main part of the FIR is not an artifact of signal penetration loss. We conclude that the addition of melt to the lower crust along the trace of the plume apparently did not produce strong physical property contrasts in the lower crust, where little reflectivity is apparent. Perhaps this was because the entire crust was hot at the time of formation. In contrast, igneous intrusion into preexisting continental crust (at the Færoe Islands end of the FIR) and into older igneous crust (at the Iceland end of the FIR) produces significant lower-crustal reflectivity. Strong lower-crustal reflectivity elsewhere beneath the northwestern European continental margins may have a similar intrusive origin.

Seismic survey probes urban earthquake hazards in Pacific Northwest

A multidisciplinary seismic survey earlier this year in the Pacific Northwest is expected to reveal much new information about the earthquake threat to U.S. and Canadian urban areas there. A disastrous earthquake is a very real possibility in the region.The survey, known as the Seismic Hazards Investigation in Puget Sound (SHIPS), engendered close cooperation among geologists, biologists, environmental groups, and government agencies. It also succeeded in striking a fine balance between the need to prepare for a great earthquake and the requirement to protect a coveted marine environment while operating a large airgun array.

Processing of pre-critical wide-angle seismic reflection data from the BABEL project

In addition to using the now widely recognised technique of simultaneously acquiring deep reflection profiles at sea and long-range refraction profiles using three-component land stations, the BABEL seismic experiment also recorded P- and S-wave energy from the large airgun array at four linear 38-component land spreads. The pioneering nature of this work led, in hindsight, to several deficiences in design, but also pointed the way towards substantial advances in analysis and interpretation of integrated seismic observations. The multifold nature of the recordings at offsets providing pre-critical reflections from Moho depths resulted in interleaved receiver gathers with trace spacings of 4–5 m, thus providing some of the most densely spaced deep-seismic wide-angle sections to date. With such a trace density, the signal-to-noise ratio was substantially increased by summation of adjacent traces, without degrading spatial resolution. Stacking the land array data using conventional common midpoint (CMP) gather techniques over source-receiver offsets of 7–78 km doubled the depth range constrained by numerically precise stacking velocity analysis. Offsets > 10 km provide normal moveouts > 1 s, although the 1.5 km of differential moveout available here limited velocity discrimination and required additional constraints to define zero-offset traveltimes. For one array, stacking velocity errors of ±200 m s−1 imposed a resulting error in zero-offset time of up to ±400 ms without use of any marine data. With receiver response functions matched, stacked sections using data from geophone arrays exhibit similar features to stacks made using hydrophone arrays at 0–3 km offsets in the same area, but also significant differences. The differences in reflection amplitudes cannot be simply attributed to theoretical amplitude-offset variations and probably represent variation in the reflectivity of the crustal rocks of the Baltic shield.