Wide-angle Seismic Reflection Data Research Articles

The lithosphere–asthenosphere boundary structure at 11–21 Ma from wide-angle seismic data in the equatorial Atlantic Ocean

SUMMARYThe lithosphere–asthenosphere boundary (LAB) separates the rigid lithospheric plate above with the ductile and convective asthenosphere below and plays a fundamental role in plate tectonic processes. The LAB has been imaged using passive geophysical methods, but these methods only provide low-resolution images. Recently, seismic reflection imaging method has provided high-resolution images of the LAB, but imaging of the LAB at younger ages has been difficult. Here, we present the image of the LAB using wide-angle seismic reflection data covering 11–21 Ma old lithosphere in the equatorial Atlantic Ocean. Using ocean bottom seismometers (OBSs), we have observed wide-angle reflections between 150 and 400 km offsets along with crustal and mantle refraction arrivals. We first performed traveltime tomography to obtain the velocity in the crust and upper mantle. The Pn arrivals provide the information about P-wave velocity down to 4 km below the Moho. The disappearance of Pn arrivals beyond 130 km offset suggests that vertical P-wave velocity gradient is negligible or negative below this depth. We extended these velocities down to 90 km depth and then applied two imaging techniques to wide-angle reflection data, namely traveltime mapping of picked reflection arrivals and pre-stack depth migration of full wavefield data. We find that these reflections originate between 34 and 67 km depth, possibly from the LAB system. We have carried out extensive modelling to show that these reflections are real and not artefacts of imaging. Comparison of our results with coincident passive seismological and magnetotelluric results suggests that wide-angle imaging technique can be successfully used to study the lithosphere and the LAB system. We find that the LAB gradually deepens with age, but becomes very deep at 17–19 Ma, which we interpret to be due to the anomalous geology along this part of the profile.

Wide-angle seismic reflections reveal a lithosphere-asthenosphere boundary zone in the subducting Pacific Plate, New Zealand.

New wide-angle seismic reflection data from offshore New Zealand show that the lithosphere-asthenosphere boundary (LAB) is more structured than previously thought. Three distinct layers are interpreted within a 10- to 12-km-thick LAB zone beginning at a depth of ≈70 km: a 3 (±1)–km-thick layer at the bottom of the lithosphere with a P-wave (VP) azimuthal anisotropy of 14 to 17% and fast azimuth subparallel to the direction of absolute plate motion and a 9 (±2)–km-thick, low VP channel with a P-wave–to–S-wave velocity ratio (VP/VS) of >2.8 in the upper 7 km of the channel and 1.8 to 2.6 in the lower 2 km of the channel. The high VP/VS ratios indicate that this channel may contain 3 to 20% partial melt that facilitates decoupling of the lithosphere from the asthenosphere and reduces resistance for plate motion. Furthermore, the strong azimuthal anisotropy above the low-velocity layer suggests localization of strain due to melt accumulation.

Reflection and transmission coefficients of spherical waves at an interface separating two dissimilar viscoelastic solids

SUMMARY Spherical wave reflection and transmission (R/T) coefficients at an interface are not only of theoretical significance but also play an important role in the amplitude variation with offset (AVO) analysis of wide-angle reflection seismic data and cross-borehole surveys. For sources close to the interface the resulting wavefields cannot be adequately described in terms of a single incident plane wave. Rather, the spherical waves must be viewed as the superposition of an infinite number of plane waves. Moreover, the R/T coefficients for each individual plane wave in viscoelastic media have proven to be more complicated than expected due to the difficulty in selecting the correct vertical slowness. In such attenuating media the R/T coefficients cannot be properly determined by simply replacing the real elastic parameters with their complex viscoelastic counterparts. In this study, the reflection and transmission coefficients of spherical waves at a plane interface separating two dissimilar viscoelastic solids are rigorously investigated. The difficulty in selecting the vertical slowness is shown to be circumvented if the spherical wavefields are calculated from the plane wavefields using the Sommerfeld integral appropriate for the dissipative materials. However, some resulting phase curves of the complex spherical wave R/T coefficients tend to be of opposite sign to the corresponding phase curves of plane waves due to non-uniqueness of the latter for post-critical wave incidence. In this contribution we propose a new definition of spherical wave R/T coefficients for viscoelastic media which differs from the conventional one. Its advantages are that it is not explicitly expressed as a function of the R/T angles, it is valid for both P and S waves, yet it is consistent with the existing definitions of spherical wave R/T coefficients but is more robust. By way of examples we compute both spherical wave reflection coefficients (SWRC) and spherical wave transmission coefficients (SWTC) for two different viscoelastic models. Unlike plane waves, both the SWRC and the SWTC of converted PS waves are found to be non-zero at vertical incidence and may be drastically affected by the existence of longitudinal PS waves which are confirmed by full waveform calculations for the converted PS waves.

The extent of continental material in oceans: C-Blocks and the Laxmi Basin example

SUMMARYWe propose a tectonic interpretation for the outer-SDRs (SDRs: Seaward-Dipping Reflectors) and Pannikar central ridge in the aborted Laxmi Basin west of India from wide-angle seismic reflection data. The outer-SDRs comprise syn-tectonic extrusives (lavas and/or volcaniclastics) emplaced above passively exhumed mid-to-lower mafic crust of continental origin. They erupted following sudden lithosphere weakening associated with isolation of a continental block (a ‘C-Block’). Continuous magmatic addition during crustal extension allowed stretching of the lower crust whilst maintaining constant or even increasing thickness. A similar process occurred at both conjugate margins allowing bulk, pure-shear plate separation and formation of linear magnetic anomalies. The Laxmi example can explain enigmatic features observed in mature oceans such as presence of distal buoyant plateaus of thick continental crust away from the margins.

Seismogenic structure in the source zone of the 1918 M7.5 NanAo earthquake in the northern South China Sea

As a typical rifted margin, the coastal region of the northern South China Sea has experienced many large crustal earthquakes in the past 400 years. For example, two destructive earthquakes with ~M7.0 in 1600 and ~M7.5 in 1918 occurred close to the NanAo Island. However, the mechanism of these earthquakes and seismotectonics of the region are still unclear. In this work we make the first study of the detailed three-dimensional crustal structure in the source zone of the 1918 NanAo earthquake (M7.5) using newly acquired wide-angle seismic reflection data. Our results show that current seismicity mostly occurred in a midcrustal low-velocity zone or its nearby high-to-low velocity transitional areas. We ascertain locations of the ENE-strike Littoral fault zone and the NW-strike Huanggangshui fault based on the seismic reflection profiles. These faults have caused a large-scale vertical dislocation of the sedimentary basement. The structure of the upper crust is very heterogeneous, containing many high or low velocity anomalies. However, the lower crustal structure is less heterogeneous, mainly characterized by low velocity mostly in the northern part of the study area and high velocity mainly in the southern part. The crust in the seismicity zone shows strong heterogeneity, reflecting highly cracked structural features possibly caused by destructive earthquakes. We infer that the midcrustal low-velocity zone and the intersection between the Littoral fault zone and the Huanggangshui fault could reflect weak structures in the crust and form a local stress buildup, which strongly affected or controlled the generation of intraplate seismicity. The 1918 NanAo earthquake very likely occurred within the current seismicity zone with a depth range of 15–25 km estimated from our tomographic images and previous geological and geophysical results.

Accurate Jacobian Matrix Using the Exact Zoeppritz Equations and Effects on the Inversion of Reservoir Properties in Porous Media

The analysis of amplitude variation with offset (AVO) plays a significant role in fluid detection and lithology discrimination in hydrocarbon reservoirs. The Zoeppritz equations are part of the basic theory of AVO analysis which describes the relationship between seismic reflection and transmission coefficients and elastic rock properties (e.g., P- and S-wave velocities and density). Currently, most AVO inversion methods are based on approximations of the exact Zoeppritz equations, which not only limit the accuracy of AVO inversion, but also restrict its application to wide-angle seismic reflection data. In addition, the most difficult part of linear AVO inversion obtaining an accurate Jacobian matrix (partial derivatives of reflection coefficients with respect to inverted parameters). Based on our previous study on the accurate gradient calculation of seismic reflection coefficients for the inversion of rock properties, we further combine the exact Zoeppritz equations with Biot–Gassmann equations to compute the gradients of seismic reflection coefficients with solid density and reservoir properties (e.g., porosity, water/gas/oil saturations) in porous media. In this paper, the partial derivative expressions of the Zoeppritz matrix elements with respect to solid density and reservoir properties are simplified to simple algebraic equations, which are readily calculated. By comparing reflection coefficients and partial derivative curves with those obtained by classic Shuey and Aki–Richards approximations, we show that our proposed method can be used to accurately obtain reservoir properties in AVO inversion.

Density structure of the crust in the Emeishan large igneous province revealed by the Lijiang- Guiyang gravity profile

The Emeishan large igneous province (hereafter named by its acronym ELIP) is the first accepted large igneous region in China. The current study tries to reconstruct the density structure of the crust in this region. For this purpose, we conducted the gravity survey along an 800-km-long profile, which stretched laterally along the latitude 27°N from Lijiang (Yunnan province) to Guiyang (Guizhou province). The fieldwork included 338 gravity measurements distributed from the inner zone to the outer zone of the mantle plume head. After a series of gravity reductions, we calculated the Bouguer gravity anomaly and then constructed the density model for ELIP by iterative forward modeling from an initial density model tightly constrained by wide-angle seismic reflection data. The topography of the Moho, here physically interpreted as a density discontinuity of ~0.4 g·cm–3, gradually rises from the inner zone (~50 km deep) to the outer zone (~40 km), describes a thicker crust in the inner zone than in any other segment of the profile and largely reproduces the shape of the Bouguer gravity anomaly curve. Both the Bouguer gravity and the density structure show significant differences with respect to the inner zone and the other two zones of ELIP according to the commonly accepted partition of the Emeishan area. A thicker and denser middle-lower crust seems to be the main feature of the western section of the profile, which is likely related to its mafic magmatic composition due to magmatic underplating of the Permian mantle plume.

Thinned continental crust intruded by volcanics beneath the northern Bay of Bengal

Since the early Cretaceous, the Bay of Bengal was formed during rifting between India and Antarctica and then by subsequent seafloor spreading. The nature of the crust underlying the Bay of Bengal is oceanic south of 15°N, but remains unknown (thinned continental crust, serpentinized mantle or oceanic crust) north of this limit. In order to better define the nature of the crust in the northern Bay of Bengal, three wide-angle reflection seismic and refraction profiles were acquired during the multichannel seismic reflection Bengal cruise. Nine ocean-bottom seismometers were deployed alternatively on three profiles. A seismic source consisting of 64 air guns with a volume of 6180 in3 was used simultaneously with a 10.05-km long streamer to acquire both seismic reflection and refraction data. Tomographic and forward modelings of the three refraction profiles reveal a 20-km thick crust north of the Bengal delta front beneath a minimum of 13 km thick sedimentary cover. The crust thins to about 10 km immediately south of the EW trending delta front and the thickness of sediments reaches a minimum of 7 km. Crustal velocities and velocity gradients are consistent with a continental origin of the crust in this area. At the base of the crust, high seismic velocities (>7.2 km/s) are interpreted as magmatic underplating. Wide-angle seismic reflection and refraction data cannot resolve the nature of the upper 4–5 km of crust (oceanic crust, exhumed mantle or thinned continental crust). But coincident seismic reflection profiles show the emplacement of a volcanic intrusion, sills and some seaward dipping reflectors (SDRs) located close to the northern prolongation of the Ninety East ridge before 70 Ma (Maastrichtian). However, most of the fan-shaped reflectors identified in the northern Bay of Bengal are synrift features. We conclude that the crust in the northern Bay of Bengal is thinned continental crust intruded by volcanic products with the presence of a minor amount of underplating material at its base. Such a crustal structure probably extends from the northern Bay of Bengal (19°N) to the Shillong Plateau (25°N). These new findings are critical for the oil and gas exploration presently very active in the northern Bay of Bengal area.

The structure and nature of the Moho beneath Central Iberia

The Moho is the most conspicuous feature in the ALCUDIA deep normal incidence and wide-angle seismic reflection datasets acquired across the Central Iberia Zone. This discontinuity appears to be sub-horizontal with a slight dip toward the center of the Iberian Peninsula, beneath the Tajo Basin. Densely spaced wide-angle seismic reflection data reveal conspicuous PmP and SmS reflections in the shot records. These reflections, arriving over a 1.5s time window and characterized by a relatively low frequency content, are used to infer the internal structure and characteristics of this crustal boundary in an effort to place constraints on its nature beneath the studied area. The wide-angle shot records were processed in order to generate low-fold stacks that reveal gradual deepening of Moho PmP phases from 10.2s TWT (~31km) in the Central Iberian Zone to 11.8s TWT (~36km) beneath the Tajo Basin. The Moho seismic signature was the main objective of a series of 1-D and 2-D synthetic modeling simulations. Reflectivity modeling was used as the 1-D modeling approach and an elastic finite difference scheme was used for the 2-D modeling. The Moho can be considered to be a self-similar/fractal heterogeneous media with a bimodal velocity distribution, alternating crustal and mantle values, with horizontal correlation lengths that range between 1 and 4km and vertical correlation lengths that range from 0.2 to 1km. The depth of this boundary is most probably the result of late to post-variscan re-equilibration processes affecting a previously deformed and laminated lower crust. The mechanisms that took part in this process might have differed to the north and south of the sampled Central Iberian Zone, being more thermally induced to the north, near the Central System. Processes leading to the layered structure of the Moho boundary, either tectonic or magmatic, should be further investigated.

New numerical and theoretical model to characterize the upper crustal structure of the Moroccan atlas from wide-angle seismic reflection data

SIMA was funded by a grant from the Spanish Science Foundation (FECYT), involves several institutions like ICTJA-CSIC, Salamanca University and Autonomous University of Barcelona, the Scientific Institute of Rabat, University Cadi Ayyad in Marrakech, FST of Errachidia, University Sidi Mohammed ben Abdellah in Fes, and was supported as part of PICASSO by grant EAR 0808939 from the NSF Continental Dynamics Program

Seismic structure of the Central Tyrrhenian basin: Geophysical constraints on the nature of the main crustal domains

AbstractIn this work we investigate the crustal and tectonic structures of the Central Tyrrhenian back‐arc basin combining refraction and wide‐angle reflection seismic (WAS), gravity, and multichannel seismic (MCS) reflection data, acquired during the MEDOC (MEDiterráneo OCcidental)‐2010 survey along a transect crossing the entire basin from Sardinia to Campania at 40°N. The results presented include a ~450 km long 2‐D P wave velocity model, obtained by the traveltime inversion of the WAS data, a coincident density model, and a MCS poststack time‐migrated profile. We interpret three basement domains with different petrological affinity along the transect based on the comparison of velocity and velocity‐derived density models with existing compilations for continental crust, oceanic crust, and exhumed mantle. The first domain includes the continental crust of Sardinia and the conjugate Campania margin. In the Sardinia margin, extension has thinned the crust from ~20 km under the coastline to ~13 km ~60 km seaward. Similarly, the Campania margin is also affected by strong extensional deformation. The second domain, under the Cornaglia Terrace and its conjugate Campania Terrace, appears to be oceanic in nature. However, it shows differences with respect to the reference Atlantic oceanic crust and agrees with that generated in back‐arc oceanic settings. The velocities‐depth relationships and lack of Moho reflections in seismic records of the third domain (i.e., the Magnaghi and Vavilov basins) support a basement fundamentally made of mantle rocks. The large seamounts of the third domain (e.g., Vavilov) are underlain by 10–20 km wide, relatively low‐velocity anomalies interpreted as magmatic bodies locally intruding the mantle.

Study on the limitations of travel-time inversion applied to sub-basalt imaging

Abstract. The difficulties of seismic imaging beneath high velocity structures are widely recognised. In this setting, theoretical analysis of synthetic wide-angle seismic reflection data indicates that velocity models are not well constrained. A two-dimensional velocity model was built to simulate a simplified structural geometry given by a basaltic wedge placed within a sedimentary sequence. This model reproduces the geological setting in areas of special interest for the oil industry as the Faroe-Shetland Basin. A wide-angle synthetic dataset was calculated on this model using an elastic finite difference scheme. This dataset provided travel times for tomographic inversions. Results show that the original model can not be completely resolved without considering additional information. The resolution of nonlinear inversions lacks a functional mathematical relationship, therefore, statistical approaches are required. Stochastic tests based on Metropolis techniques support the need of additional information to properly resolve sub-basalt structures.

Shifted-elliptical nonstretch moveout correction of wide-angle seismic data in the τ-p domain, using an example from the Faeroe-Shetland Basin

We developed a method of moveout correction in the [Formula: see text] domain to tackle some of the problems associated with processing wide-angle seismic reflection data, including residual moveout and normal-moveout stretching. We evaluated the concept of the shifted ellipse in the [Formula: see text] domain as an alternative to the well-known concept of the shifted hyperbola in the [Formula: see text] domain. We used this shifted-ellipse concept to address the problem of residual moveout caused by vertical heterogeneity in the subsurface. We also addressed the stretching problem associated with dynamic corrections by combining selected strips from a set of constant-moveout stacks generated using a shifted-ellipse equation. Application of this method to a wide-angle data set from the Faeroe-Shetland Basin provided an enhanced image of the subbasalt structure.

Abrupt change in the dip of the subducting plate beneath north Chile

No large tsunamigenic earthquake has occurred in north Chile since 1877 and the region has been largely recognized as a mature seismic gap1, 2, 3, 4, 5, 6, 7, 8, 9. At the southern end of the seismic gap, the 2007 Mw 7.7 Tocopilla earthquake ruptured the deeper seismogenic interface, whereas the coupled upper interface remained unbroken4, 6, 7. Seismological studies onshore show a gently varying dip of 20° to 30° of the downgoing Nazca plate3, 6, which extends from the trench down to depths of 40–50 km. Here, we study the lithospheric structure of the subduction zone of north Chile at about 22° S, using wide-angle seismic refraction and reflection data from land and sea, complemented by hypocentre data recorded during the 2007 Tocopilla aftershocks7. Our data document an abrupt increase in the dip of the subducting plate, from less than 10° to about 22°, at a depth of approximately 20 km. The distribution of the 2007 aftershocks indicates that the change in dip acted as a barrier for the propagation of the 2007 earthquake towards the trench, which, in turn, indicates that the subduction megathrust is not only segmented along the trench, but also in the direction of the dip. We propose that large-magnitude tsunamigenic earthquakes must cross the barrier and rupture the entire seismogenic zone.

Geophysical model of the lithosphere across the Variscan Belt of SW-Iberia: Multidisciplinary assessment

A multidisciplinary geophysical study along a large seismic transect in the SW-Iberian Peninsula has been carried out. This study integrates the crustal structure, geometry and composition obtained from normal incidence and wide-angle seismic reflection data with other observables (geoid, gravity and topography). The internal architecture of the lithosphere across the Variscan Orogen of SW-Iberia is constrained by the 300 km long high resolution deep normal seismic reflection IBERSEIS Transect. The most prominent feature imaged by the seismic survey is the Iberseis Reflective Body (IRB), a 140 km long high amplitude reflective body located in the middle crust of the northern half of the transect. The seismic velocity (Vp) distribution within the crust and the upper mantle is constrained by two wide-angle seismic transects acquired in the same area. The velocity models show a complex crust, with a specially complex middle crust, which features higher velocities than the average continental crust. Also the wide-angle data revealed that the IRB is characterized by high velocities. This feature was then interpreted as sill-like structure built up by a series of mafic intrusions. Therefore, a key issue is to study if this relatively mafic crust is consistent with other geophysical observables. Based on the velocity models, two lithospheric density models have been derived along the IBERSEIS wide-angle transects. The geoid, gravity and topography response of these models have been calculated using a finite elements code that solves, simultaneously, the geopotential, lithostatic, and heat flow equations. The resulting values are then compared with the measured observables and the crustal and lithospheric mantle geometry and density is modified until the best fit is obtained. The initial density models calculated from the seismic data adjust quite well to the real potential field data. However, minor modifications have been required in order to properly fit the observables. The final density models are consistent with the existence of relatively high density bodies in the mid-crust providing further support to the seismic interpretation. In addition, they place new constraints on the location of the lithosphere–asthenosphere boundary and on the tectonic evolution of SW-Iberia.

Paleoproterozoic subduction in northwestern Canada from near-vertical and wide-angle seismic reflection data

Near-vertical incidence and refraction – wide-angle reflection seismic data, recorded as part of Lithoprobe studies in the Paleoproterozoic–Archean domains of Canada’s Northwest Territories, show remarkable reflections from within the upper mantle. A parallel pair of reflectors imaged by the near-vertical data can be traced from Moho levels (∼33 km) down to ∼70 km depth. In a previous study, the reflectors were interpreted as the top and bottom of an ∼1.8 Ga subducted oceanic crust beneath the Hottah terrane. Further inboard, where the seismic line changes its direction from east–west to nearly north–south, another pair of reflectors extends subhorizontally for about 100 km at ∼70 km depth before dipping downward. The subhorizontal reflectors were not correlated with the dipping slab; instead they were interpreted as a separate feature. However, they roughly coincide with a horizontal interface modeled from wide-angle data by an earlier study. Considering the crooked line acquisition geometry, we re-examined both near-vertical incidence and wide-angle reflection data using 2-dimensional (2-D) and 3-D forward and inverse modeling algorithms. Our results demonstrate that the subhorizontal reflectors are the continuation of the relict subducted slab, which now extends laterally for 300 km. Its base is the source of the wide-angle data. The apparent flattening for the near-vertical data is most likely an artifact of projecting a 3-D geometry onto a 2-D cross section. The shallowly subducted slab probably contributed to the thickening and stabilization of the subcrustal lithosphere below the Wopmay orogen.

Estimation of elastic contrasts in a layered model from seismicPS-to-PPamplitude ratios

SUMMARY We present a method to estimate seismic velocity and density contrasts at a given interface in a 1-D layered model using PS-to-PP reflection amplitude ratios. The velocity structure above the reflector is constrained by traveltime modelling, and the amplitude ratios are determined using the same source–receiver pair for the measured PP and PS amplitudes (commonoffset geometry). Thereby, source and receiver site effects are cancelled, and the remaining propagation effects are included in the ray theoretical forward modelling of theoretical PS-toPP ratios. A minimization of the least-squares misfit between observed and modelled ratios provides the remaining elastic parameters below the reflector of interest (P velocity, P-to-S velocity ratio, density). 1-D examples and a 2-D synthetic case study with a dipping reflector and a laterally varying overburden demonstrate the possibilities and limitations of the method. An application of the method to a 0.6 km deep reflector below the Campi Flegrei caldera, Italy, reveals a strong contrast with a P-velocity increase from less than 2 to 3.5 km s −1 and a decrease of the P-to-S velocity ratio from 3.6 to 1.75. The proposed PS-to-PP amplitude ratio analysis is applicable for wide-angle seismic reflection data, especially when strong elastic parameter contrasts are expected and when source amplitudes or site effects are poorly known.

Interactive analysis tools for the wide-angle seismic data for crustal structure study (Technical Report)

The analysis of wide-angle seismic reflection and refraction data plays an important role in lithospheric-scale crustal structure study. However, it is extremely difficult to develop an appropriate velocity structure model directly from the observed data, and we have to improve the structure model step by step, because the crustal structure analysis is an intrinsically non-linear problem. There are several subjective processes in wide-angle crustal structure modelling, such as phase identification and trial-and-error forward modelling. Because these subjective processes in wide-angle data analysis reduce the uniqueness and credibility of the resultant models, it is important to reduce subjectivity in the analysis procedure. From this point of view, we describe two software tools, PASTEUP and MODELING, to be used for developing crustal structure models.PASTEUP is an interactive application that facilitates the plotting of record sections, analysis of wide-angle seismic data, and picking of phases. PASTEUP is equipped with various filters and analysis functions to enhance signal-to-noise ratio and to help phase identification. MODELING is an interactive application for editing velocity models, and ray-tracing. Synthetic traveltimes computed by the MODELING application can be directly compared with the observed waveforms in the PASTEUP application. This reduces subjectivity in crustal structure modelling because traveltime picking, which is one of the most subjective process in the crustal structure analysis, is not required. MODELING can convert an editable layered structure model into two-way traveltimes which can be compared with time-sections of Multi Channel Seismic (MCS) reflection data. Direct comparison between the structure model of wide-angle data with the reflection data will give the model more credibility. In addition, both PASTEUP and MODELING are efficient tools for handling a large dataset. These software tools help us develop more plausible lithospheric-scale structure models using wide-angle seismic data.

Lessons from a joint interpretation of vibroseis wide-angle and near-vertical reflection data in the northeastern Yilgarn, Western Australia

A wide-angle reflection seismic experiment was carried out in the Eastern Goldfields granite–greenstone terrane of the Archaean Yilgarn Craton during 2001. This was the first time in Australia that wide-angle data were collected using a vibrator source and with a high density of observations. Unlike other wide-angle surveys carried out in other parts of the world, our survey used both a smaller number of sweeps, and shorter sweeps. We recorded three sweeps (each with its own frequency range) at each vibration point. The experiment demonstrated that the sum of three 12 s sweeps using 3 large vibrators provides enough energy to record signal at offsets up to up to 60–70 km. A comparison of individual shot gathers from near-vertical data and receiver gathers from wide-angle data demonstrated higher reflectivity in near-vertical data. This may be due to differences in the frequency bands of the recording equipment. The after stack section obtained from dense wide-angle data is different from that obtained from conventional near-vertical reflection data. The conventional reflection section provides higher quality image of the crust compared to the wide-angle section. This could be explained by the low-fold in wide-angle data and differences in the acquisition and processing methodology. The wide-angle survey, which was coincident with a regional vibroseis seismic reflection transect, was focused on the Leonora–Laverton region. The survey was designed to supplement the deep seismic reflection studies with velocity information. This also created an opportunity to compare velocity model derived from wide-angle reflection seismic data with a structural image obtained from the deep common mid-point seismic reflection data, and thus refine our geological understanding of the area. A high velocity body reaching a maximum thickness of 2 km was identified exclusively from the seismic velocity model derived from wide-angle study. This body is interpreted as mafic rocks within the Archaean Granite–Greenstone Belt. The joint interpretation also shows that structural boundaries do not always follow lithological boundaries in our study area. The combination of wide-angle reflection and near-vertical reflection data has facilitated a more complete geological interpretation of the seismic data.

Seismic stratigraphy of the southern Rockall Basin: a comparison between wide-angle seismic and normal incidence reflection data

A wide-angle reflection/refraction profile from the RAPIDS 3 (Rockall and Porcupine Irish Deep Seismic) experiment is compared to a coincident high-quality normal incidence reflection line, both of which traverse the southern part of the Rockall Basin on the Atlantic continental margin, offshore Ireland. The positions and geometries of the independently-derived layers show a generally close comparison between the two data sets. The normal incidence seismic reflection data show a greater degree of detail on seismic facies while the wide-angle seismic profile displays greater penetration of the entire sedimentary succession and of the crust. The wide-angle geometries and the normal incidence seismic stratigraphy provide complementary data that help constrain the geology and evolution of the lightly explored Rockall region. A seismic stratigraphy is presented, based on correlation between the wide-angle and normal incidence data, shallow seismic site surveys, shallow borehole cores and tectonostratigraphic studies. Within the central part of the Rockall Basin, the sedimentary layers are generally flat-lying and have a maximum thickness of 7 km. The succession is interpreted as Jurassic to Recent in age. Some of the layers are confined to the basin whilst others extend onto the Rockall and Porcupine highs. Marginal basins at the western (Conall Basin) and eastern (Macdara Basin) edges of the Rockall Basin contain up to 4 km of pre-Tertiary sedimentary rocks, some possibly as old as Permo-Triassic.