Wide-angle Seismic Reflection Data Research Articles

VARNET MT: Interpretation of Magnetotelluric Data over the Killarney–Mallow Fault Zone in South-West Ireland

The south-west of Ireland is an area in the European Variscides where the surface geology can provide constraints on geophysical models of deep Variscan (and Caledonian) tectonic structures. The VARNET (VARiscan NETwork) project has acquired new wide-angle seismic reflection, magnetotelluric (MT) and gravity data collected in south-west Ireland in order to integrate these with surface geological observations and tectonic models from Ireland and other parts of the European Variscides. This paper is concerned with the interpretation of VARNET MT data collected across the major tectonic structure seen at the surface: the Killarney-Mallow Fault Zone. The MT data have identified a near-surface conductive zone associated with the east-west-trending Killarney-Mallow Fault Zone. The inversion of these data along three profiles across the Killarney-Mallow Fault Zone and the eastern end of Dingle Bay indicates the presence of steep, probably north-dipping structures, extending to depths of at least 2km. There is also prima facie MT evidence for a steep structure near the Cork-Kenmare Line and a steep, north-dipping structure associated with the Dingle Bay-Galtee Mountains Line. The locations of these structures appear to be consistent with those interpreted from the wide-angle seismic reflection data.

Varnet MT: Interpretation of Magnetotelluric Data over the Killarney-Mallow Fault Zone in South-West Ireland

The south-west of Ireland is an area in the European Variscides where the surface geology can provide constraints on geophysical models of deep Variscan (and Caledonian) tectonic structures. The VARNET (VARiscan NETwork) project has acquired new wide-angle seismic reflection, magnetotelluric (MT) and gravity data collected in south-west Ireland in order to integrate these with surface geological observations and tectonic models from Ireland and other parts of the European Variscides. This paper is concerned with the interpretation of VARNET MT data collected across the major tectonic structure seen at the surface: the Killarney-Mallow Fault Zone. The MT data have identified a near-surface conductive zone associated with the east-west-trending Killarney-Mallow Fault Zone. The inversion of these data along three profiles across the Killarney-Mallow Fault Zone and the eastern end of Dingle Bay indicates the presence of steep, probably north-dipping structures, extending to depths of at least 2km. There is also prima facie MT evidence for a steep structure near the Cork-Kenmare Line and a steep, north-dipping structure associated with the Dingle Bay-Galtee Mountains Line. The locations of these structures appear to be consistent with those interpreted from the wide-angle seismic reflection data.

Modelling and imaging the Moho transition: the case of the southern Urals

Synthetic seismic modelling is used to test different possible structures and forming processes of the lower crust and Moho. One and two dimensional synthetic seismic modelling of the Moho reflection is carried out to constrain the internal structure of a crust–mantle transition. A reflectivity algorithm was used to calculate the synthetic seismograms for models of the crust–mantle transition consisting of gradient zones and layered sequences. The synthetic seismograms for models consisting of laterally variable velocity structure were calculated by an explicit finite difference wave equation algorithm. The laterally heterogeneous transition models can be achieved by assuming a Moho with different degrees of variable topography or laterally discontinuous layering. The effects of a low velocity surface cover on the seismic signature of deep signals is also analysed. The seismic modelling coupled with stacked images of wide-angle seismic reflection data constrain the case history of the Moho transition beneath the southern Urals. A smooth gradational crust–mantle transition (gradient velocity–depth function) does not predict the seismic features observed in the shot records. The coda of the PmP and the amplitude behaviour of this phase with offset are modelled with a 6 km thick transition zone which consists of approximately 600 m thick, laterally variable layers with a bimodal velocity distribution. The horizontal component stacks of the wide-angle data feature arcuate events suggesting boudin like structures or a boundary with topographic relief. High amplitude sub-Moho reflections support that the structural complexity of the crust–mantle transition can be followed to the upper mantle suggesting eastward dipping structures indicating, either, remnants of an old subduction zone, trapped crustal material within the mantle, or active crust–mantle interactions.

Lithospheric structure north of Scothland--I. P-wave modelling deep reflection profiles and gravity

The W-reflector and the Flannan reflector are two enigmatic planar structures in the lithospheric mantle north of Scotland, mapped in three dimensions using deep reflection profiling. The Flannan reflector dips eastwards from the Moho to a depth of 80 km at an angle of ∼30°, whilst the W-reflector is subhorizontal, lying 10–20 km beneath the Moho and terminating westwards against the Flannan. Both reflectors have a strong impedance contrast and are laterally coherent over tens of kilometres. Wide-angle seismic reflection and refraction data are used here to constrain the P-wave velocity structure of the lithosphere in this region and to investigate the physical properties and geometry of these reflective structures. We report the results from an integrated seismic survey in which a series of explosive shots fired at sea were recorded by ocean bottom seismographs, land stations and a streamer towed by a second ship in an expanding spread configuration. A well resolved P-wave velocity structure for the lithosphere above the mantle reflectors is derived by inverting the seismic traveltimes to optimize the fit of the model to the data, and by generating synthetic seismograms from the model to match the relative amplitudes of the observed seismic phases. A high-amplitude post-critical reflection indicates the existence of a discrete reflecting interface dipping gently westwards between 40 and 50 km depth in the lithospheric mantle. The P-wave velocity model is converted to two-way reflection time and correlated with normal-incidence reflection data: the Moho structure agrees well, and the reflecting interface defined by the wide-angle data is coincident with the subhorizontal W-reflector. Less distinct later arrivals observed in the data are consistent with modelled reflections from the Flannan reflector in the mantle and from the extrapolated position of the Outer Isles Fault in the lower crust. Modelling of the relative amplitudes and critical distances of the wide-angle mantle reflection shows that the reflecting layer must be at least 3 km thick and that it has considerably higher velocity (8.5 km s−1) and density (3.5 g cm−3) than normal mantle. Further modelling tests the vertical structure within the layer, and constrains the sharpness of the upper interface and whether the reflection amplitude could have been enhanced by internal layering. Gravity data measured over the region are consistent with the velocity model and with the high-density layer in the mantle, which is constrained by the gravity modelling to be less than 10 km thick. Published laboratory measurements of the physical properties of mantle rocks indicate that mafic eclogite with high velocity and density satisfies all the observations from the mantle layer. Our preferred hypothesis for the origin for such a layer is that it is a fragment of oceanic crust that has been subducted and metamorphosed to eclogite facies.

Acoustic Structure and Seismic Velocities in the Carnarvon Basin, Australian North West Shelf: Towards an Integrated Study

High quality refraction and wide-angle reflection seismic data recorded by ocean-bottom seismographs (OBSs) deployed by the Australian Geological Survey Organisation along the 700 km long transect in the Carnarvon Basin effectively supplement results obtained by means of the conventional reflection technology. Velocity information can now be derived from both CDP (nearvertical reflection) and OBS (refraction/wide-angle reflection) data. Generally, CDP-derived average velocities are lower than OBS-derived velocities and this deviation increases with depth: from ~0.1 km/s at 8 s two way time (TWT) to 0.8-1.6 km/s at 16 s TWT. If the CDP-derived velocities are used to depth convert reflection data, then depth to these TWTs would be underestimated by 0.4 to 6.4-12.8 km respectively. Some local anomalies (at ~6s TWT CDP-derived velocities may be more than 0.1 km/s higher than the OBS-derived velocities) distort this general trend. These would result in ~0.3 km local overestimates of the depth equivalent of 6s TWT. Co-analysis of the interval velocity field reconstructed from the travel time-based interpretation of the OBS data and the conventional reflection image of the crust in some cases shows their poor correlation. Prominent reflectivity seen in the conventional reflection data at depth 13-17 km does not correspond to any significant velocity increase imaged by refraction/wide-angle techniques. These reflections were taken in our pre-OBS interpretation to mark the top of the underplated layer in the crust. Velocity below that level estimated from the OBS data is in the range 5.7-6.4 km/s. This velocity range is considerably lower than anticipated for mafic rocks under appropriate PTconditions. Quite significant local velocity increase in the depth range 15-23 km does not produce high-amplitude near-vertical reflections. Only a combination of reflection and refraction/wideangle techniques provides a clue to a consistent geological interpretation of seismic data. Improved velocity estimation in the crust is crucial for accurate depth conversion of near-vertical reflection data.

Seismic velocities in the North West Shelf Region, Australia, From Near-vertical and Wide-angle Reflection and Refraction Studies

A significant limitation of conventional seismic reflection technology is that it poorly constrains seismic velocities in the deep part of the section. These velocities are needed for seismic migration, to convert from time to depth, and to estimate the likely rock composition at depth. Seismic observations at large offsets can compensate for this limitation. The Australian Geological Survey Organisation recorded high-quality refraction and wide-angle reflection seismic data using ocean-bottom seismographs (OBS) along 5 profiles on Australia’s North West Shelf, to maximum offsets of 300 km. The OBS transects coincide with previously acquired deep-crustal seismic-reflection profiles. Consequently, velocity information can now be derived from two independent data sets: common-depth point (near-vertical) reflection data and refraction/wide-angle reflection data. Residual (difference between these two velocity data sets) velocity images can be produced and analysed. Even within the top 5 km of the section residuals between the OBS- and CDP-derived interval velocities vary from -0.5 to +3.5 km/s and increase further with depth. Consequently, the depth equivalent of 4 s two-way time will vary by up to 1.2 km or 10%. The practical implications of co-processing of seismic velocity information are: improved seismic velocity estimation, more accurate depth migration and depth conversion of seismic data and less distortion of amplitudes and shapes of geological structures after depth conversion.

Structure of the lithospheric mantle beneath the Trans-Hudson Orogen, Canada

Refraction and wide-angle reflection seismic data collected in northern Saskatchewan and Manitoba in 1993 by LITHOPROBE provide new glimpses of the deep structure of the lithospheric mantle beneath the Trans-Hudson Orogen. Velocity models obtained through ray-tracing modelling techniques revealed the presence of complicated mantle structures down to a depth of 160 km beneath the orogen. Like the crust, the lithospheric mantle appears to show a seismically transparent upper and a highly reflective lower part, suggesting that the rheological environment responsible for development of reflections in the lithospheric mantle and of that in the crust are similar. This appears to be consistent with the observation that the strong and brittle upper 30 km of the mantle contain remnants of former tectonic processes in a form of velocity anisotropy, and the lower, more ductile zone seems highly reflective and isotropic. The suspected beginning of the lithosphere-asthenosphere transition zone at a depth of 160 km, imaged as gradual disappearance of mantle reflections, coincides well with models established using other geophysical methods.

Direct Calculation of Interval Velocities and Layer Thicknesses From Seismic Wide-Angle Reflection Times

SUMMARY Wide-angle seismic reflection traveltimes generated by large explosions are used extensively to obtain both velocity and regional structure of the crust and upper mantle. The method most commonly used to calculate interval velocities and thicknesses (layer parameters) of a multilayered earth. supposed to consist of homogeneous, isotropic and horizontal layers, is based on Dix’s ( 1955) interval velocity formula, which needs a prior estimate of the rms velocity and traveltimes at zero offset. Use of Dix’s hyperbolic approximation to the non-hyperbolic wide-angle reflection times causes large errors in the determination of layer parameters. Currently, no direct method exists to extract the velocity information from such wide-angle seismic data. Here we propose a layer-stripping method, using the ray parameter, to calculate the layer parameters directly in a vertically heterogeneous earth from a set of wide-angle seismic reflection data. Synthetic reflection times contaminated by some realistic errors have been used to demonstrate the efficiency and reliability of the algorithms. Field examples using well-identified wide-angle reflection times illustrate the practical feasibility of the proposed method.

The Edoras Bank margin; continental break-up in the presence of a mantle plume

Abstract Edoras Bank lies on the eastern continental margin of the North Atlantic about 900 km south of the present-day Iceland plume. We studied the continent-ocean transition of the rifted margin using normal-incidence and wide-angle seismic reflection data. Seaward dipping reflectors are well-developed over much of the margin and are underlain by a high velocity (7.3–7.5 km s −1 ) lower crust. This is interpreted as either igneous underplating or as existing crust heavily intruded at the time of break-up by melts with high MgO contents and suggests that the thermal anomaly associated with the Iceland plume was well-developed in this area at the time of break-up.

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.

Determination of Poisson's ratio from pre-critical wide-angle seismic reflection data from the BABEL project

The pre-critical wide-angle seismic reflection data collected during the BABEL survey using multireceiver spreads contain high-amplitude P-wave and mode-converted S-wave reflected arrivals. The two phases can be partially separated and their signal-to-noise ratio emphasized by exploiting the multifold nature and the increased sensitivity of velocity analysis afforded by these data to produce zero-offset-corrected stacked sections. Using these sections, a method has been developed to derive estimates of Poisson's ratio by optimizing the cross-correlation of P-wave and S-wave arrivals, and hence to constrain petrological models of the crust sampled by these data. Although the apparently high degree of temporal and spatial resolution of reflected arrivals that are visible within the stacked sections must be reduced to achieve a satisfactory degree of correlation robustness, the results shed some light on crustal petrology, and concur with existing estimates of Poisson's ratio for the Baltic shield.

Thrusting of the central California margin over the edge of the Pacific plate during the transform regime

Wide-angle seismic reflection and refraction data, mapped geologic structures, and marine magnetic anomalies provide evidence that oceanic crust underlies the coastal margin of California from Morro Bay northward beyond San Francisco. We interpret the presence of this oceanic slab to be at least partly the result of thrusting of North America over the Pacific plate during Pliocene to Quaternary time, well after the principal subduction regime ended. Likely causes of this thrusting are the change in plate motions at about 3.5 Ma and extension in the Basin and Range province.

A gravity model of the crust beneath the Tertiary Piemonte basin (northwestern Italy)

A gravity model of a crustal section crossing the Tertiary Piemonte basin from north to south is presented. It was constructed by combining geological data and the results of the application of several geophysical methods, with particular reference to the seismic findings. A model is proposed for the morphology of the deep crust (crust/mantle transition) identified by the wide angle reflection seismic data from the European Geotraverse (EGT) project and its geological consequences are discussed. The existence of substantial displacements at the crust/mantle boundary is stressed. They do not appear to be directly related to superficial structures but underscore the wedging of the Ligurian mantle on the Insubrian crust. Compared with the observed Bouguer gravity field, gravity modelling of the section has supplied a calculated anomaly with a persistent short wavelength negative residual anomaly curve in the area of the Asti basin. It is suggested that this anomaly is due to a wedge of sedimentary rocks within the crust. The wedge consists of both Apennine and Padane units and is located along a major structural boundary where metamorphic crust and Tertiary sediments belonging to the Alpine domain overthrust Apennine and Padane units during the Tertiary

Migration of wide-angle seismic reflection data from the Grenville front in Lake Huron

Migration of wide-angle seismic reflection data from the Grenville front in Lake Huron

Mapping the Moho of the Western Alps by wide-angle reflection seismics

A new picture of the Moho beneath the Western Alps could be obtained using wide-angle reflection seismic data. This picture, in addition to its increased sharpness, clearly shows how the Moho deepens down to the root zone of the chain (55 km). A 25–30 km deep reflector with upper mantle characteristics is discovered under the Briançonnais zone where it overlies the autochthonous Moho. This unit does not seem to connect to the so-called “Ivrea body”, situated much shallower and farther into the chain. It supports a hypothetical flaking of the European lithosphere beneath the Western Alps. Farther east, the Moho beneath the Po Plain is clearly mapped too, with a crustal thickness increasing in steps from 25 to 35 km. This P-wave Moho picture is compared with an S-wave picture, and major differences appearing in the middle crust are discussed. Finally, the seismic response of the Moho is investigated, and is shown to be shifted towards low frequencies in the inner zones of the chain. This could partly explain why the ECORS-CROP vertical seismic line, equipped with high-frequency geophones, faltered over the identification of the deep Alpine Moho.

A seismic investigation of the crustal and upper mantle structure within the Central Volcanic Region of New Zealand

Abstract The structure of the crust and upper mantle of the Central Volcanic Region, New Zealand, are deduced from long-range seismic refraction and wide-angle seismic reflection data. Three shots were detonated in the Bay of Plenty and one in Lake Taupo; seismographs were located at approximately 10 km intervals between these shots. The geometry of the experiment was such that there was a partially reversed coverage for the crustal phases, but correlatable upper mantle phases were only recorded from the Bay of Plenty shots. A nondipping, plane-layer model to account for the refraction observations consists of a 15 ± 2 km thick crust of seismic velocity 5.5–6.15 km/s, underlain by an upper mantle with an anomalously low velocity of 7.5 ± 0.2 km/s. Wide-angle upper mantle reflections were also recorded from the Lake Taupo shot; a simple T2 versus X2 analysis of these reflections gives a depth of about 16±1 km to the upper mantle with a root-mean-square (r.m.s.) velocity for the crust of 5.94 ± 0.2 km/s. Th...