Controlled Source Seismic Data Research Articles

3-D imaging of the Balmuccia peridotite body (Ivrea–Verbano zone, NW-Italy) using controlled source seismic data

SUMMARY We provide new results from a controlled-source seismic experiment on the deepest part of the Val Sesia crust–mantle section of the Ivrea–Verbano zone (IVZ) in the Italian Alps. The IVZ is a tilted, almost complete section through the continental crust and exposes gabbros and peridotites in the structurally deepest level, coinciding with high-resolution gravity anomalies imaging the Ivrea geophysical body. The seismic experiment SEIZE (SEismic imaging of the Ivrea ZonE) was conducted along two crossing profiles: an NNE-SSW profile of ∼11 km length and an E-W profile of ∼16 km length. 432 vibration points were recorded with 110 receivers resulting in 24 392 traveltime picks. Inversion methods using Markov chain Monte Carlo techniques have been used to derive an isotropic 3-D P-wave velocity model based on first break traveltimes (refracted phases) from controlled source seismic data. Resulting seismic P-wave velocities (Vp ) range from 4.5 to 7.5 km s−1, with an expected general trend of increasing velocities with depth. A sharp velocity change from low Vp in the West to high Vp in the East marks the Insubric Zone (ISZ), the Europe–Adria plate boundary. The most prominent feature of the 3-D tomography model is a high-velocity body (Vp increases from 6 to 7.5 km s−1) that broadens downwards. Its pointy shape peaks the surface East of Balmuccia at a location coincident with the exposed Balmuccia peridotite. Considering rock physics, high-resolution gravity and other geophysical data, we interpret this high-velocity body as dominantly composed of peridotite. The dimension of this seismically imaged peridotite material is far bigger than interpreted from geological cross-sections and requires a revision of previous models. The interpretation of ultramafic bodies in the IVZ as fragments of mantle peridotites interfingered in the crust during pre-Permian accretion is not supported by the new data. Instead, we revive a model that the contact between the Balmuccia peridotite and the Permian mafic magmas might represent a fossil continental crust–mantle transition zone.

Highly heterogeneous upper-mantle structure in Fennoscandia from finite-frequency P-body-wave tomography

SUMMARY We present a P-wave velocity model of the upper mantle, obtained from finite-frequency body-wave tomography, to analyse the relationship between deep and surface structures in Fennoscandia, one of the most studied cratons on the Earth. The large array aperture of 2000 km × 800 km allows us to image the velocity structure to 800 km depth at very high resolution. The velocity structure provides background for understanding the mechanisms responsible for the enigmatic and strongly debated high topography in the Scandinavian mountain range far from any plate boundary. Our model shows exceptionally strong velocity anomalies with changes by up to 6 per cent on a 200 km scale. We propose that a strong negative velocity anomaly down to 200 km depth along all of Norway provides isostatic support to the enigmatic topography, as we observe a linear correlation between hypsometry and uppermost mantle velocity anomalies to 150 km depth in central Fennoscandia. The model reveals a low-velocity anomaly below the mountains underlain by positive velocity anomalies, which we explain by preserved original Svecofennian and Archaean mantle below the Caledonian/Sveconorwegian deformed parts of Fennoscandia. Strong positive velocity anomalies to around 200 km depth around the southern Bothnian Bay and the Baltic Sea may be associated with pristine lithosphere of the present central and southern Fennoscandian craton that has been protected from modification since its formation. However, the Archaean domain in the north and the marginal parts of the Svecofennian domains appear to have experienced strong modification of the upper mantle. A pronounced north-dipping positive velocity anomaly in the southern Baltic Sea extends below Moho. It coincides in location and dip with a similar north-dipping structure in the crust and uppermost mantle to 80 km depth observed from high-resolution, controlled source seismic data. We interpret this feature as the image of a Palaeoproterozoic boundary that has been preserved for 1.8 Gy in the lithosphere.

The crustal structure in the Northwest Atlantic region from receiver function inversion – Implications for basin dynamics and magmatism

The Labrador Sea and Baffin Bay form an extinct Palaeogene oceanic spreading system, divided by a major continental transform, the Davis Strait, with the whole region defined as the Northwest Atlantic. The Davis Strait hosts the Ungava Fault Zone and is the central structural element of the Davis Strait Large Igneous Province (DSIP) that formed broadly coeval with continental breakup to its north and south. While constraints on the crustal structure in this region primarily exist in the offshore, crustal models are limited onshore, which makes an interpretation of regional structures as well as the extent, and therefore origin of the DSIP extremely difficult to ascertain. Here, we have collected all available teleseismic data from the Northwest Atlantic margins and applied a receiver function inversion to retrieve station-wise velocity models of the crust and uppermost mantle. We integrate the outcomes with published controlled-source seismic data and regional crustal models to make inferences about the crustal structure and evolution of the Northwest Atlantic. In particular, we focused on constraining the spatial extent and origin of high velocity lower crust (HVLC), and determining whether it is generically related to the Davis Strait Igneous Province, syn-rift exhumed and serpentinised mantle, or pre-existing lower crustal bodies such as metamorphosed lower crust or older serpentinised mantle rocks. The new results allow us to better spatially constrain the DSIP and show the possible spatial extent of igneous-type HVLC across Southwest Greenland, Northwest Greenland and Southeast Baffin Bay. Similarly, we are able to relate some HVLC bodies to possible fossil collision/subduction zones/terrane boundaries, and in some instances to exhumed and serpentinised mantle.

Physical conditions and frictional properties in the source region of a slow-slip event

Recent geodetic studies have shown that slow-slip events can occur on subduction faults, including their shallow (<15 km depth) parts where tsunamis are also generated. Although observations of such events are now widespread, the physical conditions promoting shallow slow-slip events remain poorly understood. Here we use full waveform inversion of controlled-source seismic data from the central Hikurangi (New Zealand) subduction margin to constrain the physical conditions in a region hosting slow slip. We find that the subduction fault is characterized by compliant, overpressured and mechanically weak material. We identify sharp lateral variations in pore pressure, which reflect focused fluid flow along thrust faults and have a fundamental influence on the distribution of mechanical properties and frictional stability along the subduction fault. We then use high-resolution data-derived mechanical properties to underpin rate–state friction models of slow slip. These models show that shallow subduction fault rocks must be nearly velocity neutral to generate shallow frictional slow slip. Our results have implications for understanding fault-loading processes and slow transient fault slip along megathrust faults. A shallow slow-slip source region has laterally variable elastic properties and pore pressure, and near-velocity-neutral frictional properties, according to seismic imaging of part of the Hikurangi subduction margin and data-constrained modelling.

A new 3-D P-wave velocity model for the Gulf of Cadiz and adjacent areas derived from controlled-source seismic data: application to nonlinear probabilistic relocation of moderate earthquakes

SUMMARYThe SW Iberian margin is well known for its complex tectonic setting and crustal structure and by the occurrence of moderate magnitude earthquakes and some great tsunamigenic earthquakes. Fortunately, many seismic reflection and refraction profiles have been carried out, providing detailed information about the crustal structure of the main geologic domains in this region. These studies show a first-order variation due to the transition from oceanic to continental domain, large-scale heterogeneities within the crust and an irregular Moho topography. Routine earthquake locations in this area have been usually computed using a general 1-D velocity model which is clear that cannot account for such a heterogeneous structure. In addition, regional seismic stations used to locate the Gulf of Cadiz seismicity are on land and far away to the east, implying large azimuthal gaps and distances. In this context, a 3-D approach seems necessary to properly solve the crustal velocity field and improve earthquake location in this area. With this purpose, we present a new digital 3-D P-wave velocity distribution for the crust and uppermost mantle derived from previously published controlled-source seismic experiments carried out in SW Iberia and the Gulf of Cadiz over the last 40 yr. We have reviewed more than 50 wide-angle and multichannel seismic reflection and refraction profiles and digitized the most significant published 2-D seismic velocity models, performing an updated compilation of crustal parameters (P-wave velocities and geometry and depth of the main crustal interfaces). These velocities as a function of position and depth have been interpolated using ordinary kriging algorithm to obtain, in the form of a regular georeferenced 20 × 20 × 1 km grid spacing, a high-resolution 3-D P-wave velocity distribution for the crust and uppermost mantle and a continuous Moho depth map of the whole area of this study (33°N–41°N latitude and 15°W–5°W longitude). Since current seismic location tools allow the implementation of 3-D grid structures, we have applied our 3-D model to relocate a selection of moderate earthquakes occurred in the studied region using a probabilistic nonlinear method. In the Gulf of Cadiz area the probabilistic approximation provides maximum likelihood hypocentres located within the uppermost mantle with the majority of depths ranging between 20 and 45 km. This model would subsequently be implemented at the Spanish Seismic Network for the routine relocation of the seismicity of the area.

Links between sediment consolidation and Cascadia megathrust slip behaviour

At sediment-rich subduction zones, megathrust slip behaviour and forearc deformation are tightly linked to the physical properties and in situ stresses within underthrust and accreted sediments. Yet the role of sediment consolidation at the onset of subduction in controlling the downdip evolution and along-strike variation in megathrust fault properties and accretionary wedge structure is poorly known. Here we use controlled-source seismic data combined with ocean drilling data to constrain the sediment consolidation and in situ stress state near the deformation front of the Cascadia subduction zone. Offshore Washington where the megathrust is inferred to be strongly locked, we find over-consolidated sediments near the deformation front that are incorporated into a strong outer wedge, with little sediment subducted. These conditions are favourable for strain accumulation on the megathrust and potential earthquake rupture close to the trench. In contrast, offshore Central Oregon, a thick under-consolidated sediment sequence is subducting, and is probably associated with elevated pore fluid pressures on the megathrust in a region where reduced locking is inferred. Our results suggest that the consolidation state of the sediments near the deformation front is a key factor contributing to megathrust slip behaviour and its along-strike variation, and it may also have a significant role in the deformation style of the accretionary wedge. Consolidated sediments in the Cascadia subduction zone may create conditions favourable for megathrust earthquake ruptures over long distances and close to the trench, according to analyses of seismic velocity of sediments from the region. Less-consolidated sediments instead may promote aseismic slip of the plate boundary.

A strong contrast in crustal architecture from accreted terranes to craton, constrained by controlled-source seismic data in Idaho and eastern Oregon

Crustal structure was derived from EarthScope Idaho-Oregon (IDOR) controlled-source seismic data across the Precambrian continental margin in the Idaho and Oregon region of the U.S. Cordillera. Refraction and wide-angle reflection traveltimes were inverted to derive a seismic velocity model that constrains the contact between oceanic accreted terranes and craton. The seismic data reveal that the boundary is a near-vertical, through-going feature of the crust, represented by the transpressional western Idaho shear zone (WISZ). The WISZ separates crust with different seismic velocities at all depths, implying a contrast in lithology, and extends to an ∼7 km offset of the Moho. The thinner, ∼32-km-thick accreted terrane crust to the west is characterized by faster seismic velocities that correspond to an intermediate composition. We interpret a high-velocity layer below a high-amplitude seismic reflection as mafic magmatic underplating associated with the feeder system of the Columbia River Basalts. The cratonic crust east of the WISZ is 37–40 km thick, with a felsic composition to ∼29 km subsurface depth, underlain by an intermediate-composition layer above the Moho. The strong contrasts in lithology and crustal thickness across the WISZ have influenced subsequent magmatism and extension in the region. The northwestern extent of the Archean Grouse Creek cratonic block beneath the Atlanta lobe of the Idaho batholith is interpreted based on continuity of crustal architecture in the seismic model. The velocity structure and crustal thickness east of the WISZ are consistent with the Atlanta lobe melting within a thickened crust.

Seismic imaging of a megathrust splay fault in the North Chilean subduction zone (Central Andes)

Prominent trench-parallel fault systems in the arc and fore-arc of the Chilean subduction zone can be traced for several thousand kilometers in north–south direction. These fault systems possibly crosscut the entire crust above the subduction megathrust and are expected to have a close relationship to transient processes of the subduction earthquake cycles. With the motivation to image and characterize the structural inventory and the processes that occur in the vicinity of these large-scale fault zones, we re-processed the ANCORP'96 controlled-source seismic data set to provide images of the faults at depth and to allow linking geological information at the surface to subsurface structures. The correlation of the imaging results with observed hypocenter locations around these fault systems reveals the origin and the nature of the seismicity bound to these fault systems. Active and passive seismic data together yield a picture of a megathrust splay fault beneath the Longitudinal Valley at mid-crustal level, which can be observed from the top of the subduction plate interface and which seems to be connected to the Precordilleran Fault System (PFS) known at the surface. This result supports a previously proposed tectonic model where a megathrust splay fault defines the Western Altiplano as a crustal-scale fault-bend-fold. Furthermore, we clearly imaged two branches of the Uyuni-Kenayani Fault (UKF) in a depth range between 0 and 20km. In summary, imaging of these faults is important for a profound understanding of the tectonic evaluation and characterization of the subduction zone environment, for which the results of this study provide a reliable basis.

A new Moho boundary map for the northern Fennoscandian Shield based on combined controlled-source seismic and receiver function data

We present a new Moho map for the Early-Proterozoic northwestern part of the Fennoscandian Shield, where the POLENET/LAPNET passive seismic array was located. The map is based on previously published and re-evaluated controlled source seismic data and P-wave receiver functions as well as new estimates of the Moho depth obtained by our analysis of P-wave receiver functions at broadband stations of the POLENET/LAPNET array. We estimated individual data quality for all input data and combined them into a new Moho map using CRUST3D software. The software seeks the simplest (smoothest) Moho surface that is consistent with all seismic data within their individual uncertainty limits. The new Moho map indicates that the crustal thickness in the study region varies between 42km and 58km, with the greatest thickness being reached in two separate areas in the northeast and the southeast. Two areas with relatively flat and shallow Moho, with an average Moho depth of c. 44km, are located in the eastern and south-western parts of the study area. These two areas are separated by the Moho depression, with a maximum depth of 58km. They can be associated with the Archean core of the Karelian craton and with the part of it that was reworked during the Early Proterozoic, respectively. A region with an average Moho depth of c. 47km can be seen in the northern part of our study area, deepening to c. 55km in the northeastern corner.

Tomographic Vp and Vs structure of the California Central Coast Ranges, in the vicinity of SAFOD, from controlled-source seismic data

A seismic reflection/refraction survey across the San Andreas fault near Parkfield, California, has refined our knowledge of the upper crustal structure of the central California Coast Ranges at the San Andreas Fault Observatory at Depth (SAFOD). The survey consisted of a 46-km-long line of seismographs (25–50 m spacing) and 63 explosions (25–200 kg; nominal spacing of 500 m, with some gaps). The traveltimes of refracted P and S waves from the explosions constitute independent data sets of relatively high quality that were inverted to produce P- and S-wave velocity models (Vp, Vs) along the profile, extending to as much as 5 km depth. The Vp and Vs models show a prominent lateral drop in velocities a few hundred metres northeast of SAFOD, between the drill hole and the San Andreas fault. The Vp model shows particularly well a southwest-dipping velocity inversion beneath SAFOD, the top of which correlates with a fault penetrated by the drill hole that separates granitic rocks above from sedimentary rocks below. In addition to Vp and Vs models, a Vp/Vs model was derived. A Vp/Vs ratio lower than 1.73 is seen only at depth, in a narrow zone beginning at the target earthquakes for SAFOD and extending downward and northeastward into the North America Plate. Clusters in the parameter space spanned by Vp/Vs ratios and Vp can be identified by two different methods, one more intuitive analytical method and one more abstract method based on neural network techniques. These clusters are correlated to different rock types, based on laboratory and in situ data. These clusters are remapped back into xߞz plane along the profile. Prominent features mapped this way include Salinian granitic rocks beneath and west of SAFOD, and a body of sedimentary rocks faulted beneath these granitic rocks along what we and others interpret to be a branch of the Buzzard Canyon Fault (BCF) system. These sedimentary rocks extend from this fault to the San Andreas fault system. Unfortunately, our cluster analysis shows no significant discontinuity at the San Andreas fault, owing presumably to the fact that the San Andreas fault is located within sedimentary rocks having similar elastic properties. This paper is an attempt to ‘downward’ continue a geological map by geophysical means based on elastic properties of rock samples from the region.

An integrated analysis of controlled and passive source seismic data across an Archean‐Proterozoic suture zone in the Rocky Mountains

We conducted a new integrated analysis of the controlled and passive source seismic data from the Continental Dynamics of the Rocky Mountains (CD‐ROM) experiment in the western United States. A specific goal of our study was to establish a stronger tie between the CD‐ROM and Deep Probe experiments that together form a transect that extends almost 3000 km from northern New Mexico to the southern Northwest Territories of Canada. We created a new P wave velocity and interface model from the controlled source seismic (CSS) data on the basis of an advanced picking strategy that produced substantially more arrivals whose traveltimes could be picked. In addition, we were able to identify a substantial set of S wave arrivals and to establish an independent S wave model, which allowed us to estimate the Vp/Vs and Poisson's ratios in the crust. An integrated analysis of the CSS data with recently generated receiver function results and gravity data allowed us to construct a well‐constrained multiparameter structural model. This model resolves the structural framework of the crust and uppermost mantle of the transition from the Wyoming craton to the north, across the Cheyenne belt suture zone, and into the Proterozoic terranes to the south. Our tectonic interpretation that crustal‐scale crocodile structures are present provides an explanation for the south dip of the Cheyenne belt in the upper crust and the north dipping slab in the mantle revealed by earlier tomographic results. The very distinct crustal structures north and south of the suture zone are clearly shown in our model and document that the blocks that collided at ∼1.8 Ga to form the Cheyenne belt suture zone have retained their basic crustal and uppermost mantle structure since then.

Crustal structure of the South American–Caribbean plate boundary at 67°W from controlled source seismic data

We present the results of new seismic reflection and wide‐angle data across the SE Caribbean plate boundary. The 550 km long N–S profile crosses the structures involved in the active 55 Ma long continent‐arc oblique collision between the Caribbean (CAR) and the South American (SA) plate. From the north to the south these structures include the accretionary prism, the extinct volcanic arc (Leeward Antilles arc), the Tertiary Bonaire basin, the continental‐size dextral strike‐slip fault system (San Sebastián–El Pilar fault), the allochthonous exhumed terranes, and the authocthonous fold and thrust belt (Caribbean Mountain system) and foreland basin. The wide‐angle data show that these elements are characterized by different velocity structures and that they are separated by sharp lateral velocity variations. The Leeward Antilles arc exhibits a velocity structure similar to that of the Lesser Antilles active volcanic arc, indicating that the extinct arc has not been modified by the collision with the SA plate. The data show a ∼20 km change in crustal thickness across the San Sebastián fault, suggesting that the dextral strike‐slip fault is a crustal feature that likely continues in the mantle as a primary strand of the plate boundary between the South American and the Caribbean plates. South of the strike‐slip fault and beneath the exhumed eclogitic terranes, the data image a north dipping, high‐velocity (&gt;6.5 km/s) anomaly in the upper crust (3–11 km), indicating that high‐pressure/low‐temperature rocks are the likely lithologies responsible for the high seismic velocities and suggesting that exhumation of these assemblages is enabled by the strike‐slip fault.

The Krasnouralsky profile in the Middle Urals, Russia: A tomographic approach to vintage DSS data

The Middle Urals region has been widely studied with geophysical methods over the past decades. An integrated program is in progress to summarize this knowledge, including modern reprocessing of controlled-source seismic data. This work is devoted to the Krasnouralsky DSS profile. We applied modern tomography inversion algorithms in 2D and 2.5D on first break traveltime picks from an archive catalogue. A number of initial models and various smoothing constraints were used to investigate the influence of starting models on the final model. Robustness and uncertainty of the recovered models were estimated with hypothesis testing and checkerboard tests. The recovered velocity structure shows a thicker crust below the contact of the West Uralian Zone and the Central Uralian Zone and below the Tagil–Magnitogorsk Zone. Deep high velocity anomalies on both sides of this zone are interpreted as crustal thinning or alteration of the crust by intrusions of mantle material. Our results suggest that it is worthwhile reinterpreting DSS traveltime data with modern inversion techniques.

Layered crust–mantle transition zone below a large crustal intrusion in the Norwegian–Danish Basin

The crust and uppermost mantle below the large positive gravity anomaly in the central part of the Norwegian–Danish Basin, the Silkeborg Gravity High (SGH), is investigated using controlled source seismic data. A more than 80 km long and ~ 20 km thick intrusion is interpreted. The seismic velocity within the intrusion varies from 6.8 km/s at the top (12 km depth) to 7.2–7.7 km/s at the Moho level (30–31 km depth). The characteristics of the Moho reflectivity vary along the profile. Below the central part of the intrusion, the Moho reflectivity is very weak, due to a small velocity contrast between the lowermost crust (7.7 km/s) and the uppermost mantle (7.9–8.0 km/s). The seismic data show a “ ringing” Moho below the western part of the intrusion. The coda trailing the main P mP reflection is about 1.0 s long and is composed of 4–5 wavelets. We demonstrate that this feature may be explained by a layered transition zone between 30 and 35 km depth, where high-velocity layers of mantle affinity (7.9–8.05 km/s) alternate with layers of typical lower crustal velocity (6.7–7.3 km/s). The characteristics of this layering, which causes the P mP coda, are modelled by matching synthetic seismograms to the observed data. Synthetic seismograms, calculated using the reflectivity method for 1 D models, constrain the nominal layer thickness to 100–500 m. Waveform modelling in 2 D constrains the layered internal structure of the transition zone and the length of the heterogeneous zone along the profile. We interpret the layered crust–mantle transition zone as solidified melts of mantle affinity, which were intruded into the lowermost crust. The layered zone is proposed to be associated with the extensional tectonism that occurred in the area in Late Carboniferous–Early Permian, which also caused the large crustal intrusion. The subsequent cooling may have initiated the formation of the Danish Basin.

Identification of crustal and upper mantle heterogeneity by modelling of controlled-source seismic data

High-frequency controlled-source seismic sections with dense spatial sampling show the existence of heterogeneity at different depth levels of the continental crust and upper mantle. Our sources of information are the Peaceful Nuclear Explosion (PNE) seismic data sets recorded to large offsets in the former Soviet Union supplemented by recordings from the North American Early Rise deep seismic experiment and normal-incidence reflection seismic sections collected in northwest Europe. Heterogeneity in the crust and upper mantle can be uniquely identified in reversed high-frequency (2–10 Hz) PNE seismic sections collected with dense spatial sampling (nominal receiver spacing of 10–15 km) out to 4000 km offset. We document pronounced seismic scattering from three heterogeneous zones: The lower crust from ∼20 km to ∼40 km depth, an ∼80 km thick low-velocity zone below ∼100 km depth, and the ∼320–460 km depth interval around the top of the mantle transition zone. We calculate the full seismic wavefield in heterogeneous crust–mantle models with a two-dimensional finite-difference algorithm. We represent the heterogeneous layers by random fluctuations of the elastic parameters and Q-values. The spatial (horizontal and vertical) correlation lengths and the standard deviation of the scattering media are constrained by comparison of observed and calculated seismic sections. The lower crustal heterogeneity causes a coda to the upper mantle arrivals at all recorded frequencies. This coda is a prominent feature for whispering-gallery phases (teleseismic Pn), which travel as multiply reflected refractions below the Moho to more than 3000 km offset from the PNE sources. The heterogeneous mantle low-velocity zone causes a scattered coda trailing the first arrivals in the ∼800–1400 km offset range. The best fit to the observations along profile Kraton in Siberia is obtained by an 80 km thick heterogeneous low-velocity zone below 100 km depth, represented by fluctuations described by a von Karman distribution function with a Hurst number of 0.5 and spatial correlation lengths of 5–10 km (horizontally) and 3–5 km (vertically). Scattered arrivals, which trail the reflection from the ‘410’ discontinuity and a reflection from a shallower depth of ∼320 km, constrain heterogeneity around the top of the transition zone. This heterogeneity is modelled by fluctuations in the 320–460 km depth interval with correlation lengths on the order of 20–40 km by 5–10 km. Scattering from the two shallower heterogeneous zones improves the description of the phases from around the transition zone.

Statistical inversion of controlled-source seismic data using parabolic wave scattering theory

Summary Statistical parameters describing heterogeneity in the Proterozoic basement of the Baltic Shield were estimated from controlled-source seismic data, using a statistical inversion based on the theory of wave propagation through random media (WPRM), derived from the parabolic wave approximation. Synthetic plane-wave seismograms generated from models of random media show consistency with WPRM theory for forward propagation in the weak-scattering regime, whilst for two-way propagation a discrepancy exists that is due to contamination of the primary wave by backscattered energy. Inverse modelling of the real seismic data suggests that the upper crust to depths of ∼ 15 km can be characterized, subject to the range of spatial resolution of the method, by a medium with an exponential spatial autocorrelation function, an rms velocity fluctuation of 1.5 ± 0.5 per cent and a correlation length of 150 ± 50 m. Further inversions show that scattering is predominantly occurring in the uppermost ∼ 2 km of crust, where rms velocity fluctuation is 3 − 6 per cent. Although values of correlation distance are well constrained by these inversions, there is a trade-off between thickness of scattering layer and rms velocity perturbation estimates, with both being relatively poorly resolved. The higher near-surface heterogeneity is interpreted to arise from fractures in the basement rocks that close under lithostatic pressure for depths greater than 2 − 3 km.

Elastic properties of hydrate‐bearing sediments using effective medium theory

Accurate and detailed models of the seismic velocity structure of gas hydrate‐bearing sediments may be determined by careful analysis of controlled source seismic data. However, interpretation of these velocities in terms of hydrate saturation of the pore space has hitherto relied on semiempirical formulae and/or simple effective medium theory. We develop a rigorous theoretical scheme to relate the seismic properties of a clay‐rich hydrate‐bearing sediment to its porosity, mineralogy, microstructural features and hydrate saturation. We consider separately the two possible end‐members for the distribution of hydrate in the pore space: (1) hydrates are unconnected and located in the pore voids without appreciable grain contact and (2) connected hydrates are forming cement binding around the grains. The scheme is transversely isotropic, to allow for anisotropy due to alignment of clay platelets, and is based on a combination of a self‐consistent approximation, a differential effective medium theory, and a method of smoothing for crystalline aggregates. We have applied the scheme to lithological and seismic velocity data from Ocean Drilling Program Site 995 on the Blake Ridge (southeastern U.S. continental margin) to make estimates of the hydrate saturation. It was found that the hydrates are probably unconnected, and their volume concentration varies between ∼0% at 100 m below the seabed and ∼9% at 400 m depth, just above the “bottom simulating reflector”, if the clay platelet orientation distribution resembles the function we have used.

Three-dimensional interface modelling with two-dimensional seismic data: the Alpine crust-mantle boundary

SUMMARY We present a new approach to determine the 3-D topography and lateral continuity of seismic interfaces using 2-D-derived controlled-source seismic reflector data. The aim of the approach is to give the simplest possible structure consistent with all reflector data and error estimates. We define simplicity of seismic interfaces by the degree of interface continuity (i.e. shortest length of oVsets) and by the degree of interface roughness (least surface roughness). The method is applied to structural information of the crust‐mantle boundary (Moho) obtained from over 250 controlled-source seismic reflection and refraction profiles in the greater Alpine region. The reflected and refracted phases from the Moho interface and their interpretation regarding crustal thickness are reviewed and their reliability weighted. Weights assigned to each reflector element are transformed to depth errors considering Fresnel volumes. The 2-D-derived reflector elements are relocated in space (3-D migration) and interpolation is performed between the observed reflector elements to obtain continuity of model parameters. Interface oVsets are introduced only where required according to the principle of simplicity. The resulting 3-D model of the Alpine crust‐mantle boundary shows two oVsets that divide the interface into a European, an Adriatic and a Ligurian Moho, with the European Moho subducting below the Adriatic Moho, and with the Adriatic Moho underthrusting the Ligurian Moho. Each sub-interface depicts the smoothest possible (i.e. simplest) surface, fitting the reflector data within their assigned errors. The results are consistent with previous studies for those regions with dense and reliable controlledsource seismic data. The newly derived Alpine Moho interface, however, surpasses earlier studies by its lateral extent over an area of about 600 km by 600 km, by quantifying reliability estimates along the interface, and by obeying the principle of being consistently as simple as possible.