Normal Incidence Reflectance Data Research Articles

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.

Optical characterization of β-FeSi 2 thin films prepared on fused quartz by femtosecond laser ablation

Single-phase β-FeSi 2 thin films have been grown on quartz substrates using femtosecond laser deposition (800 nm, 50 fs, 1 KHz) under gas pressure of 3.0×10 −4 Pa. X-ray diffraction (XRD) and field-emission scanning electron microscopy (SEM) were used to determine the structural properties and surface images of the films. Typical XRD patterns of the film showed that no other diffraction peak except β-FeSi 2 was found. The SEM results indicated that the films were composed of well-distributed grains, in the range 50–150 nm in diameter. In addition, normal incidence spectral transmittance and reflectance data suggested that the β-FeSi 2 film has a direct energy gap of about 0.85 eV. The thickness of the layer and the refractive index of the film were determined by performed calculation in the wavelength range 1.9–2.7 μm. Furthermore, the Raman spectra of the films were also discussed.

Optical Characterization of β-FeSi2 Thin FilmsPrepared by Femtosecond Laser Ablation

Iron disilicide thin films are prepared on fused quartz usingfemtosecond laser deposition (FsPLD) with a FeSi2 alloy target.X-ray diffraction results indicate the films are single-phase,orthorhombic, β-FeSi2. Field scanning electron microscopy, highresolution transmission electron microscopy, UV–VIS–NIR spectroscopyand Raman microscope are used to characterize the structure,composition, and optical properties of the β-FeSi2 films. Normalincidence spectral transmittance and reflectance data indicate aminimum, direct energy gap of 0.85 eV. The two most intense lines ofRaman scattering peaked at 181.3 cm−1 and 235.6 cm−1 forthe film on fused quartz, and at 191.2 cm−1 and 243.8 cm−1for the film on Si (100), are observed.

Facies Architecture of Flexural Margin Lowstand Delta Deposits in Lake Edward, East African Rift: Constraints from Seismic Reflection Imaging

Abstract Lake Edward is single half-graben rift basin unique in the East African Rift System as a result of its connection to Lake George via the Kazinga Channel, a paleo–river valley which is incised into the flexural margin of the lake. Using recently acquired normal-incidence seismic reflection data and thirty-three lake-bottom sediment samples, we examine the consequences of this hydrologic connection for understanding depositional processes and facies development along the flexural margin of Lake Edward. Three depositional sequences were defined in the study area based on top and bottom discordances of seismic reflections. Climatically induced changes in lake level appear to have forced sequence development. Sequence I deposits indicate that highstand conditions prevailed earlier in the Pleistocene. Evidence for high-frequency lake-level change comes from lowstand delta clinoforms in two separate stratigraphic intervals on the flexural margin. An older lowstand event occurred during Sequence II time and reduced lake level by at least ~ 37 m. A younger lowstand event occurred during the deposition of Sequence III and reduced lake level by ~ 18 m. Clastic sedimentation on the flexural margin of Lake Edward is strongly modified by inflow from the Kazinga Channel and Lake George during periods of lowered lake level. We suggest that the presence of an incised valley at the margin of the rift controls the stratigraphic development of clinoforms diagnostic of lowstand deltaic sedimentation. The morphology and orientation of these clinoforms suggest that discharge from Lake George during arid periods utilized the Kazinga Channel to transport sediment toward the exposed flexural margin of Lake Edward. Dipping reflections in sigmoid-oblique clinoforms suggest that these flows were capable of transporting sediment loads that included coarse grain sizes. Deposition by this process occurred several times during the late-Quaternary history of Lake Edward, and represents an important modern analogue for linked incised valley-lacustrine delta sequences in the stratigraphic record.

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.

Remote Estimates of Physical and Acoustic Sediment Properties in the South China Sea Using Chirp Sonar Data and the Biot Model

A chirp sonar acquires sea-bed reflection data in the South China Sea, generates imagery of the sediment layering, and estimates sediment properties along the acoustic propagation paths selected for the Asian Sea International Acoustics Experiment (ASIAEX). Normal incidence reflection data, collected by the chirp sonar, are processed with an inversion technique based on the Biot model. The inputs for the inversion are reflection coefficient and attenuation rolloff measurements extracted from chirp sonar reflection data. The inversion provides the acoustic and physical properties of the sediments for the top layer of the sea bed. Porosity, predicted by the inversion, agrees with directly measured porosity. Estimates of porosity, mean grain size, permeability, fast wave velocity, and attenuation (in decibels/meter) are given for the uppermost sediment layer along each of the acoustic propagation lines.

A Method for Estimating the Physical and Acoustic Properties of the Sea Bed Using Chirp Sonar Data

This paper proposes a method, based on the Biot model, for estimating the physical and acoustic properties of surficial ocean sediments from normal incidence reflection data acquired by a chirp sonar. The inversion method estimates sediment porosity from reflection coefficient measurements and, using the estimated porosity and the measured change in fast wave attenuation with frequency, estimates the permeability of the top sediment layer. The spectral ratio of echoes from the interface at the base of the upper sediment layer and from the sediment-water interface provides a measure of the change in attenuation with frequency. Given the porosity and permeability estimates, the Kozeny-Carman equation provides the mean grain size and the inversion method yields the acoustic properties of top sediment layer. The inversion technique is tested using chirp sonar data collected at the 1999 Sediment Acoustics Experiment (SAX-99) site. Remote estimates of porosity, grain size, and permeability agree with direct measurements of those properties.

Techniques for estimating sediment attenuation from chirp sonar data

Recent broadband (50–500 Hz) matched-field inversions of geoacoustic parameters indicated that sediment attenuation is difficult to estimate especially at short source-receiver distances. On the other hand, geoacoustic inversions become less practical at long source-receiver distances due to the range-dependency of both ocean bottom and water column. Several inversion techniques are investigated to estimate in-situ sediment attenuation from normal incidence reflection data, collected by chirp sonars. Both frequency-shift and attenuation roll-off measurement techniques showed nonlinear frequency dependency of attenuation for sandy sediments at the 2-12-kHz frequency band. Estimated attenuation values are extrapolated to lower frequencies by using the Biot theory. The extrapolated results are consistent with those of the matched-field inversions. Several advantages and limitations of the frequency-shift and attenuation roll-off measurement techniques are also discussed. [Work supported by ONR.]

A simple spectrophotometric method for determination of the optical constants and band gap energy of multiple layer TiO 2 thin films

Using sol–gel processing, TiO 2 films have been prepared on microscopic glass slides by dip-coating technique. From X-ray diffractometric (XRD) and X-ray photoelectron spectroscopic (XPS) measurements, it has been confirmed that the deposited films are an anatase form of stoichiometric TiO 2. Using either normal incidence transmittance or normal incidence reflectance data, a simple method for determining both film refractive index, n( λ), and film thickness, d, has been proposed for a transparent film on a non-absorbing flat substrate. At same wavelength within visible region, a correlation between T S and T B has been established and verified. T S and T B represent the transmittances of single side and both sides coated films of same thickness, respectively, prepared at identical conditions. Average film thickness per coating at various withdrawal speeds (58–146 mm min −1) has been estimated. An empirical dispersion equation that describes the variation of n( λ) of TiO 2 films in the wavelength ( λ) range of ∼390–800 nm has been deduced. Band gap energy ( E g) has also been estimated for these films. It has been observed that E g changes from ∼3.35 to 3.16 eV as the film thickness varies from ∼100 to 300 nm and it does not depend significantly upon film withdrawal speeds in the range 58–146 mm min −1.

Techniques for estimating sediment properties from chirp sonar data

The chirp sonar is a towed wideband FM reflection profiler developed to collect normal incidence reflection data suitable for sediment property inversions; the sonar also generates detailed reflection profiles of the seabed. The chirp sonar system transmits FM pulses with high time-bandwidth products to attain high pulse energy; consequently, the acoustic data have a high signal-to-ambient-noise ratio after correlation processing. The vertical resolution of the images is about 1 cm when transducer bandwidth is approximately 40 kHz. Data processing methods have been developed to automatically map the locations of sediment layer interfaces, and to estimate compressional wave attenuation, acoustic impedance, and compressional wave velocity of sediment layers using normal incidence FM reflection data. The height of sand ripples and the phase dispersion of compressional waves can also be measured from the acoustic imagery. Field data collected by other investigators during the High Frequency Acoustics DRI experiments off Fort Walton Beach were used to verify the acoustic property estimates. Comparisons show that chirp sonar estimates of acoustic properties agree with direct measurements of the properties based on in situ acoustic probe data and sediment core data collected by investigators from Naval Research Laboratory, Stennis Space Center, and APL, Washington State.

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.

Virtual substrate model for in situ thin film thickness determination

A computational system has been developed for in situ monitoring of the optical properties of transparent thin films, grown by the spray pyrolysis technique on specular semi-infinite substrates, using the virtual substrate model for normal incidence reflectance. The system consists of a PC, a custom made data acquisition board, and software capable of in situ thickness monitoring and post-deposition analysis of optical parameters using the R = R(t) curve. The Levenberg– Marquardt numerical procedure was used in order to fit theoretical and experimental values of the normal incidence reflectance data with a small percentage error.

Pre-Zechstein structures around the MONA LISA deep seismic lines in the southern Horn Graben area

Seismic reflection data from the Horn Graben area in the southeastern part of the North Sea, off-shore Denmark, have been interpreted to illustrate the upper crustal structures around the MONA LISA deep seismic lines. The study area comprises the southern Horn Graben area and the eastern part of East North Sea High, where the Caledonian collision suture between Baltica and Eastern Avalonia bends such that the strike direction changes from ESE in the south to NNW in the north. Integrated interpretation of normal-incidence reflection data and wide-angle refraction data reveals substantial occurrences of lower and upper Palaeozoic strata in the area, thickest below the Horn Graben. This may indicate that Horn Graben developed as a graben structure during late Palaeozoic in the former Caledonian foredeep. On the northern and eastern parts of the MONA LISA deep seismic reflection lines 1 and 3, the main E- dipping boundary fault of the southern Horn Graben segment appears to be listric at depth with a sub-horizon-tal detachment at the top of the reflective lower crust. We have mapped the lateral extent of the lower Permian, volcanic Rotliegend reflector in the study area on the basis of seismic lines from the RTD-81 survey. Dipping reflections observed in the sedimentary strata below the Rotliegend reflector are interpreted as Cal-edonian structures generated by folding and deformation in Lower Palaeozoic Baltica shelf sediments in the Caledonian foreland basin. A sequence of S- and W-dipping reflections above 4 s twt are interpreted as preserved Caledonian thrusts in the upper crustal frontal part of the SW-dipping Caledonian Deformation Front.

MONA LISA — Deep seismic investigations of the lithosphere in the southeastern North Sea

The MONA LISA collaborative project has collected 1112 km of seismic normal-incidence reflection data (recorded to 26 s) and wide-angle data from 26 onshore and 2 offshore locations along 4 profiles in the southeastern North Sea. The seismic data clearly image structures in the crust and uppermost mantle that may be related to Caledonian collision and Late Palaeozoic to Mesozoic rifting and basin formation. For the first time, dipping and subhorizontal reflections from the mantle have been observed to 24 s twt in seismic normal-incidence reflection sections. Strong sub-horizontal reflections are observed on two perpendicular profiles at c. 21 s twt, which is near the base of the lithosphere. These deep reflections are sharper than expected from a thermal–rheological transition suggesting that they are generated at lithological or tectonic interfaces. The Moho is reflective along all four profiles. Particularly strong and sharp normal-incidence and wide-angle reflections were recorded from the Moho south of the Caledonian Deformation Front. The normal-incidence reflection sections show unusually low intra-crustal reflectivity. Lower crustal, subhorizontal reflectivity is only observed west of the Central Graben and in a short profile segment at the base of the interpreted Caledonian Deformation Front. The crustal suture between Avalonia and Baltica is interpreted to be south- to west-dipping. SSW- and N-dipping reflections to 20 s twt are observed from the uppermost mantle. Two models of Tornquist Sea subduction may explain these events: either southward subduction and later extensional reactivation or northward subduction with later extension along a SSW-dipping shear zone. The crust is c. 5 km thinner below the Central Graben than below the surrounding parts of the Mid North Sea–Ringkøbing-Fyn High. The lower crust is reflective on the western flank of the Central Graben. East-dipping reflections from the lower crust and the upper mantle to the east of the graben may indicate a component of simple shear in the evolution of the rift structure.

Seismic image of the Cantabrian Mountains in the western extension of the Pyrenees from integrated ESCIN reflection and refraction data

Integrated analysis of normal-incidence and large-aperture seismic reflection data collected in 1992 and 1993 within the Spanish ESCIN and complementary projects provide a first complete NS crustal transect across the Northern Iberian Peninsula and continental margin. Images of the crustal structure of the Cantabrian Mountains and their transition to the Duero basin and to the Cantabrian margin are obtained from: (a) a 65-km-long vertical reflection profile ESCIN-2 on land; (b) a 200-km-long reversed refraction profile; and (c) wide-angle recordings of the marine ESCIN-4 profile. Consistent results between reflectivity pattern and velocity-depth distribution reveal important lateral variations in the deep structure. The reflective crust imaged in the ESCIN-2 profile changes its attitude from sub-horizontal beneath the Duero basin to north-dipping beyond the Mountain front. Basement thrusts are observed in the upper crust merging into a detachment at 6 s (TWT) and may have triggered the Alpine uplift of the range. The Moho is identified at the bottom of the reflective lower crust and deepens from 12 to 15 s at the northern end of the profile, about 35 km inland. Modelling of the refraction data laterally extends the seismic image and provides evidence for Variscan crustal features beneath the Duero basin. Northwards, the velocity in the lower crust decreases and the Moho, constrained by the wide-angle data from profile ESCIN-2, deepens to about 60 km ending abruptly at the shoreline. The velocity-depth model is constrained along the Asturian platform up to the continental slope, where the crust-mantle boundary is located at 24 km depth. This ‘margin Moho’ shows a progressive deepening southwards, and extends to the coast where it is found at 30 km depth. The present seismic data support an important Alpine reworking and thickening of the crust under the Cantabrian Mountains. The onshore/offshore transition is marked by an imbrication of two crusts of very different thicknesses. This signature offers a strong parallelism with the one previously observed further east across the Pyrenees in the ECORS seismic profile.

Reflectivity characteristics of the crust from modelling refraction/wide-angle and normal-incidence reflection data on BABEL Line 1

Marine normal-incidence reflection and P- and S-wave refraction/wide-angle reflection data from the BABEL project, line 1, in the Bothnian Sea have been used for investigating the nature of the reflectivity in the area. The data are available along the same line, and therefore the same piece of crust is probed by two seismic methods. We use this favourable circumstance for model calculations concerning both data sets by analysing the waveforms of reflections from normal-incidence traveltimes at approximately 4, 12 and 16 s TWT. Strong sub-horizontal reflections are observed on the normal-incidence reflection section at about 4 s TWT. These reflections have been modelled using the reflectivity method (1-D modelling) utilising the water bottom reflection as an estimate of the source wavelet and show a positive polarity. Thickness and P-wave velocity consistent with these reflections are 50 m and 7 km/s, respectively, with a host rock velocity of 6 km/s. From the polarity, velocity and thickness, these reflections are attributed to dolerite sills in the upper crust. Strong lower-crustal reflections are observed on the wide-angle and normal-incidence reflection sections. However, the corresponding S-wave reflections from the lower crust are considerably weaker. The implied high positive impedance contrast for the P-wave lower-crustal reflection and the weak S-wave reflectivity suggests the presence of a highly mafic sheet at about 40 km depth. The wide-angle P- and S-wave sections show strong reflections at the Moho. Coincident normal incidence data show only weak Moho reflectivity indicating a gradient in velocity at the crust/mantle boundary. Dipping events onlap the strong sub-horizontal lower crustal and the P-wave Moho reflections on the wide-angle data. These are explained by dipping structures above the lower crustal reflector and below the Moho, with the latter also consisting of a gradient in velocity.

Simultaneous Inversion of Reflection and Refraction Seismic Data and Application to Field Data From the Northern Rio Grande Rift

SUMMARY An inversion scheme that determines a velocity model simultaneously from normalincidence reflection and refraction/wide-angle reflection seismic data has been developed, tested, and applied to real seismic data. Simultaneous inversion minimizes the non-uniqueness of inversion results. A common midpoint (CMP) stacked reflection section provides a detailed image of the structural configuration in time. Refraction or wide-angle reflection data can be used to derive a velocity model. If both refraction data and normal-incidence reflection data are available along coincident profiles, simultaneous inversion of the data sets can produce an improved model. With the extra control of the interface depth provided by reflection data, it is also possible to avoid ambiguity of phase identification. The commonly used damped-least-squares (DLS) inversion method often results in significant inversion artefacts. We provide effective methods for reducing inversion artefacts using both damping and smoothing. From tests using synthetic data, we conclude that the simultaneous inversion scheme can improve resolution and can be applied to velocity models with a reasonably complex structure. The simultaneous inversion scheme has been successfully applied to coincident seismic reflection and refraction data recorded by the SAGE program in 1990-1991 in the western boundary area of the Rio Grande rift near Abiquiu, New Mexico. In this example, the simultaneous inversion derived a velocity model in which stratigraphic units and extensional fault blocks can be resolved.

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.

The Bragg condition limitation on inversion of normal incidence reflection data

A simple analytic expression is derived for the impulse response of a continuously stratified sediment. The expression, which neglects multiple scattering, allows a straightforward calculation of both the forward and inverse problem, but more importantly, it clearly establishes the ‘‘Bragg condition’’ as a fundamental limitation on the inversion of acoustic data dominated by single scattering. (For normal incidence backscatter, the Bragg condition states that an acoustic probe signal of wavelength λ senses or ‘‘filters out’’ only the Fourier component of the impedance profile having wavelength λ/2, i.e., λmedium=λacoustic/2.) The validity of the analytic expression is demonstrated by comparing it with exact numerical calculations for both the forward and inverse problem. In particular, it is shown that to obtain trends in impedance that occur over meters, requires a probe signal with wavelengths of approximately twice the desired trend distance. Many high‐frequency inversions reported in the literature show trends with wavelengths that are orders of magnitude larger than any wavelength in the probe signal. Consequently, it is worthwhile to ask whether the long wavelength trends, which cannot be acoustically sensed by the short wavelength probe signals, are physically meaningful, or whether they are artifacts of arbitrary underlying assumptions in the signal processing method. [Work supported by NRL and ONR.]

The polarity of deep seismic reflections from the lithospheric mantle: Evidence for a relict subduction zone

BIRPS offshore deep seismic reflection profiles to the north of Scotland have revealed two bright continuous reflectors in the continental lithospheric upper mantle, the dipping Flannan and sub-horizontal W reflectors. The polarity of reflections from the Flannan has been determined from normal-incidence reflection data, using the far-field source wavelet as a starting model. The effective far-field wavelet was calculated by adding a receiver ghost and the effects of recording filters, attenuation during transmission and sea bottom multiples to the source wavelet. Reflection polarity was determined in a blind test by comparing the modelled wavelet with stacks of the Flannan reflection. In eight out of ten stacks, the reflection was picked as positive polarity, two out of ten were unknown. To verify this result, the test was repeated for Moho reflections; in this case nine out of ten stacks were picked as positive. The modelling also demonstrated that the Flannan reflection is apparently from a simple interface; complex interlayering is not required to explain the waveforms, and they are not consistent with reflections from a single thin layer. Seismic data acquired at wide-angle across the W mantle reflector show sub-crustal high-amplitude arrivals, which can only be explained as post-critical reflections. Only a high-velocity eclogitic layer, contained within normal mantle, with a sharp upper boundary and a diffuse base can explain all our observations. We suggest that the Flannan reflector represents the top of a relict oceanic and eclogitic component of a pre-Caledonian subduction zone within the lithospheric mantle.