Near-vertical Reflection Data Research Articles

Structure of the crust/mantle transition beneath the Variscan foreland in SW Poland from coincident wide-angle and near-vertical reflection data

Lower crustal and Moho reflectivity vary significantly across the transition from Trans-European Suture Zone to the Variscan Orogen of Central Europe along the SUDETES 2003 S01 profile. One 50-km segment of the 270-km profile was a detailed reflection–refraction survey called GRUNDY 2003, in which the shot points were also recorded along the Polish part of the S01 profile to produce dense recordings of wide-angle reflections. Processed near-vertical incidence (NVI) data recorded along the GRUNDY 2003 G01 line revealed highly reflective, laminated lower crust starting at ca. 7 s TWT (20–23 km depth). Moho was inferred as the base of the crustal reflectivity at ca. 11.5–12 s (33–37 km depth). Wide-angle data recorded along the S01 profile, processed and stacked using CDP method with up to 41-fold, revealed reflectivity associated with the lower crust and the Moho in two areas: east of the Odra Fault zone and close to the Dolsk Fault. Moho boundary deduced from both the NVI and wide-angle stacks fits well to the “refraction” Moho constrained by the ray-tracing modelling and joint refraction/reflection tomography of P g and P mP arrivals performed along the S01 profile. Part of the Moho boundary imaged along the S01 profile (close to the Dolsk Fault zone) is proposed to represent a metamorphic front of lower crustal rocks partly undergone eclogite transformation. This observation is consistent with the proposed southward underthrusting of Baltica lower crust below the Avalonian acretionary prism. The postulated thin (up to 10 km) high-velocity eclogite layer overlying “normal” velocity mantle would also help to reconcile the apparent contradiction of a high heat-flow observed over the area with the high P n velocities.

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.

The Global Offset seismic approach: advantages and limitations

Seismic acquisition and processing in thrust belt areas represent challenging tasks in geophysics. Rough topography, geological complexity and sharp velocity variations increase the difficulties of recording seismic data of good quality and decrease the possibility of obtaining satisfactory imaging. In these circumstances, application of pre-stack depth migration (PSDM) can help in improving the imaging. However, the success of PSDM depends on the range of offset used and on the accuracy of the adopted background velocity model (Jin and Madariaga, 1994). When significant lateral velocity variations are predominant (Lynn and Claerbout, 1982), the derivation of an accurate velocity model by standard velocity analysis can fail. In these cases, accurate tomographic techniques can be extremely helpful, especially if applied to redundant data sets recorded in an extended range of offsets. In this framework, the advantages and difficulties of recording useful data at offsets larger than 15-20 km, or more, are well known from refraction seismic widely applied for studies of the middle-deep portion of the terrestrial crust. Super-critical reflections and long offset turning rays generally contain a lot of information about the elastic properties of the investigated medium. On the other hand, the risk of introducing artefacts when stacking and migrating wide angle reflections can be high, especially in the case of complex geological settings. Typical artefacts can be produced by lateral events introduced in the final migrated section, or by reflected energy migrated in the wrong position due to wrong velocity field definition. Other artefacts can be created when the wide angle reflections are not properly distinguished and separated from refracted energy. How to reduce these risks without renouncing the benefits offered by long offset data is still an open question. During the last decade many experiments have been performed using the Global Offset seismic approach (Buia et al., 2002; Colombo et al., 2004a; Colombo et al., 2004b; Colombo et al., 2003; Dell’Aversana, 2003), in order to verify the contribution offered by this methodology for improving the velocity field definition and seismic imaging. The Global Offset acquisition layout is set in order to guarantee the same shot and receiver spacing typical of the standard near-vertical reflection seismic, but for an extended range of offsets. Experiments with offsets larger than 30 km and shotreceiver density comparable with that of the industrial reflection seismic have been performed in the last few years (Dell’Aversana et al., 2001). In this sense Global Offset bridges the gap between commercial reflection seismic (commonly used for exploration in the hydrocarbon industry) and long offset refraction seismic (often applied for academic studies, focused mainly on the middle-deep portions of the terrestrial crust). Near-critical and post-critical reflections and long-offset turning rays are included in the Global Offset data set. The advantage is that the wide-angle reflections can show a high signal-to-noise ratio even in those cases where complex overburden can prevent to record good near-vertical reflection data. Moreover, using long offsets, it is possible to undershoot high velocity ‘shallow’ layers that can produce a shield effect for the propagating wave-field (Dell’Aversana et al. 2003). In general, massive application of transmission-reflection tomography is the first step aimed at producing a reliable velocity model (Dell’Aversana et al., 2003; Improta et al., 2000a, 2000b; Operto et al., 2004; Ravaut et al., 2004;). This can be successively used for improving the pre-stack depth migration process (Dell’Aversana et al, 2002). In this paper we show, with real examples, how including high-fold long offset data can really improve the seismic imaging and interpretation process. At the same time we also discuss the pitfalls that can be caused if long offset data are used indiscriminately without the necessary limitations. Finally, we explain how these problems can be controlled and avoided using a series of ‘consistency’ criteria.

Multiscale imaging of complex structures from multifold wide-aperture seismic data by frequency-domain full-waveform tomography: application to a thrust belt

An application of full-waveform tomography to dense onshore wide-aperture seismic data recorded in a complex geological setting (thrust belt) is presented. The waveform modelling and tomography are implemented in the frequency domain. The modelling part is solved with a finite-difference method applied to the visco-acoustic wave equation. The inversion is based on a local gradient method. Only the P-wave velocity is involved in the inversion. The inversion is applied iteratively to discrete frequency components by proceeding from low to high frequencies. This defines a multiscale imaging in the sense that high wavenumbers are progressively incorporated in images. The linearized waveform tomography requires an accurate starting velocity model that has been developed by first-arrival traveltime tomography. After specific pre-processing of the data, 16 frequency components ranging between 5.4 and 20 Hz were inverted. Ten iterations were computed per frequency component leading to 160 tomographic models. The waveform tomography has successfully imaged southwest-dipping structures previously identified from other geophysical data as being associated with high-resistivity bodies. The relevance of the tomographic images is locally demonstrated by comparison of a velocity–depth function extracted from the waveform tomography models with a coincident vertical seismic profiling (VSP) log available on the profile. Moreover, comparison between observed and synthetic seismograms computed in the (starting) traveltime and waveform tomography models demonstrates unambiguously that the waveform tomography successfully predicts for wide-angle reflections from southwest-dipping geological structures. This study demonstrates that the combination of first-arrival traveltime and frequency-domain full-waveform tomographies applied to dense wide-aperture seismic data is a promising approach to quantitative imaging of complex geological structures. Indeed, wide-aperture acquisition geometries offer the opportunity to develop an accurate background velocity model for the subsequent waveform tomography. This is critical, because the building of the macromodel remains an open question when only near-vertical reflection data are considered.

Integration loop of of ‘global offset’ seismic, continuous profiling magnetotelluric and gravity data

In this paper an approach aimed at integrating many different geophysical/geological data is discussed. It is based on a recursive process of forward and inverse modelling of seismic and non-seismic data. The experimental data set of the Enhanced Seismic In Thrust belts (ESIT) research project, funded by Eni E&P, Enterprise Oil Italiana and the European Union, was used in order to apply the approach to a real case of complex geological setting. Near-vertical reflection seismic, long-offset seismic, high-resolution magnetotelluric, gravity, borehole and surface geological data were involved in the process. We demonstrate how a ‘self-feeding’ integration loop is an efficient way to produce a unique geophysical model that responds to several basic requirements, such as optimized inversion/modelling in each parameter space, best fit in each geophysical domain, best seismic imaging, reliable geological meaning and cost/benefit ratio optimization. Introduction Integration of multiple data sets in complex geological settings represents one of the most challenging objectives in geophysics. This is true especially in the case of geophysical projects based on the acquisition of highly redundant data sets and characterized by many different sources of information. In fact, particularly in complex areas where the quality of standard seismic is poor, alternative non-seismic approaches are required. An exploration strategy based on many different and complementary methodologies always produces a complex data set, and integrating all the information in consistent and reliable models can fail if an appropriate integration strategy is not applied. In previous work (Dell'Aversana & Morandi 2000, 2002; Dell'Aversana 2001), we introduced an integration approach based on recursive forward and inverse modelling of seismic, magnetotelluric and gravity data. We showed that the so- called ‘global offset’ seismic approach (which also includes high-fold, long-offset data) can improve significantly the process of building reliable models by a quantitative integration with MT and gravity data and with the support of borehole information. In a subsequent paper (Dell'Aversana et al. 2002b), we demonstrated how, by applying prestack depth migration to global offset data, it is possible to improve the depth imaging in difficult geological settings, also when the S/N ratio of the conventional near-vertical reflection data is very low. This result can be obtained especially if non-seismic data are used for defining accurate multi-parametric models. These models can contribute to the definition of an appropriate velocity field for seismic data migration, as will be clarified in this paper. In fact, recent experiments and applications showed how the continuous profiling magnetotelluric method can produce reliable resistivity sections that can support both the velocity field definition and the geological interpretation in case of low-quality seismic sections (Zerilli & Dell'Aversana 2002). Here, we continue the discussion about the integration of many data sets, but also introduce several important additional concepts. We take the opportunity offered by the large multiple data set collected during the ESIT research project (Buia et al. 2002; Dell'Aversana et al. 2002b). We demonstrate that an appropriate quantitative integration of global offset seismic, continuous profiling high-resolution magnetotelluric (HRMT), gravity, borehole and geological data is a reliable and cost-effective process. Each source of information contributes to different aspects of the process, due to the varying benefits and limitations of each of the different methodologies used. Based on many different geophysical parameters, the final result is a well-calibrated model that is consistent with the best seismic imaging. The geological consistency is considered as a fundamental requirement at each step of the process.

Assessment of crustal velocity models using seismic refraction and reflection tomography

Two tomographic methods for assessing velocity models obtained from wide-angle seismic traveltime data are presented through four case studies. The modelling/inversion of wide-angle traveltimes usually involves some aspects that are quite subjective. For example: (1) identifying and including later phases that are often difficult to pick within the seismic coda, (2) assigning specific layers to arrivals, (3) incorporating pre-conceived structure not specifically required by the data and (4) selecting a model parametrization. These steps are applied to maximize model constraint and minimize model non-uniqueness. However, these steps may cause the overall approach to appear ad hoc, and thereby diminish the credibility of the final model. The effect of these subjective choices can largely be addressed by estimating the minimum model structure required by the least subjective portion of the wide-angle data set: the first-arrival times. For data sets with Moho reflections, the tomographic velocity model can be used to invert the PmP times for a minimum-structure Moho. In this way, crustal velocity and Moho models can be obtained that require the least amount of subjective input, and the model structure that is required by the wide-angle data with a high degree of certainty can be differentiated from structure that is merely consistent with the data. The tomographic models are not intended to supersede the preferred models, since the latter model is typically better resolved and more interpretable. This form of tomographic assessment is intended to lend credibility to model features common to the tomographic and preferred models. Four case studies are presented in which a preferred model was derived using one or more of the subjective steps described above. This was followed by conventional first-arrival and reflection traveltime tomography using a finely gridded model parametrization to derive smooth, minimum-structure models. The case studies are from the SE Canadian Cordillera across the Rocky Mountain Trench, central India across the Narmada-Son lineament, the Iberia margin across the Galicia Bank, and the central Chilean margin across the Valparaiso Basin and a subducting seamount. These case studies span the range of modern wide-angle experiments and data sets in terms of shot—receiver spacing, marine and land acquisition, lateral heterogeneity of the study area, and availability of wide-angle reflections and coincident near-vertical reflection data. The results are surprising given the amount of structure in the smooth, tomographically derived models that is consistent with the more subjectively derived models. The results show that exploiting the complementary nature of the subjective and tomographic approaches is an effective strategy for the analysis of wide-angle traveltime data.

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.