Seismic Reflection Imaging Research Articles

Deep reflection seismic imaging of iron‐oxide deposits in the Ludvika mining area of central Sweden

ABSTRACTReflection seismic data were acquired within two field campaigns in the Blötberget, Ludvika mining area of central Sweden, for deep imaging of iron‐oxide mineralization that were known to extend down to 800–850 m depth. The two surveys conducted in years 2015 and 2016, one employing a seismic landstreamer and geophones connected to wireless recorders, and another one using cabled geophones and wireless recorders, aimed to delineate the geometry and depth extent of the iron‐oxide mineralization for when mining commences in the area. Even with minimal and conventional processing approaches, the merged datasets provide encouraging information about the depth continuation of the mineralized horizons and the geological setting of the study area. Multiple sets of strong reflections represent a possible continuation of the known deposits that extend approximately 300 m further down‐dip than the known 850 m depth obtained from historical drilling. They show excellent correlation in shape and strength with those of the Blötberget deposits. Furthermore, several reflections in the footwall of the known mineralization can potentially be additional resources underlying the known ones. The results from these seismic surveys are encouraging for mineral exploration purposes given the good quality of the final section and fast seismic surveys employing a simple cost‐effective and easily available impact‐type seismic source.

Imaging the Shallow Subsurface Structure of the North Hikurangi Subduction Zone, New Zealand, Using 2‐D Full‐Waveform Inversion

AbstractThe northern Hikurangi plate boundary fault hosts a range of seismic behaviors, of which the physical mechanisms controlling seismicity are poorly understood, but often related to high pore fluid pressures and conditionally stable frictional conditions. Using 2‐D marine seismic streamer data, we employ full‐waveform inversion (FWI) to obtain a high‐resolution 2‐D P wave velocity model across the Hikurangi margin down to depths of ~2 km. The validity of the FWI velocity model is investigated through comparison with the prestack depth‐migrated seismic reflection image, sonic well data, and the match between observed and synthetic waveforms. Our model reveals the shallow structure of the overriding plate, including the fault plumbing system above the zone of slow‐slip events to theoretical resolution of a half seismic wavelength. We find that the hanging walls of thrust faults often have substantially higher velocities than footwalls, consistent with higher compaction. In some cases, intrawedge faults identified from reflection data are associated with low‐velocity anomalies, which may suggest that they are high‐porosity zones acting as conduits for fluid flow. The continuity of velocity structure away from International Ocean Discovery Program drill site U1520 suggests that lithological variations in the incoming sedimentary stratigraphy observed at this site continue to the deformation front and are likely important in controlling seismic behavior. This investigation provides a high‐resolution insight into the shallow parts of subduction zones, which shows promise for the extension of modeling to 3‐D using a recently acquired, longer‐offset, seismic data set.

Angle-Domain Differential Reflection Traveltime Tomography

Seismic reflection tomography based on prestack depth migration has been widely used for seismic imaging in the presence of complex geology. However, it still has some well-known and unfavorable features, such as slow convergence and requirements of many rounds of migration and residual moveout picking. To avoid the dependence on the unknown true depths of the reflection interfaces and to mitigate the ill-condition of the inverse problem, we propose a differential reflection tomography approach based on the angle-domain common-image gathers. The objective of our method is to iteratively update the velocity model so that the depth deviations between pairs of adjacent incident angles are minimized. Eigenvalue analysis of the tomographic matrices indicates that the conditioning is improved by our double-difference measurement instead of the commonly used float-datum criterion. Aiming at a stable and efficient velocity update, we introduce a structure-oriented model regularization and multi-level grid refinement in each round of residual moveout picking. Synthetic and field data examples show that the proposed approach can successfully retrieve the velocity structures for seismic imaging using fewer rounds of prestack depth migration and residual moveout picking.

Automation methodology to determine the inclination angle of non-parallel BSR on seismic reflection images

Based on the Radon transform, this work presents an automation methodology to determine the inclination angle of the non-parallel BSRs. The non-parallel BSR pattern recognition system was tested with synthetic and real seismic reflection images (from offshore the southern Baja California Peninsula) localizing the BSRs properly in all the cases. The pattern recognition system shows that low time is used to process the seismic images, the methodology is easy to implement in desktop or laptop machines. Therefore, it would be a useful tool to work in situ.

Distribution of Crustal Melt Bodies at the Hot Spot‐Influenced Section of the Galápagos Spreading Centre From Seismic Reflection Images

AbstractAccretion of the lower crust at mid‐ocean ridges is a debated topic, with modern seismic observations pointing to a complex magmatic system that includes an axial multisill system of middle‐ and lower‐crustal melt lenses and near‐ and off‐axis melt bodies. Here we revisit the hot spot‐influenced section of the western Galápagos Spreading Centre and reprocess multichannel seismic reflection data using a wide‐angle seismic tomography model. Our new images show that the magma reservoir in the lower crust at this ridge section is intruded with partially molten melt lenses. The images also show evidence for off‐axis melt lenses, magmatic‐hydrothermal interactions and Moho reflections in this region. We conclude that the similarities between the axial crustal structure of this hot spot‐influenced mid‐ocean ridge and the multisill magmatic structure imaged at the East Pacific Rise indicate that these features are common along the global mid‐ocean ridge system where seafloor spreading is dominated by magmatic accretion.

3D Seismic Reflection Imaging with Uncontrolled Sources using Large N Arrays

3D Seismic Reflection Imaging with Uncontrolled Sources using Large N Arrays

Burial-Related Compaction Modifies Intrusion-Induced Forced Folds: Implications for Reconciling Roof Uplift Mechanisms Using Seismic Reflection Data

Space for shallow-level sills and laccoliths is commonly generated by bending and uplift of overlying rock and sediment. This so-called “roof uplift” produces forced folds, the shape and amplitude of which reflect the geometry of underlying intrusions. The surface expression of forced folds can therefore be inverted to constrain intruding magma body properties, whilst ancient forced folds provide a record of sill and laccolith emplacement. Deciphering how shallow-level intrusion translates into roof uplift is thus critical to enhancing our understanding and forecasting of magma emplacement. To-date, emplacement models and surface deformation inversions are underpinned by the consideration that roof uplift is, to a first-order, an elastic process. However, several studies have suggested inelastic processes can accommodate significant magma volumes, implying first-order roof uplift may be a function of elastic and inelastic deformation. In particular, seismic reflection images of forced folds above ancient sills and laccoliths have been used to argue that final fold amplitudes can be substantially less (by up to 85%) than the underlying intrusion thickness. Although these seismic-based observations imply elastic and inelastic deformation accommodated intrusion, these studies do not consider whether burial-related compaction has reduced the original fold amplitude. Here, we use geological (e.g., lithology) and geophysical (e.g., seismic velocity) information from the Resolution-1 borehole offshore eastern New Zealand, which intersects a forced fold and upper ∼50 m of a sill imaged in 2D seismic reflection data, to decompact the folded sequence and recover its original geometry. We show the Resolution Sill is likely ∼117–187 m thick, depending on the interval velocity for the entire intrusion, whereas the forced fold has an apparent maximum amplitude of ∼127 m, corresponding to a sill thickness-fold amplitude discrepancy of up to 47%. Decompaction indicates the original maximum forced fold amplitude likely ranged from ∼131–185 m, suggesting post-emplacement, burial-related compaction of this and other forced folds may be the source of apparent discrepancies between fold amplitude and intrusion thickness. Whilst seismic reflection data can provide fundamental insights into how shallow-level emplacement translates into roof uplift and ground displacement, we show decompaction and backstripping are required to recover the original fold geometry. To assess the relative importance of elastic and inelastic space-making processes during the formation of seismically imaged sills and forced folds, we demonstrate that our method should be applied to remove any post-emplacement, burial-related compaction signature.

Faulting processes during early-stage rifting: seismic and geodetic analysis of the 2009–2010 Northern Malawi earthquake sequence

SUMMARYIn December, 2009, a rare sequence of earthquakes initiated within the weakly extended Western Rift of the East African Rift system in the Karonga province of northern Malawi, providing a unique opportunity to characterize active deformation associated with intrabasinal faults in an early-stage rift. We combine teleseismic and regional seismic recordings of the largest events, InSAR imagery of the primary sequence, and recordings of aftershocks from a temporary (4-month) local network of six seismometers to delineate the extent and geometry of faulting. The locations of ∼1900 aftershocks recorded between January and May 2010 are largely consistent with a west-dipping normal fault directly beneath Karonga as constrained by InSAR and CMT fault solutions. However, a substantial number of epicentres cluster in an east-dipping geometry in the central part of the study area, and additional west-dipping clusters can be discerned near the shore of Lake Malawi, particularly in the southern part of the study area. Given the extensive network of hanging wall faults mapped in the Karonga region on the surface and in seismic reflection images, the distribution of events is strongly suggestive of multiple faults interacting to produce the observed deformation, and the InSAR data permit this but do not require it. We propose that fault interaction contributed to the seismic moment release as a series of Mw 5-to-6 events instead of a normal main shock–aftershock sequence. We find the depth of fault slip during the main shocks constrained by InSAR peaks at less than 6 km, while the majority of recorded aftershocks are deeper than 6 km. This depth discrepancy appears to be robust and may be explained by fault interaction. Structural complexities associated with fault interaction may have limited the extent of coseismic slip during the main shocks, which increased stress deeper than the coseismic slip zone on the primary fault and synthetic faults to the east, causing the energetic aftershock series. There is no evidence of deformation at the Rungwe volcanic province ∼50 km north of the earthquake sequence between 2007 and 2010, consistent with previous interpretations of no significant magmatic contribution during the sequence.

Deep onshore reflection seismic imaging of the chalk group strata using a 45 kg accelerated weight-drop and combined recording systems with dense receiver spacing

The Chalk Group forms important hydrocarbon reservoirs offshore and water aquifers onshore Denmark. Within a day of fieldwork, a 450 m long reflection seismic profile was acquired onshore in an area in southeast Denmark, where the Chalk Group extends almost to the surface and is approximately 900 m thick. The main objective of the study was to image the complete Chalk Group in high resolution and to study the origin of reflectivity within the different chalk units. A 45 kg accelerated weight-drop source, in combination with dense receiver spacing using microelectromechanical sensors mounted on a streamer and 48 planted geophones, was used for data acquisition. The profile runs subparallel to the cliffs of Stevns, and the recorded signal reaches the base of the Chalk Group at approximately 600 ms. The fully cored 443 m-deep Stevns-1 borehole, which is located at the recorded seismic line, provides excellent control on lithologic and facies changes. Comparison with the borehole data demonstrates that our seismic data set provides a high-resolution image of the internal layering of the Chalk Group. We find that the internal reflection coefficients of the Chalk Group are, in general, small based on wireline-log data. However, the reflected amplitudes are just big enough to be recorded with the receiver setup used, even from the pure chalk beds of the Chalk Group. The reflectivity seen on the high-resolution seismic profile is influenced by occurrences of clay-enriched chalk layers. Flint bands consisting of numerous flint nodules are a characteristic of the uppermost part of the Chalk Group at Stevns. The flint nodules appear to produce significant scattering of the seismic signals, and flint-rich layers appear with diffuse internal reflectivity characteristics. Outcrop-scale mound structures in Danian and Upper Cretaceous outcrops are for the first time seismically resolved.

Subsurface fault investigation in Chiang Rai province, northern Thailand by integrated geophysical surveys

A magnitude 6.3 earthquake, the biggest instrumentally recorded earthquake in Thailand, occurred in Chiang Rai Province on 5 May 2014 and caused large damages in the affected area. The earthquake generated numerous aftershocks that could portray the location of the fault plane beneath the ground. In this work, we conducted integrated geophysical surveys consisting of 2D seismic reflection and 2D resistivity imaging surveys to explore for active faults that could have caused this earthquake. A seismic reflection survey line with a total length of 3,750 metres and resistivity survey with a total length of 1,975 metres were conducted along the Chiang Rai earthquake’s aftershock locations. The subsurface fault geometry was imaged from this integrated geophysical survey. Numerous subsurface discontinuities detected from both seismic reflection and 2D resistivity imaging survey were interpreted as potential faults along the survey line with depths from a few meters to around 500 metres. These subsurface discontinuities correspond well with the aftershock locations which could suggest a fault rupture plane.

Reflection seismic images and amplitude ratio modelling of the Chilean subduction zone at 38.25°S

Active source near-vertical reflection (NVR) data from the interdisciplinary project TIPTEQ were used to image and identify structural and petrophysical properties within the Chilean subduction zone at 38.25°S, where in 1960 the largest earthquake ever recorded (Mw 9.5) occurred. Reflection seismic images of the subduction zone were obtained using the post-stack depth migration technique to process the three components of the NVR data, allowing to present P- and S-stacked time sections and depth-migrated seismic reflection images. Next, the reflectivity method allowed to model traveltimes and amplitude ratios of pairs of reflections for two 1D profiles along the studied transect. The 1D seismic velocities that produced the synthetic seismograms with amplitudes and traveltimes that fit the observed ones were used to infer the rock composition of the different layers in each 1D profile. Finally, an image of the subduction zone is given. The Chilean subduction zone at 38.25°S underlies a continental crust with highly reflective horizontal, as well as dipping events. Among them, the Lanalhue Fault Zone (LFZ), interpreted to be east-dipping, is imaged to very shallow depths for the first time. In terms of seismic velocities, the inferred composition of the continental crust is in agreement with field geology observations at the surface along the profile. Furthermore, no measurable amounts of fluids above the plate interface in the continental crust in this part of the Chilean subduction zone are necessary to explain the results. A large-scale anisotropy in the continental crust and upper mantle is qualitatively proposed. However, quantitative studies on this topic in the continental crust of the Chilean subduction zone at 38.25°S do not exist to date.

SH-Mode Seismic-Reflection Imaging of Earthfill Dams

Abstract Assessing subsurface characteristics and imaging geologic features (e.g., faults, cavities, low-velocity layers, etc.) are typical problems in near-surface geophysics. These questions often have adverse geotechnical engineering implications, and can be especially acute when associated with high-hazard structures such as large earthen flood-control dams. Dam-related issues are becoming more frequent in the United States, because a large part of this major infrastructure was designed and constructed in the early- to mid-twentieth century; these dams are thus passing into the latter stages of their design life, where minute flaws that were overlooked or thought to be insignificant in design/construction are now proving problematic. The high-hydraulic heads associated with these structures can quicken degradation of weak areas and compromise long-term integrity. Addressing dam-related problems solely with traditional invasive drilling techniques is often inadequate (i.e., lack of lateral resolution) and/or economically exorbitant at this scale. However, strategic geotechnical drilling integrated with the broad utility of near-surface geophysics, particularly the horizontally polarized shear-wave (SH-mode) seismic-reflection technique for imaging the internal structural detail and geological foundation conditions of earthfill embankment dams can cost-effectively improve the overall subsurface definition needed for remedial engineering. Demonstrative evidence for this supposition is provided in the form of SH-wave seismic-reflection imaging of in situ and engineered as-built components of flood-control embankment dams at two example sites in the central United States.

Migration-based microseismic event location in the Schlema-Alberoda mining area

Mining operations in the Schlema-Alberoda uranium mine located in south-west Saxony (Germany) stopped in 1990, and the mine was then allowed to flood. The region is seismically active and has been monitored for a long time. We have located some of the microseismic events which occurred between 2005 and 2012 in the mining area to investigate the nature of the seismicity, as well as the long-term role of mining-induced and triggered seismicity. In this study, we used pseudo-synthetic data obtained from the vertical component records of a local seismic network, to accurately locate the hypocenters. The location procedure is based on a Kirchhoff Prestack Depth Migration approach. In order to assess the accuracy of the migration results, a homogeneous and a 3-D velocity model are tested in the migration algorithm. Both of these velocity models were previously used to migrate data acquired by a 3-D reflection seismic survey in this area. The resulting images were comparable, with minor differences to the reflectors in our area of interest. In contrast, we found that the errors in the velocity model can strongly affect the migration-based earthquake localization. The located hypocenters are compared to the 3-D reflection seismic image of the area to determine possible correlations between the seismicity and reflectivity. This comparison shows that some of reflectors can be directly correlated with the induced and triggered seismicity of the area. The results also indicate that the main causes of the post-mining seismicity are changes in the mine flooding level and the consequent changes in the pore pressure of the subsurface structures. Finally, we confirm that the hypocenters are often bound to preexisting subsurface structures like faults and fractures in the area. In particular, the spatial pattern of the hypocenters proves the existence of a previously unmapped extension of the Ruhmvoll fault. This pattern also suggests a connection between this structure and the Roter Kamm fault, a major component of the larger Gera-Jàchymov fault zone.

2D multiresolution automated system for detecting BSRs on seismic reflection images

2D multiresolution automated system for detecting BSRs on seismic reflection images

Controls on submarine canyon head evolution: Monterey Canyon, offshore central California

The Monterey submarine canyon, incised across the continental shelf in Monterey Bay, California, provides a record of the link between onshore tectonism, fluvial transport, and deep-marine deposition. High-resolution seismic-reflection imaging in Monterey Bay reveals an extensive paleocanyon unit buried below the seafloor of the continental shelf around Monterey and Soquel canyon heads. Paleocanyons shifted position through numerous phases of cut-and-fill in response to Salinas, Pajaro, and San Lorenzo river extensions and avulsions across the continental shelf during high-frequency Pleistocene sea-level and climatic variations. Five seismic facies within the Monterey paleocanyon unit and below the modern canyon are defined to interpret canyon evolution during the Pleistocene. Repeated sea-level oscillations appear to have switched the main fairway(s) of sediment transport. Large-scale erosion and fill occurred in marine environments. Paleocanyon fill is characterized by paleo-axial channel deposits and mass transport deposits, followed by canyon head abandonment and marine sedimentation. The upper portion of the paleocanyon unit contains relatively small channels that were likely incised by erosion in the paleo-Salinas and Pajaro rivers and filled with a mix of nonmarine and marine deposits. Shifting position of submarine canyons over time is characteristic of Monterey Bay, east of the Monterey Bay Fault Zone, and is likely unidentified in other submarine canyon head regions that lack dense high-resolution seismic-reflection subbottom images. We show that canyon heads can be areas of sediment accumulation linked to sea-level oscillations, providing new insights into submarine canyon evolution and sequence stratigraphy.

Automatic velocity analysis using high-resolution hyperbolic Radon transform

Velocity analysis is crucial in reflection seismic data processing and imaging. Velocity picking is widely used in the industry for building the initial velocity model. When the size of the seismic data becomes extremely large, we cannot afford the corresponding human endeavor that is required by the velocity picking. In such situations, an automatic velocity-picking algorithm is highly demanded. We have developed a novel automatic velocity-analysis algorithm that is based on the high-resolution hyperbolic Radon transform. We formulate the automatic velocity-analysis problem as a constrained optimization problem. To solve the optimization problem with a hard constraint on the sparsity and distribution of the velocity spectrum, we relax it to a more familiar L1-regularized optimization problem in two steps. We use the iterative preconditioned least-squares method to solve the L1-regularized problem, and then we apply the hard constraint of the target optimization during the iterative inversion. Using synthetic and field-data examples, we determine the successful performance of our algorithm.

How, where, and when do radial faults grow near salt diapirs?

We examine 3-D seismic reflection data from the Santos Basin, offshore Brazil to determine how, where and when do radial faults grow near a sub-circular salt diapir (stock). We show roof stretching alone cannot account for the large heights and lengths of the kilometre-scale radial faults, suggesting stock widening (‘stem push’), a mechanism implied in numerical models but not yet documented in natural examples, played a pivotal role in radial fault formation. We suggest that, when a diapir is covered by a roof, radial faults form in its overburden due to roof stretching, extending no further than the limit of the drape folding. The roof may then be shouldered aside and the faults buried along the stock flanks, exposing these strata to stem push-related stresses that may then reactivate pre-existing or form new radial faults. Radial faults, irrespective of how they formed, may dip-link with or offset one-another as salt continues to rise. We suggest the causal mechanism for radial fault formation will likely change as roof thickness varies during diapirism, with this reflecting the ratio between sedimentation rate and salt volumetric flux. Our findings are likely applicable to other diapirs, helping us not only to interpret the paleo-stress state of salt-bearing sedimentary basins, but also advancing our understanding of fracture distributions, potential fluid flow pathways, and reservoir compartmentalization around salt diapirs in basins where seismic reflection imaging is poor.

High-resolution diapycnal mixing map of the Alboran Sea thermocline from seismic reflection images

Abstract. The Alboran Sea is a dynamically active region where the salty and warm Mediterranean water first encounters the incoming milder and cooler Atlantic water. The interaction between these two water masses originates a set of sub-mesoscale structures and a complex sequence of processes that entail mixing close to the thermocline. Here we present a high-resolution map of the diapycnal diffusivity around the thermocline depth obtained using acoustic data recorded with a high-resolution multichannel seismic system. The map reveals a patchy thermocline, with spots of strong diapycnal mixing juxtaposed with areas of weaker mixing. The patch size is of a few kilometers in the horizontal scale and of 10–15 m in the vertical one. The comparison of the obtained maps with the original acoustic images shows that mixing tends to concentrate in areas where internal waves, which are ubiquitous in the surveyed area, become unstable and shear instabilities develop, enhancing energy transfer towards the turbulent regime. These results are also compared with others obtained using more conventional oceanographic probes. The values estimated based on the seismic data are within the ranges of values obtained from oceanographic data analysis, and they are also consistent with reference theoretical values. Overall, our results demonstrate that high-resolution seismic systems allow the remote quantification of mixing at the thermocline depth with unprecedented resolution.

Revisiting Dix’s RMS approximation for Normal Move-Out Velocity

The RMS velocity approximation continues to be used in seismic data processing and interpretation while its limitations are not well understood because its derivation is opaque to most modern geophysicists. One of the reasons for this is because Dix describes in (Dix, 1955) NMO velocity functions determined by T2-X2 analysis of shot records, which very few current geophysicists have ever performed. A clearer understanding of T2-X2 analysis and the derivation of the RMS formula will enable NMO-derived velocities to be better understood and so better used in reflection seismic imaging, inversion and interpretation. Dix’s derivation closely follows the process of correcting the apparent depth from say refraction of objects in water when viewed from the air. So it is useful to search for this subject and follow the development which relies on the observer’s eye separation being small relative to the depth of the object and also relies on refraction being towards the normal of any interface so that angles of incidence will reduce and remain small. This is true for light because the speed of light in water is much slower in water than in air. Reflection seismic uses sound which in general bends away from the normal at each interface as sound velocities tend to increase with depth so that angles of incidence increase which causes a problem with the method which we will see later.

A low-angle detachment fault revealed: Three-dimensional images of the S-reflector fault zone along the Galicia passive margin

A new 3-D seismic reflection volume over the Galicia margin continent–ocean transition zone provides an unprecedented view of the prominent S-reflector detachment fault that underlies the outer part of the margin. This volume images the fault's structure from breakaway to termination. The filtered time-structure map of the S-reflector shows coherent corrugations parallel to the expected paleo-extension directions with an average azimuth of 107°. These corrugations maintain their orientations, wavelengths and amplitudes where overlying faults sole into the S-reflector, suggesting that the parts of the detachment fault containing multiple crustal blocks may have slipped as discrete units during its late stages. Another interface above the S-reflector, here named S′, is identified and interpreted as the upper boundary of the fault zone associated with the detachment fault. This layer, named the S-interval, thickens by tens of meters from SE to NW in the direction of transport. Localized thick accumulations also occur near overlying fault intersections, suggesting either non-uniform fault rock production, or redistribution of fault rock during slip. These observations have important implications for understanding how detachment faults form and evolve over time. 3-D seismic reflection imaging has enabled unique insights into fault slip history, fault rock production and redistribution.