Velocity Contrast Research Articles

Can Higher-Order Finite-Difference Operators Be Applied across a Material Interface?

ABSTRACT It is well known that higher-order and thus longer-stencil finite-difference operators (FDOs) can be advantageously used for evaluating spatial derivatives in the finite-difference schemes applied to smoothly heterogeneous media. This is because they reduce spatial grid dispersion. However, realistic models often include sharp material interfaces. Can high-order long-stencil FDOs be applied across such material interface? We address this question by comparing exact spatial derivatives against derivatives approximated by FDOs with respect to the interface representation, velocity contrast, and order of the FDO. The interface is considered in an arbitrary position with respect to the spatial grid. The material interface exactly represented by the Heaviside step function causes a large error of the FDO spatial derivative near the interface. The maximum error near the interface practically does not depend on the order of the FDO. There are only small differences in errors among FDOs of different orders elsewhere. The larger the velocity contrast, the larger the error. If the material interface is represented using a wavenumber band-limited Heaviside function, the error is smoothed and several times smaller. The error in the wavenumber band-limited model decreases with an increasing order of the FDO. Our findings combined with those by Moczo et al. (2022) lead to the important conclusion: The wavenumber band-limited representation of the material interface is not only a necessary consequence of discretization of the original physical model but also significantly reduces the error in evaluating a spatial derivative using the FDO.

Mapping crustal structure across southern Australia using seismic ambient noise tomography

The rocks of southern Australia record over three billion years of Earth’s evolution, but the basement geology is veiled by sediment. Geophysical data are needed to unveil the geology. The 2018–2022 Lake Eyre Basin and 2020–2022 AusArray SA seismic arrays expand seismic coverage in South Australia. In conjunction with permanent and preceding temporary arrays, we extracted Rayleigh wave dispersion data from ambient noise recordings at a total of 501 seismic stations spanning the transition from Precambrian to Phanerozoic Australia. These data were used to develop Rayleigh wave phase velocity maps of southern Australia at periods 3–20s, and in turn, a shear wave velocity model to 20km depth. At upper-crustal depths, low velocity structure tracks Phanerozoic sedimentary accumulations. The Moyston Fault, regarded as the boundary between the Delamerian and Lachlan Orogens, has an intermittent expression in the shear velocity model: it has no obvious expression at depths shallower than ∼10km, but in the mid-crust is marked by a velocity contrast in southern Victoria and a velocity contrast tracing the southern edge of the Darling Basin in western New South Wales. An arcuate velocity contrast characterising the western edge of deep, sediment-filled troughs of the Darling Basin is a candidate for the transition from Precambrian to Phanerozoic crust. Fluid derived from neotectonic metamorphic devolatilization and/or remnant hydrated mantle is our preferred hypothesis for explaining seismicity and coincident seismic and conductivity anomalies in the mid-to-lower crust beneath the Ikara–Flinders Ranges. Our model suggests that the Olympic Dam and Carrapateena IOCG deposits reside above the margin of a mid-crustal low seismic velocity zone. We surmise that this might reflect past metalliferous fluid movement associated with the Olympic Cu-Au Province, akin to low reflectivity and low resistivity zones evident in 2D reflection seismic and magnetotelluric profiles, respectively.

Direct seismic modeling: day surface topography and shallow subsurface anisotropy

Introduction. The article is devoted to one of the problems in the development of oil and gas fields - the construction of correct geological models of the subsurface space. Researchers from various scientific groups around the world have proposed various ways to improve the accuracy of the computer simulations used in this process. The purpose of this study is to assess the degree of influence of the day surface relief and the anisotropy of the upper part of the geological section on the recorded seismic signal using a realistic model of the Orenburg field as an example.Materials and methods. A seismogeological model describing the Lower Permian interval of the Orenburg geological section is considered. According to well data, the elastic properties of geological formations were estimated: density and propagation velocities of longitudinal and transverse waves. There is a high contrast of P-wave velocities estimated from sonic logs. The reservoir in this model is confined to the lower layers. It is composed of sulfate-carbonate media, uniform in density and acoustic properties. Using the grid-characteristic method, zero-offset synthetic seismograms were calculated. The choice of structural curvilinear computational grids made it possible to correctly consider the relief of the day surface.Research results. In this work, two different models were compared. The first model included the anisotropy of the upper part of the section and the topography of the day surface. In the second model, the upper boundary of the computational domain was flat, and the entire medium was considered within the framework of an isotropic linear elastic model. The analysis of synthetic seismograms showed that the anisotropy inherent in this model does not significantly affect the recorded seismic wave field. However, considering the relief of the day surface significantly shifts the times of arrival of reflected waves.Discussion and Conclusion. The algorithm presented in the paper can be used to verify the field data processing graph, since the assessment of the anisotropy of the medium is a standard step in building a velocity model. The presented approach can be extended to 3D models of realistic dimensions.

Detection of fault zone head waves and the fault interface imaging in the Xianshuihe–Anninghe Fault zone (Eastern Tibetan Plateau)

SUMMARY Fault zone head waves (FZHWs) are an essential diagnostic signal that provides high-resolution imaging of fault interface properties at seismogenic depth. In this study, we validate the existence of a bi-material interface in the Xianshuihe–Anninghe Fault (XAF) zone around their intersection and determine the cross-fault velocity contrast. We employ a semi-automatic workflow to detect and pick FZHWs and direct P waves. In addition, to improve the identification ability of potential FZHWs in the automatic picking process, we adopt a ‘forward-detecting and backward-picking’ strategy combining the short-term average/long-term average (STA/LTA) algorithm with a kurtosis detector. The polarization and characteristic periods of the waveforms are then used to manually refine the picks and evaluate the quality. The results indicate that the average velocity contrast along the southern Xianshuihe Fault is 3–5 per cent, with the northeast side characterizing a faster P-wave velocity, in agreement with tomographic results. A systematic moveout between FZHWs and the direct P waves over a 100 km long fault segment reveals a single continuous interface in the seismogenic zone. The single bi-material fault structure might be conducive to the preparation of large earthquakes and further influences the corresponding dynamic rupture processes.

Salt reconstruction in full-waveform inversion using topology optimization techniques

SUMMARY With the increasing advances in the oil and gas industry, seismic imaging near or under salt structures has become an important point in deep-water exploration. Detailed velocity models of these areas are particularly interesting not only to characterize hydrocarbon reservoirs but also to identify potential sites for hydrogen and carbon dioxide storage in offshore salt caverns. Thus, we study the full-waveform inversion for the salt reconstruction in acoustic media with constant density considering the time-harmonic wave propagation in a finite element formulation using the topology optimization (TO) method. This problem is challenging due to the strong velocity contrast between salt bodies and the sedimentary background, in addition to the lack of low-frequency data and the inherent ill-posedness of the inverse problem. In this context, we incorporate techniques from the TO field, usually used in design applications, to overcome or reduce these known problems. We initially defined the squared slowness as a combination of two fields, one related to the salt shape and the other to the background. An interpolation rule based on the solid isotropic material with penalization method, combined with filtering and projection schemes, is used to find the shape of the salt bodies with increased sharpness interfaces. A Helmholtz-type filter is applied to modify the gradient aimed to regularize the problem and provide a more stable way for the salt shape to evolve during the inversion process. In particular, we demonstrate that the proposed approach may be relevant for reconstructing media with salt bodies when a suitable starting model is unavailable, and sharp interfaces are required. In addition, we present inversion results from synthetic data generated by a variable density model to demonstrate the approach capability when subjected to a reconstruction application.

Internal Structure of the Central Garlock Fault Zone From Ridgecrest Aftershocks Recorded by Dense Linear Seismic Arrays

AbstractWe provide high‐resolution seismic imaging of the central Garlock fault using data recorded by two dense seismic arrays that cross the Ridgecrest rupture zone (B4) and the Garlock fault (A5). Analyses of fault zone head waves and P‐wave delay times at array A5 show that the Garlock fault is a sharp bimaterial interface with P waves traveling ∼5% faster in the northern crustal block. The across‐fault velocity contrast agrees with regional tomography models and generates clear P‐wave reflections in waveforms recorded by array B4. Kirchhoff migration of the reflected waves indicates a near‐vertical fault between 2 and 6 km depth. The P‐wave delay times imply a ∼300‐m‐wide transition zone near the Garlock fault surface trace beneath array A5, offset to the side with faster velocities. The results provide important constraints for derivations of earthquake properties, simulations of ruptures and ground motion, and future imaging studies associated with the Garlock fault.

A local earthquake tomography on the EAF shows dipping fault structure

The East Anatolian Fault Zone (EAFZ) is a left-lateral transform fault zone located between the Anatolian and Arabian plates. In this study, in order to image the upper crustal structure beneath the eastern segments of EAFZ, 3D seismic velocity variations are computed using local earthquake tomography. The initial catalog for the tomography process consists of 2200 well-located earthquakes recorded at 49 seismic stations around the study region between 2007 and 2020. 1D initial velocity model is constructed based on previous studies in the region. The maximum number of iterations and the velocity perturbations which sustain the linearity of the inversion are determined based on the detailed tests. Reliable zones of the final model are decided based on the Derivative Weighted Sum and Hit Count distribution. The resulting velocity model displays a clear velocity contrast across the surface trace of the EAFZ down to a depth of 12 km. While the Anatolian side of the fault displays higher velocities associated with the ophiolitic units in the region, the south of the fault zone is represented by lower velocities due to sedimentary deposits. The vertical cross-sections of tomographic models show a north dipping fault between Palu and Çelikhan. The complete earthquake catalog is relocated using the 3D velocity model. Together with the obtained velocity model, the relocated hypocenters indicate that the dip of the EAFZ is not uniform, the Palu segment dips to the north with an angle of ~80°, while the Pütürge and Erkenek segments dip to the north with a lower angle of ~60?70°.

Low-velocity anomaly in the lithosphere of eastern Central Tianshan imaged with seismic waveform fitting

As one of the most important active intracontinental orogenic belts in the world, the Tianshan has experienced the closure of the Paleo-Asian Ocean, rejuvenation, and intracontinental orogeny caused by the far-field effect of the India-Eurasia collision. Here, based on the delayed seismic waveforms recorded by the China National Seismic Network (CNSN) from local and teleseismic earthquakes, we find an obvious low-velocity anomaly (LVA) of 6% at depths of 35–110 km, which is just beneath the eastern Central Tianshan, near the highest peak of Yilianhabierga Mountain. We constrain the lateral distribution of the LVA with arrival delays of teleseismic P waves, and the vertical distribution and velocity contrast of the LVA with waveform fitting of local seismic P waves. Combined with previous results, the LVA could be related to the rejuvenation of the Tianshan caused by the India-Eurasia plate collision. After the bidirectional subduction of the Tarim lithosphere and Junggar lithosphere into the Tianshan upper mantle, the hot upwelling asthenosphere filled the space in which the delaminated lithosphere was previously occupied. Then, the hot upwelling asthenosphere heated the lithosphere and resulted in the partial melting of the accretionary wedge, which is normally believed to have high water content. Our results can provide important insight into the lithospheric deformation processes and tectonic evolution in the Tianshan.

Study on particle cluster dynamics behavior in settling and the influence by fiber barrier

The dispersion of particle clusters is relevant to a high concentration carrying fluid such as fracturing and drilling fluid, and the fiber-containing fluid could improve the carrying performance of particles. In the present study, the settling dynamics behavior of clusters is investigated using the CFD-DEM method. The numerical simulation method employed was able to accurately predict the deformation and velocity change rules, which express the cluster settling dynamics behavior. The velocity variation of particles is affected by hindrance and contact inside the cluster, and the velocity contrast leads to particle leakage and cluster deformation. The cluster will have regular shape changes and break up in the settlement process under gravity and fluid drag force. According to the sensitivity parameter analysis, the deformation of clusters is mainly affected by the fluid rheological properties. Different from glycerol, the settling cluster in viscoelastic HPG has a more complex dynamic behavior and a better fiber barrier effect. The fiber barrier between particles in the cluster can be regarded as a weakly constrained fiber grid structure. Due to the contact force, fiber grid structure formed in the cluster could effectively restrain the shape change and break up trend in the settlement process. The design also considers the influence of fiber and HPG concentration, and we analyze the settling velocity of the mixed-fiber cluster. Overall, the research on the cluster settling dynamics behavior is helpful in analyzing the effect of fiber-containing fluid in the carrying performance of particles.

Enigmatic crustal and upper mantle structure in the NE Sino-Korean Craton based on nuclear explosion seismic data

We interpret the crustal and upper mantle structure along ∼2500 km long seismic profiles in the northeastern part of the Sino-Korean Craton (SKC). The seismic data with high signal-to-noise ratio were acquired with a nuclear explosion in North Korea as source. Seismic sections show several phases including Moho reflections (PmP) and their surface multiple (PmPPmP), upper mantle refractions (P), primary reflections (PxP, PL, P410), exceptionally strong multiple reflections from the Moho (PmPPxP), and upper mantle scattering phases, which we model by ray-tracing and synthetic seismograms for a 1-D fine-scale velocity model. The observations require a thin crust (30 km) with a very low average crustal velocity (ca. 6.15 km/s) and exceptionally strong velocity contrast at the Moho discontinuity, which can be explained by a thin Moho transition zone (< 5 km thick) with strong horizontal anisotropy. We speculate that this anisotropy was induced by lower crustal flow during delamination dripping. An intra-lithospheric discontinuity (ILD) at ∼75 km depth with positive velocity contrast is probably caused by the phase transformation from spinel to garnet. Delayed first arrivals followed by a long wave train of scattered phases of up to 4 s duration are observed in the 800–1300 km offset range, which are modelled by continuous stochastic velocity fluctuations in a low-velocity zone (LVZ) below the Mid-Lithospheric Discontinuity (MLD) between 120 and 190 km depth. The average velocity of this LVZ is about 8.05 km/s, which is much lower than the IASP91 standard model. This LVZ is most likely caused by rocks which are either partially molten or close to the solidus, which explains both low velocity and the heterogeneous structure.

Seismic imaging of the crust and uppermost mantle structure in the Qinling orogenic belt and its surroundings: Geodynamic implications

The Qinling orogenic belt and surrounding areas are a junction zone in Central China that connects the North China craton, Yangtze craton and northeastern Tibetan plateau. This area is in a key position within the more general framework where there is eastward extrusion of the Tibetan plateau and westward subduction of the Pacific plate. There is an ongoing scientific debate on how these two main processes are driving the geodynamic evolution of the Qinling orogenic belt and surrounding areas. Specific mechanisms have been proposed, such as lower crustal flow, crustal shortening and big mantle wedge tectonics. Knowledge of the seismic velocity structure helps us understand how these two driving forces affect the tectonic blocks of the study area, as well as test how well the proposed mechanisms to fit the regional velocity structure. For this purpose, a high-resolution P-wave velocity model of the crust and uppermost mantle was obtained by inverting arrival times from local earthquakes that occurred in the Qinling and surrounding region. Our three-dimensional (3D) model suggests that crustal channel flow cannot exist in the whole Qinling orogenic belt, and the localized low-velocity zone generated by the horizontal compressive stress caused by the collision of the Indian and Eurasian plates, is a possible explanation for the growth of the northeastern Tibetan plateau. The contrast in seismic velocities found under the east-Qinling and the west-Qinling orogenic belts implies a different response of these two terranes to the eastward extrusion of the Tibetan plateau. Our velocity model points to relatively stable structural features in the east-Qinling and the Weihe graben, while a low-velocity anomaly in the uppermost mantle of the southernmost Ordos plateau suggests that the lithosphere of the southern Ordos plateau is undergoing destruction.

D" Reflection Polarities Inform Lowermost Mantle Mineralogy

Polarities of seismic reflections at the discontinuity atop the D" region (PdP and SdS) indicate the sign of the velocity contrast across the D" reflector. Recent studies found PdP polarities matching and opposite those of P and PcP. While anisotropy could explain this behavior, we find that the ratio of the change in S wave velocity over change in P wave velocity (R-value) can influence polarity behavior of D" reflected P waves. For R-values exceeding 3, the P wave reverses polarity in the absence of anisotropy while S wave polarity is not influenced by the R-value. Using sets of one million models for normal mantle and MORB with varying minerals and processes across the boundary, we carry out a statistical analysis (Linear Discriminant Analysis) finding that there is a marked difference in mantle mineralogy to explain R-values larger and smaller than 3, respectively. Based on our results we can attribute different mineralogy to a number of cases. In particular, we find that when velocities increase across D" and polarities of PdP and SdS are opposite the post-perovskite phase transition is still the best explanation whereas MORB is the best explanation when PdP and SdS are the same. When the velocities are decreasing, the post-perovskite phase transition within MORB is the best explanation if PdP and SdS polarities are the same but if PdP and SdS are opposite, our results indicate that primordial material or mantle enriched in bridgmanite can explain the polarity behavior, further constraining mineralogy within the large low seismic velocity provinces.

Combined analysis of active and passive surface waves for 2D characterization of a landfill

Active and passive surface wave methods have been effective tools for near-surface investigation. For the cases both the near-surface resolution and investigation depth are required, the incorporation of active and passive surface wave methods is necessary to measure the dispersion curve over a broad frequency band. In this work, we carry out the active and passive surface wave methods to characterize a landfill, mainly for delineating its base. Both active and passive surface wave data are collected using a linear array and the roll-along acquisition mode. To analyze the passive data, we use the seismic interferometry technique to generate the virtual source gather. According to the dispersion analysis results, the frequency bands for available phase velocity estimations of active and passive data are different but overlapped. We then employ the multiscale window analysis of surface waves (MWASW) method to extract several dispersion curves from active shot gather and passive virtual source gather. The use of the MWASW method is based on the consideration of heterogeneity in landfills. Once the dispersion curves are extracted, the two kinds of phase velocity data are combined to produce composite dispersion curves. By simultaneously inverting the composite dispersion curves of multisize spatial windows, a 2D shear-wave velocity (Vs) model is generated that shows a clear velocity contrast between the refuse and the underlying stratum at a depth around 5 m consistent with the result of five in-line boreholes. The successful investigation of refuse depth demonstrates that the combination of active and passive surface waves has a good ability to resolve the Vs model of landfills from surface to deep formations.

On the correlation between earthquake coda horizontal-to-vertical spectral ratios and amplification functions at the KiK-net network

The Japanese KiK-net network comprises about 700 stations spread across the whole territory of Japan. For most of the stations, VP and VS profiles were measured down to the bottom borehole station. Using the vast dataset of earthquake recordings from 1997 to 2020 at a subset of 428 seismic stations, we compute the horizontal-to-vertical spectral ratio of earthquake coda, the S-wave surface-to-borehole spectral ratio, and the equivalent outcropping S-wave amplification function. The de facto equivalence of the horizontal-to-vertical spectral ratio of earthquake coda and ambient vibration is assessed on a homologous Swiss dataset. Based on that, we applied the canonical correlation analysis between amplification information and the horizontal-to-vertical spectral ratio of earthquake coda across all KiK-net sites. The aim of the correlation is to test a strategy to predict local earthquake amplification basing the inference on site condition indicators and single-station ambient vibration recordings. Once the correlation between frequency-dependent amplification factors and amplitudes of horizontal-to-vertical coda spectral ratios is defined, we predict amplification at each site in the selected KiK-net dataset with a leave-one-out cross-validation approach. In particular, for each site, three rounds of predictions are performed, using as prediction target the surface-to-borehole spectral ratio, the equivalent of a standard spectral ratio referred to the local bedrock and to a common Japanese reference rock profile. From our analysis, the most effective prediction is obtained when standard spectral ratios referred to local bedrock and the horizontal-to-vertical spectral ratio of earthquake coda are used, whereas a strong mismatch is obtained when standard spectral ratios are referred to a common reference. We ascribe this effect to the fact that, differently from amplification functions referred to a common reference, horizontal-to-vertical spectral ratios are fully site-dependent and then their peak amplitude is influenced by the local velocity contrast between bedrock and overlying sediments. Therefore, to reduce this discrepancy, we add in the canonical correlation as a site proxy the inferred velocity of the bedrock, which improves the final prediction.

High‐Resolution Imaging of Fault Zone Structure Along the Creeping Section of the Haiyuan Fault, NE Tibet, From Data Recorded by Dense Seismic Arrays

AbstractHigh‐resolution imaging of fault zone structures is essential for understanding earthquake physics and fault mechanics. As a major left‐lateral strike‐slip fault in northeastern Tibet, fine structures of the damage zone in the creeping (Laohushan) section of the Haiyuan fault remain unclear. To resolve geometry and velocity reduction of the damage zone, we deployed a dense temporary network of 110 seismic stations around the creeping section of the Haiyuan fault. Travel time delays from teleseismic P arrivals suggest an approximately 1‐km‐wide low‐velocity zone, likely illuminating a broader damage zone around the Haiyuan fault. A catalog is constructed for local earthquakes based on phase picks identified from a machine learning technique. The amplification of waveforms from these local events and waveform modeling of fault zone trapped waves indicate a narrower inner damage zone with depth‐dependent width (ranging from 150 to 50 m) that extends to a depth of approximately 4 km. These values are generally consistent with those found on other noncreeping faults in California, suggesting that these damage zone properties are not affected by fault slip behaviors at shallow depth. In addition, a clear bi‐material velocity contrast across the fault is revealed by the analysis of teleseismic P arrivals. Assuming the contrast extends to a depth of 15 km, we find that P‐wave velocity is approximately 5% slower in the crustal block north of the fault. Our study shows that a temporary dense seismic network is effective in illuminating cross‐fault velocity contrast and fault geometry.

Lower crustal intraplate seismicity in Kachchh region (Gujarat, India) triggered by crustal magmatic infusion: Evidence from shear wave velocity contrast across the Moho

The genesis of intraplate seismicity remains a topic of debate among seismologists. Here, we address this issue by investigating shear wave velocity contrast across the Moho (δβM) in the Deccan volcanic-covered Kachchh seismogenic region, located in the Gujarat state of western India. This region, having complex geology, has witnessed several large damaging intraplate earthquakes since historical times. Most of the shallow focused earthquakes clustered in the eastern part of Kachchh often occurred due to the fluid pressure in the fractured rock matrix at the intersection of a fault network consisting of mainly three prominent faults, viz. Gedi, Kachchh Mainland, and North Wagad. However, the causes of lower crustal earthquakes are not well understood – whether it is due to the reactivation of faults or processes related to the fluid-filled fracked rock matrix of mafic/ultramafic material or magmatic infusion from the underlying mantle. Our results show that the δβM in the study area varies widely from ∼0.15 to 0.86 km/s. The lowest value of δβM (∼0.15 km/s) coincide with lower crustal seismicity. Further, the scaling relation between δβM and crustal thickness is negative, i.e., the thick crust has a low δβM, implying that the magmatic material has enveloped the lower crust, which ascended through the deep rifts during Deccan volcanism at ∼66 Ma. Higher values of δβM below a few stations have been ascribed to the pockets of trapped magmatic material or fluid below the lower crust. We suggest that the presence of mafic/ultramafic material in the crust-mantle transition zone, together with the fluid-filled fractured rock matrix in the intersecting fault geometry might accumulate enough strain to generate seismic activity in the lower crust. The presence of volatile influx or fluid pressure facilitates the triggering of deep-seated faults, and CO2-rich carbonatite-type magma percolated from the shallow asthenosphere.

Seismic Tomography of the Trans-North China Orogen and Its Dynamic Implications

We present a high-resolution S wave velocity model of the Trans-North China Orogen (TNCO) and its adjacent areas derived from S wave traveltime residuals of teleseismic waveforms recorded by the dense ChinArray seismic network. Our model reveals S wave velocity anomalies beneath the study area, suggesting the strong lateral heterogeneities of the upper mantle structures. The upper mantle of the Datong volcano is dominated by the significant low velocity anomalies. In addition to the low-velocity zone rooted in the mantle transition zone beneath the Datong volcano region, two low velocity layers extend northwestward to the Hetao graben and southwestward to the southern TNCO within depths of 100–300 km, respectively. Based on these low-velocity anomalies, we provide new insights into the origin of the Datong volcano, which may be related to the asthenospheric upwelling originated from the water-rich mantle transition zone and horizontal mantle flows driven by the extrusion northeastward of the Tibetan plateau. A clear velocity contrast in the upper mantle above 300 km depth beneath the Ordos block and the TNCO implies that the Ordos block still preserves thick cratonic root while the lithosphere of the TNCO is modified. We propose that the asthenospheric upwelling and northeastward push of the Tibetan plateau account for the lithospheric deformation of the TNCO and its adjacent areas.

Unlocking the Hydrocarbon Potential of the Mannar Basin (Sri Lanka) Based on New Data and New Ideas

The Mannar Basin is a failed rift basin located between Sri Lanka and India. Massive flood basalts have been deposited during latest Cretaceous/earliest Paleocene magmatic activity that is also represented by numerous igneous sill intrusions. The two discoveries are associated with this magmatism: Barracuda and Dorado, both discovered in 2011, have been drilled in the northern part. One of the key challenges for hydrocarbon exploration in volcanic provinces is sub-basalt imaging due to the velocity contrast between basalts and siliciclastics. To unlock hidden play types and identify geological features within Mesozoic intervals we utilized both infill 2D seismic data acquired in 2018 and vintage 2D seismic data reprocessed in 2021. These provide new insights for hydrocarbon exploration and the tectono-stratigraphic evolution of Sri Lanka’s offshore basins. We integrated these new study results into a regional 2D petroleum system modelling study. Modelling results indicate hydrocarbon potential for several play types: Cretaceous reef, turbidite and fan plays. Since the tectonic evolution of the Mannar Basin is still a matter of debate, especially related to timing and duration of extension and Sri Lanka’s position during Gondwana breakup, we created different heat flow scenarios to validate the impact on the hydrocarbon prospectivity.

Enhanced solvent extraction in a serial converging-diverging microchannel at high injection ratio

A microchannel with serial converging–diverging geometries, inspired by porous structures of rock, was designed to enhance the cross-interfacial mass transfer of two-phase flow at a high phase injection ratio up to 200:1. It was experimentally demonstrated that the design enhanced mass transfer coefficient by approximately four times compared to classic microchannel design. The superior performance of the microchannel was attributed to the retention of dispersed fluid before the converging geometry due to the capillary pressure barrier that resulted in velocity contrast between the two fluids and reduced the effective mass-transfer distance in continuous phase. Additionally, the deformation and dispersal of droplets at the diverging geometry further increased the interfacial area. The flow patterns in this geometry were experimentally identified. The serial converging–diverging design can be used for high phase ratio extraction applications.

Elastic Transmitted Wave Reverse Time Migration for Imaging Earth’s Interior Discontinuities: A Numerical Study

ABSTRACT Seismic imaging is crucial in investigating Earth’s interior structures and understanding its tectonics and evolution. The reflected, converted, and scattered waves have attracted considerable attention in the previous studies, whereas the directly transmitted waves are less used in seismic imaging. In this study, we present a novel passive source elastic transmitted wave reverse time migration (T-RTM) method to characterize major discontinuities in Earth’s interior using transmitted P or S waveforms. By extrapolating and then cross correlating the wavefields from the sources with the transmitted wavefields from the receivers using flood velocity models, the velocity discontinuities can be clearly imaged. The advantages and potential applications of the proposed T-RTM method are demonstrated with three synthetic imaging experiments. First, with distributed acoustic sensing using submarine cables, we show that the proposed method can be used to image the Moho with teleseismic S waves recorded in a single axial component, which is difficult for other imaging methods. Second, using direct P waveforms with a single P-wave velocity model, we show that the proposed method can be used to image 3D Moho topography without relying on the VP/VS ratio like in the traditional receiver function imaging. Third, the proposed method can also be used to image a fault plane with a sharp velocity contrast using local earthquakes. We discuss the limitations of the proposed method and some potential issues in field-data applications with additional numerical experiments. The proposed T-RTM method could provide many new opportunities for utilizing transmitted waveforms in the study of oceanic and continental structures.