P-wave Velocity Model Research Articles

Upper mantle velocity structure beneath the eastern South China Block and implications for late Mesozoic magmatism

The eastern South China Block (ESCB) is characterized by intense magmatism in the late Mesozoic. To further investigate the mechanism of late Mesozoic magmatism, we apply the tomographic method to construct a P-wave velocity model down to 750 km by using 40,989 P-wave arrival times collected from 687 teleseismic events recorded by 191 stations. Our model shows that (1) a high-Vp anomaly is observed beneath the lower Yangtze Block (YZB) and northern Cathaysia Block (CAB) in the crust and uppermost mantle, which could represent the present lithosphere; (2) two high-Vp anomalies are observed in the upper mantle beneath the YZB, which could be the detached lithosphere and the relict of the subducted SCB; (3) the break-off of the subducted Eurasian Plate has penetrated the 410 km discontinuity beneath central and northern Taiwan Island; (4) the stagnant Paleo-Pacific Plate in the MTZ is mainly beneath the lower YZB; (5) the low-Vp anomaly in the upper mantle that is immediately above the stagnant Paleo-Pacific Plate may represent the thermal asthenosphere caused by plate dehydration; and (6) the low-Vp anomaly beneath the southern CAB extends from the crust downward to the MTZ, which may represent an ‘upward channel’ for thermal materials. Based on our results and those of previous studies, we suggest that late Mesozoic magmatism in the ESCB could be highly associated with the subduction of the Paleo-Pacific Plate and its eastward rollback. We also suggest that thermal/chemical erosion may be responsible for lithospheric thinning in the ESCB.

Integrated Geophysical Study of the Collision Between the North China Craton and the Yangtze Craton and Its Links With Craton Lithospheric Thinning

Unlike stable cratons elsewhere in the world, the lithosphere is strongly thinned in the east of both the North China Craton (NCC) and the Yangtze Craton (YZC) compared with the west. We deployed four active-source onshore-offshore wide-angle seismic survey lines in the eastern NCC and YZC from 2010 to 2016 with the aim of revealing the mechanism of lithospheric thinning and the process of the collision between the NCC and YZC. We obtained high-resolution crustal P-wave velocity models for the eastern NCC and YZC based on seismic forward modeling, travel-time tomography, and finite-difference wave-field modeling. Based on our integrated geophysical study and previous work, we propose a five-stage model for the collision between the YZC and NCC, with different dominant geodynamic mechanisms in each stage. Our collision model shows that lithospheric thinning in the eastern NCC and YZC is a consequence of the NCC-YZC collision and subduction of the Pacific plate.

Structural complexities and tectonic barriers controlling recent seismic activity in the Pollino area (Calabria–Lucania, southern Italy) – constraints from stress inversion and 3D fault model building

Abstract. We reconstruct the 3D fault model of the structures causative of the 2010–2014 Pollino seismic activity by integrating structural–geological and high-resolution seismological data. We constrained the model at the surface with fault-slip data, and at depth, by using the distributions of selected high-quality relocated hypocenters. Relocations were performed through the non-linear Bayloc algorithm, followed by the double-difference relative location method HypoDD applied to a 3D P-wave velocity model. Geological and seismological data highlight an asymmetric active extensional fault system characterized by an E- to NNE-dipping low-angle detachment, with high-angle synthetic splays, and SW- to WSW-dipping, high-angle antithetic faults. Hypocenter clustering and the time–space evolution of the seismicity suggest that two sub-parallel WSW-dipping seismogenic sources, the Rotonda–Campotenese and Morano–Piano di Ruggio faults, are responsible for the 2010–2014 seismicity. The area of the seismogenic patches obtained projecting the hypocenters of the early aftershocks on the 3D fault planes, are consistent with the observed magnitude of the strongest events (Mw=5.2, and Mw=4.3). Since earthquake-scaling relationships provide maximum expected magnitudes of Mw=6.4 for the Rotonda–Campotenese and Mw=6.2 for the Morano–Piano di Ruggio faults, we may suppose that, during the sequence, the two structures did not entirely release their seismic potential. The reconstructed 3D fault model also points out the relationships between the activated fault system and the western segment of the Pollino Fault. The latter was not involved in the recent seismic activity but could have acted as a barrier to the southern propagation of the seismogenic faults, limiting their dimensions and the magnitude of the generated earthquakes.

Surface and underground seismic characterization at Terziet in Limburg—the Euregio Meuse–Rhine candidate site for Einstein Telescope

We present a detailed characterization of surface and underground seismic noise measured at Limburg in the south of the Netherlands. This location is the Euregio Meuse–Rhine candidate for hosting Einstein Telescope, a future observatory for gravitational waves. Seismic noise measurements were performed with an array of seismometers installed on the surface. Passive seismic methods like beamforming were used to extract the propagation wave types of ambient seismic noise and the Rayleigh-wave dispersion in the region. Subsurface shear-wave models sensitive to depths of 300 m were derived by using the Rayleigh-wave dispersion and ellipticity. Subsurface P-wave velocities to depths of 200 m were obtained from an active seismic survey. Wavepath Eikonal tomography was used on the source-receiver refracted-wave travel-times to obtain a subsurface P-wave velocity model. Both the passive and the active seismic data analysis point to the presence of a layered geology with a soft-soil to hard-rock transition occurring at a shallow depth of about 25 to 40 m. The surface arrays are complemented by two permanent tri-axial seismometers installed on the surface and in a borehole at 250 m depth. Their data are used to interpret the surface-wave and body-wave contributions to the observed seismic noise. We use a cross-correlation analysis and compute the theoretical surface-wave eigenfunctions to understand the contributions of the different wave types at different frequencies. We observe that below 4 Hz in the horizontal component and 9 Hz in the vertical component, the seismic noise at depth is dominantly due to surface waves. Above these frequencies a significant contribution can be attributed to both nearby and far-away body-wave sources. At a depth of 250 m we find that the surface noise power has been damped by up to a factor 104 above about 2 Hz. The Limburg geology with soft-soil on top of hard-rock efficiently damps the anthropogenic noise produced at the surface. This implies that Einstein Telescope’s test masses are shielded from anthropogenic seismic noise and construction at greater depth will not bring significant further improvements in this regard. A body-wave background has been identified that contributes about half of the total underground seismic noise at 250 m depth for frequencies above 4 Hz. It remains to be studied if subtraction schemes for Newtonian noise originating from this body-wave background will be necessary. Finally, we estimate an interferometer downtime of about 3% due to regional and teleseismic earthquakes. We believe this is acceptable as it is comparable to current experience at the LIGO and Virgo interferometer sites.

Toward a Practical Appraisal for Waveform Tomography of Band- and Offset-Limited Marine Seismic Data

We present a generalized workflow to retrieve high-resolution P-wave velocity ( <formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex>$V_{ P}$</tex> </formula> ) models of complex Earth's subsurface structures from traditional marine near-vertical seismic reflection experiments. These records have typically offsets too short to map refraction phases and lack low-frequency information. The workflow is composed of three steps: 1) downward continuation (DC) of seismic records to the seafloor to recover diving wave information; 2) travel-time tomography (TTT) of first arrivals obtained from DC data, to retrieve a kinematically correct model; and 3) full-waveform inversion (FWI) of the original streamer dataset, starting with the model obtained with TTT and sequentially introducing higher wavenumber details into the model. We show that the TTT allows overcoming the issues associated with the nonlinearity intrinsic to FWI. We also disentangle envelope and phase from the waveform to choose the objective function most suitable for FWI. We assess the accuracy of initial models and predict the quality of the FWI results by quantifying the early arrival cycle skipping between original and simulated data. The efficiency of the workflow is tested with a challenging synthetic target model, containing vertical boundaries with strong velocity contrasts and velocity inversions embedded in a checkerboard-like pattern. We show that workflow steps 1) and 2) provide a TTT model that is not cycle skipped at the frequencies available in most marine seismic experiments and thus allow step 3) FWI to obtain high-resolution <formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex>$V_{ P}$</tex> </formula> models of the subsurface using band- and offset-limited field datasets, traditionally collected in marine airgun and streamer acquisitions.

A regularization by denoising (RED) scheme for 3-D FWI model updates in large-contrast media

SUMMARY Full waveform inversion (FWI) endeavours to estimate high-resolution physical properties of subsurface structures. The technique minimizes the data misfit between observed and modelled seismograms. Despite its success, the application of FWI in areas with high-velocity contrasts remains a challenging problem. Often, quadratic regularization methods are chosen to stabilize inverse problems. Unfortunately, quadratic regularization does not preserve edges and sharp discontinuities adequately. Conversely, a regularization term that uses the l1 norm of the gradient of model parameters can preserve discontinuities. The latter leads to edge-preserving methods based on total variation regularization. This work adopts the framework named regularization by denoising (RED) to solve the FWI problem in high-contrast media. The RED technique only requires an image denoising engine, which, in our case, is a modified weighted total variation filter. One advantage of adopting the RED algorithm for solving FWI problems is its simplicity in the numerical implementation and selection of trade-off parameters. We have benchmarked our algorithm via the 2-D BP/EAGE model, a model with significant velocity contrasts and complex salt bodies. We have also tested the proposed regularization method with the 3-D SEG/EAGE overthrust P-wave velocity model. We also compare the proposed RED method and FWI with total variation regularization.

Structural characteristics of the Chiwei Platform and their implications for the back-arc rifting of the Okinawa Trough, East China Sea

The Chiwei Platform is an isolated submarine topographic high located on the northwestern slope of the southern Okinawa Trough. It is separated from the East China Sea Shelf by a wedge-shaped submarine valley corresponding to a deep basin (Chiwei Depression). Newly acquired seismic reflection profiles and P-wave velocity models demonstrate that the acoustic basement of the platform is shallow and only covered by thin sediments or is even exhumed. Similarities in seismic reflection and P-wave velocity structures between the Chiwei Platform, East Chiwei Seamount and Diaoyudao Uplift suggest that the Chiwei Platform is a high-standing relict basement high formed during the back-arc rifting of the Okinawa Trough. It lies in the transition zone in which the directions of the back-arc rifting centers clockwise rotated from the NE-SW to E-W southwestwardly. Southwest of the transition zone, the southern Diaoyudao Uplift is left-laterally shifted relative to the middle Diaoyudao Uplift and might be disconnected from the latter due to the presence of the Chiwei Depression. The original southern Diaoyudao Uplift exhibited a basement high resembling the middle and northern Diaoyudao Uplift. However, this NE-SW-trending basement high was destroyed and collapsed due to oblique dissections of the E-W-trending rifting centers along with westward propagation of the southern Okinawa Trough. Since nonmagnetic prerifted basement rocks have not been drilled on the Diaoyudao Uplift, the Chiwei Platform is a preferential drilling site where the prerifted basement rocks can readily be obtained due to its high-standing position and thin sedimentary cover. This work can provide supplementary constraints on the basement properties of the Ryukyu back-arc region and help to better understand the evolution of the southern Okinawa Trough.

Seismogenic crustal structure affected by the Hainan mantle plume

Mantle plumes are one of the most influential first-order tectonic systems on Earth, playing a prominent role in controlling local magmatic activity and seismicity. Recent studies have shown that the Hainan mantle plume originates from the lower mantle and its volcanic activity is still intense during the Holocene. However, the effect of the Hainan plume on local seismicity is still unclear. In this study, we conducted the first wide-angle seismic survey in the source area of the 1969 earthquake doublet (M 5.1 and 5.2) offshore Hainan Island. A high-resolution 3-D P-wave velocity model of the crust is obtained using a large amount of Pg and PmP arrival times recorded by a portable array consisting of 17 ocean bottom seismographs (OBSs). Our results show that the 1969 earthquake doublet occurred in high-to-low velocity transitional zones which correspond to active faults. Prominent low-velocity anomalies are revealed in the crust of the study area, which may reflect magmatic intrusion channels. On the basis of multiple geophysical evidence including multi-channel seismic reflection profiles, geothermal data, and seismic tomography results, we infer that the low-velocity anomalies in the source area may reflect magmas and fluids from the Hainan plume, and high-angle normal faults in the crust serve as conduits for ascending of the magmas and fluids. The 1969 earthquake doublet offshore Hainan Island can be attributed to the magma-hydrothermal alteration processes in the fault zones, which led to the significant reduction of the fault strength and friction coefficient, facilitating the shear failure propagation. Furthermore, the seismicity in and around Hainan Island exhibits distinct swarm-like behaviors, which is also evidence for the Hainan plume affecting the crustal seismicity.

Full waveform inversion based on the non-parametric estimate of the probability distribution of the residuals

SUMMARY In an attempt to overcome the difficulties of the full waveform inversion (FWI), several alternative objective functions have been proposed over the last few years. Many of them are based on the assumption that the residuals (differences between modelled and observed seismic data) follow specific probability distributions when, in fact, the true probability distribution is unknown. This leads FWI to converge to an incorrect probability distribution if the assumed probability distribution is different from the real one and, consequently it may lead the FWI to achieve biased models of the subsurface. In this work, we propose an objective function which does not force the residuals to follow a specific probability distribution. Instead, we propose to use the non-parametric kernel density estimation technique (KDE) (which imposes the least possible assumptions about the residuals) to explore the probability distribution that may be more suitable. As evidenced by the results obtained in a synthetic model and in a typical P-wave velocity model of the Brazilian pre-salt fields, the proposed FWI reveals a greater potential to overcome more adverse situations (such as cycle-skipping) and also a lower sensitivity to noise in the observed data than conventional L2- and L1-norm objective functions and thus making it possible to obtain more accurate models of the subsurface. This greater potential is also illustrated by the smoother and less sinuous shape of the proposed objective function with fewer local minima compared with the conventional objective functions.

Can sub-slab low-velocity anomalies be an artifact caused by anisotropy? A case study from the Alboran slab area in the western Mediterranean

Although it is well understood that mantle flow in a subduction setting can develop seismic anisotropy, many tomographic studies still assume an isotropic mantle. In this study, we explore how unaccounted-for seismic anisotropy contributes to P-wave anomalies, focusing on apparent sub-slab low velocities. We use data from the westernmost Mediterranean, where the Alboran slab has subducted beneath the Gibraltar Arc. Our isotropic P-wave velocity model shows a low-velocity anomaly at depths of 75–225 km to the west of the high-velocity-anomaly that represents the Alboran slab. We explore whether this low-velocity anomaly could be an anisotropy-induced artifact as a significant shear-wave-splitting (SWS) along the arc is observed. We consider five different hypothetical anisotropy models representing different mantle configurations. The anisotropy models are based on 1) the SWS observations, 2) the depth distribution of the low-velocity anomaly and 3) geodynamic modeling of the development of entrained and toroidal mantle flow in generic subduction zones. Given the underdetermined nature of an anisotropic inversion, we include the anisotropy models as a priori constraints on the tomography. The tomographic results including anisotropy models have smaller travel time residuals and isotropic anomalies in the region of interest while fitting the SWS observation better than the isotropic model. We propose that unaccounted-for seismic anisotropy can produce artifacts in isotropic tomography of real datasets, and this is at least partially the case for the low-velocity anomaly below the Alboran slab. Further, we show that not taking this into account can lead to erroneous conclusions about mantle temperature and the presence of melt. From the insights that our anisotropy models provide, we also suggest that toroidal mantle flow is currently dominant below and around the Alboran slab.

Crustal extension and magmatism along the northeastern margin of the South China Sea: Further insights from shear waves

The northern continental margin of the South China Sea (SCS) is a unique rifted margin that experienced weak magmatism in the syn-rift stage and intense magmatism in the post-rift stage. This area also exhibits a complicated relationship between crustal extension and magmatism. We present the tomographic inversion of seismic P-wave and S-wave velocity models, as well as the Vp/Vs ratio model, in order to delineate the crustal extension and post-rift magmatic features. The velocity structure is created by forward modelling (RayInvr) and travel-time tomographic inversion (Tomo2D). The results suggest that Mesozoic strata near the Dongsha area has a maximum thickness of ~4.6 km with a Vp of 3.5–5.5 km/s, a Vs of 1.9–3.1 km/s, and Vp/Vs ratios of 1.71–1.76, indicating a low porosity and high degree of diagenesis. High-velocity lower crust (HVLC) is imaged in our model, with a Vp of 7.0–7.5 km/s ± 0.05–0.25 km/s and Vp/Vs ratios of 1.70–1.82 ± 0.05. The composition of the HVLC is mainly mafic, we considered it is related to magmatic underplating due to decompression melting caused by crustal extension. The crustal anomalies with high Vp/Vs ratios of 1.80–1.85 ± 0.04 are identified, which are the product of post-rift magmatic intrusions. The seaward increase in continental lower crustal Vp/Vs ratios, suggests an increasing level of mafic intrusion into the continental crust. We attributed crustal hyperextension to increasing fault density, which caused intense magmatic intrusion and thinner HVLC beneath the continent-ocean transition (COT).

Feasibility study on joint tomography of refraction and reflection seismic waves for geotechnical purposes: a preliminary result

Many developing regions are subject to infrastructure magnification thus green-sustainable infrastructure has always been a relevant concern for most regulators and engineers. In order to support better urban planning, civil engineering operations, and environmental studies, an ample supply of information regarding the neighbouring environment is a must. This includes the information from the Earth’s subsurface, particularly the mechanical properties of soil and rock, on which the planning or action is executed. Complementarily, geophysical measurements utilizing the active seismic method can effectively provide an accurate approximation of the properties. This method includes generating seismic waves using the non-invasive artificial seismic source which then be collected on the Earth’s surface by a number of detectors called geophones. The recorded seismic waves bring information about the medium through which they propagate. They can provide the mechanical properties of the medium after applying some sequences of data interpretation techniques. Conventionally, this is done using only one type of seismic wave, either the refracted or reflected waves. In this study, we build a new procedure incorporating these two types of seismic waves to refine the inferred model. The result shows a promising improvement of the P-wave velocity model in terms of its resolution.

Multiparameter model building for the Qiuyue structure using 4C ocean-bottom seismometer data

Thin sand-mud-coal interbedded layers and multiples caused by shallow water pose great challenges to conventional 3D multichannel seismic techniques used to detect the deeply buried reservoirs in the Qiuyue field. In 2017, a dense ocean-bottom-seismometer (OBS) acquisition program acquired a 4C data set in the East China Sea. To delineate the deep reservoir structures in the Qiuyue field, we have applied a full-waveform inversion (FWI) workflow to this dense 4C OBS data set. After preprocessing, including receiver geometry correction, moveout correction, component rotation, and energy transformation from three dimensions to two dimensions, preconditioned first-arrival traveltime tomography based on an improved scattering-integral algorithm is applied to construct an initial P-wave velocity model. To eliminate the influence of the wavelet estimation process, a convolutional-wavefield-based objective function for the preprocessed hydrophone component is used during acoustic FWI. By inverting the waveforms associated with early arrivals, a relatively high-resolution underground P-wave velocity model is obtained, with updates at a depth of 2.0 and 4.7 km. The initial S-wave velocity and density models are then constructed based on their prior relationships to the P-wave velocity, accompanied by a reciprocal source-independent elastic FWI to refine both velocity models. Compared with a traditional workflow, guided by stacking velocity analysis or migration velocity analysis, and using only the pressure component or other single component, the workflow presented in this study represents a good approach for inverting the 4C OBS data set to characterize subseafloor velocity structures.

An Adaptive Material Interpolation for the Reconstruction of P-Wave Velocity Models with Sharp Interfaces using the Topology Optimization Method

A topology optimization (TO) approach is used to reconstruct P-wave velocity models with sharp interfaces. The concept of material model (interpolation), usually applied in TO to design structures and multi-physics devices, is explored in this work to solve this inverse problem. An adaptive interpolation rule is proposed to deal with the reconstruction problem as a transition from a single- to a multi-material approach combining the Solid Isotropic Material with Penalization (SIMP) and peak function material models. Data collected during the optimization process is used to find material candidates by means of a curve fitting strategy based on generalized simulated annealing (GSA), if this information is not available. The numerical analysis is carried out using a finite element (FE) approach in the frequency domain. Both forward and adjoint problems are solved aided by an open source Domain-Specific Language (DSL) framework and automated derivation tool, while the optimization problem is solved by using a BFGS algorithm. Numerical results for 2D examples demonstrated that proposed material interpolation can lead to solutions with sharper interfaces and improved resolution without including any type of regularization or extra constraint in the optimization problem.

Undercover karst imaging using a Fuzzy c-means data clustering approach (Costa Brava, NE Spain)

Identification of undercover karst areas can be essential for engineering or hydrogeological studies. Geological reconnaissance of karst areas can be challenging due to their complex and highly heterogeneous internal structure. Herein, we present a case study carried out at the carbonate coastal Montgrí Massif in Costa Brava (Spain), which, in the study area, is characterized by a well-developed karst system covered by Quaternary sediments. The objectives of the work are to delineate and characterize karst structures using geophysical techniques and to produce a subsurface geological model that goes beyond the individual physical properties determined independently by each geophysical method. We acquired electrical resistivity and seismic datasets for five collocated profiles. We analyzed the seismic datasets to retrieve P-wave (Vp) and S-wave (Vs) velocity models. Clay-filled karst formations may produce seismic and electrical signatures that are difficult to interpret due to the wide range of values that can be present depending on the properties of the karst's infill. In this type of environment, the Fuzzy c-means (FCM) data clustering approach is a way of producing an integrated model that can help to constrain the lithological interpretation. In this study, FCM is first applied to an electrical resistivity-Vs dataspace in order to characterize near surface sediments. Four different groups are recognized corresponding to different lithologies, fluid conductivities, and saturations in the unconsolidated sediments. Secondly, FCM analysis of the resistivity-Vp datasets provides integrated models that help to delineate the karst areas. The interpretive constraints used derive from the results of the first FCM application and the lithological description of a single borehole that reaches the karst below the Quaternary sediments. In this way, karst is identified in all five profiles. This study shows that the integration of seismic and electrical resistivity information using the FCM approach can extend borehole information throughout the study area and thus provide comprehensive geological models in karst environments.

Advances in surface-wave tomography for near-surface applications

Surface-wave (SW) tomography is a technique that has been widely used in the field of seismology. It can provide higher resolution relative to the classical multichannel SW processing and inversion schemes that are usually adopted for near-surface applications. Nevertheless, the method is rarely used in this context, mainly due to the long processing times needed to pick the dispersion curves as well as the inability of the two-station processing to discriminate between higher SW modes. To make it efficient and to retrieve pseudo-2D/3D S-wave velocity (VS) and P-wave velocity (VP) models in a fast and convenient way, we develop a fully data-driven two-station dispersion curve estimation, which achieves dense spatial coverage without the involvement of an operator. To handle higher SW modes, we apply a dedicated time-windowing algorithm to isolate and pick the different modes. A multimodal tomographic inversion is applied to estimate a VS model. The VS model is then converted to a VP model with the Poisson's ratio estimated through the wavelength-depth method. We apply the method to a 2D seismic exploration data set acquired at a mining site, where strong lateral heterogeneity is expected, and to a 3D pilot data set, recorded with state-of-the-art acquisition technology. We compare the results with the ones retrieved from classical multichannel analysis.

Estimation of Heat Flow Using a Bottom Simulating Reflection Based on 3D Seismic Data, West Africa

DOI:10.17014/ijog.8.3.297-311A Bottom Simulating Reflection (BSR) has been identified using 3D seismic data from the Cameroon continental slope margin. The BSR covers an area of c. 350 km2 in water depths ranging between 940 m to 1,750 m across an area characterized by high- and low-gradient slopes, gullies, scours, and fans. The thickness of the Gas Hydrate Stability Zone (GHSZ) is ~100 - 250 m, assuming an average velocity of 1,800 m/s. Pockmarks are intensively developed across the slope and most of them are observed in the BSR area. Geothermal estimation is based on hydrate stability conditions for pure methane - seawater system, hydrostatic pressure model, and a range of P-wave velocity models for the GHSZ, ranging from 1,600 -1,800 to 2,000 - 2,200 m/s. Geothermal gradient is calculated showing the range and distribution of thermal gradients in the BSR area from 0.046 C/m to 0.094 oC/m with assumed GHSZ velocity of 1,800 m/s. Thermal gradient anomalies have been observed in association with gullies, vertically stacked channels and in some individual pockmarks. The highest anomalies of 0.08 C/m – 0.094 C/m are found in the depression areas of pockmark trains, within seafloor gullies. These positive anomalies are most likely controlled by active or recently active fluid advection and expulsion through the Cameroon slope.

Crustal structure beneath the Zhongsha Block and the adjacent abyssal basins, South China Sea: New insights into rifting and initiation of seafloor spreading

Continental rifting, break-up, and onset of seafloor spreading are inherently controlled by the segmentation and structure of the continental domain suffering from extension. Today, the Zhongsha Atoll (ZS) is wedged between the Northwest Sub-basin (NWSB) and the Southwest Sub-basin (SWSB), two oceanic abyssal basins of the South China Sea (SCS). The nature of the crust and the structure of the transition from continental to oceanic domain are key to revealing the processes and dynamics during the rifting and break-up of the Zhongsha block. In this paper, we present a P-wave velocity model obtained from both forward modeling and tomographic inversion of wide-angle seismic line OBS2017-2. The results support the continental nature of the Zhongsha Block with a thickness of up to ~25 km. However, the transition from the thick continental domain of the ZS into both adjacent abyssal basins shows clear differences. To the north, a ~120 km wide domain of extended continental crust was observed. Farther north, the NWSB is characterized as a narrow basin with typical oceanic crust. The transitional domain between the continental and oceanic crust shows a ~30–40 km wide region with a high-velocity lower crust reflecting excessive magmatism. In contrast, the SWSB is characterized by a sharp transition from the thick continental crust of the ZS to thin oceanic crust which is probably underlain by serpentinized mantle. The strong rheological properties of the pre-rift crust in the western part of the SCS margin may be the reason that rifting concentrated on narrow rifts and thinning focused on necking domains, while the ZS avoided any intense extension. The configuration of rigid blocks thereafter affected the break-up position and the style of oceanic crust.

Challenges in shallow target reconstruction by 3D elastic full-waveform inversion — Which initial model?

Elastic full-waveform inversion (FWI) is a powerful tool for high-resolution subsurface multiparameter characterization. However, 3D FWI applied to land data for near-surface applications is particularly challenging because the seismograms are dominated by highly energetic, dispersive, and complex-scattered surface waves (SWs). In these conditions, a successful deterministic FWI scheme requires an accurate initial model. Our study, primarily focused on field data analysis for 3D applications, aims at enhancing the resolution in the imaging of complex shallow targets, by integrating devoted SW analysis techniques with a 3D spectral-element-based elastic FWI. From dispersion curves, extracted from seismic data recorded over a sharp-interface shallow target, we build different initial S-wave ([Formula: see text]) and P-wave ([Formula: see text]) velocity models (laterally homogeneous and laterally variable), using a specific data transform. Starting from these models, we carry out 3D FWI tests on synthetic and field data, using a relatively straightforward inversion scheme. The field data processing before FWI consists of band-pass filtering and muting of noisy traces. During FWI, a weighting function is applied to the far-offset traces. We test 2D and 3D acquisition layouts, with different positions of the sources and variable offsets. The 3D FWI workflow enriches the overall content of the initial models, allowing a reliable reconstruction of the shallow target, especially when using laterally variable initial models. Moreover, a 3D acquisition layout guarantees a better reconstruction of the target’s shape and lateral extension. In addition, the integration of model-oriented (preliminary monoparametric FWI) and data-oriented (time windowing) strategies into the main optimization scheme has produced further improvement of the FWI results.

Multimodal surface-wave tomography to obtain S- and P-wave velocities applied to the recordings of unmanned aerial vehicle deployed sensors

Exploration seismic surveys in hard-to-access areas such as foothills and forests are extremely challenging. The Multiphysics Exploration Technologies Integrated System (METIS) research project was initiated to design an exploration system, facilitating the acquisition in these areas by delivering the receivers from the sky using unmanned aerial vehicles. Air dropping of the sensors in vegetated areas results in an irregular geometry for the acquisition. This irregularity can limit the application of conventional surface wave methods. We have developed a surface wave workflow for estimating the S-wave velocity ([Formula: see text]) and P-wave velocity ([Formula: see text]) models and that supports the irregular geometry of the deployed sources and receivers. The method consists of a multimodal surface-wave tomography (SWT) technique to compute the [Formula: see text] model and a data transform method (the wavelength/depth [W/D] method) to determine the Poisson’s ratio and [Formula: see text] model. We applied the method to the METIS’s first pilot records, which were acquired in the forest of Papua New Guinea. Application of SWT to the data resulted in the first 90 m of the [Formula: see text] model. The W/D method provided the Poisson’s ratio averaged over the area and the [Formula: see text] model between 10 and 70 m from the surface. The impact of the acquisition scale and layout on the resolution of the estimated model and the advantages of including the higher modes of surface waves in the tomographic inversion are assessed in detail. The presence of shots from diverse site locations significantly improves the resolution of the obtained model. Including the higher modes enhances the data coverage and increases the investigation depth.