scholarly journals Three-dimensional gravity anomaly inversion in the Pyrenees using compressional seismic velocity model as structural similarity constraints

2020 ◽  
Author(s):  
Roland Martin ◽  
Jérémie Giraud ◽  
Vitaliy Ogarko ◽  
Sébastien Chevrot ◽  
Stephen Beller ◽  
...  
Keyword(s):  
Seismic Tomography ◽  
Seismic Velocity ◽  
Thermal State ◽  
Gravity Data ◽  
Waveform Inversion ◽  
Gravity Anomalies ◽  
Velocity Model ◽  
Density Model ◽  
Real Field ◽  
Structural Constraints

Summary We explore here the benefits of using constraints from seismic tomography in gravity data inversion and how inverted density distributions can be improved by doing so. The methodology is applied to a real field case in which we reconstruct the density structure of the Pyrenees along a southwest-northeast transect going from the Ebro basin in Spain to the Arzacq basin in France. We recover the distribution of densities by inverting gravity anomalies under constraints coming from seismic tomography. We initiate the inversion from a prior density model obtained by scaling a pre-existing compressional seismic velocity Vp model using a Nafe-Drake relationship : the Vp model resulting from a full-waveform inversion of teleseismic data. Gravity data inversions enforce structural similarities between Vp and density by minimizing the norm of the cross-gradient between the density and Vp models. We also compare models obtained from 2.5D and 3D inversions. Our results demonstrate that structural constraints allow us to better recover the density contrasts close to the surface and at depth, without degrading the gravity data misfit. The final density model provides valuable information on the geological structures and on the thermal state and composition of the western region of the Pyrenean lithosphere.

Temperature, strain rates, and rheology: the key parameters controling strength variations in the Australian lithosphere

2020 ◽  
Author(s):  
Magdala Tesauro ◽  
Mikhail Kaban ◽  
Alexey Petrunin ◽  
Alan Aitken
Keyword(s):  
Heat Flow ◽  
Seismic Tomography ◽  
Seismic Velocity ◽  
Gravity Data ◽  
Geophysical Methods ◽  
The Other ◽  
Crust And Upper Mantle ◽  
Large Variability ◽  
Other Hand ◽  
Australian Plate

<p>The Australian plate is composed of tectonic features showing progression of the age from dominantly Phanerozoic in the east, Proterozoic in the centre, and Archean in the west. These tectonic structures have been investigated in the last three decades using a variety of geophysical methods, but it is still a matter of debates of how temperature and strength are distributed within the lithosphere. We construct a thermal crustal model assuming steady state variations and using surface heat flow data, provided by regional and global database, and heat generation values, calculated from existing empirical relations with seismic velocity variations, which are provided by AusREM seismic tomography model. The lowest crustal temperatures are observed in the eastern part of the WAC and the Officer basin, while Central and South Australia are regions with anomalously elevated heat flow values and temperatures caused by high heat production in the crustal rocks. On the other hand, the mantle temperatures, estimated in a previous study, applying a joint interpretation of the seismic tomography and gravity data, show that the Precambrian West and North Australian Craton (WAC and NAC) are characterized by thick and relatively cold lithosphere that has depleted composition (Mg# > 90). The depletion is stronger in the older WAC than the younger NAC. Substantially hotter and less dense lithosphere is seen fringing the eastern and southeastern margin of the continent. Both crustal and mantle thermal models are used as input for the lithospheric strength calculation. Another input parameter is the crustal rheology, which has been determined based on the seismic velocity distribution, assuming that low (high) velocities reflect more sialic (mafic) compositions and thus weaker (stiffer) rheologies. Furthermore, we use strain rate values obtained from a global mantle flow model constrained by seismic and gravity data. The combination of the values of the different parameters produce a large variability of the rigidity of the plate within the cratonic areas, reflecting the long tectonic history of the Australian plate. The sharp lateral strength variations are coincident with intraplate earthquakes location. The strength variations in the crust and upper mantle is also not uniformly distributed: In the Archean WAC most of the strength is concentrated in the mantle, while the Proterozoic Officer basin shows the largest values of the crustal strength. On the other hand, the younger eastern terranes are uniformly weak, due to the high temperatures.</p>

Unraveling the upper mantle heterogeneity from integrated multi-observable inversions

2020 ◽  
Author(s):  
Javier Fullea ◽  
Sergei Lebedev ◽  
Zdenek Martinec ◽  
Nicolas Celli
Keyword(s):  
Seismic Tomography ◽  
Upper Mantle ◽  
Plate Tectonics ◽  
Seismic Velocity ◽  
Gravity Data ◽  
Density Field ◽  
Seismic Velocities ◽  
Satellite Gravity ◽  
The Earth ◽  
Multiple Data Sets

<p>The lateral and vertical thermochemical heterogeneity in the mantle is a long standing question in geodynamics. The forces that control mantle flow and therefore Plate Tectonics arise from the density and viscosity lateral and vertical variations. A common approach to estimate the density field for geodynamical purposes is to simply convert seismic tomography anomalies sometimes assuming constraints from mineral physics. Such converted density field does not match in general with the observed gravity field, typically predicting anomalies the amplitudes of which are too large. Knowledge on the lateral variations in lithospheric density is essential to understand the dynamic/residual isostatic components of the Earth’s topography linking deep and surface processes. The cooling of oceanic lithosphere, the bathymetry of mid oceanic ridges, the buoyancy and stability of continental cratons or the thermochemical structure of mantle plumes are all features central to Plate Tectonics that are dramatically related to mantle temperature and composition.</p><p><br>Conventional methods of seismic tomography, topography and gravity data analysis constrain distributions of seismic velocity and density at depth, all depending on temperature and composition of the rocks within the Earth. However, modelling and interpretation of multiple data sets provide a multifaceted image of the true thermochemical structure of the Earth that needs to be appropriately and consistently integrated. A simple combination of gravity, petrological and seismic models alone is insufficient due to the non-uniqueness and different sensitivities of these models, and the internal consistency relationships that must connect all the intermediate parameters describing the Earth involved. In fact, global Earth models based on different observables often lead to rather different, even contradictory images of the Earth.</p><p><br> Here we present a new global thermochemical model of the lithosphere-upper mantle (WINTERC-grav) constrained by state-of-the-art global waveform tomography, satellite gravity (geoid and gravity anomalies and gradiometric measurements from ESA's GOCE mission), surface elevation and heat flow data. WINTERC-grav is based upon an integrated geophysical-petrological approach where all relevant rock physical properties modelled (seismic velocities and density) are computed within a thermodynamically self-consistent framework allowing for a direct parameterization of the temperature and composition variables.</p>

Basin architecture and density structure beneath the Strait of Georgia, British Columbia

2003 ◽  
Vol 40 (7) ◽  
pp. 965-981 ◽  
Author(s):  
C Lowe ◽  
S A Dehler ◽  
B C Zelt
Keyword(s):  
Seismic Velocity ◽  
Gravity Data ◽  
Velocity Model ◽  
Seismic Structure ◽  
Strait Of Georgia ◽  
3 Dimensional ◽  
The North ◽  
Georgia Basin ◽  
Geologic Interpretation ◽  
Velocity Information

Georgia Basin is located within one of the most seismically active and populated areas on Canada's west coast. Over the last decade, geological investigations have resolved important details concerning the basin's shallow structure and composition. Yet, until recently, relatively little was known about deeper portions of the basin. In this study, new seismic velocity information is employed to develop a 3-dimensional density model of the basin. Comparison of the calculated gravity response of this model with the observed gravity field validates the velocity model at large scales. At smaller scales, several differences between model and observed gravity fields are recognized. Analysis of these differences and correlation with independent geoscience data provide new insights into the structure and composition of the basin-fill and underlying basement. Specifically, four regions with thick accumulations of unconsolidated Pleistocene and younger sediments, which were not resolved in the velocity model, are identified. Their delineation is particularly important for studies of seismic ground-motion amplification and offshore aggregate assessment. An inconsistency between the published geology and the seismic structure beneath Texada and Lasqueti Islands in the central Strait of Georgia is investigated; however, the available gravity data cannot preferentially validate either the geologic interpretation or the seismic model in this region. We interpret a northwest-trending and relatively linear gradient extending from Savory Island in the north to Boundary Bay in the south as the eastern margin of Wrangellia beneath the basin. Finally, we compare Georgia Basin with the Everett and Seattle basins in the southern Cascadia fore arc. This comparison indicates that while a single mechanism may be controlling present-day basin tectonics and deformation within the fore arc this was not the case for most of the Mesozoic and Tertiary time periods.

New gravity data and 3-D density model constraints on the Ivrea Geophysical Body (Western Alps)

2020 ◽  
Vol 222 (3) ◽  
pp. 1977-1991 ◽  
Author(s):  
M Scarponi ◽  
G Hetényi ◽  
T Berthet ◽  
L Baron ◽  
P Manzotti ◽  
...  
Keyword(s):  
Gravity Anomaly ◽  
Sea Level ◽  
Seismic Velocity ◽  
Gravity Data ◽  
Western Alps ◽  
Density Model ◽  
Density Contrast ◽  
Surface Rock ◽  
Spatial Coverage ◽  
Density Map

SUMMARY We provide a high-resolution image of the Ivrea Geophysical Body (IGB) in the Western Alps with new gravity data and 3-D density modelling, integrated with surface geological observations and laboratory analyses of rock properties. The IGB is a sliver of Adriatic lower lithosphere that is located at shallow depths along the inner arc of the Western Alps, and associated with dense rocks that are exposed in the Ivrea-Verbano Zone (IVZ). The IGB is known for its high seismic velocity anomaly at shallow crustal depths and a pronounced positive gravity anomaly. Here, we investigate the IGB at a finer spatial scale, merging geophysical and geological observations. We compile existing gravity data and we add 207 new relative gravity measurements, approaching an optimal spatial coverage of 1 data point per 4–9 km2 across the IVZ. A compilation of tectonic maps and rock laboratory analyses together with a mineral properties database is used to produce a novel surface rock-density map of the IVZ. The density map is incorporated into the gravity anomaly computation routine, from which we defined the Niggli gravity anomaly. This accounts for Bouguer Plate and terrain correction, both considering the in situ surface rock densities, deviating from the 2670 kg m–3 value commonly used in such computations. We then develop a 3-D single-interface crustal density model, which represents the density distribution of the IGB, including the above Niggli-correction. We retrieve an optimal fit to the observations by using a 400 kg m–3 density contrast across the model interface, which reaches as shallow as 1 km depth below sea level. The model sensitivity tests suggest that the ∼300–500 kg m–3 density contrast range is still plausible, and consequently locates the shallowest parts of the interface at 0 km and at 2 km depth below sea level, for the lowest and the highest density contrast, respectively. The former model requires a sharp density discontinuity, the latter may feature a vertical transition of densities on the order of few kilometres. Compared with previous studies, the model geometry reaches shallower depths and suggests that the width of the anomaly is larger, ∼20 km in west–east direction and steeply E–SE dipping. Regarding the possible rock types composing the IGB, both regional geology and standard background crustal structure considerations are taken into account. These exclude both felsic rocks and high-pressure metamorphic rocks as suitable candidates, and point towards ultramafic or mantle peridotite type rocks composing the bulk of the IGB.

SeisAndes: A High Resolution Crust and Upper Mantle Seismic Velocity Model beneath the Central Andes from 16° S to 30° S from Full Waveform Inversion

2020 ◽  
Author(s):  
Yajian Gao ◽  
Frederik Tilmann ◽  
Dirk-Philip van Herwaarden ◽  
Sölvi Thrastarson ◽  
Andreas Fichtner ◽  
...  
Keyword(s):  
Upper Mantle ◽  
Seismic Velocity ◽  
Waveform Inversion ◽  
Velocity Model ◽  
Central Andes ◽  
Full Waveform Inversion ◽  
Low Velocity ◽  
Nazca Plate ◽  
Middle Crust ◽  
Full Waveform

<div> <div> <div> <p>We present a new seismic tomography model from multi-scale full seismic waveform Inversion for the crustal and upper-mantle structure beneath the Central Andes (16°-30° S), where the oceanic Nazca plate is subducting beneath the South American continent. The Central Andes is characterized by significant along-strike changes in crustal shortening and thickening, arc migration, subduction erosion and catastrophic earthquakes (e.g. the 2014 Iquique M8.1 earthquake). A high resolution seismic velocity model would bring new insights into the geodynamics of this region, especially for the effects on the seismicity and volcanic arc from the serpentinization in the mantle wedge and dehydration effects from the subducting oceanic crust. </p> <p>Our model is derived from multi-scale full waveform inversion, including multiple time period stages (40-80 s, 30-80 s, 20-80 s, 15-80 s and 12-60 s). In order to avoid the risk of falling into the local minima of optimization, we started our inversion from the lowest frequency signals costing lower computational resources. Specifically, the forward and adjoint simulation based on a 3D model are accomplished with Salvus (Afanasiev et al., 2018), which is a suite of spectral-element method solver of the seismic wave equation. We invert waveforms from 117 events, which are carefully selected for good data coverage of the study region and depth range. We take advantage of the adjoint methodology coupled with conjugate gradients and L-BFGS optimization scheme to update the velocity model. We adopt a time-frequency phase shift as misfit functional with adjoint sources in the first four period-stages, and cross-correlation coefficient in the final stage after most of the phase shifts has been eliminated. The cross-correlation coefficient can capture distorted body wave seismograms, not only the phase shift. We also provide a resolution analysis through the computation of the point-spreading functions and validation dataset with a misfits evolution chart, demonstrating the robustness of our final model.</p> <p>Through full-waveform inversion, we provide a new higher resolution P and S wave velocity model from the middle crust to the upper mantle around 300 km depth. The subducted Nazca slab in the upper mantle beneath the Central Andes is fully imaged, with dip angle variations from the north to the south. We could also observe a strong low velocity band in the middle crust and uppermost mantle from 80 to 100 km beneath the volcanic arc, correlating with the volcano distributions and recent intermediate depth seismicity relocation results. An offset of this low velocity band between 20°-21°S is conspicuous, both in the middle crust and uppermost mantle, indicating a weak extent of the dehydration from 20°-21°S, resulting in the weak intermediate depth seismicity and absent volcanic activity in the same latitude range. We also imaged strong low velocity anomalies in the middle crust beneath the Altiplano-Puna Volcanic Complex and South Puna, giving strong evidence supporting the magmatic underpinnings and reservoirs. Meanwhile, low velocity beneath Puna tectonic units down to 100 km may represent the lithospheric detachment, resulting in the melting and upwelling fluids from the Nazca plate.</p> </div> </div> </div>

Coupling USArray and satellite gravity data – an integrated conductivity, density and seismic velocity model of the western USA

2020 ◽  
Author(s):  
Bernhard Weise ◽  
Max Moorkamp ◽  
Stewart Fishwick
Keyword(s):  
Seismic Velocity ◽  
Joint Inversion ◽  
Gravity Data ◽  
Velocity Model ◽  
Data Sets ◽  
Satellite Gravity ◽  
Consistent Model ◽  
The Usa ◽  
The Individual ◽  
Tectonic Boundaries

<p>The EarthScope USArray project provides high quality magnetotelluric and seismic observations, which have been used to identify tectonic boundaries of the USA. Combining these data sets together with satellite gravity observations, we investigate how the different data sets can complement each other in order to find a consistent model of the subsurface. Using a cross-gradient constraint, we first invert the magnetotelluric and gravity data sets in order to demonstrate the feasibility of our approach and to identify any difficulties. Once a joint conductivity and density model is found, we perform a full joint inversion of all three data sets. By comparison with models derived from separate inversions of the individual observables we can show how the different data sets interact. Examining the magnitude of the cross-gradient lets us distinguish parts of the model where a good agreement of the recovered structures has been achieved from those where differing patterns are necessary in order to achieve an acceptable data fit. In this presentation we will give an overview of our approach, highlight our strategy and show results from individual and joint inversions.</p>

scholarly journals 2D density model of the Chinese continental lithosphere along a NW-SE transect

2015 ◽  
Vol 45 (2) ◽  
pp. 135-148
Author(s):  
Barbora Šimonová ◽  
Miroslav Bielik ◽  
Jana Dérerová
Keyword(s):  
Seismic Velocity ◽  
Qaidam Basin ◽  
Gravity Data ◽  
Junggar Basin ◽  
Density Model ◽  
Low Velocity ◽  
The North ◽  
Deep Seismic Soundings ◽  
Qilian Orogen ◽  
North Western

Abstract This paper presents a 2D density model along a transect from NW to SE China. The model was first constructed by the transformation of seismic velocity to density, revealed by previous deep seismic soundings (DSS) investigations in China. Then, the 2D density model was updated using the GM-SYS software by fitting the computed to the observed gravity data. Based on the density distribution of anomalous layers we divided the Chinese continental crust along the transect into three regions: north-western, central and south-eastern. The first one includes the Junggar Basin, Tianshan and Tarim Basin. The second part consists of the Qilian Orogen, the Qaidam Basin and the Songpan Ganzi Basin. The third region is represented by the Yangtze and the Cathaysia blocks. The low velocity body (vp =5.2 – 6.2 km/s) at the junction of the North-western and Central parts at a depth between 21 – 31 km, which was discovered out by DSS, was also confirmed by our 2D density modelling.

scholarly journals Seismic Velocity Structure Beneath the Tofino Forearc Basin Using Full Waveform Inversion

Energies ◽  
2021 ◽  
Vol 14 (11) ◽  
pp. 3099
Author(s):  
Subbarao Yelisetti ◽  
George D. Spence
Keyword(s):  
Continental Shelf ◽  
Seismic Velocity ◽  
Waveform Inversion ◽  
Velocity Model ◽  
Vancouver Island ◽  
Pacific Rim ◽  
Full Waveform Inversion ◽  
Seismic Velocities ◽  
Full Waveform ◽  
Accreted Terranes

Given the effects of steep dips and large lateral variations in seismic velocity beneath the Vancouver Island continental shelf, seismic processing and travel time inversion are inadequate to obtain a detailed velocity model of the subsurface. Therefore, seismic full waveform inversion is applied to multichannel seismic reflection data to obtain a high-resolution velocity model beneath the Tofino fore-arc basin under the continental shelf off Vancouver Island margin. Seismic velocities obtained in this study help in understanding the shallow shelf sediment structures, as well as the deeper structures associated with accreted terranes, such as Pacific Rim and Crescent terranes. Shallow high velocities, as large as ∼5 km/s, were modeled in the mid-shelf region at ∼1.5–2.0 km depth. These coincide with an anticlinal structure in the seismic data, and possibly indicate the shallowest occurrence of the volcanic Crescent terrane. In general, seismic velocities increase landward, indicating sediment over-consolidation related to the compressional regime associated with the ongoing subduction of the Juan de Fuca plate and the emplacement of Pacific Rim and Crescent accreted terranes. Seismic velocities show a sharp increase about 10 km west of Vancouver Island, possibly indicating an underlying transition to the Pacific Rim terrane. A prominent low velocity zone extending over 10 km is observed in the velocity model at 800–900 m below the seafloor. This possibly indicates the presence of a high porosity layer associated with lithology changes. Alternatively, this may indicate fluid over-pressure or over-pressured gas in this potential hydrocarbon environment.

scholarly journals Full-Waveform Inversion for Imaging Faulted Structures: A Case Study from the Japan Trench Forearc Slope

2021 ◽  
Author(s):  
Ehsan Jamali Hondori ◽  
Chen Guo ◽  
Hitoshi Mikada ◽  
Jin-Oh Park
Keyword(s):  
Seismic Data ◽  
Initial Velocity ◽  
Waveform Inversion ◽  
Velocity Model ◽  
Control Measures ◽  
Japan Trench ◽  
Full Waveform Inversion ◽  
Full Waveform ◽  
Time Migration ◽  
Shallow Sediments

AbstractFull-waveform inversion (FWI) of limited-offset marine seismic data is a challenging task due to the lack of refracted energy and diving waves from the shallow sediments, which are fundamentally required to update the long-wavelength background velocity model in a tomographic fashion. When these events are absent, a reliable initial velocity model is necessary to ensure that the observed and simulated waveforms kinematically fit within an error of less than half a wavelength to protect the FWI iterative local optimization scheme from cycle skipping. We use a migration-based velocity analysis (MVA) method, including a combination of the layer-stripping approach and iterations of Kirchhoff prestack depth migration (KPSDM), to build an accurate initial velocity model for the FWI application on 2D seismic data with a maximum offset of 5.8 km. The data are acquired in the Japan Trench subduction zone, and we focus on the area where the shallow sediments overlying a highly reflective basement on top of the Cretaceous erosional unconformity are severely faulted and deformed. Despite the limited offsets available in the seismic data, our carefully designed workflow for data preconditioning, initial model building, and waveform inversion provides a velocity model that could improve the depth images down to almost 3.5 km. We present several quality control measures to assess the reliability of the resulting FWI model, including ray path illuminations, sensitivity kernels, reverse time migration (RTM) images, and KPSDM common image gathers. A direct comparison between the FWI and MVA velocity profiles reveals a sharp boundary at the Cretaceous basement interface, a feature that could not be observed in the MVA velocity model. The normal faults caused by the basal erosion of the upper plate in the study area reach the seafloor with evident subsidence of the shallow strata, implying that the faults are active.

Sign in / Sign up

Export Citation Format

Share Document