3D Density Model Research Articles

3D lithosphere-scale density model of the Central European Basin System and adjacent areas

Abstract To analyze the first-order structural features characterizing the crust and the lithospheric mantle below the Central European Basin System and adjacent areas, a new lithosphere-scale 3D structural/density model has been derived that integrates published knowledge with 3D gravity modeling. With this study we aim to integrate previous results on sub-domains of the study area and regional results of limited resolution in conjunction with 3D gravity analysis to derive the configuration of the lower crust and lithospheric mantle that is consistent with the known density distribution of the sediment fill and with observed deep seismic data as well as with observed gravity. The derived 3D density model resolves the internal configuration of the crystalline crust below the Central European Basin System and adjacent areas at the regional scale and shows that the crystalline crust is characterized by a layered structure. The upper to middle crystalline crust is characterized by relatively low seismic p-wave velocities and densities. It shows strong variations in thickness across the study area that partly correlate spatially with the inherited segmentation in response to crustal amalgamation. This upper to middle crystalline crust is underlain by a continuous high-density/high-velocity lower crustal layer, locally more than 30 km thick beneath the East European Craton and generally decreasing in thickness from the older to the younger tectonic domains. Though the thickness of this high-density lower crust is generally small below the Permo-Cenozoic Central European Basin System, it can reach locally more than 18 km below the Northeast German and the Norwegian–Danish basins. The derived depth to the lithosphere–asthenosphere boundary confirms previously proposed geometries and is up to 230 km deep beneath the Precambrian domain, but less than 100 km deep beneath the Phanerozoic domains, implying that the average thermal gradient is higher in the Phanerozoic lithosphere than in the Precambrian part.

Combined geological and gravimetric mapping and modelling for an improved understanding of observed high-resolution gravity variations: a case study for the Global Geodynamics Project (GGP) station Moxa, Germany

Geodynamic observatories around the globe continuously monitor signals like gravity, tilt and strain as a function of time. However, global signals are often masked by local effects, caused by the direct surroundings of the station, including the local geological setting. This link is well established for superconducting gravimeters (SG) that observe the gravity field variations at very high resolution. An enhancement of the SG time series by the application of local correction is exemplarily shown here in a very practicable procedure for the Geodynamic Observatory Moxa, Germany. We show how the combination of geological and gravimetrical mapping and modelling around Moxa results in a significant correction of the original gravity data, as can be proven by comparison with the satellite-derived gravity field. Detailed geological mapping of the observatory surroundings, including measurements of fold axes, foliations and joints, was the basis of the present study. The fold structure in the area of interest was interpreted by geometrical 3D modelling. The complete representation of different rock types in space also provides a better understanding of the local hydro-geological situation. Using a combination of the 3D geological model with the high-resolution Bouguer map of the observatory surroundings, we developed a 3D density model. This model enables the correction of small gravity effects caused by mass variations close to the SG, in particular local hydro-geological mass changes. Thus, the procedure presented here, based on a close connection of geology and gravimetry, is a powerful tool for the reduction of local gravity effects on SG recordings. It should be applicable to SG stations worldwide, where similar hydrologically driven mass changes can be assumed.

Crustal density structure from 3D gravity modeling beneath Himalaya and Lhasa blocks, Tibet

The Himalaya and Lhasa blocks act as the main belt of convergence and collision between the Indian and Eurasian plates. Their crustal structures can be used to understand the dynamic process of continent–continent collision. Herein, we present a 3D crustal density model beneath these two tectonic blocks constrained by a review of all available active seismic and passive seismological results on the velocity structure of crust and lower lithosphere. From our final crustal density model, we infer that the present subduction-angle of the Indian plate is small, but presents some variations along the west–east extension of the orogenic belt: The dip angle of the Moho interface is about 8–9° in the eastern and western part of the orogenic belt, and about 16° in the central part. Integrating crustal P-wave velocity distribution from wide-angle seismic profiling, geothermal data and our crustal density model, we infer a crustal composition model, which is composed of an upper crust with granite–granodiorite and granite gneiss beneath the Lhasa block; biotite gneiss and phyllite beneath the Himalaya, a middle crust with granulite facies and possible pelitic gneisses, and a lower crust with gabbro–norite–troctolite and mafic granulite beneath the Lhasa block. Our density structure (<3.2g/cm3) and composition (no fitting to eclogite) in the lower crust do not be favor to the speculation of ecologitized lower crust beneath Himalaya and the southern of Lhasa block.

Incorporating the concept of equivalent freshwater head in successive horizontal simulations of seawater intrusion in the Nile Delta aquifer, Egypt

Summary A new approach to study seawater intrusion problems in coastal aquifers is presented. The approach is demonstrated for the case of the Nile Delta aquifer in Egypt. FEFLOW, a 3D finite element variable density model, is employed, however, because of the lack of 3D data, and to demonstrate the proposed approach, the simulations are performed in 2D horizontal views. The concept of equivalent freshwater head (usually implemented in 2D vertical simulations) is adapted in the horizontal (areal) simulations. After calibration against field observations, the simulations are conducted at four horizontal sections located at different levels (100, 200, 300 and 400 m) below the mean seawater level. The depth of the horizontal section is identified through assigning an appropriate pressure “equivalent freshwater” head at the boundaries. The study domain is modified for the horizontal sections at 300 and 400 m, respectively, to account for the aquifer geometry at these depths. The effect of freshwater recharge from the Nile River on the seawater intrusion is observed in the upper layer around its two main branches. The results of the horizontal simulations clearly demonstrate the variation of water concentration in the vertical direction. As the depth increases, the transition zone (in which the concentration varies from the seawater to the freshwater concentration) is shifted toward the landside and become more extensive. At the lower levels of the Nile Delta aquifer, the seawater migrates much further inland as compared to the shallower levels. The concept of horizontal simulations at different levels is further developed to produce meaningful concentration distributions in the vertical sections. This approach allows for a better realization of seawater intrusion in coastal aquifers.

Deep structure of the crust beneath the Sea of Okhotsk inferred from 3D seismic density modeling

Potential field anomalies of the Sea of Okhotsk region are analyzed for compiling a map of the basement’s tectonic structures. A 3D density model of the Earth’s crust is constructed using seismogeological and experimental-petrophysical data, which made it possible to obtain a visual image of main structures of the region reflecting the observable geophysical anomalies. The obtained data allow a domain located in the central part of the Sea of Okhotsk beyond the limits of the exclusive economic zone of the Russian Federation to be considered as a natural continuation of the continental shelf since the latter is structurally similar to western Kamchatka. The deep structural boundaries rise beneath the large sedimentary Deryugin and Tinro basins, which is characteristic of petroliferous basins.

Gravity data inversion for the lithospheric density structure beneath North China Craton from EGM 2008 model

The density structures of the crust and upper mantle beneath the North China Craton (NCC) are reconstructed in this paper by using the Bouguer gravity anomaly derived from EGM08 model. The Occam’s inversion method and preconditioned conjugate gradient (PCG) are adopted in our paper for solving the 3D density inversion problems. Then the depth weighted function is incorporated into model parameters to avoid the skin effects in inversion. In our study, the Bouguer gravity anomaly derived from EGM08 model is used to study the crust and upper mantle structures beneath the NCC. For reducing the non-uniqueness of our inversion problem, we use the inverted density distribution from 0.5° × 0.5° P-wave tomographic results as our initial model. Finally, the high-resolution 3D density model is established down to 250 km depth for the first time in this region. The density images at eight representative layers (10, 25, 40, 60, 80, 120, 160, 200 km) and eight vertical cross-sections (linear profiles along latitude 35°, 36°, 37°, 38°, 39°, 40°, longitude 108°, 119°) are displayed and compared with the P-wave velocity tomographic images. The 3D-dimensional density model indicates that lateral heterogeneities are widely distributed in different units within the NCC, while their density patterns and depth extensions are significantly different for three main units of NCC, suggesting different tectonic mechanisms that have dominated the evolution of these regions in the Phanerozoic. In particular, our inverted density images demonstrate that the Phanerozoic lithospheric reactivation and thinning may not have influenced the central and western NCC to the same extent as the eastern NCC, most of which preserves relatively thick cratonic lithosphere today. However, localized lithospheric thinning may exist in the circular-Ordos rift systems, whose ancient tectonic belts around the Ordos plateau may have been affected by multi-phase tectonic events in its long evolution history.

Stochastic inversion of a gravity field on multiple scale parameters using surface and borehole data

ABSTRACTA 3D stochastic inversion method based on a geostatistical approach is presented for three‐dimensional inversion of gravity on multiple scale parameters using borehole density and gravity and surface gravity. The algorithm has the capability of inverting data on multiple supports. The method involves four main steps: i) upscaling of borehole densities to block densities, ii) selection of block densities to use as constraints, iii) inversion of gravity data with selected block densities as constraints and iv) downscaling of inverted densities to small prisms. Two modes of application are presented: estimation and simulation. The method is first applied to a synthetic stochastic model. The results show the ability of the method to invert surface and borehole data simultaneously on multiple scale parameters. The results show the usefulness of borehole data to improve depth resolution. Finally, a case study using gravity measurements at the Perseverance mine (Quebec, Canada) is presented. The recovered 3D density model identifies well three known deposits and it provides beneficial information to analyse the geology of massive sulfide for the domain under study.

Three-dimensional phase separation and identification in granite

Typical granite is mainly composed of quartz, plagioclase, and alkali feldspars, together with some smaller fractions of ferromagnesian phases and trace phases. The occurrence and abundance of these minerals can be determined by destructive methods, but the three-dimensional (3D) visualization of these mineral structures is very difficult. For geological samples, X-ray microcomputed tomography (mu CT) is a very powerful tool to visualize 3D structures. However, mu CT imaging is based on the X-ray linear attenuation coefficient, which depends on atomic composition, density, and X-ray energy, sometimes making it hard to identify different phases inside the sample. This problem can be overcome by combining mu CT with other techniques that provide chemical information. The combination of mu CT with micro-X-ray fluorescence (mu XRF) and X-ray diffraction (XRD) allows segmentation and identification of the different minerals at the surface of the sample. Combining this information with the 3D density model obtained through mu CT scanning of the granite allows 3D phase identification. In this paper the results of the analysis of a representative Precambrian granite by these three complementary techniques are discussed and combined with the results of more traditional techniques like thin-section petrography. Although the granite sample is shown to be very heterogeneous, correct phase identification in 3D is obtained.

The 3-D gravity inversion package GROWTH2.0 and its application to Tenerife Island, Spain

We present the gravity inversion software GROWTH2.0 and its application to recently obtained gravity data from the volcanic island of Tenerife (Canary Islands, Spain) to inform on its subsurface density structure. GROWTH2.0 is an inversion tool which enables the user to obtain, in a nearly automatic and nonsubjective mode, a 3D model of the subsurface density anomalies based on observed gravity anomaly data. The package is composed of three parts: (a) GRID3D to generate a 3D partition of the subsurface volume into parallelepiped elements, (b) GROWTH to perform the inversion routine and to obtain a 3D anomalous density model, and (c) VIEW for visual representation of the input data, the inversion model, and modeling residuals. The current version of the tool has been developed from an earlier code ( Camacho et al., 2002) and now incorporates several novelties: (1) a Graphical User Interface (GUI), (2) an optional automated routine for determination of parameter λ, which controls the balance between model fitness and smoothness, (3) optional determination of values for minimum density contrast, (4) a robust handling of outlier data, and (5) improved automated data reduction for terrain effects based on anticorrelation with topographic data. The new capabilities and applicability of GROWTH2.0 for 3-D gravity inversion are demonstrated by a case example using new gravity data from the volcanic island of Tenerife. In a nearly automatic approach, the software provides a 3-D model informing on the location and shape of the main structural building blocks of the island. Our model results allow us to shed light on the low-density structure of the islands dominant Pico Viejo–Pico Teide (PV–PT) volcanic complex and the identification of an intrusive structure (the east bulge volcano) embedded in Teide's east flank. A low-density body located at around 5.8 km depth beneath PT's summit may represent a current magma or hybrid reservoir.

Estimation of the gravitational potential energy of the earth based on different density models

The estimation of the Earth’s gravitational potential energy E was obtained for different density distributions and rests on the expression E = − (Wmin + ΔW) derived from the conventional relationship for E. The first component Wmin expresses minimum amount of the work W and the second component ΔW represents a deviation from Wmin interpreted in terms of Dirichlet’s integral applied on the internal potential. Relationships between the internal potential and E were developed for continuous and piecewise continuous density distributions. The global 3D density model inside an ellipsoid of revolution was chosen as a combined solution of the 3D continuous distribution and the reference PREM radial piecewise continuous profile. All the estimates of E were obtained for the spherical Earth since the estimated (from error propagation rule) accuracy σE of the energy E is at least two orders greater than the ellipsoidal reduction and the contribution of lateral density inhomogeneities of the 3D global density model. The energy E contained in the 2nd degree Stokes coefficients was determined. A good agreement between E = EGauss derived from Gaussian distribution and other E, in particular for E = EPREM based on the PREM piecewise continuous density model and E-estimates derived from simplest Legendre-Laplace, Roche, Bullard and Gauss models separated into core and mantle only, suggests the Gaussian distribution as a basic radial model when information about density jumps is absent or incomplete.

A Constrained 3D Density Model of the Upper Crust from Gravity Data Interpretation for Central Costa Rica

The map of complete Bouguer anomaly of Costa Rica shows an elongated NW-SE trending gravity low in the central region. This gravity low coincides with the geographical region known as the Cordillera Volcánica Central. It is built by geologic and morpho-tectonic units which consist of Quaternary volcanic edifices. For quantitative interpretation of the sources of the anomaly and the characterization of fluid pathways and reservoirs of arc magmatism, a constrained 3D density model of the upper crust was designed by means of forward modeling. The density model is constrained by simplified surface geology, previously published seismic tomography and P-wave velocity models, which stem from wide-angle refraction seismic, as well as results from methods of direct interpretation of the gravity field obtained for this work. The model takes into account the effects and influence of subduction-related Neogene through Quaternary arc magmatism on the upper crust.

3D density model of the Central Andes

We developed a 3D density model of the continental crust, the subducted plate and the upper mantle of the Central Andes between 20–29°S and 74–61°W through the forward modelling of Bouguer anomaly. The goal of this contribution is to gain insight on the lithospheric structure integrating the available information (geophysical, geologic, petrologic, and geochemical) in a single model. The geometry of our model is defined and constrained by hypocentre location, reflection and refraction on and offshore seismic lines, travel time and attenuation tomography, receiver function analysis, magnetotelluric studies, thermal models and balanced structural cross-sections. The densities allocated to the different bodies are calculated considering petrologic and geochemical data and pressure and temperature conditions. The model consists of 31 parallel E–W vertical planes, where the continental crust comprises distinct bodies, which represent the different morphotectonic units of the Central Andes. We include a partial melting zone at midcrustal depths under the Altiplano-Puna (low-velocity zone) and consider the presence of a rheologically strong block beneath the Salar de Atacama basin, according to recent seismic studies. Contour maps of the depth of the continental Moho, the thickness of the lower crust and the depth to the bottom of the lithosphere below South America are produced. The possible percentage of partial melt in the Central Andes low-velocity zone is estimated. The residual anomaly is calculated by subtracting from the Bouguer anomaly the gravimetric effect of the modelled subducted slab and of the modelled Moho. Isostatic anomalies are calculated from regional and local isostatic Mohos calculated with and without internal loads, derived from our gravity model, which are then compared to the modelled continental Moho. This study contributes to a more detailed knowledge of the lithospheric structure of this region of the Andes and provides an integrated 3D density model, which may be used by the geoscientific community as a tool that can help to understand and interpret the geodynamic features and processes acting along the Central Andes.

A 3D finite-difference modeling study of seismic imaging challenges in Bintuni Bay, Irian Jaya Barat

In this paper, we describe how we designed and built two 3D finite difference (FD) models for Bintuni Bay, Irian Jaya Barat to help us understand key subsurface components adversely affecting seismic data quality for clastic reservoirs below thick and karstified carbonates. Our modeling efforts consisted of four main steps: creation of realistic geologic models based on seismic interpretation; construction of 3D velocity and density models based on the geologic model and well data; generation of 3D FD shot records with dense, areal, receiver grids; and depth migration of subsets of shots and receivers corresponding to various acquisition geometries.

The ice age paleo-channel Ellerbeker Rinne an integrated 3D gravity study

In the framework of a joint field work with partners of the Landesamt fur Landwirtschaft, Umwelt und landliche Raume (LLUR, the former LANU; Flintbek, Germany) and the Leibniz-Institut fur Angewandte Geophysik (LIAG; Hannover, Germany) gravity measurements have been conducted in July 2005 by members of a student course Geophysical field exercise in the area of the Ellerbek paleo-channel near Tangstedt. At approximately 450 stations complete Bouguer anomalies have been calculated. The error of gravity anomaly accounts for 2 x 10(-7) m/s(2) (max.) to -8 x 10(-7) m/s(2) (min.). The gravity field of the region is affected by a strong regional gravity field, which is caused by salt structures. Therefore, a regional field was calculated which based on a regional survey of the State Geodetic Survey. It was subtracted from the Bouguer field and a slightly negative residual field (-1.0 to -1.5 x 10(-5) m/s(2)) resulted in the area of the Ellerbek paleo-channel. It was analysed and interpreted by curvature algorithms, Euler deconvolution and 3D modelling. Euler deconvolution and curvature result in a depth of max. 400 m for this part of the Ellerbek paleo-channel; clusters of Euler source points were also calculated for depths of 130 m and 220 m which points to rather inhomogeneous infill of the channel. The compiled 3D density model was constrained by the seismic profile P3 of LIAG and borehole information. The model explains the positive anomaly north of Tangstedt (> 1.5 x 10(-5) m/s(2)) With a high density body (Geschiebemergel; boulder clay) of 300 kg/m(3) density contrast to the sedimentary infill of the paleo-channel which has its bottom at 200 m depth.

Three-dimensional deep density model of the Pautovaya Horst and its geological interpretation (Northeast Russia)

The quantitative interpretation of gravity anomalies reveals a concealed granitoid pluton within the core of the Pautovaya Horst (southeastern extremity of the Iniyali-Debin Synclinorium, northeast Russia). Using a new interpretative gravimetry method, a 3D density model is developed for the study area with its geological-petrological explanation. The model satisfies the necessary condition that the solution of inverse gravimetry problems provides the true solution.

High-resolution gravity study of the Gray Fossil Site

The Gray Fossil Site, Washington County, Tennessee, has produced a remarkable Mio-Pliocene fauna and flora with no known correlative in the Appalachian region. After its discovery in 2000, a series of auger holes were drilled by the Tennessee Department of Transportation (TDOT) to determine the areal extent of the site. Drilling indicated that the fossils occurred in fill material within a paleokarst basin, but the distribution of boreholes does not permit details of sinkhole topography, and therefore its formation and fill history, to be adequately resolved. To better image the sinkhole basin, a high-resolution gravity survey, which included 1104 gravity measurement stations, was conducted. These data were used to create complete Bouguer and residual gravity anomaly maps and a 3D density model via inversionmethods. The residual gravity anomaly map compares favorably with 29 TDOT auger holes drilled to basement, but contains significantly more detail. The residual gravity anomaly map reveals the presence of seven separate sinkholes. However, 3D inverse modeling constrained by drill-hole depths and density data indicates that there are 11 separate sinkholes formed within the Knox Group carbonates. These sinkholes, which range between 20 and [Formula: see text] in depth, are aligned along northwest and northeast trending linear features that correlate to structural features formed during the Appalachian orogenies. It is possible that the overall sinkhole basin formed as the result of partial coalescence of multiple sinkhole structures controlled by a joint system and that the sinkholes then acted as a natural trap for the Gray Fossil Site fauna and flora.

Intrusion versus inversion—a 3D density model of the southern rim of the Northwest German Basin

An unsolved problem of regional importance for both the evolution and structure of the Northwest German Basin is the existence or non-existence of the so-called Bramsche Massif. Explaining the nature of this massif and the cause of a related strong, positive Bouguer anomaly (Bramsche Anomaly) is critical. In the study described here, we tested an existing “intrusion model” against a newer “inversion model” in the southern Northwest German Basin. In the intrusion model, the strongly-positive Bouguer anomaly represents the gravity effect of an intrusion at depths between 6 and 10 km. More recent interpretations invoke tectonic inversion rather than intrusion to explain increased burial and the low level of hydrocarbon maturity found in boreholes. We tested these different interpretations by constructing 3D forward density models to 15 km depth. The intrusion model was updated and adjusted to incorporate recent data and we also modelled pre-Zechstein structures using different scenarios. The final model has a very good fit between measured and modelled gravity fields. Based on currently available seismic and structural models, as well as borehole density measurements, we show that the positive Bouguer anomaly cannot be modeled without a high-density, intrusive-like body at depth. However, further in-sight into the crustal structures of the Bramsche region requires more detailed investigations.

Comparison of gravimetric and seismic constraints on the structure of the Aegean lithosphere in the forearc of the Hellenic subduction zone in the area of Crete

The island of Crete is located in the forearc of the Hellenic subduction zone, where the African lithospheric plate is subducting beneath the Eurasian one. The depth of the plate contact as well as the internal structure of the Aegean plate in the area of Crete have been a matter of debate. In this study, seismic constrains obtained by wide-angle seismic, receiver function and surface wave studies are discussed and compared to a 3D density model of the region. The interface between the Aegean continental lithosphere and the African one is located at a depth of about 50 km below Crete. According to seismic studies, the Aegean lithosphere in the area of Crete is characterised by strong lateral, arc–parallel heterogeneity. An about 30 km thick Aegean crust is found in central Crete with a density of about 2850 kg/m3 for the lower Aegean continental crust and a density of about 3300 kg/m3 for the mantle wedge between the Aegean crust and the African lithosphere. For the deeper crust in the area of western Crete two alternative models have been proposed by seismic studies. One with an about 35 km thick crust and another one with crustal velocities down to the plate contact. A grid search is performed to test the consistency of these models with gravimetric constraints. For western Crete a model with a thick lower Aegean crust and a density of about 2950 kg/m3 is favoured. The inferred density of the lower Aegean crust in the area of Crete correlates well with S-wave velocities obtained by surface wave studies. Based on the 3D density model, the weight of the Aegean lithosphere is estimated along an E–W oriented profile in the area of Crete. Low weights are found for the region of western Crete.

Are solar maximum fan streamers a consequence of twisting sheet structures?

Context. Fan streamers are often observed at low to mid latitudes in the corona at solar maximum, appearing narrow in latitudinal extent near the Sun, and fanning out with height, adopting an approximately linear, but not necessarily radial, configuration above ∼3 R� . Aims. We offer arguments to support the conjecture that such structures may sometimes consist of high density, non-uniform sheets, viewed edge-on near the Sun, and twisting to a more face-on alignment by 3 R� . Methods. EUV and white light observations of a fan streamer observed on 2000/12/05 are analyzed. A simple 3D density model is used to recreate the streamer structure. Results. EIT images show a thin bright sheet at the base of the streamer. The continuation of this structure through the EIT, MLSO MKIV coronameter, and LASCO C2 fields of view, suggests that this sheet is formed mostly of open magnetic field lines. The overall large-scale appearance of the streamer is well simulated by a simple model of a twisting high-density sheet. If the twisting-sheet conjecture is valid, there is a correlation between the distribution of enhanced rays within the streamer viewed in white light, and the distribution of small regions of enhanced brightness seen on the disk in EIT 171 A at the position of the streamer base. Conclusions. We suggest that the apparent poleward divergence of equatorial coronal rays, or threads, seen during solar maximum above active regions, may sometimes be a consequence of such a twisting sheet topology.

Kimberlite exploration using integrated airborne geophysics

Airborne magnetic and electromagnetic surveys together with kimberlite indicator mineral geochemistry have been standard practice in the search for kimberlites. The recent advent of the airborne gravity gradiometer (AGG) showed that airborne gravity gradiometry could also be a successful tool in kimberlite exploration. The installation of a digital AGG system on a helicopter led to the first airborne gravity gradient cum magnetic cum electromagnetic survey. The survey was flown over the central part of the Ekati tenement within the Lac de Gras kimberlite province. Each of these three geophysical methods relies on a different physical property contrast for its success. A selected sub-area shows that no one method would have identified all known pipes. But all known pipes would have been discovered by integrating all three data sets. The pipes in the selected area are associated with conductivity and/or gravity gradient anomalies; a few with magnetic anomalies. New data alone are not sufficient to guarantee success in a mature exploration environment. The geophysical data were individually inverted to create 3D density, magnetic susceptibility and electrical conductivity models. Integrating and applying classification techniques to the three 3D models was used in the generation of new targets. Drill testing of the targets has begun leading to the discovery of a new pipe.