Joint Inversion Of Gravity Research Articles

Generalized joint inversion of multimodal geophysical data using Gramian constraints

We introduce a new approach to the joint inversion of multimodal geophysical data using Gramian spaces of model parameters and Gramian constraints, computed as determinants of the corresponding Gram matrices of the multimodal model parameters and/or their attributes. We demonstrate that this new approach is a generalized technique that can be applied to the simultaneous joint inversion of any number and combination of geophysical datasets. Our approach includes as special cases those extant methods based on correlations and/or structural constraints of the multimodal model parameters. As an illustration of this new approach, we present a model study relevant to exploration under cover for iron oxide copper‐gold (IOCG) deposits, and demonstrate how joint inversion of gravity and magnetic data is able to recover alteration associated with IOCG mineralization.

Separation of sources and 3D inversion of gravity and magnetic data for the Thuringian Basin, Germany

Separation of sources and 3D inversion of gravity and magnetic data for the Thuringian Basin, Germany We propose a novel methodology for separation of potential field sources and its 3D inversion. New approaches are developed to separate sources: i) in depth using a succession of upward and downward continuation; ii) in the lateral direction by means of approximation with the field of 3D line segments; iii) according to density and magnetization contrast based on pseudo-gravity calculation. Our original inversion algorithms allow the recovery of unknown 3D geometry both for a restricted body of arbitrary shape and for a contact surface. For the first time, we apply our algorithms to joint inversion of gravity and magnetic data for a large area (the Thuringian Basin in central Germany). We separate in depth sources of both gravitational and magnetic anomalies for the whole territory of Thuringia and compare corresponding components. A 3D model of the main sources is presented based on approximation with 3D line segments and their further transforming into a restricted body or a contact surface with the same field.

Joint inversion of gravity and seismic data along a profile across the seismogenic fault of 2010 Yushu Ms7.1 earthquake

Yushu M s7.1 earthquake occurred on the Ganzi-Yushu fault zone, across which we carried out a joint relative-gravity and seismic-reflection survey, and then performed a gravity inversion constrained by the seismic-reflection result. Based on the data of complete Bouguer gravity anomaly and seismic reflection, we obtained a layered interface structure in deep crust down to Moho. Our study showed that the inversion could reveal the interfaces of strata along the survey profile and the directions of regional faults in two-dimension. From the characteristics of the observed topography of the Moho basement, we tentatively confirmed that the uplift of eastern edge of Qinghai-Tibet plateau was caused by the subduction of the Indian plate.

Gravity–geoelectric joint inversion over the Potiguar rift basin, NE Brazil

Abstract A joint inversion of gravity and geoelectrical data has been performed along a 100 km long profile across the oil-bearing Potiguar rift basin in NE Brazil. The integrated approach is based on the connection between density and resistivity of a sedimentary sequence through the porosity. Seventy-one gravity stations and twenty-nine vertical electrical soundings were carried out across the central part of the Early Cretaceous basin in the Precambrian Borborema Province to apply the proposed joint inversion. Both the physical properties and geometry of the multilayer deep model were well constrained by a wide set of prior information obtained by surface mapping, geophysical logs and seismic sections. The results reveal a rift architecture formed by a 5000 m thick half-graben structure separated by an intervening basement high and an extensive adjacent platform with a sedimentary infill of about 300 m thick. The calculated model shows geometries for the sedimentary density/resistivity interfaces in agreement with the seismostratigraphic sequences revealed by seismic section, representing a substantial improvement in comparison with previously published gravity models.

Pareto optimal 2D joint inversion of gravity and magnetic data

Building models of a geological system using potential field is almost a routine exercise, where both gravity and magnetic data are used to interpret the architectural framework of a geological setting. Although nonuniqueness in potential field interpretation is inevitably a major issue (Skeels, 1947, Roy, 1962), it is argued by many authors (Pedersen, 1979; Bosch et al., 2006) that the use of data redundancy, for example, joint use of gravity and magnetic data as well as appropriate constraints in the interpretation/inverse modelling overcomes the ambiguity limitation of potential field interpretation. However, joint use of gravity and magnetic data especially for geometric inversion is a non-trivial task. By geometric inversion of potential field data we mean determining the geometric configuration of the modelled body whose physical properties, such as density/susceptibility are known. Such potential field inversions are inherently nonlinear. The presence of nonlinearity between the data and model space often results in an apparent paradoxical situation, offering a less stable and thus a less reliable inverse model while exercising the data redundancy by joint use of gravity and magnetic data. We address the issue of producing a stable solution for a 2D joint geometric inversion of gravity and magnetic data via the ?Pareto optimal? criterion. To this end, we state that the joint inversion of gravity and magnetic data can be realised via a multi-objective optimisation problem (MOP), where the objective functions related to the best fitting gravity and magnetic data are satisfied simultaneously. Very recently, MOP has been introduced by Kozlovskaya et al. (2007) in seismic anisotropy studies of lithosphere by jointly inverting shear wave splitting and P-wave residual data. With regard to the MOP our goal is not to determine a unique, possibly globally optimal, solution, but to determine a class of optimal solutions defining a compact set. Such a class of solutions is known as a Pareto set or Pareto front, where each solution is optimal in a Pareto sense; meaning there exists no other solution which will improve one objective function without degrading the other objective function (Coello Coello et al., 2002). To determine a Pareto front we use the epsilon-constraint method, where one objective is minimised while the other one is constrained with a bound by a pre-selected value of epsilon. To minimise the objective function we use a Particle Swarm Optimization (PSO) scheme, which is a global stochastic optimisation scheme. The rationale in selecting PSO is that it is easy to implement and is computationally inexpensive since its memory and its CPU speed requirements are low (Jones, 2006). PSO has been used for both continuous and discrete optimisation problems (Clerc and Kennedy, 2002). The application of PSO in geophysical inversion is demonstrated by Shaw and Shrivastava, (2007). In this study we discuss the application of MOP and PSO in the joint inversion of gravity/magnetic data as well as testing the method through numerical experiment using synthetically generated gravity and magnetic data for a simple but realistic geological model.

Joint inversion of gravity and magnetotelluric data

An emerging field in geophysics is joint inversions that aim to improve the reliability of subsurface modelling (Haber & Oldenberg 1997, Gallardo & Meju 2004 ). Such inversions are where two complementary geophysical data sets are simultaneously inverted to yield a unified model. Individual inversions can produce highly varied models that fit the data equally well. Joint inversions may help to reduce this ambiguity without having to introduce external constraints. The integrated models also allow for more rigorous interpretations. The difficulty with joint inversion is how to link different data sets. There are two approaches to linking the different data sets, the petrophysical approach and the structural approach. The petrophysical approach links geophysical parameters through rock properties such as porosity. The structural approach is based on the premise that the geological conditions that control changes in one parameter will also affect the other parameter. This means changes in one parameter should coincide with changes in the other parameter. We look to explore both joint inversion approaches in the first instance by combining gravity and magnetotelluric (MT) data. Both techniques offer cost effective ways of determining the Earth?s subsurface properties. Gravity is already widely accessible and MT data is being increasingly acquired. This creates many opportunities to apply the inversions.

Joint inversion of gravity and magnetic data for a two-layer model

Based on the synchronous joint gravity and magnetic inversion of single interface by Pilkington and the need of revealing Cenozoic and crystalline basement thickness in the new round of oil-gas exploration, we propose a joint gravity and magnetic inversion method for two-layer models by concentrating on the relationship between the change of thickness and position of the middle layer and anomaly and discuss the effects of the key parameters. Model tests and application to field data show the validity of this method.

Joint Inversion of Gravity and Seismic Data Based on Common Gridded Model with Random Density and Velocity Distributions

AbstractIn view of the problems existing in joint inversion of gravity & seismic data and the published research results, we study a model construction method based on common gridded model with random density and velocity distributions to meet the needs of joint inversion and the complicated model with large density and velocity variations. 2.5 dimensional gravity forward modeling is fulfilled in accordance with this gridded model. By improving the ray‐tracing method in 2 dimensional seismic travel‐time computing to suit the gridded media with random velocity distribution, we realize the synchronous joint inversion of gravity & seismic data based on this kind of common gridded model in accordance with the improved very fast simulated annealing algorithm. The model test shows that the joint inversion could accurately determine the density and velocity structures of complicated model with uncommon interface and large variations of density and velocity. Moreover, joint inversion method is clearly superior to the single inversion of gravity data. The joint inversion of the observed data with a priori constraining information also gives good effects, which make it clear that this method is effective and practicable in improving inversion accuracy and reducing ambiguity.

Rifting and lower crustal reflectivity: A case study of the intracratonic Dniepr‐Donets rift zone, Ukraine

Intracratonic rifting, caused by late Devonian extensional stresses in the East European Craton, created the largest rift zone in Europe, the Pripyat‐Dniepr‐Donets rift (southeast Ukraine). The rift basin is approximately 2000 km long, up to 170 km wide, and 22 km deep. Wide‐angle refraction and reflection seismic data from the Donbas Basin deep seismic Refraction and Reflection Experiments (DOBRE'99) project together with gravity and magnetic data are analyzed for the structure and evolution of the Donbas Fold Belt, which is the uplifted and deformed part of the Dniepr‐Donets Basin. The seismic data are used for identification of large‐scale crustal structures and modeling of the seismic velocities of the crust and uppermost mantle. A ray‐trace‐based velocity and density model is derived by joint inversion of gravity and traveltime data. The inversion result reveals a zone of high density and velocity beneath the basin at middle to lower crustal levels, slightly offset to the NE of the rift axis. Full waveform synthetic seismograms, matching the observed data, show high‐amplitude and low‐frequency arrivals from this high‐density body as well as from the Moho. We interpret the high‐amplitude, low‐frequency signals as reflections from layered magmatic rocks, which intruded into the ductile lower crust during the main rift phase and subsequently were sorted by fractional crystallization. The intrusive material thickened the lower crust by approximately 50%. This may explain the enigmatic flat Moho topography across the rift zone which has been significantly stretched (β = 1.3). The rifting initiated in the late Devonian (Frasnien) as a consequence of back‐arc extension in relation to subduction of the Paleo‐Tethys Ocean. The subducting oceanic slab may have enriched the mantle with volatiles and created convection, leading to strong partial melting, upwelling, and continued rifting in the Famennien. We interpret the asymmetrical rift geometry as a combination of pure and simple shear tectonic processes.

Joint inversion of gravity and geoelectrical data for groundwater and structural investigation: application to the northwestern part of Sinai, Egypt

SUMMARY The simulated annealing method was applied to jointly invert gravity and resistivity data to obtain the geometry of the density/resistivity interfaces and subsurface electrical resistivity distribution. The results obtained from the inversions of synthetic data indicate that the joint inversion significantly improves the solution decreasing the ambiguity of the models. The method was applied to gravity and resistivity data carried out in Sinai (northwestern Egypt). The results obtained revealed the geometry of the water-bearing zone with thickness ranging from 80 to 180 m and of the limestone‐bedrock interface dipping northwards.

Joint inversion of gravity and magnetic data for two-layer models

Gravity and magnetic data are inverted jointly in terms of a model consisting of an interface separating two layers having a constant density and magnetization contrast. A damped least-squares inversion is used to determine the topography of the interface. The inversion requires knowledge of the physical property contrasts across the interface and its average depth. Since the relationship between model parameters and data is weakly nonlinear, a constant damped least-squares inverse is used during the iterative solution search. The effect of this inverse is closely related to a downward continuation of the field to the average interface depth. The method is used to map the base of the Sept-Iles mafic intrusion, Quebec, Canada, and the shape of the central uplift at the Chicxulub impact crater, Yucatan, Mexico. At Sept-Iles, the intrusion reaches a thickness of [Formula: see text], coincident with the maximum gravity anomaly, south of the intrusion center. At Chicxulub, the top of the central uplift is modeled to be [Formula: see text] deep and has a single peak form.

Exploration and Modelling of Basaltic Rocks and a Maar Depression

Magnetic and gravimetric measurements have been carried out in Lusatia in Eastern Germany. Here a old geological massiv comprising granodiorits, graywacks and lower paleocoic sediments exist. The boundaries are characterized by big tectonic faults in western, northern and southern parts. In Tertiaery basaltic vulcanism appears in connection with young movements. The objective of our research was to explore known and hidden basaltic intrusions by magnetic methods. Gravity measurements were carried out to find the thickness of overlying loose sediments. An estimation of the magnetic susceptibility from basaltic rocks reaching the surface was made as well. We found the figure and the depth of the basaltic intrusions by 2-3/4D forward modelling. Most of the rocks are showing remanent magnetic behaviour. A maar depression was detected and interpreted by joint inversion of gravity and magnetic anomalies.

Structures on Lithospheric Density and Geomagnetism of the Tianshan Orogenic Belt and Their Geodynamic Implications

AbstractThe geoscience transect from Korla to Jimsar runs through the northern margin of the Tarim basin, the Tianshan orogenic belt and the southern margin of the Junggar basin. Along the profile, a joint inversion of gravity with geomagnetism has been carried out and 2‐D density structure and 2‐D geomagnetic structure have been obtained. The result shows that both the Tarim basin and the Junggar basin under thrust to the Tianshan orogenic belt in opposite directions. In the crust, the average density of the northern margin of the Tarim basin and the southern margin of the Junggar basin is relatively high, while that of the Tianshan orogenic belt is relatively low. However, the Tianshan orogenic belt bears a high geomagnetic intensity, especially in the entire crust from the southern margin of the Junggar basin to the middle of the Tiansha orogenic belt. This indicates that the southern margin and the northern margin of the Tianshan orogenic belt may possess different tectonic evolution histories, different tectonic movement patterns and different tectonic intensities. Some lowdensity bodies located at the top part of the upper mantle are found on the boundaries between the Tarim basin and the Tianshan orogenic belt and that between the Junggar basin and the Tianshan orogenic belt. This may be inferred as a result of subduction of the Tarim block northward and that of the Junggar block southward to the Tianshan orogenic belt. During this process, the mass of the lower crust on the northern margin of the Tarim basin and the southern margin of the Junggar basin has been carried down to the top part of the upper mantle beneath the Tianshan orogenic belt. The 2‐D density structure and the 2‐D geomagnetic structure of the geoscience transect from Korla to Jimsar provide some lithospheric evidence for the tectonic segmentation of the Tianshan orogenic belt.

New Results at Mayon, Philippines, from a Joint Inversion of Gravity and Deformation Measurements

In this paper, we detail the combination of the genetic algorithm (GA) inversion technique with the elastic-gravitational model originally developed by Rundle and subsequently refined by Fernandez and others. A sensitivity analysis is performed for the joint inversion of deformation and gravity to each of the model parameters, illustrating the importance of proper identification of both the strengths and limitations of any source model inversion, and this technique in particular. There is a practical comparison of the theoretical results with the inversion of geodetic data observed at the Mayon volcano in the Philippines, where there are gravity changes without significant deformation, after the 1993 eruption.

Study of Volcanic Sources at Long Valley Caldera, California, Using Gravity Data and a Genetic Algorithm Inversion Technique

We model the source inflation of the Long Valley Caldera, California, using a genetic algorithm technique and micro-gravity data. While there have been numerous attempts to model the magma injection at Long Valley Caldera from deformation data, this has proven difficult given the complicated spatial and temporal nature of the volcanic source. Recent work illustrates the effectiveness of considering micro-gravity measurements in volcanic areas. A genetic algorithm is a problem-solving technique which combines genetic and prescribed random information exchange. We perform two inversions, one for a single spherical point source and another for two-sources that might represent a more spatially distributed source. The forward model we use to interpret the results is the elastic-gravitational Earth model which takes into account the source mass and its interaction with the gravity field. The results demonstrate the need to incorporate more variations in the model, including another source geometry and the faulting mechanism. In order to provide better constraints on intrusion volumes, future work should include the joint inversion of gravity and deformation data during the same epoch.

Joint inversion of gravity and arrival time data from Parkfield: New constraints on structure and hypocenter locations near the SAFOD drill site

Taking advantage of large datasets of both gravity and elastic wave arrival time observations available for the Parkfield, California region, we generated an image consistent with both types of data. Among a variety of strategies, the best result was obtained from a simultaneous inversion with a stability requirement that encouraged the perturbed model to remain close to a starting model consisting of a best fit to the arrival time data. The preferred model looks essentially the same as the best‐fit arrival time model in areas where ray coverage is dense, with differences being greatest at shallow depths and near the edges of the model where ray paths are few. Earthquake locations change by no more than about 100 m, the general effect being migration of the seismic zone to the northeast, closer to the surface trace of the San Andreas Fault.

Correction to “Deep structure of the Baikal rift zone revealed by joint inversion of gravity and seismology”

Correction to “Deep structure of the Baikal rift zone revealed by joint inversion of gravity and seismology”

Deep structure of the Baikal rift zone revealed by joint inversion of gravity and seismology

The question of plate boundary forces and deep versus shallow asthenospheric uplift has long been debated in intracontinental rift areas, particularly in the Baikal rift zone, Asia, which is colder than other continental rifts. As previous gravity and teleseismic studies support the dominance of opposing mechanisms in the Baikal rift, we reconsidered both data sets and jointly inverted them. This more effective approach brings insight into location of the perturbing bodies related to the extension in this region. Our new joint inversion method allows for inverting the velocity‐density relationship with independent model parametrization. We obtain velocity and density models that consistently show (1) crustal heterogeneities that coincide with the main tectonic features at the surface, (2) a faster and denser cratonic mantle NW of Lake Baikal that we relate to the thermal contrast between old and depleted Archean (Siberian platform) and Paleozoic orogenic belt (Sayan‐Baikal belt), (3) three‐dimensional topographic variations of the crust‐mantle boundary with well‐located upwarpings, and (4) the lithosphere‐asthenosphere boundary uplift up to 70 km depth with a NW dip. Our resulting velocity and density models support the idea of a combined influence of lithospheric extension and inherited lithospheric heterogeneities for the origin of the Baikal rift zone.

Joint inversion of gravity and magnetic data under lithologic constraints

Interpreting exploration data requires combining different types of information to solve the geologic puzzle. It implies bringing together all data components into an image that makes conceptual sense in terms of the geology of the exploration area. The identification of geologic objects and the inference of a spatial description of the lithology—consistent with all available information—are the objectives of the process.

Joint inversion of gravity and magnetic anomalies of eastern Canada

The power spectra and degree correlation of the surface topography and free-air gravity anomalies of eastern Canada show that the gravity anomalies are subdivided into three parts. The short-wavelength components (30-170 km, shorter than 30 km are not well resolved) largely arise from density perturbations in the crust and to a lesser extent from the surface topography and Moho undulation, whereas the contribution of intracrustal sources to the intermediate-wavelength components (170-385 km) is comparable with that of the topography. The long-wavelength components (385-1536 km) are overcompensated at the Moho. We present a crustal model for the intermediate- and long-wavelength components which takes into account the surface topography, density perturbations in the crust, and Moho undulation with a certain degree of isostatic compensation. The general characteristics of this model resemble the crustal structure revealed from seismic measurements. The reduced-to-pole magnetic anomalies of eastern Canada show no pronounced correlation with the topography and with the vertical gradient of the gravity anomalies, suggesting that the source bodies are within the crust and Poisson's relationship does not hold over the entire area. Assuming that the magnetic anomalies arise from induced magnetization, lateral variations of magnetic susceptibility of the crust are determined while taking into account the effects of the surface topography and the Moho undulation of our crustal model. The intermediate- and long-wavelength components of the susceptibility contrasts delineate major collision zones as low-susceptibility regions. We interpret this in terms of thermal demagnetization of the high-magnetic crustal roots beneath the collision zones.