Gravity and magnetic inversion with minimization of a specific functional

Geophysics ◽  
1984 ◽  
Vol 49 (8) ◽  
pp. 1354-1360 ◽  
Author(s):  
A. Guillen ◽  
V. Menichetti
Keyword(s):  
Physical Properties ◽  
Magnetic Data ◽  
Constant Density ◽  
Minimum Volume ◽  
Data Inversion ◽  
Gravity And Magnetic ◽  
Magnetic Inversion ◽  
Line Passing ◽  
Susceptibility Contrast ◽  
The Moment

The nonuniqueness of gravity or magnetic data inversion is well known. In order to remove ambiguity, some authors have sought solutions minimizing a functional describing geometrical or physical properties. Last and Kubik (1983), in particular, developed a method explaining the observed anomaly by structures of minimum volume. In this method the domain where anomalous sources are searched is divided into elementary prisms of a constant density or susceptibility contrast. Each elementary contrast is allowed to vary individually. Thus a contrast distribution is computed. The search for this kind of solution leads in general to geologically more appropriate bodies, but exceptions do occur. In this paper, the technique is broadened to include the search for solutions minimizing the moment of inertia with respect to the center of gravity or with respect to a given dip line passing through it. The resulting structures are both deeper and more compact, precisely as is required in specific cases. Theoretical and actual examples illustrate this flexible inversion technique.

scholarly journals Improving the use of the randomized singular value decomposition for the inversion of gravity and magnetic data

Geophysics ◽  
2020 ◽  
Vol 85 (5) ◽  
pp. G93-G107
Author(s):  
Saeed Vatankhah ◽  
Shuang Liu ◽  
Rosemary Anne Renaut ◽  
Xiangyun Hu ◽  
Jamaledin Baniamerian
Keyword(s):  
Singular Value Decomposition ◽  
Spectral Properties ◽  
Regularization Parameter ◽  
Gravity Data ◽  
Singular Value ◽  
Magnetic Data ◽  
Gravity And Magnetic ◽  
Magnetic Inversion ◽  
Model Matrix ◽  
Value Decomposition

The focusing inversion of gravity and magnetic potential-field data using the randomized singular value decomposition (RSVD) method is considered. This approach facilitates tackling the computational challenge that arises in the solution of the inversion problem that uses the standard and accurate approximation of the integral equation kernel. We have developed a comprehensive comparison of the developed methodology for the inversion of magnetic and gravity data. The results verify that there is an important difference between the application of the methodology for gravity and magnetic inversion problems. Specifically, RSVD is dependent on the generation of a rank [Formula: see text] approximation to the underlying model matrix, and the results demonstrate that [Formula: see text] needs to be larger, for equivalent problem sizes, for the magnetic problem compared to the gravity problem. Without a relatively large [Formula: see text], the dominant singular values of the magnetic model matrix are not well approximated. We determine that this is due to the spectral properties of the matrix. The comparison also shows us how the use of the power iteration embedded within the randomized algorithm improves the quality of the resulting dominant subspace approximation, especially in magnetic inversion, yielding acceptable approximations for smaller choices of [Formula: see text]. Further, we evaluate how the differences in spectral properties of the magnetic and gravity input matrices also affect the values that are automatically estimated for the regularization parameter. The algorithm is applied and verified for the inversion of magnetic data obtained over a portion of the Wuskwatim Lake region in Manitoba, Canada.

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

Geophysics ◽  
2006 ◽  
Vol 71 (3) ◽  
pp. L35-L42 ◽  
Author(s):  
Mark Pilkington
Keyword(s):  
Least Squares ◽  
Joint Inversion ◽  
Physical Property ◽  
Magnetic Data ◽  
Model Parameters ◽  
Constant Density ◽  
Central Uplift ◽  
Gravity And Magnetic ◽  
Damped Least Squares ◽  
Gravity And Magnetic Data

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.

Joint gravity and magnetic inversion in 3D using Monte Carlo methods

Geophysics ◽  
2006 ◽  
Vol 71 (4) ◽  
pp. G153-G156 ◽  
Author(s):  
Miguel Bosch ◽  
Ronny Meza ◽  
Rosa Jiménez ◽  
Alfredo Hönig
Keyword(s):  
Monte Carlo ◽  
Magnetic Susceptibility ◽  
Prior Information ◽  
3D Model ◽  
Mass Density ◽  
Magnetic Data ◽  
Gravity And Magnetic ◽  
Magnetic Inversion ◽  
Gravity And Magnetic Data ◽  
Model Layer

We jointly invert gravity and magnetic data following a Monte Carlo method that provides estimation for a 3D model of the structure and physical properties of the medium. In particular, the model layer geometry and the density and magnetic susceptibility fields within layers are estimated, and their uncertainties are described with posterior probabilities. This method combines the gravity and magnetic data with prior information of the mass density and magnetic susceptibility statistics, and statistical constraints on the model interface positions. The resulting model realizations jointly comply with the observations and the prior statistical information.

Magnetic and gravity investigation of kimberlites in north-central Botswana

Geophysics ◽  
2020 ◽  
pp. 1-51
Author(s):  
Kitso matende ◽  
Kevin Mickus
Keyword(s):  
Gravity Anomaly ◽  
Kimberlite Pipe ◽  
Magnetic Data ◽  
Magnetic Survey ◽  
Density Anomaly ◽  
Gravity And Magnetic ◽  
Magnetic Inversion ◽  
Magnetic Analysis ◽  
Lateral Extent ◽  
The Difference

The Orapa Kimberlite field of Botswana is one of the world’s major diamond producing regions. Within this field, there are several small kimberlite pipes that have not been completely explored in terms of the lateral extent, depth and diamond potential. Two such pipes, BK54 and BK55, were found during a ground gravity and magnetic survey and subsequent drilling confirmed the presence of kimberlite material. To determine the physical properties of these pipes, their lateral extent, depth and thickness were estimated using a gravity and magnetic analysis and 2.5D and 3D modeling. Tilt derivatives of the magnetic data indicated that BK54 has a northwest-trending elliptical shape and BK55 has a roughly circular shape. Residual gravity anomaly maps indicate that BK54 does not have a density anomaly while BK55 is associated with a gravity maximum. 3D gravity and magnetic inversion modeling constrained by magnetic susceptibility measurements indicate that BK54 is smaller in volume than BK55 and neither pipe is thicker than 125 m. The difference in shape and the lack of a gravity anomaly over BK54 implies a different formation mechanism for each kimberlite pipe. While several mechanisms are suggested, BK54 may have formed by a more explosive eruption producing more tuffistic material in both the crater and diatreme facies. The gravity and magnetic analysis also found that the kimberlite pipes, while small, are larger in extent than determined by drilling and warrant additional drilling to determine their economic potential.

scholarly journals Utilisation of probabilistic MT inversions to constrain magnetic data inversion: proof-of-concept and field application

2021 ◽  
Author(s):  
Jeremie Giraud ◽  
Hoël Seillé ◽  
Mark D. Lindsay ◽  
Gerhard Visser ◽  
Vitaliy Ogarko ◽  
...  
Keyword(s):  
Field Application ◽  
Magnetic Data ◽  
Structural Constraints ◽  
Proof Of Concept ◽  
Bound Constraints ◽  
Probabilistic Information ◽  
Data Inversion ◽  
Magnetic Inversion ◽  
Interval Bound ◽  
Spatially Varying

Abstract. We propose, test and apply a methodology integrating 1D magnetotelluric (MT) and magnetic data inversion, with a focus on the characterization of the cover-basement interface. It consists of a cooperative inversion workflow relying on standalone inversion codes. Probabilistic information about the presence of rock units is derived from MT and passed on to magnetic inversion through constraints combining such structural constraints with petrophysical prior information. First, we perform the 1D probabilistic inversion of MT data for all sites and recover the respective probabilities of observing the cover-basement interface, which we interpolate to the rest of the study area. We then calculate the probabilities of observing the different rock units and partition the model into domains defined by combinations of rock units with non-zero probabilities. Third, we combine such domains with petrophysical information to apply spatially-varying, disjoint interval bound constraints to least-squares magnetic data inversion. We demonstrate the proof-of-concept using a realistic synthetic model reproducing features from the Mansfield area (Victoria, Australia) using a series of uncertainty indicators. We then apply the workflow to field data from the prospective mining region of Cloncurry (Queensland, Australia). Results indicate that our integration methodology efficiently leverages the complementarity between separate MT and magnetic data modelling approaches and can improve our capability to image the cover-basement interface. In the field application case, our findings also suggest that the proposed workflow may be useful to refine existing geological interpretations and to infer lateral variations within the basement.

scholarly journals Unravelling the internal architecture of the Alnö alkaline and carbonatite complex (central Sweden) using 3D models of gravity and magnetic data

2017 ◽  
Author(s):  
Magnus Andersson ◽  
Alireza Malehmir
Keyword(s):  
Magnetic Susceptibility ◽  
Magnetic Anomaly ◽  
Bouguer Anomaly ◽  
Magnetic Data ◽  
Common Source ◽  
Gravity Gradients ◽  
Aeromagnetic Data ◽  
3D Inversion ◽  
Gravity And Magnetic ◽  
Magnetic Inversion

Abstract. The Alnö complex in central Sweden is one of the largest alkaline and carbonatite ring-shaped intrusions in the world. Presented here is the 3D inversion of ground gravity and aeromagnetic data that confirms some of the previous ideas about the 3D geometry of the complex but also suggests that the complex may continue laterally further to north than earlier expected. The gravity and aeromagnetic data show the complex as (i) a strong positiver Bouguer anomaly, around 20 mGal, one of the strongest gravity gradients observed in Sweden, and (ii) a strong positive magnetic anomaly, exceeding 2000 nT. Magnetic structures are clearly discernible within the complex and surrounding area. Petrophysical measurements (density, bulk magnetic susceptibility, and magnetic remanence) were used to constrain the 3D inversion. Both gravity and magnetic inversion models suggest that dense (> 2850 kg/m3) and magnetic (> 0.05 SI) rocks extend down to about 3.5–4 km depth. Previous studies have suggested a solidified magma reservoir at this approximate depth. The inversion models further suggest that two apparently separate regions within the intrusion with gravity and magnetic highs are likely connected at depth, starting from 800–1000 m, implying a common source for the rocks observed in these two regions. The modelling of the aeromagnetic data indicates that a more than 3 km wide ring-shaped magnetic high in the bay that can be a hidden part of the complex, linking a satellite intrusion in Söråker on the northern side of the bay to the main intrusion on the Alnö Island. While the rim of the ring must consist of highly susceptible rocks to support the magnetic anomaly, the centre has a relatively low magnetisation and is probably made up of low-susceptible wall-rocks or metasomatised wall-rocks down to about 2 km. Below this depth the 3D susceptibility model shows higher magnetic susceptibility values. From these observations the solidified magma chamber is interpreted to extend further to north than has previously been suggested.

INVESTIGATION OF MAGNETIZATION AND DENSITY OF A NORTH ATLANTIC SEAMOUNT USING POISSON’S THEOREM

Geophysics ◽  
1971 ◽  
Vol 36 (5) ◽  
pp. 919-937 ◽  
Author(s):  
Lindrith Cordell ◽  
Patrick T. Taylor
Keyword(s):  
Physical Properties ◽  
North Atlantic ◽  
Remanent Magnetization ◽  
Fourier Transforms ◽  
Magnetic Data ◽  
Total Magnetization ◽  
The North ◽  
Gravity And Magnetic ◽  
Special Cases ◽  
The North Atlantic

The relationship between the gravitational and magnetic potentials caused by a uniform distribution of mass and magnetization may be used to obtain independent information about these physical properties. The general relationship in the frequency domain between the Fourier transforms of the gravity and magnetic anomaly fields is established through the Poisson theorem. The discrete Fourier transforms of the sampled continuous functions are used in an analysis which leads to a system of linear equations involving terms in density, magnetization, and calculated finite Fourier‐series coefficients. A least squares solution of the system yields the three components of the total magnetization vector divided by the density. From these results, the direction of total magnetization and the minimum of the Koenigsberger ratio Q can be determined uniquely. The remanent magnetization direction and certain other information can be derived for special cases in which the value of one or more of the physical property terms can be assigned. Accurate results were obtained in the analysis of data from a theoretical model. Analysis of gravity and magnetic data from the North Atlantic Gilliss seamount indicates the presence of a significant component of remanent magnetization and leads to derived physical properties which are in fairly close agreement with dredged sample data. The calculated direction of remanent magnetization indicates a paleomagnetic pole position in eastern Siberia, in general agreement with the predicted position for a Cretaceous source in the North Atlantic. The seamount example illustrates certain contingent problems to be considered in practical application of the method.

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