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We analyzed magnetic data of a deep-water area in the Eastern Mediterranean, offshore Egypt where there are numerous salt domes. Regional-residual separation of the reduced to pole (RTP) magnetic anomalies was performed with the discrete wavelet transform, and the residual magnetic anomalies were interpreted as mainly due to the susceptibility contrast between sediments and salt bodies. A multiscale boundary analysis of the anomalies yielded meaningful lineaments at both large and short scales. Salt bodies lineaments are very well defined in the short-scale map. Then, we inverted the data by a nonlinear and non-iterative 3D inversion technique, to delineate the salt bodies and estimate their magnetization contrast. We used the depths obtained from the Euler deconvolution to form a set of maxima and minima constraints in the inversion. Our procedure yielded an interesting map of the top of salt in the deep-water region, entirely based on the magnetic data, which agrees well with the seismic top of salt. This is a somewhat surprising result, in some respects not obvious, demonstrating that magnetic prospecting can be an important and low-cost tool for the exploration of salt diapirs, especially when seismic data are partially or completely inaccessible or of poor quality. [ABSTRACT FROM AUTHOR]

We performed magnetic and GPR measurements to image the buried ruins of the Middle Age abbey San Pietro in Crapolla, on the Sorrento‐Amalfi Coast (Massa Lubrense, Southern Italy). The site represents an important religious location, which is nowadays partially buried along the cliff. An integrated study was necessary to map the buried structures and address the archaeological excavation. For this reason, we carried out the surveys on two main grids in order to reconstruct the structures of the abbey and of its related church. The magnetic data were filtered through the discrete wavelet transform (DWT) and then transformed to total gradient maps. The obtained maps were interpreted with depth from extreme points (DEXP) imaging method to assess the horizontal and depth positions of the top. The GPR data were processed and time‐depth converted. Results from the integrated interpretation of these data suggest the possible presence of different vaulted rooms and an elongated structure at 0.3‐m depth from ground surface. This latter is interpretable in terms of perimetral and internal walls of the abbey and its church. These outcomes were crucial to successfully address archaeological excavations, which targeted one of the modelled areas and unearthed a wall at the predicted depths. [ABSTRACT FROM AUTHOR]

We modeled gravity data to explore Mercury’s internal structure and show the presence of crustal heterogeneities in density. We first evaluated the lithospheric flexure occurring in the spherical harmonic degree range 5–80, according to the flexural isostatic response curve. We thus estimated a mean elastic lithosphere thickness of about 30 ± 10 km and modeled the crust-mantle interface, which varies from 19 to 42 km depth, according to a flexural compensation model. The isostatic gravity anomalies were then obtained as the residual field with respect to the contributions from topography and lithospheric flexure. Isostatic anomalies are mainly related to density variations in the crust: gravity highs mostly correspond to large-impact basins suggesting intra-crustal magmatic intrusions as the main origin of these anomalies. Isostatic gravity lows prevail, instead, above intercrater plains and may represent the signature of a heavily fractured crust. [ABSTRACT FROM AUTHOR]

In the inversion of geophysical data, an attempt is made to obtain a model with the best fit on the observed data. Unfortunately, the results are usually accompanied by non-uniqueness and ambiguity. These inversion problems can be reduced by inverting different geophysical datasets. Sequential inversion is one of the most common ways to integrate two or more geophysical datasets, to obtain a model that is compatible with all geophysical data, thus reducing the amount of ambiguity. This paper presents separate inversions of DC resistivity and magnetic data and sequential inversion of DC resistivity constrained by magnetic data. Here, the inverse model of magnetic data is considered the initial model for the sequential inversion of DC resistivity data. At first, the algorithm is applied to a synthetic model composed of four conductive and magnetized bodies, and the results show notable improvement for the resistivity model after sequential inversion, compared with the separate resistivity inversion model. Finally, encouraged by the results obtained in the synthetic case, the algorithm was applied to DC resistivity and magnetic datasets that were collected in the archeological area of old Pompeii city nearby Naples, Italy. The result of the sequential resistivity inversion model was notably superior to the corresponding resistivity model obtained from standard separate inversion. [ABSTRACT FROM AUTHOR]

In this study, we describe new advances in the multiscale methodology to allow a more realistic interpretation of volcanic deformation fields by investigating geometrically irregular bodies and multi-source scenarios. We propose an integrated approach to be applied to InSAR measurements, employing the Multiridge and ScalFun methods and the Total Horizontal Derivative (THD) technique: this strategy provides unconstrained information on the source geometrical parameters, such as the depth, position, shape, and horizontal extent. To do this, we start from conditions where the biharmonic deformation field satisfies Laplace's equation and homogeneity law. We test the use of the multiscale procedures to model single and multisource scenarios with irregular geometries by retrieving satisfactory results for a set of simulated sources. Finally, we employ the proposed approach to the 2004–2009 uplift episode at the Yellowstone Caldera (U.S.) measured by ENVISAT InSAR to provide information about the volcanic plumbing system. Our results indicate a single ~ 50 × 20 km2 extended source lying beneath the caldera at around 10 km b.s.l. (depth to the center), which is shallower below both the resurgent domes (6–7 km b.s.l. depth to the top). [ABSTRACT FROM AUTHOR]

Different geophysical methods use different model parameterizations and inversion algorithms. Thus, combining these different inversion systems and yet adding the nonlinear cross‐gradient constraint in a joint inversion framework might be a big challenge, for instance, as explained further by Moorkamp et al. in 2011, there is a complex interaction between the data misfit terms, regularization and cross‐gradient terms and an imperfect fit to the data is expected. In this paper, we use a sequential algorithm for a two‐dimensional joint inversion of gravity and magnetic data, which tries to avoid these issues by decoupling the gravity inversion, the magnetic inversion and the cross‐gradient minimization processes. The efficiency of the algorithm and developed code is demonstrated by the joint inversion of noisy synthetic data. The results show a significant improvement in the respective models obtained by introducing the cross‐gradient joint inversion over the models obtained by separate inversions for synthetic data and then for field data targeting potash ore source in the AjiChai salt deposit in north‐western Iran. In this application case, the lower density of salt minerals such as potash, compared to its surrounding sedimentary sequences, motivates a gravity study. Furthermore, the relative lower susceptibility of these salt minerals, alongside their diamagnetic effect, makes them a suitable target for magnetic surveys. Separate gravity and magnetic studies had been performed over the deposit; however, a constitutive relationship between density and magnetization within the area of interest supporting a joint inversion had not been established. In this paper, we apply the sequential cross‐gradient approach to perform the first full joint inversion for the AjiChai salt deposit. The magnetic inversion here is performed to recover the magnetization amplitudes rather than the magnetization vector. In fact, we assume there is no remanent magnetization and, therefore, that the magnetization vector is constant and parallel to the geomagnetic field direction. The constructed density and magnetization models are of high concordance with available geological information and previous studies including drilling results. In addition, unlike previous separate inversion models, the models are structurally and geometrically similar. [ABSTRACT FROM AUTHOR]

In this paper we perform a 2-D joint inversion of DC resistivity and magnetic data, constrained by cross-gradients. Inspired by methods developed for potential fields, we introduce into both the separate and joint inversion algorithms also compactness and depth weighting functions, under the form of a model weighting-function. These constraints, usually not considered for DC resistivity inversion, reveal to be decisive for its joint inversion with magnetic data. A linear approximated forward problem of the resistivity is used for the joint inversion so that both the resistivity and magnetic problems are expressed as a linear integral equation under the form of a Fredholm integral of 1st kind. To examine the feasibility of the joint inversion algorithm, we first test the method with two synthetic cases: a thick dyke in a two-layered medium and a cavity located above a conductor. A third synthetic case involves a multisource model. The results are encouraging, revealing that the cross-gradient constraint is an effective tool to improve the separate inversions of DC resistivity and magnetic data. The joint inversion algorithm is also applied to data in the archeological area of the old Pompeii city, nearby Naples. Comparing the results of joint and separate inversions, we obtain a significant improvement in the interpretation of both kind of data in terms of buried walls of an ancient roman villa. In all the studied cases, the cross-gradient constraint appears to be a key-diagnostic tool to assess whether actual coherence is gained among DC resistivity and magnetic susceptibility models. [ABSTRACT FROM AUTHOR]

Inversion of magnetic data is complicated by the presence of remanent magnetization, and it provides limited information about the magnetic source because of the insufficiency of data and constraint information. We propose a Fourier domain transformation allowing the separation of magnetic anomalies into the components caused by induced and remanent magnetizations. The approach is based on the hypothesis that each isolated source is homogeneous with a uniform and specific Koenigsberger ratio. The distributions of susceptibility and remanent magnetization are subsequently recovered from the separated anomalies. Anomaly components, susceptibility distribution and distribution of the remanent and total magnetization vectors (direction and intensity) can be achieved through the processing of the anomaly components. The proposed method therefore provides a procedure to test the hypotheses about target source and magnetic field, by verifying these models based on available information or a priori information from geology. We test our methods using synthetic and real data acquired over the Zhangfushan iron‐ore deposit and the Yeshan polymetallic deposit in eastern China. All the tests yield favourable results and the obtained models are helpful for the geological interpretation. [ABSTRACT FROM AUTHOR]

Both imaging and inversion of potential fields allow the estimation of the source–property distribution. Here, we compare these methods in order to assess their relative advantages and performances. Specifically, we use an iterative imaging algorithm, which is based on the compact depth from extreme points (CDEXP), and the data-space inverse algorithm. This choice was determined because both the methods use a depth weighting function and a compacting function, i.e., they yield a compact source solution. Inverted and imaged solutions are compared with each other, for two sets of noise-corrupted synthetic data, one relative to a simple prism and the other to two oppositely dipping dikes. In both cases, the two models show a noticeable similarity. However, the execution times are substantially different, with the inversion times being an order of magnitude greater. Finally, we interpret two real gravity data sets by using both the approaches: gravity data sets acquired over 1) the Galinge iron-ore deposit of Northwest China and 2) Jiaodong gold deposit of East China. We found that the source models obtained by imaging and inversion methods are once again similar. [ABSTRACT FROM AUTHOR]

In this work, the crustal volume struck by the 2016–2017 seismic sequence in Central and Northern Apennines is investigated using constrained 3‐D inversion of the gravity anomalies. In order to focus on the area comprising the two mainshocks of the sequence, we perform a regional field removal on the data as a preprocessing step. This residual data set is then inverted into a 3‐D density contrast model. We perform a series of inversions to test different geological scenarios and parameters, with increasing complexity in the reference geometries. We first test a model comprising turbidites, carbonates and evaporites, and basement and then introduce a low‐density layer at the top of the basement. The geometries are obtained in agreement with the available geological and geophysical information in the area. We found that the density distribution with depth is compatible with previous models. Moreover, results support the hypothesis based on borehole evidence of a low‐density upper basement across the entire area, possibly consisting of low‐grade metamorphic rocks (phyllites). Finally, we compare our modeling results to the spatial distribution at depth of major seismic events between August and November 2016. These events appear to be concentrated within the denser units at both shallow and deep locations, while the deeper events often occur in a region of major density contrast corresponding to the top of the basement.Key Points: We produce 3‐D inversions of gravity anomalies to investigate the relations between lithology changes and seismic eventsWe report a spatial relation between events distribution and density changes in the upper crustThe derived models support a layered basement limiting the depth propagation of seismic events [ABSTRACT FROM AUTHOR]

In the European region, the magnetic field at satellite altitudes (∼350 km) is mainly defined by a long-wavelength magnetic low, called the Central European Magnetic Low (CEML) here, located to the southwest of the Trans-European Suture Zone (TESZ). We studied this area through a joint analysis of magnetic and total gradient (∇T) anomaly maps for a range of different altitudes of 5, 100 and 350 km. Tests on synthetic models showed the usefulness of the joint analysis at various altitudes to identify reverse dipolar anomalies and to characterize areas in which magnetization is weak. This way we identified areas where either reversely or normally magnetized sources are locally dominant. At a European scale these anomalies are sparse, with a low degree of coalescence effect. The ∇T map indeed presents generally small values within the CEML area, indicating that the Paleozoic Platform is weakly magnetized. At 350 km of altitude, the TESZ effect is largely dominant: with intense ∇T highs above the East European Craton (EEC) and very small values above the Paleozoic Platform, this again denotes a weakly magnetized crust. Small coalescence effects are masked by the trend of the TESZ. Although we identified sparsely located reversely magnetized sources in the Paleozoic Platform of the CEML, the joint analysis does not support a model of a generally reversely magnetized crust. Instead, our analysis strongly favors the hypothesis that the CEML anomaly is mainly caused by a sharp contrast between the magnetic properties of the EEC and Paleozoic Platform. [ABSTRACT FROM AUTHOR]

To investigate the crustal magnetic structure, it is important to assess the susceptibility and remanence properties of rocks and ores. In this paper, we propose a method to extract the contributions of induced and remanent magnetization from modeling of magnetic anomalies. We first estimate the direction of the total magnetization vector by studying the reduced‐to‐pole anomaly and its correlation with different magnitude magnetic transforms. Then we invert the magnetic data to obtain the volumetric distribution of the magnetization intensity. As the third step, based on a priori information about the Koenigsberger ratio derived from petrophysical measurements, we extract the distributions in the source volume of the induced and remanent magnetization intensities, based on a generalized relationship involving the total and remanent magnetizations, and the true susceptibility. In this way, we are able to produce separate maps of the anomaly fields attributed to the physical magnetic source parameters: remanent and induced magnetization. After validating the method with synthetic data, we analyze the data relative to the Mesozoic and Cenozoic igneous rocks in Yeshan region, eastern China. The analysis of the separated magnetization components reveals that the intrusion of dioritic and basaltic rocks occurred at different geological periods, and the basaltic rocks were magnetized by a reversed geomagnetic field. The uncertainty analysis shows that a larger Koenigsberger ratio is beneficial to extract more reliable remanence and susceptibility information. Key Points: An inversion method of magnetic data in the presence of remanent magnetization is proposedInduced and remanent magnetizations are estimated for each block of the source domain, and corresponding magnetic anomalies are extractedYeshan region (eastern China) is studied, where Cenozoic basalts were formed during a period of reversed direction of the geomagnetic field [ABSTRACT FROM AUTHOR]

Natural remanent magnetization and self-demagnetization on high-susceptibility bodies are two important factors affecting magnetic data inversion. We propose a framework for the inversion and interpretation of magnetic anomalies, in which significant remanent magnetization and self-demagnetization are present simultaneously. The framework is based on the assumptions that the external applied field and internal self-demagnetization field are uniform and the deflection of self-demagnetization in the total magnetization direction is negligible. First, the magnetization vector distributions are obtained from magnetic data by estimating the magnetization direction, then inverting for the magnetization intensity distribution, using the inferred magnetization direction as a constraint. Based on a priori information about the Koenigsberger ratio derived from petrophysical measurements, the direction and intensity of the remanent magnetization are obtained. The self-demagnetization factor is then computed using the finite volume method. Finally, the true-susceptibility distribution is achieved by correcting for the self-demagnetization effect. The method is first applied to synthetic magnetic data produced by a prism-shaped source model that has significant remanent magnetization and high susceptibility. In a case study of the Daye iron-ore deposit, Hubei province, China, the true susceptibility and remanent magnetization are reconstructed. The remanence direction information reveals that local geological activities such as synclines and faults lead to changes in the remanence directions at different local deposits. [ABSTRACT FROM AUTHOR]

ABSTRACT: We present a non‐linear method to invert potential fields data, based on inverting the scaling function (τ) of the potential fields, a quantity that is independent on the source property, that is, the mass density in the gravity case or the magnetic susceptibility in the magnetic case. So, no a priori prescription of the density contrast is needed, and the source model geometry is determined independently on it. We assume Talwani's formula and generalise the Multi‐HOmogeneity Depth Estimation method to the case of the inhomogeneous field generated by a general two‐dimensional source. The scaling function is calculated at different altitudes along the lines defined by the extreme points of the potential fields, and the inversion of the scaling function yields the coordinates of the vertices of a multiple source body. Once the geometry is estimated, the source density is automatically computed from a simple regression of the scaling function of the gravity data versus that generated from the estimated source body with a unit density. We solve the above non‐linear problem by the very fast simulated annealing algorithm. The best performance is obtained when some vertices are constrained by either reasonable bounds or exact knowledge. In the salt dome case, we assumed that the top of the body is known from seismic observations, and we solved for the lateral and bottom parts of the body. We applied the technique on three synthetic cases of complex sources and on the gravity anomalies over the Mors salt dome (Denmark) and the Godavari basin (India). In all these cases, the method performed very well in terms of both geometrical and source property definition. [ABSTRACT FROM AUTHOR]

ABSTRACT We propose a fast method for imaging potential field sources. The new method is a variant of the 'Depth from Extreme Points,' which yields an image of a quantity proportional to the source distribution (magnetization or density). Such transformed field is here transformed into source-density units by determining a constant with adequate physical dimension by a linear regression of the observed field versus the field computed from the 'Depth from Extreme Points' image. Such source images are often smooth and too extended, reflecting the loss of spatial resolution for increasing altitudes. Consequently, they also present too low values of the source density. We here show that this initial image can be improved and made more compact to achieve a more realistic model, which reproduces a field consistent with the observed one. The new algorithm, which is called 'Compact Depth from Extreme Points' iteratively produces different source distributions models, with an increasing degree of compactness and, correspondingly, increasing source-density values. This is done through weighting the model with a compacting function. The compacting function may be conveniently expressed as a matrix that is modified at any iteration, based on the model obtained in the previous step. At any iteration step the process may be stopped when the density reaches values higher than prefixed bounds based on known or assumed geological information. As no matrix inversion is needed, the method is fast and allows analysing massive datasets. Due to the high stability of the 'Depth from Extreme Points' transformation, the algorithm may be also applied to any derivatives of the measured field, thus yielding an improved resolution. The method is investigated by application to 2D and 3D synthetic gravity source distributions, and the imaged sources are a good reconstruction of the geometry and density distributions of the causative bodies. Finally, the method is applied to microgravity data to model underground crypts in St. Venceslas Church, Tovacov, Czech Republic. [ABSTRACT FROM AUTHOR]

Magnetic and electromagnetic surveying are effective techniques frequently used in archaeology because the susceptibility and the electric resistivity contrast between the cover soil and several buried finds often lead to detectable anomalies. Significant advances were recently achieved by 3D imaging methods of potential field data that provide an estimate of the magnetization distribution within the subsurface. They provide a high-resolution image of the source distribution, thanks to the differentiation of the field and to the stability of the process. These techniques are fast and quite effective in the case of a compact, isolated, and depth-limited source, i.e., just the kind of source generally occurring in archaeological investigations. We illustrate the high-resolution imaging process for a geophysical study carried out at Torre Galli ( Vibo Valentia, Calabria, Italy), one of the most significant sites of the early Iron Age in Italy. Multi-scale derivative analysis of magnetic data revealed the trends of anomalies shaped and aligned with a regular geometry. This allowed us to make an outline of the buried structures, and then to characterize them in terms of size, shape, and depth by means of the imaging technique. Targeted excavations were therefore addressed to the locations selected by our analysis, revealing structures showing exactly the predicted features and confirming the archaeological hypothesis concerning the settlement organization partitioned in terms of functional differentiation: an intermediate area occupied mostly by defensive structures placed between the village, westward, and the necropolis, eastward. [ABSTRACT FROM AUTHOR]

We describe a multihomogeneity theory for source-parameter estimation of potential fields. Similar to what happens for random source models, where the monofractal scaling-law has been generalized into a multifractal law, we propose to generalize the homogeneity law into a multihomogeneity law. This allows a theoretically correct approach to study real-world potential fields, which are inhomogeneous and so do not show scale invariance, except in the asymptotic regions (very near to or very far from their sources). Since the scaling properties of inhomogeneous fields change with the scale of observation, we show that they may be better studied at a set of scales than at a single scale and that a multihomogeneous model is needed to explain its complex scaling behaviour. In order to performthis task,we first introduce fractional-degree homogeneous fields, to show that: (i) homogeneous potential fields may have fractional or integer degree; (ii) the source-distributions for a fractional-degree are not confined in a bounded region, similarly to some integer-degree models, such as the infinite line mass and (iii) differently from the integer-degree case, the fractional-degree source distributions are no longer uniformdensity functions. Using this enlarged set of homogeneous fields, real-world anomaly fields are studied at different scales, by a simple search, at any local window W, for the best homogeneous field of either integer or fractional-degree, this yielding a multiscale set of local homogeneity-degrees and depth estimations which we call multihomogeneous model. It is so defined a new technique of source parameter estimation (Multi-HOmogeneity Depth Estimation, MHODE), permitting retrieval of the source parameters of complex sources. We test the method with inhomogeneous fields of finite sources, such as faults or cylinders, and showits effectiveness also in a real-case example. These applications showthe usefulness of the new concepts, multihomogeneity and fractional homogeneity-degree, to obtain valid estimates of the source parameters in a consistent theoretical framework, so overcoming the limitations imposed by global-homogeneity to widespread methods, such as Euler deconvolution. [ABSTRACT FROM AUTHOR]

In Cutrofiano, in the southern part of the Salento Peninsula, Apulia, Italy, a Pleistocene calcarenitic sequence was quarried by digging extensive networks of galleries along the geological succession most suitable for the quarrying activity. These caves represent a potential hazard for the built-up environment due to the occurrence of underground instability that may propagate upward and eventually reach the surface, causing sinkholes. In this work we propose integrated interdisciplinary methods for cavities detection. The methodology was applied at a test area located along a major road near Cutrofiano using geological and electrical-resistivity tomography and microgravity geophysical methods. [ABSTRACT FROM AUTHOR]

An integrated review of existing geological and geophysical data – partly acquired during oil and gas exploration – combined with new data provided by deep geothermal studies of selected key areas, was used for the 3D modeling and mapping of the top of the geothermal reservoir developed at a regional scale in Sicily (Central Mediterranean). The resulting 1:500,000 scale map covers the area of the whole Sicily (about 25,700 km2) and is devoted to represent the main input for both the thermal modeling and the evaluation of geothermal potential at a regional scale. As the map indicates the distribution at depth of a likely target for geothermal drilling, it can be also used as a rough indicator of expected drilling cost for geothermal projects. Such a map can be seen as a useful planning tool for any geothermal project, and related exploration to be carried out in the Sicily region in the future. [ABSTRACT FROM PUBLISHER]

We illustrate a new imaging method to estimate the depth to the sources of potential fields and the structural index. The method consists of applying the Depth from EXtreme Point (DEXP) transformation to the ratio (ℜ) between two different-order partial derivatives of the field. While the scaling function of the potential field depends on the structural index, we show that the scaling function of ℜ merely depends on the difference between the two used orders of differentiation. This allows three main features to be established for the DEXP transformation of ℜ: (1) it is independent from the structural index; (2) the estimation of the source depths is fully automatic, simply consisting in the search of position of the extreme points of the DEXP image and (3) the structural index of each source is finally determined from the scaling function or the extreme points using the estimated depth. Besides the well-known characteristics of the DEXP transformation, such as high-resolution and stability, the DEXP transformation of ℜ enjoys one more relevant feature: it can be applied to multisource cases, yielding simultaneously correct estimations of structural index and depth for each source in the same image. However, while the DEXP transformation is a linear transformation of the field, the DEXP transformation of ℜ is non-linear, and a procedure is described to circumvent the non-linear effects. The method is tested with synthetic examples and the estimated source parameters show a good agreement with the true values. The method was applied also to real magnetic data from the Pima copper mine, Arizona, USA, Hamrawien area, Egypt and Cataldere, Bala district of Turkey. The results are consistent with the known information about the causative sources. [ABSTRACT FROM AUTHOR]

We illustrate a new imaging method to estimate the depth to the sources of potential fields and the structural index. The method consists of applying the Depth from EXtreme Point (DEXP) transformation to the ratio (ℜ) between two different-order partial derivatives of the field. While the scaling function of the potential field depends on the structural index, we show that the scaling function of ℜ merely depends on the difference between the two used orders of differentiation. This allows three main features to be established for the DEXP transformation of ℜ: (1) it is independent from the structural index; (2) the estimation of the source depths is fully automatic, simply consisting in the search of position of the extreme points of the DEXP image and (3) the structural index of each source is finally determined from the scaling function or the extreme points using the estimated depth. Besides the well-known characteristics of the DEXP transformation, such as high-resolution and stability, the DEXP transformation of ℜ enjoys one more relevant feature: it can be applied to multisource cases, yielding simultaneously correct estimations of structural index and depth for each source in the same image. However, while the DEXP transformation is a linear transformation of the field, the DEXP transformation of ℜ is non-linear, and a procedure is described to circumvent the non-linear effects.The method is tested with synthetic examples and the estimated source parameters show a good agreement with the true values. The method was applied also to real magnetic data from the Pima copper mine, Arizona, USA, Hamrawien area, Egypt and Cataldere, Bala district of Turkey. The results are consistent with the known information about the causative sources. [ABSTRACT FROM PUBLISHER]

A detailed gravity survey with 235 measurements was carried out at the King Fahd University of Petroleum and Minerals campus, which is located at the crest of the oil-bearing Dammam Dome (Eastern Province of Saudi Arabia). This survey allows us to better understand the geometry of the underlying Dammam Dome and its tectonic regime. The acquired data were processed using conventional gravity data reduction techniques. The effectiveness of terrain correction was evaluated using several recently developed algorithms. Afterward, processed data were subject to geophysical filters for edge detection (terracing transformation and horizontal gradient) and depth estimation (tilt derivative and 3D inversion). 3D Bouguer maps were generated and compared to the proposed geological models for the Dammam Dome. The results show the existence of ENE-WSW striking tectonic lines, where two nearly vertical, km long tectonic lines were predominant. The orientation of these tectonic lines defines an NNW-SSE trend for the least principal stress axis (σ3) and an ENE-WSW trend for the σ1σ2 stress plane of the driving stress regime, fitting well with the transtension stress regime recently suggested for the area. More importantly, the results of this study demonstrate that the Dammam Dome was affected by the intraplate stresses transferred from the convergence between the Arabian and Eurasian plates along the Zagros orogeny. [ABSTRACT FROM AUTHOR]

We describe a multiscale method to estimate the excess mass of gravity anomaly sources, based on the theory of source moments. Using a multipole expansion of the potential field and considering only the data along the vertical direction, a system of linear equations is obtained. The choice of inverting data along a vertical profile can help us to reduce the interference effects due to nearby anomalies and will allow a local estimate of the source parameters. A criterion is established allowing the selection of the optimal highest altitude of the vertical profile data and truncation order of the series expansion. The inversion provides an estimate of the total anomalous mass and of the depth to the centre of mass. The method has several advantages with respect to classical methods, such as the Gauss’ method: (i) we need just a 1-D inversion to obtain our estimates, being the inverted data sampled along a single vertical profile; (ii) the resolution may be straightforward enhanced by using vertical derivatives; (iii) the centre of mass is also estimated, besides the excess mass; (iv) the method is very robust versus noise; (v) the profile may be chosen in such a way to minimize the effects from interfering anomalies or from side effects due to the a limited area extension. The multiscale estimation of excess mass method can be successfully used in various fields of application. Here, we analyse the gravity anomaly generated by a sulphide body in the Skelleftea ore district, North Sweden, obtaining source mass and volume estimates in agreement with the known information. We show also that these estimates are substantially improved with respect to those obtained with the classical approach. [ABSTRACT FROM PUBLISHER]

ABSTRACT Potential field interpretation can be carried out using multiscale methods. This class of methods analyses a multiscale data set, which is built by upward continuation of the original data to a number of altitudes conveniently chosen. Euler deconvolution can be cast into this multiscale environment by analysing data along ridges of potential fields, e.g., at those points along lines across scales where the field or its horizontal or vertical derivative respectively is zero. Previous work has shown that Euler equations are notably simplified along any of these ridges. Since a given anomaly may generate one or more ridges we describe in this paper how Euler deconvolution may be used to jointly invert data along all of them, so performing a multiridge Euler deconvolution. The method enjoys the stable and high-resolution properties of multiscale methods, due to the composite upward continuation/vertical differentiation filter used. Such a physically-based field transformation can have a positive effect on reducing both high-wavenumber noise and interference or regional field effects. Multiridge Euler deconvolution can also be applied to the modulus of an analytic signal, gravity/magnetic gradient tensor components or Hilbert transform components. The advantages of using multiridge Euler deconvolution compared to single ridge Euler deconvolution include improved solution clustering, increased number of solutions, improvement of accuracy of the results obtainable from some types of ridges and greater ease in the selection of ridges to invert. The multiscale approach is particularly well suited to deal with non-ideal sources. In these cases, our strategy is to find the optimal combination of upward continuation altitude range and data differentiation order, such that the field could be sensed as approximately homogeneous and then characterized by a structural index close to an integer value. This allows us to estimate depths related to the top or the centre of the structure. [ABSTRACT FROM AUTHOR]

ABSTRACT Nonparametric inverse methods provide a general framework for solving potential-field problems. The use of weighted norms leads to a general regularization problem of Tikhonov form. We present an alternative procedure to estimate the source susceptibility distribution from potential field measurements exploiting inversion methods by means of a flexible depth-weighting function in the Tikhonov formulation. Our approach improves the formulation proposed by , differing significantly in the definition of the depth-weighting function. In our formalism the depth weighting function is associated not to the field decay of a single block (which can be representative of just a part of the source) but to the field decay of the whole source, thus implying that the data inversion is independent on the cell shape. So, in our procedure, the depth-weighting function is not given with a fixed exponent but with the structural index N of the source as the exponent. Differently than previous methods, our choice gives a substantial objectivity to the form of the depth-weighting function and to the consequent solutions. The allowed values for the exponent of the depth-weighting function depend on the range of N for sources: 0 ≤ N≤ 3 (magnetic case). The analysis regarding the cases of simple sources such as dipoles, dipole lines, dykes or contacts, validate our hypothesis. The study of a complex synthetic case also proves that the depth-weighting decay cannot be necessarily assumed as equal to 3. Moreover it should not be kept constant for multi-source models but should instead depend on the structural indices of the different sources. In this way we are able to successfully invert the magnetic data of the Vulture area, Southern Italy. An original aspect of the proposed inversion scheme is that it brings an explicit link between two widely used types of interpretation methods, namely those assuming homogeneous fields, such as Euler deconvolution or depth from extreme points transformation and the inversion under the Tikhonov-form including a depth-weighting function. The availability of further constraints, from drillings or known geology, will definitely improve the quality of the solution. [ABSTRACT FROM AUTHOR]

ABSTRACT Downward continuation is a useful transformation, mainly used to enhance measured gravity or magnetic field anomalies. It is known to be an unstable transformation that should be strictly used only in the harmonic region, apparently preventing any meaningful application to continuations inside the source volume. Despite these well-known theoretical and practical limitations it has been used to recover source parameters by different methods, here referred to as normalized full gradient methods. Such methods show that downward continuation may be extended to the source volume, which is assumed to contain one-point, isolated singularities, which is a quasi-harmonic region. We modify the normalized full gradient method focusing our attention to the way the downward continuation is normalized. Differently from normalized full gradient methods, we study the effect of the normalization not only on the analytical signal modulus of the downward continued field but also on the downward continuation of the gravity or magnetic fields themselves. With our method, called normalized downward continuation, several statistically meaningful normalizations are considered, some of them yielding improved, more resolved depth estimations for synthetic as well as measured total-field anomalies. From a statistical point of view, the downward continued field tends to have right-skewed histograms at shallow depths, while becoming symmetrically distributed at greater depths. This occurs because, as the depth of continuation increases, the intrinsic error propagation of the downward continuation allows the error to dominate with respect to the source-related signal. For non-isolated anomalies, consistent results are also obtained but the normalizing factors must be computed within windows centred to the studied anomaly. [ABSTRACT FROM AUTHOR]

Terra Nova, 23, 127-133, 2011 Abstract We analyse the pattern of magnetic anomalies in the Tyrrhenian back-arc basin by a simple and objective method that is able to simplify the readability of the maps and to safely identify reversely magnetized sources. This analysis is useful to better understand the reliability of recent studies indicating the presence of striped, 'mid-ocean like', magnetic anomalies in this small basin, from which spreading velocities have been calculated. Our results show that in the Vavilov and Marsili sub-basins reversely magnetized bodies are actually present, but their areal distribution is rather irregular. The resulting pattern of normally and reversely magnetized bodies does not fit with an ocean-like central expansion model. Instead, the distribution of magnetic sources may suggest that the crustal tearing induced by extension processes is not concentrated only beneath the seamounts, but may have in different times involved different areas of these sub-basins. [ABSTRACT FROM AUTHOR]

The way potential fields convey source information depends on the scale at which the field is analysed. In this sense a multiscale analysis is a useful method to study potential fields particularly when the main field contributions are caused by sources with different depths and extents. Our multiscale approach is built with a stable transformation, such as depth from extreme points. Its stability results from mixing, in a single operator, the wavenumber low-pass behaviour of the upward continuation transformation of the field with the enhancement high-pass properties of n-order derivative transformations. So, the complex reciprocal interference of several field components may be efficiently faced at several scales of the analysis and the depth to the sources may be estimated together with the homogeneity degrees of the field. In order to estimate the source boundaries we use another multiscale method, the multiscale derivative analysis, which utilizes a generalized concept of horizontal derivative and produces a set of boundary maps at different scales. We show through synthetic examples and application to the gravity field of Southern Italy that this multiscale behaviour makes this technique quite different from other source boundary estimators. The main result obtained by integrating multiscale derivative analysis with depth from extreme points is the retrieval of rather effective information of the field sources (horizontal boundaries, depth, structural index). This interpretative approach has been used along a specific transect for the analysis of the Bouguer anomaly field of Southern Apennines. It was set at such scales, so to emphasize either regional or local features along the transect. Two different classes of sources were individuated. The first one includes a broad, deep source with lateral size of 45∼50 km, at a depth of 13 km and having a 0.5 structural index. The second class includes several narrower sources located at shallowest depths, ranging from 3–6 km, with lateral size not larger than 5 km and structural indexes ranging from 1–1.5. Within a large-scale geological framework, these results could help to outline the mean structural features at crustal depths. [ABSTRACT FROM AUTHOR]

Geophysical well-logs often show a complex behavior which seems to suggest a multifractal nature. Multifractals are highly intermittent signals, with distinct active bursts and passive regions which cannot be satisfactorily characterized in terms of just second-order statistics. They need a higher-order statistical analysis. In contrast with monofractals which have a homogeneous scaling, multifractals may include singularities of many types. Here we describe how a multiscale analysis can be used to describe the magnetic susceptibility data scaling properties for a deep well (KTB, Germany), down to about 9000 m. A multiscale analysis describes the local and global singular behavior of measures or distributions in a statistical fashion. The global analysis allows the estimation of the global repartition of the various Holder exponents. As such, it leads to the definition of a spectrum, D(α), called the singularity spectrum. The local analysis is related to the possibility of estimating the Lipschitz regularity locally, i.e., at each point of the support of a multifractal signal. The application of both approaches to the KTB magnetic susceptibility data shows a meaningful correlation between the sequence of Holder exponents vs. depth and the lithological units. The Holder exponents reach the highest values for gneiss units, intermediate ones for amphibolite units and the lowest values for variegated units. Faults are found to correspond to changes for H also when they are of intra-lithological type. [ABSTRACT FROM AUTHOR]

ABSTRACT The problem of removing directional trends frequently occurs in the processing of magnetic data and also in the subsequent steps of data interpretation. The so-called corrugations are typical directional trends occurring in levelled data, which may be removed in several ways. Classical techniques are based on high-pass filtering of the data and successively filtering these transformed data with directional cosine filters. Other linear features are due to real sources, such as pipelines in shallow surveys or dike swarms in regional surveys. They should, nevertheless, be considered as noise, due to the fact that their effect is strong and tends to hide the field features related to structures of more interest. We deal with both kinds of problem, presenting the results of a study in an archaeological area of southern Italy. Decorrugation of magnetic field anomalies is performed using a method based on the excellent space–frequency localization properties of wavelet bases, allowing a very sharp filtering of the field along a selected direction. We compare this technique with the classical one in a synthetic case and find that the wavelet decorrugation is simpler and produces low distortion maps. Besides the field decorrugation, the wavelet approach was also shown to be useful in the subsequent enhancement of the measured field. In fact, we show that the wavelet analysis offers a unique framework where various filtering problems (directional, isotropic, global or local as well) may be easily solved. As regards the archaeological case, strong noisy effects from elongated sources (pipelines) were successfully removed in a sharp and local way. [ABSTRACT FROM AUTHOR]

Examines the properties of the scaling exponent of potential field power spectra. Types of density and magnetization distributions with spectral exponents in the fractal range; Application of the fractal theory to magnetic and gravity anomalies; Calculation of the source power spectrum.

A method based on the discrete wavelet transform was applied to the regional-residual separation of potential fields and to the filtering of local anomalies. A specific space-scale wavelet analysis, called multiresolution analysis, allowed decomposition of the signal with respect to a vast range of scales. Different analysing wavelets were applied to anomalies in both synthetic and real cases, but the more appropriate basis needed to be chosen by requiring the maximum compactness for the multiresolution analysis. Moreover, since such analysis was found not to be shift-invariant, the same criterion was applied to choosing the best signal shift. Application of the technique to both synthetic and real cases produced an optimal space-scale representation of the fields and a consistent regional-residual separation. Furthermore, the space localization allowed a variety of filtered signals to be obtained, each one with a specific scale and local area content. Fourier and wavelet analyses were both applied to the filtering out of the intense Etna anomaly from the aeromagnetic field of Sicily. The wavelet method was more powerful, suppressing only the Etna volcano anomaly and leaving the rest of the map practically unchanged. [ABSTRACT FROM AUTHOR]

The estimation of the depth to the top and bottom of a magnetic source from magnetic data defines a nonlinear inverse problem, while the evaluation of the distribution of magnetization determines a linear inverse problem. In this paper, these interpretation problems are resolved in the continuous case of 2D magnetized bodies with lateral magnetization variations. A formulation of the magnetic problem accounting for different directions of remanent and total magnetization vectors and including a more general definition of apparent susceptibility is presented. Differences between 2D and 2D formulations are stressed, as regards the anomaly amplitude, shape and zero-level. In order to utilize well-known continuous linear inverse methods, Fréchet derivatives of the data functionals with respect to the depth of the source top and bottom, are analytically described. Thus, using the spectral expansion inverse method ( Parker, 1977) and linearizing the problem at several steps of an iterative process, the source depth is obtained within a few iterations, although the starting model is distant from the final solution. The interpretation of an anomaly in the Italian region shows the usefulness of the method. [ABSTRACT FROM AUTHOR]

Uturuncu volcano (southwestern Bolivia) is localized within one of the largest updoming volcanic zones, the Altiplano Puna Volcanic Complex (APVC). In several geodetic studies the observed uplift phenomenon is analyzed and modeled by considering a deep source, related to the Altiplano Puna Magma Body (APMB). In this framework, we perform a multiscale analysis on the 2003–2010 ENVISAT satellite data to investigate the existence of a multi-source scenario for this region. The proposed analysis is based on Cross-correlation and Multiridge method, pointing out the spatial and temporal multiscale properties of the deformation field. In particular, we analyze the vertical component of ground deformation during two time interval: within the 2005–2008 time interval an inflating source at 18.7 km depth beneath the central zone of the APVC is retrieved; this result is in good agreement with those proposed by several authors for the APMB. Between August 2006 and February 2007, we identify a further inflating source at 4.5 km depth, beneath Uturuncu volcano; the existence of this latter, located just below the 2009–2010 seismic swarm, is supported by petrological, geochemical, and geophysical evidence, indicating as a possible interpretative scenario the action of shallow, temporarily trapped fluids. [ABSTRACT FROM AUTHOR]