Search

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]

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]

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]

In potential field inversion problems we usually solve underdetermined systems and realistic solutions may be obtained by introducing a depth-weighting function in the objective function. The choice of the exponent of such power-law is crucial. It was suggested to determine it from the field-decay due to a single source-block; alternatively it has been defined as the structural index of the investigated source distribution. In both cases, when k -order derivatives of the potential field are considered, the depth-weighting exponent has to be increased by k with respect that of the potential field itself, in order to obtain consistent source model distributions. We show instead that invariant and realistic source-distribution models are obtained using the same depth-weighting exponent for the magnetic field and for its k -order derivatives. A similar behavior also occurs in the gravity case. In practice we found that the depth weighting-exponent is invariant for a given source-model and equal to that of the corresponding magnetic field, in the magnetic case, and of the 1st derivative of the gravity field, in the gravity case. In the case of the regularized inverse problem, with depth-weighting and general constraints, the mathematical demonstration of such invariance is difficult, because of its non-linearity, and of its variable form, due to the different constraints used. However, tests performed on a variety of synthetic cases seem to confirm the invariance of the depth-weighting exponent. A final consideration regards the role of the regularization parameter; we show that the regularization can severely affect the depth to the source because the estimated depth tends to increase proportionally with the size of the regularization parameter. Hence, some care is needed in handling the combined effect of the regularization parameter and depth weighting. [ABSTRACT FROM AUTHOR]

The estimation of the structural index and of the depth to the source is the main task of many popular methods used to analyze potential field data, such as Euler deconvolution. However, these estimates are unstable even in the presence of a weak amount of noise, and Euler deconvolution of noisy data leads to an underestimation of structural index and depth. We have studied how the structural index and depth estimates are affected by applying low-pass filtering to the data. Physically based low-pass filters, such as upward continuation and integration, have been shown to be the best choice over a range of altitudes (upward continuation) or orders (integration filters), mainly because their outputs have a well-defined physical meaning. In contrast, mathematical low-pass filters require that the filter parameters be tuned carefully by means of several trial tests to produce optimally smoothed fields. The C-norm criterion is a reliable strategy to produce a stabilized vertical derivative, and we discourage Butterworth filters because they tend to a vertical integral filter, for a high cutoff wavenumber, thus complicating the interpretation of the estimated structural index. We found that the estimated structural index and depth to source increase proportionally with the amount of smoothing, unless in the case of overfiltering. In that case, the severe distortion of the original field may cause a decrease of the estimated structural index and depth to source. [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]

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]

The author offers a commentary on a paper which presented an interpretation of self-potential data recorded at the ground surface of the earth using the depth from extreme points (DEXP) method. The author outlines the incorrect assumptions outlined in the imaging algorithm presented by the paper for the fast localization of ore bodies from self-potential (SP) data recorded at the ground surface of the earth.

We used a fast method to interpret self-potential data: the depth from extreme points (DEXP) method. This is an imaging method transforming self-potential data, or their derivatives, into a quantity proportional to the source distribution. It is based on upward continuing of the field to a number of altitudes and then multiplying the continued data with a scaling law of those altitudes. The scaling law is in the form of a power law of the altitudes, with an exponent equal to half of the structural index, a source parameter related to the type of source. The method is auto-consistent because the structural index is basically determined by analyzing the scaling function, which is defined as the derivative of the logarithm of the sell-potential (or of its pth derivative) with respect to the logarithm of the altitudes. So, the DEXP method does not need a priori information on the self-potential sources and yields effective information about their depth and shape/ typology. Important features of the DEXP method are its high-resolution power and stability, resulting from the combined effect of a stable operator (upward continuation) and a high-order differentiation operator. We tested how to estimate the depth to the source in two ways: (1) at the positions of the extreme points in the DEXP transformed map and (2) at the intersection of the lines of the absolute values of the potential or of its derivative (geometrical method). The method was demonstrated using synthetic data of isolated sources and using a multisource model. The method is particularly suited to handle noisy data, because it is stable even using high-order derivatives of the self-potential. We discussed some real data sets: Malachite Mine, Colorado (USA), the Sariyer area (Turkey), and the Bender area (India). The estimated depths and structural indices agree well with the known information. [ABSTRACT FROM AUTHOR]

Several noniterative, imaging methods for potential field data have been proposed that provide an estimate of the 3D magnetization/density distribution within the subsurface or that produce images of quantities related or proportional to such distributions. They have been derived in various ways, using generalized linear inversion, Wiener filtering, wavelet and depth from extreme points (DEXP) transformations, crosscorrelation, and migration. We demonstrated that the resulting images from each of these approaches are equivalent to an upward continuation of the data, weighted by a (possibly) depth-dependent function. Source distributions or related quantities imaged by all of these methods are smeared, diffuse versions of the true distributions; but owing to the stability of upward continuation, resolution may be substantially increased by coupling derivative and upward continuation operators. These imaging techniques appeared most effective in the case of isolated, compact, and depth-limited sources. Because all the approaches were noniterative, computationally fast, and in some cases, produced a fit to the data, they did provide a quick, but approximate picture of physical property distributions. We have found that inherent or explicit depth-weighting is necessary to image sources at their correct depths, and that the best scaling law or weighting function has to be physically based, for instance, using the theory of homogeneous fields. A major advantage of these techniques was their speed, efficiently providing a basis for further detailed, follow-up modelling. [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]

A new method based on 3D multiridge analysis of potential fields assumes a 3D subset in the harmonic region and studies the behavior of potential field ridges, which are built by joining extreme points of the analyzed field computed at different altitudes. Three types of ridges are formed by searching for the zeros of the first horizontal and first vertical derivatives of the potential field (types I and II, respectively) and the zeros of the potential field itself (type III). This method uses a redundant set of ridges, called a multiridge set, to determine source type and location. For homogeneous potential fields generated by simple sources, all of the ridges are straight lines converging to the source position. This method analyzes the multiridges by using a geometric criterion to find the source position at the intersection of the multiridge set and by solving the three reduced Euler equations associated with ridge types I, II, and III. The reduced Euler type I and II equations are used to obtain the structural index and the vertical and horizontal source positions; equation type III estimates the horizontal and vertical source positions. Tests on synthetic as well as the Bishop model field yield good results even with noise-corrupted data. Results obtained using magnetic data collected over the wreck of a military ship in the Tyrrhenian Sea successfully determine its vertical and horizontal positions and the structural index. [ABSTRACT FROM AUTHOR]

Abstract: Volcanomagnetic anomalies have been mostly observed during strong eruptions. Our aim is to improve the geomagnetic data analysis to evidence the anomalies occurring in a larger time span, especially in the phases preceding the eruptive events. We developed a time variant statistical approach and applied it to the 2000–2002 Etna geomagnetic temporal series. It is based on an algorithm that statistically predicts the geomagnetic field at the station on the volcanic edifice by that recorded at the remote one. In such a way a number of significant changes in the time series (called statistical innovations), marking the local magnetic field change, were detected. The distribution of such statistical innovations accurately describes the Etna volcanic evolution: we note a progressive increase of the innovation occurrence as the eruptive cycles were approaching and only few and weak innovations at times between the various eruptive cycles. The significance of this analysis is further confirmed by the close agreement among the mean square prediction error, strain release and the volcanic activity behavior. On the contrary, the geomagnetic field at a single station or its difference at two stations do not have any clear correlation with other measured physical quantities. The complex pattern of the prediction error was also investigated by a multifractal analysis. We found that the Holder regularity increases with the intensification of the volcanic activity, implying that innovations tend to be less sporadic and correlated during the major volcanic phases. [Copyright &y& Elsevier]

We show that potential fields enjoy valuable properties when they are scaled by specific power laws of the altitude. We describe the theory for the gravity field, the magnetic field, and their derivatives of any order and propose a method, called here Depth from Extreme Points (DEXP), to interpret any potential field. The DEXP method allows estimates of source depths, density, and structural index from the extreme points of a 3D field scaled according to specific power laws of the altitude. Depths to sources are obtained from the position of the extreme points of the scaled field, and the excess mass (or dipole moment) is obtained from the scaled field values. Although the scaling laws are theoretically derived for sources such as poles, dipoles, lines of poles, and lines of dipoles, we give also criteria to estimate the correct scaling law directly from the data. The scaling exponent of such laws is shown to be related to the structural index involved in Euler Deconvolution theory. The method is fast and stable because it takes advantage of the regular behavior of potential field data versus the altitude z. As a result of stability, the DEXP method may be applied to anomalies with rather low SNRs. Also stable are DEXP applications to vertical and horizontal derivatives of a Newtonian potential of various orders in which we use theoretically determined scaling functions for each order of a derivative. This helps to reduce mutual interference effects and to obtain meaningful representations of the distribution of sources versus depth, with no prefiltering. The DEXP method does not require that magnetic anomalies to be reduced to the pole, and meaningful results are obtained by processing its analytical signal. Application to different cases of either synthetic or real data shows its applicability to any type of potential field investigation, including geological, petroleum, mining, archeological, and environmental studies. [ABSTRACT FROM AUTHOR]

Abstract: The Lucania Apennines form the frontal part of the Apennines in Southern Italy. This orogenic belt was formed in response to late Miocene–Early Pleistocene shortening and allochthon emplacement toward the northeast, and was subsequently affected by extensional faulting which migrated to northeast ahead of the thrust system. As a result, contraction structures are found associated to Pliocene–Early Pleistocene thrust-top basins in the NE part of the area, whereas extensional faults shape Middle Pleistocene–Holocene basin on the SW sector. The main structural grain of the belt is NW–SE, but E–W structures are widespread in the area as a result of phases of non-coaxial thrusting and strike-slip faulting. The analysis, one by one, of the components of the Gravity Gradient Tensor (GGT) has proved to yield a fine image of the structural setting of the investigated area. GGT is a second rank tensor containing the second spatial derivatives of the gravity potential and in this paper is used instead than the more traditional gravity data. The Tzz component allows an accurate description of the location and of the shape of basins and other structures, but different components of the GGT provide even more detailed insights for such structures. In particular, we found that the Tzy and Tyy locate well those structures trending closer to the E–W direction for both thrust-top and extensional basins, and their termination against the main NW–SE structures. The combined use of components of GGT provides a finer definition of the anomaly sources particularly if a good knowledge of their strike and depth is independently estimated with other geological and geophysical investigations. [Copyright &y& Elsevier]