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Downward continuation is a useful transformation, mainly used to enhance the measured field. Known limitations regard its instability and are theoretical, because the continuation level should not be deeper than the source depth. Despite of this downward continuation was used in several papers to recover source parameters with different methods here referred to as Normalized Full Gradient methods. These studies show that downward continuation may be applied to quasi-harmonic regions, i.e., volumes containing one-point isolated singularities. We modify the Normalized Full Gradient by using statistically meaningful normalizations and find improved, more resolved depth estimations. We show also that methods defined theoretically in the harmonic region, can still be meaningful when applied to downward continued fields within quasi-harmonic regions.

A microgravity survey was completed to precisely locate and better characterize the near-surface geometry of a recent fault with small throw in a mountainous area in the Southern Apennines (Italy). The site is on a segment of the Irpinia fault, which is the source of the M6.9 1980 earthquake. This fault cuts a few meter of Mesozoic carbonate bedrock and its younger, mostly Holocene continental deposits cover. The amplitude of the complete Bouguer anomaly along two profiles across the fault is about 50 μGal. The data were analyzed and interpreted according to a self-constrained strategy, where some rapid estimation of source parameters was later used as constraint for the inversion. The fault has been clearly identified and localized in its horizontal position and depth. Interesting features in the overburden have been identified and their interpretation has allowed us to estimate the fault slip-rate, which is consistent with independent geological estimates.

We have applied clustering techniques to the terracing of potential field data. We found out how [Formula: see text]-means clustering or a simple reclassification of the field values based on the minimum Euclidean distance from a set of cluster centers can produce a nicely terraced potential field map, with the degree of simplification of the original map controlled by the number of clusters. We developed a method to automatically define the number and the center value of these clusters. The gravity or magnetic maps terraced by clustering techniques are transformed and present no smooth transitions, and each terrace has a constant field value. Such a terraced map is thus suitable for computing an apparent physical property distribution. To obtain even better results, it is possible to combine clustering techniques with edge-preserving filters. We tested our method on simple and complex synthetic fields and finally applied it to the real gravity data of a mining region in Canada, finding good correspondence between the resulting apparent density distribution and a simplified geologic map.

In past decades, magnetic surveying had become popular as one of the most effective techniques supporting archaeological prospecting. This is possible because the existence of susceptibility contrasts between the cover soil and several buried finds often causes detectable anomalies. More recently, great advances were made in signal enhancement and boundary analysis of potential field anomalies, thanks to methods allowing a suitable differentiation of the field without making the process unstable. New three-dimensional (3D) imaging techniques provided an estimate of the magnetization distribution within the subsoil by means of high-resolution images of the source distribution. Most of these methods are fast and reliable in the presence of shallow and compact sources, just as in the case of the sources usually occurring in archaeological prospecting. Nevertheless, great effort was spent by the scientific community to overcome serious problems causing low signal-to-noise ratio in the measurements. This chapter provides a step-by-step description of technical solutions adopted to improve the quality of data and to perform a better interpretation of the magnetic anomalies usually associated to the presence of archaeological finds. To this end, a summary of case histories is illustrated giving a general framework of the latest progress in archaeo-magnetism.

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. © The Authors 2014. Published by Oxford University Press on behalf of The Royal Astronomical Society.

In this paper we present a new method for source parameter estimation, based on the local wavenumber function. We make use of the stable properties of the Depth from EXtreme Points (DEXP) method, in which the depth to the source is determined at the extreme points of the field scaled with a power-law of the altitude. Thus the method results particularly suited to deal with local wavenumber of high-order, as it is able to overcome its known instability caused by the use of high-order derivatives. The DEXP transformation enjoys a relevant feature when applied to the local wavenumber function: the scaling-law is in fact independent of the structural index. So, differently from the DEXP transformation applied directly to potential fields, the Local Wavenumber DEXP transformation is fully automatic and may be implemented as a very fast imaging method, mapping every kind of source at the correct depth. Also the simultaneous presence of sources with different homogeneity degree can be easily and correctly treated. The method was applied to synthetic and real examples from Bulgaria and Italy and the results agree well with known information about the causative sources.

Summary We describe a multiscale method for source parameter estimations (depth to the source, homogeneity degree) based on a new theory, which we propose for studying explicitly inhomogeneous field. The theory consists in enlarging the set of homogeneous fields with those having a fractional homogeneity-degree and in relaxing the standard homogeneous equation to a local homogeneity equation. We show the validity of the method as applied to the inhomogeneous-degree gravity fields of a finite vertical cylinder.

We interpreted selected profiles of the Gravity Bouguer anomalies in Southern Apennines by a two-step procedure. First, a high resolution boundary analysis is performed by using the MDA technique, based on the Enhanced Horizontal Derivative function (Fedi and Florio 2001). This analysis applied to Southern Italy Bouguer gravity map gave detailed information on the shallow and deep structural setting. The second step is the interpretation, by DEXP method, of the gravity field linked to a EHD lineament of interest. With Depth from Extreme Points method we can retrieve a rather effective information about the sources, namely their horizontal boundaries, depth and structural index.

In this paper we will explore the performances of the joint inversion, through the Euler reduced equations, of more ridges converging to the same source (Euler deconvolution for a multiridge set). The application to the total field due to synthetic sources demonstrate the improvements in the source parameters estimation with respect to the single ridge inversion even in the case of the simultaneous estimation of two parameters. The method yields a good clustering of the solutions and could not require any acceptance criterion or a-posteriori processing.

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.

E022 Improved SCALFUN Analysis of Potential Fields by a Criterion of Ridge–Consistency M. Fedi* (University of Napoli Federico II) G. Florio (University of Napoli Federico II) & L. Cascone (University of Napoli Federico II) SUMMARY Ridge analysis is well known in the frame of the Continuous Wavelet Transform methods and was recently proposed as a main step of the potential field interpretation method SCALFUN. The information related to more ridges converging to the same one-point source should be consistent otherwise a poor quality of the estimates of the source properties has to be expected. This can be due to an

A boundary analysis of the gravity and magnetic fields of the Phlegrean Fields volcanic area and of the surrounding Campanian Plain reveals a complex structural setting. The Campanian Plain results well defined from sharp density boundaries. A set of major E–W lineaments occurs within the Plain. We derive a framework of the structural control to the volcanological evolution of Campanian Plain, which appears strongly influenced by a set of NE–SW lineaments bordering the Acerra depression. The density and magnetization boundaries of the Phlegrean Fields are significantly consistent, both indicating for the collapsed Phlegrean caldera an area considerably less extended than that based on previous geologically-defined models. Many other structures are evidenced in the Phlegrean area, often with agreement between density and magnetization boundaries. Inside the caldera, a strong consistence between the structures we identified and recent seismicity has been noted.