V. Paoletti

Analysis of depth resolution in potential-field inversion

We study the inversion of potential fields and evaluate the degree of depth resolution achievable for a given problem. To this end, we introduce a powerful new tool: the depth-resolution plot (DRP). The DRP allows a theoretical study of how much the depth resolution in a potential-field inversion is influenced by the way the problem is discretized and regularized. The DRP also allows a careful study of the influence of various kinds of ambiguities, such as those from data errors or of a purely algebraic nature. The achievable depth resolution is related to the given discretization, regularization, and data noise level. We compute DRP by means of singular-value decomposition (SVD) or its generalization (GSVD), depending on the particular regularization method chosen. To illustrate the use of the DRP, we assume a source volume of specified depth and horizontal extent in which the solution is piecewise constant within a 3D grid of blocks. We consider various linear regularization terms in a Tikhonov (damped least-squares) formulation, some based on using higher-order derivatives in the objective function. DRPs are illustrated for both synthetic and real data. Our analysis shows that if the algebraic ambiguity is not too large and a suitable smoothing norm is used, some depth resolution can be obtained without resorting to any subjective choice of depth weighting.

The role of multilevel data in potential field interpretation

We start from the general inverse problem for potential fields and discuss the validity and suitability of using not only horizontal variations of them, but also their vertical ones. Hence multilevel data sets are considered, being they obtained either from measured data at different levels or by measuring the field just at the lower level and then generating by upward continuation the data at the remaining upper levels. In the latter case the upward continued data must be considered as the true ones plus some errors due to experimental errors and to the fact that they are generated from a discrete data set known on a finite survey area. Several forms of a priori information, such as that assuming the source can be represented by a fault or a sheet sandwich model, may allow the information contained in multilevel data to be effectively used for the interpretation of gravity and magnetic data. Recently established techniques of analysis, such as the continuous wavelet transform, are applied successfully to potential fields using implicitly the information contained in multilevel data. Assuming a block model for the source domain, the upward continuation formula may help to improve the solution of either 1D, 2D or 3D inverse problems enlarging the system with the equations related to the vertical variations of the fields, thereby reducing the algebraic ambiguity. Meaningful and computationally suitable quantities, such as weighted averages of potential fields, are also closely related to the use of multilevel data, providing useful insights for the determination of the depth distribution of the sources.

Macroseismic attenuation in the Campanian area, southern Italy

The main objective of the present study is to evaluate seismic attenuation relationships for the Campanian area (southern Italy) using the felt intensity report data obtained from comprehensive historical databases (DOM 4.1). We focused our attention on the Campania region because it is characterized by a high seismic hazard and risk, particularly in the Naples area and its suburbs. In order to derive an attenuation relationship for the area, we fitted the observed data by using several functions. We found that a linear plus logarithmic model gives the best fits for the data in the Campanian region. Most of the attenuation relationships proposed up to now for the Italian Peninsula have an isotropic behavior and do not always properly describe the macroseismic attenuation. Therefore, in order to check the possible dependence of the attenuation on the azimuth of the seismic rays, we divided our data set in octants and performed for each of them the same analysis we carried out for the whole data set. The obtained results differ from octant to octant and the differences, besides being associated with the source effects, could be interpreted as probably due to the existence of lateral variations in the lithological and physical features of the crust at different depths, which could affect the patterns of attenuation.

Susceptibility regional zonation of earthquake-induced landslides in Campania, Southern Italy

In this paper, we present a GIS-based method for regional zoning of seismic-induced landslide susceptibility and show its application to the territory of the Campania region, Southern Italy. The method employs only three factors that we believe are most significant in the susceptibility assessment: the type of outcropping rock/soil, the slope angle, and the MCS intensity. Each of the three parameters is quantified in terms of relative weight expressed as indices, and the resulting Seismic Landslide Susceptibility index of an area is given by the average of the indices of the first two factors multiplied by the index of the third factor. The result of this susceptibility zonation applied to Campania shows a good agreement between the distribution of the historical earthquake-triggered landslides and the highly susceptible zones.

Gravity modeling finds a large magma body in the deep crust below the Gulf of Naples, Italy

We analyze a wide gravity low in the Campania Active Volcanic Area and interpret it by a large and deep source distribution of partially molten, low-density material from about 8 to 30 km depth. Given the complex spatial-temporal distribution of explosive volcanism in the area, we model the gravity data consistently with several volcanological and petrological constraints. We propose two possible models: one accounts for the coexistence, within the lower/intermediate crust, of large amounts of melts and cumulates besides country rocks. It implies a layered distribution of densities and, thus, a variation with depth of percentages of silicate liquids, cumulates and country rocks. The other reflects a fractal density distribution, based on the scaling exponent estimated from the gravity data. According to this model, the gravity low would be related to a distribution of melt pockets within solid rocks. Both density distributions account for the available volcanological and seismic constraints and can be considered as end-members of possible models compatible with gravity data. Such results agree with the general views about the roots of large areas of ignimbritic volcanism worldwide. Given the prolonged history of magmatism in the Campania area since Pliocene times, we interpret the detected low-density body as a developing batholith.

Geomagnetometry for Archaeology

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.

Sessione S 32. Archaeogeophysics: requirement or choice?

Abstract from 88th Congress of the Italian Geological Society, 2016-09-07 - 2016-09-09, NaplesAbstract from 88th Congress of the Italian Geological Society, 2016-09-07, 2016-09-09, Naplesbook Edited by D. Calcaterra, S. Mazzoli, F.M. Petti, B. Carmina & A. Zuccari doi: 10.3301/ROL.2016.79

Joint Inversion of Gravity Gradient Tensor at Vredefort Impact Crater

In recent years, Gravity Gradient Tensor (GGT) has been successfully used in applied and environmental geophysics, also in light of the development of gradiometers. In this paper, we aim at analysing the inversion, either joint or separate, of different GGT components and of the sole gravity field vertical component. We perform our analysis by inspection of the Picard Plot, a well-known Singular Value Decomposition tool, and employ both synthetic data and gradiometer measurements carried out at the Vredefort structure, South Africa. We show that the main factors controlling the quality of the inversion are algebraic ambiguity and signal-to-noise ratio. Provided that algebraic ambiguity is kept low – by different combinations of GGT components and/or only gravity field data – the choice of components involved in the inversion is non-crucial to the quality of the reconstructions. Nonetheless, the use gradiometers allows a quicker and more effective way, with respect to the sole gravity field, to improve algebraic ambiguity.

A Tool for Analysing Depth Resolution in Potential-field Inversion - Application to the Neapolitan Volcanic Area

A careful management of the data errors in potential field inversion is crucial for obtaining reliable information about the source distribution with respect to depth. The Depth Resolution Plot introduced by Fedi, Hansen and Paoletti provides a convenient tool for this analysis: it allows a computational/visual analysis of how much the depth resolution in a potential-field inversion problem is influenced by the way the problem is discretized and regularized. Here we present a variant of the Depth Resolution Plot, called Approx Depth Resolution Plot, which is better suited for large-scale problems, and we employ it to study the retrievable depth resolution in the inversion of the gravity field of the Neapolitan Volcanic Area.

A Tool for Analysing Depth Resolution in Potential-field Inversion

A careful management of the ambiguities and errors in potential field inversion is crucial for obtaining reliable information about the source distribution with respect to depth. In this paper we introduce a tool, the ApproxDRP, which is based on an approximation of the singular vectors obtained by the iterative Lanczos bidiagonalization algorithm. This tool allows a computational/visual analysis of how much the depth resolution in a potential-field inversion problem is influenced by the way the problem is discretized and regularized. Since the ApproxDRP is based on an iterative method, it is suitable for large-scale problems. We show that when used in combination with a plot of the approximate SVD quantities, the ApproxDRP may successfully show the limitations of depth resolution resulting from noise in the data. This allows a reliable analysis of the retrievable depth information and effectively guides the user in choosing the optimal number of iterations, for a given problem.