Alfredo Mazzotti

First deep seismic reflection images of the Eastern Alps reveal giant crustal wedges and transcrustal ramps

[1]xa0The Alps are considered as a classical example for an orogen created by continental plate collision. In this study we present new images obtained from deep seismic reflection profiling in the Eastern Alps between Munich and Venice which give rise to examine and revise existing concepts. The seismic sections exhibit a prominent bi-verging reflection pattern at crustal scale. A major ramp-like structure, outcropping at the Inn-Valley fault, can be traced southward over 80 km into the mountain root where relics of the subducted Penninic ocean are expected. New models of the evolution of the Eastern Alps show an upper/lower crustal decoupling along transcrustal thrust faults with opposite thrust directions of both the European and the Adriatic-African continents.

Seismic exploration in complex terrains : A processing experience in the Southern Apennines

We discuss a data‐processing sequence adopted to reprocess a seismic line that crosses the Italian southern Apennines from the Tyrrhenian Sea to the Adriatic margin and investigate both the overthrust and foreland areas. We first determine the main causes of the very low S/N ratio in the field data and then propose a processing sequence aimed at exploiting the signal content, also making use of a priori geological knowledge of this area. Our work indicates a combination of causes for the very low quality of the seismic data. These include length of the spread (about 20 km) that is unfavorable because of the rapid variation in the near‐surface geology, tectonic complexity, crooked‐line acquisition, and the rough topography associated with outcropping rocks characterized by highly variable velocities. Based on the outcome of this data analysis, we present a processing sequence driven by knowledge of the regional tectonic setting and by knowledge of the shallow subsurface geology. The main effort is in remov...

European orogenic processes research transects the eastern Alps

The Alps—the youngest and most elevated mountain range in Europe—have inspired ideas about orogenic evolution for a long time. During the late 1980s, the western Alps were the site of intensive research using seismic profiling methods by Swiss, Italian, and French national programs [Rome et al., 1990; Pfiffner et al., 1997] .These investigations, some of which formed part of the European Traverse [Blundell et al., 1992], provided a great wealth of new data relevant to the Alpine orogeny. This orogeny is generally viewed in the context of the collision of the European and the Adriatic/African continental plates after the closure and subduction of the Penninic Ocean since about 40–50 Ma.

Multicomponent Velocity Analysis by means of Covariance Measures and Complex Matched filters.

Velocity analysis for multicomponent data is revised in order to improve the accuracy of the velocity estimate and to combine information from horizontal and vertical components into a single panel. Multicomponent OBC data are strongly contaminated by coherent noise such as torsional modes, mud rolls, multiples and ghosts. The reflections cannot be properly resolved by the standard semblance operator that is insensitive to coherent noises and is unable to distinguish between interfering events. Velocity estimation is improved by the use of a coherence measurement based on the decomposition into eigenstructures of the spatial covariance matrix as well as by the approximate a-priori knowledge of the wavelet amplitude spectrum. The multicomponent panel, in which the velocity analysis is performed, is obtained by adding in quadrature the horizontal and the vertical responses. One of the main advantages is that the velocity analysis carried out on a single gather allows to speed up the velocity picking that, otherwise, has to be repeated for orthogonal panels. Moreover, lithologic bounds on the Vp/Vs ratio can be checked because the trends of pure and converted waves are mapped together. Tests performed on synthetic and real data show that the multicomponent velocity analysis provides accurate velocity estimations even for data at the early steps of processing.

Estimation of a High Resolution P-wave Velocity Model of the Seabed Layers by Means of Global and a Gradient-based FWI

Summary We present a two-step procedure to full-waveform inversion (FWI) that combines a stochastic, genetic algorithm optimization and a subsequent gradient-based inversion with the aim to estimate a high-resolution P-wave velocity (Vp) model of the shallow seabed layers. In particular, we take advantage of the broad band frequency content of the seismic well-site (WSS) data to extend the frequency range up to 70 Hz. The first step is a genetic algorithm optimization aimed at deriving a reliable starting model for the subsequent gradient-based FWI. The lack of low frequencies and the limited maximum offset of the WSS acquisition, make the GA inversion particularly crucial as it provides a Vp field that contains the low-medium wavelengths of the subsurface compressional velocity field. These wavelengths are essential to attenuate the risk for the following gradient-based FWI of being trapped into local minima. The gradient-based FWI is performed in the acoustic approximation thus inverting for the Vp only. This two-step procedure yields a final Vp model characterized by an improved resolution with respect to the outcomes of GA-FWI and many fine details of the layering. The fair match between the reflections kinematics in the actual and predicted data supports the reliability of the final model.

Application of quaternion algorithms for multicomponent data analysis: a review

Summary We illustrate the applications on real data sets of three algorithms of multicomponent seismic processing implemented by means of quaternion algebra i.e.: velocity analysis, deconvolution, and Rayleigh wave extraction. First we present an application of quaternion velocity analysis on multicomponent traces, which results in an improved resolution and it allows us to discern on a single velocity panel the different wave trends (P, PS, converted waves) simultaneously. Then we present an application of quaternion deconvolution, that represents an extension of the classical Wiener deconvolution, and takes advantage of the vectorial nature of the wave-field, performing better than scalar filters. Finally, we show an application of a quaternion procedure that extracts a single Rayleigh wave mode on a multicomponent land data set.

A New AVA Attribute Based on P-wave and S-wave Reflectivities for Overpressure Prediction

Pore pressure prediction is a key step for safe well drilling operations. This prediction is usually performed deriving a velocity-pressure relationship calibrated to a reference well. However, in the last decades, several methods commonly applied in hydrocarbon exploration have been extended to predict anomalous pressure values directly from seismic data. Among these the Amplitude Versus Angle (AVA) technique has received many attentions. In this work we show that the expected pressure effect on the elastic rock properties is very different from the gas-effect. This makes the classical AVA attributes used for lithology and fluid prediction ineffective in highlighting pressure anomalies. Therefore, we propose a new AVA attribute that can be used to evidence the decrease in P and S impedances that may be associated to overpressured formations. This attribute can be easily derived from the intercept and gradient values computed by means of the Shuey equation. To demonstrate the applicability of this new attribute for pore pressure prediction we show different applications to field datasets over already drilled prospects where overpressures have been encountered. In all cases this attribute shows anomalous responses in correspondence to overpressured layers and then it proves to be effective in highlighting anomalous fluid pressure regimes.

Non-stretch fourth order nmo through iterative partial corrections and deconvolution

Source to receiver distances employed in seismic data acquisition have been steadily increasing and it is now common to work with data acquired with more than 10 km of offset. Sub-basalt exploration and seismic undershooting are just two applications where long-offset reflections are valuable. However, such reflections are often subjected to muting to avoid NMO stretch artifacts, thus causing a loss of valuable information. It is therefore of interest to find ways to avoid the distortions caused by the standard NMO correction and to retrieve these portions of the recorded wavefield for a better use in the processing. To this end we develop a non-stretch NMO correction based on a wavelet estimation and on a iterative procedure of partial NMO correction and deconvolution. To drive the corrections we make use of 4th order traveltime curves, that further extend the offset range of usable reflections. Then we estimate time and space variant wavelets, by means of SVD along the sought traveltimes, that become the desired output for the deconvolution trying to retrieve the original shapes of the partially stretched wavelets. We test our method on synthetic data and we perform a blind test on real data simulating an undershooting acquisition.

Compressional and Shear Wave Velocities Estimation from Well Log Data Using a Genetic Algorithm Approach

Geophysical measurements in bore-holes are frequently affected by gaps in the recording of one or more logs, or else, the recording of certain logs, such as S-wave velocity logs, may be performed along limited depth extensions. It is therefore of interest to be able to estimate the missing log intervals from a certain set of recorded logs. We propose to do that by making use of a Genetic Algorithm (GA) optimization that is capable of extracting the linear or exponential relations linking the sought parameter to the other available logs. We test this technique on different sets of logs (gamma ray, resistivity, density, neutron, sonic and shear sonic) from four wells drilled in different geological contexts and through different lithologies (sedimentary and intrusive). We focus particularly on P and S-wave velocity predictions. As it is demonstrated by a series of blind tests, it results that the GA optimization yields quantitative relations that are very effective and reliable in predicting the missing logs.