ALBERT

Description: We present here two methods to obtain reflection images of upper crust seismic reflectors. The techniques are based on migration and waveform coherence analysis of reflected seismic phases recorded in local earthquake seismograms and in active seismic data. The first method is a move-out and stack of reflected seismic phases in local earthquake recordings. The theoretical travel times of reflected/converted phases in a 1D medium for a given interface depth and velocity model are used to align the recordings in time. The locations and origin times of events are initially estimated from the P and S arrival times. Different seismic gathers are obtained for each reflected/converted phase at the interface under consideration, and the best interface depth is chosen as that which maximizes the value of a semblance function computed on moved-out records. This method has been applied to seismic records of microearthquakes that have occurred at the Mt. Vesuvius volcano, and it confirms the reports of an 8- to 10-km-deep seismic discontinuity beneath the volcano that was previously identified as the roof of an extended magmatic sill. The second is a non-linear 2D method for the inversion of reflection travel times aimed at the imaging of a target upper-crust reflector. This method is specifically designed for geophysical investigations in complex geological environments (oil investigations, retrieving of images of volcano structures) where the presence of complex structures makes the standard velocity analysis difficult and degrades the quality of migrated images. Our reflector is represented by nodes of a cubic-spline that are equally spaced at fixed horizontal locations. The method is based on a multiscale approach and uses a global optimization technique (genetic algorithm) that explores the whole of the parameter space, i.e. the interface position nodes. The forward problem (the modelling of reflection travel times) is solved using the finite-difference solver of Podvine & Lecomte (1991) and using an a priori known background velocity model. This non-linear method allows the automated determination of the global minimum (or maximum) without relying on estimates of the gradient of the objective function in the starting model and without making assumptions about the nature of the objective function itself. We have used two types of objective functions. The first is a least-squares L2 norm, defined as the sum of the squared differences between the observed and the calculated travel times. The second is based on coherence measures (semblance). The main advantage of using coherence measures is that they do not require travel-time picking to assess the degree of fit to the data model. Thus, the time performance of the whole procedure is improved and the subjectivity of the human operators in the picking procedure is removed. The methods are tested on synthetic models and have been applied to a subset of data that was collected during the active seismic experiments performed in September 2001 in the gulfs of Naples and Pozzuoli in the framework of what is known as the SERAPIS project.

Description: Published

Description: 157-178

Description: open

Description: We applied a type of depth migration for converted seismic phases to active and passive seismic data sets from the Campi Flegrei volcanic region in southern Italy. The migration method is based on the diffraction summation migration technique. Travel times to grid points in a volume were calculated in smooth P and S-velocity models and trace energy near the calculated converted phase time was stacked over multiple sources at one receiver. Weighting factors based on Snell's Law at an arbitrarily oriented local interface were applied to better focus trace energy. PP reflection images from the active data set provide increased detail to images of the caldera rim from other studies. The migrated images also show features near 2–3 km in depth beneath Pozzuoli City, which may be associated with an over-pressured gas volume, as suggested by other geophysical investigations. Possible deeper features near 4 km depth may be related to the presence of the carbonate basement or may image a previously undetected feature, such as a small body of strongly thermometamorphosed volcanic rock. The current passive earthquake data set from the 1984 ground uplift episode was not well suited to the converted phases analysis due to narrow P–S windows and high noise levels in the traces. However, two stations provide confirmation and extension of imaged features in the active data.

Description: Published

Description: 243-256

Description: 3.2. Tettonica attiva

Description: JCR Journal

Description: reserved

Description: We studied the ambient noise recorded at Irpinia Seismic Network (ISNet), a seismic network installed along the Campania–Lucania Apenninic chain (southern Italy), with the aim of characterizing the noise spectrum for each station as a function of time and the detection threshold of the network. For the latter purpose, we proposed a mixed indirect approach based on the signal-to-noise ratio (SNR) in the time domain, with parameterization in the frequency domain. The source signature is represented by the convolution of the Brune source time function with the Azimi attenuation curve. We found that 1.3 is the minimum magnitude an event should have to be detected at least at five stations with an SNR larger than five, wherever it occurs. We observed a space variability of the detection threshold as large as 0.3 units, ascribed to both the geometrical configuration of the network and the differences in the noise levels at the different stations. A sensitivity study indicates that the estima- tion of the detection threshold is robust for changes in the focal depths and stress drop, while it is strongly affected by the anelastic attenuation. In our case, changes of the reduced time t␣ in the range 0.015–0.035 s generate changes in the completeness threshold of 0.5 units. Noise levels were obtained by a statistical analysis on the power spectral density curves along almost three years of continuous data from 22 stations. We found that, at short periods, major time variations are generated by diurnal changes in the wind intensity and other meteorological factors. At longer periods, we retrieved the micro- seismic peak, resulting from the constructive interference of oceanic waves. We also found an additional peak between 2 and 4 s, correlated with the sea wave height along the Tyrrhenian coast.

Description: Published

Description: 574–586

Description: 4.1. Metodologie sismologiche per l'ingegneria sismica

Description: JCR Journal

Description: restricted

Description: In this paper we studied the physical properties of the Gulf of Naples (Southern Italy) for its use as a commu- nication channel for the acoustic transmission of digital data acquired by seismic instruments on the seafloor to a moored buoy. The acoustic link will be assured by high frequency acoustic modems operating with a central frequency of 100 kHz and a band pass of 10 kHz. The main operational requirements of data transmission con- cern the near horizontal acoustic link, the maximum depth of the sea being about 300 m and the planned hori- zontal distance between seismic instruments and buoy 2 km. This study constructs the signal-to-noise ratio maps to understand the limits beyond which the clarity of the transmission is no longer considered reliable. Using ray- theory, we compute the amplitudes of a transmitted signal at a grid of 21×12 receivers to calculate the transmis- sion loss at each receiver. The signal-to-noise ratio is finally computed for each receiver knowing also the trans- mitter source level and the acoustic noise level in the Gulf of Naples. The results show that the multipath effects predominate over the effects produced by the sound velocity gradient in the sea in the summer period. In the case of omnidirectional transmitters with a Source Level (SL) of 165 dB and a baud rate of 2.4 kbit/s, the results al- so show that distances of 1400-1600 m can be reached throughout the year for transmitter-receiver connections below 50 m depth in the underwater acoustic channel.

Description: Published

Description: 411-426

Description: 1.8. Osservazioni di geofisica ambientale

Description: JCR Journal

Description: open