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Local earthquake tomography (LET) is a popular method for inverting arrival time picks of local-regional earthquakes for P- and S-wave velocity and hypocenter parameters in seismically active regions. This popularity is due to some robust and well-documented open-source codes that are sometimes used as black boxes. The availability of a very complete time-pick database on the Western Alps gives us a chance to thoroughly investigate the influence of the numerous processes and parameters involved when applying LET to the Western Alps or similar targets. From a subset of high-quality manual picks (1989-2014), we compute preliminary P and S velocity models that are used to predict arrival times for later events and allow the selected fusion of picks downloaded from different seismological agencies for a consistent 33-year database (1989-2021). Different model reconstructions are proposed by selecting different subsets of the arrival time dataset. Aside data input into the LET, influence of initial stratified or three-dimensional velocity models and related initial earthquake locations is investigated together with grid discretization, Laplacian smoothing and damping parameters in the standard penalty approach commonly used by LET codes. These numerical parameters account for the expected limited resolution of seismic waves due to their finite-frequency content. Parameter selection is handled by the user, whereas frequency impact is only implicit in the onset picks. The earthquake distribution allows a reconstruction down to 40-km depth over an area of ∼ 500x500 km2. Robust features such as the high-velocity Ivrea body anomaly, and a deep low-velocity anomaly associated with crustal thickening underneath the mountain belt survive whatever the tomography strategy and parameters. Finally, a comparison with previous LET reconstructions suggests that finite-frequency content be explicitly incorporated through wave equation tomography to improve spatial resolution. This would

The ANTARES neutrino telescope is a large photomultiplier array designed to detect neutrino-induced upward-going muons by their Cherenkov radiation. Understanding the absorption and scattering of light in the deep Mediterranean is fundamental to optimising the design and performance of the detector. This paper presents measurements of blue and UV light transmission at the ANTARES site taken between 1997 and 2000. The derived values for the scattering length and the angular distribution of particulate scattering were found to be highly correlated, and results are therefore presented in terms of an absorption length;,ab, and an effective scattering length lambda(sct)(eff). The values for blue (UV) light are found to be lambda(abs) similar or equal to 60(26) m, lambda(sct)(eff similar or equal to) 265(122) m, with significant (similar to15%) time variability. Finally, the results of ANTARES simulations showing the effect of these water properties on the anticipated performance of the detector are presented. (C) 2004 Elsevier B.V. All rights reserved.

The ANTARES neutrino telescope, to be immersed depth in the Mediterranean Sea, will consist of a three-dimensional matrix of 900 large area photomultiplier tubes housed in pressure-resistant glass spheres. The selection of the optimal photomultiplier was a critical step for the project and required an intensive phase of tests and developments carried out in close collaboration with the main manufacturers worldwide. This paper provides an overview of the tests performed by the collaboration and describes in detail the features of the photomultiplier tube chosen for ANTARES. (c) 2005 Elsevier B.V. All rights reserved.

In this paper, we show how the migration of active seismic data can be used to identify and position a seismic discontinuity in a complex medium, and how the amplitude variations of the converted P to S waves can be interpreted to constrain the seismic velocities below the interface. For the application we turn our attention on Mt Vesuvius, an active volcano threatening a densely populated area. To better define its plumbing system, we investigate a mid-crust seismic discontinuity first identified by Zollo et al. (1996, Science, 274, 592-594) and assumed to be the top of a layer containing low-velocity material. We deduce a reference velocity model from previous works on first arrival times, and use it to migrate PP reflected and PS converted waves. In the migration image, the interface extends at least 20 000 m NE and 20 000 m SW of the volcano, at the depth of 8000 m, and with a mean dip less than 3 per cent. The migration of finite-difference synthetics exclude the fact that the migrated phases interpreted as 8000 m deep reflections or conversions might be multiples in the shallow layers. The PS and first arrival amplitudes are compared at a fixed station and for all the shots to determine the variations of the P-to-S conversion coefficient with the angle of incidence. It appears to vary very slowly over the whole range of incidence angle at disposal (50degrees-80degrees). This implies a dramatic drop of S-wave velocity from approximately 3600 m s(-1) above to less than 1000 m s(-1) below the interface. The smoothness of the variations indicate that the P-wave velocity also diminishes across the interface. The very low S-wave velocity, other geophysical observations at Vesuvius and in other volcanic systems, lead to the conclusion that below the 8000 m discontinuity there is a very extended layer of hot, partially molten material.

In this paper, we show how the migration of active seismic data can be used to identify and position a seismic discontinuity in a complex medium, and how the amplitude variations of the converted P to S waves can be interpreted to constrain the seismic velocities below the interface. For the application we turn our attention on Mt Vesuvius, an active volcano threatening a densely populated area. To better define its plumbing system, we investigate a mid-crust seismic discontinuity first identified by Zollo et al. (1996, Science, 274, 592-594) and assumed to be the top of a layer containing low-velocity material. We deduce a reference velocity model from previous works on first arrival times, and use it to migrate PP reflected and PS converted waves. In the migration image, the interface extends at least 20 000 m NE and 20 000 m SW of the volcano, at the depth of 8000 m, and with a mean dip less than 3 per cent. The migration of finite-difference synthetics exclude the fact that the migrated phases interpreted as 8000 m deep reflections or conversions might be multiples in the shallow layers. The PS and first arrival amplitudes are compared at a fixed station and for all the shots to determine the variations of the P-to-S conversion coefficient with the angle of incidence. It appears to vary very slowly over the whole range of incidence angle at disposal (50degrees-80degrees). This implies a dramatic drop of S-wave velocity from approximately 3600 m s(-1) above to less than 1000 m s(-1) below the interface. The smoothness of the variations indicate that the P-wave velocity also diminishes across the interface. The very low S-wave velocity, other geophysical observations at Vesuvius and in other volcanic systems, lead to the conclusion that below the 8000 m discontinuity there is a very extended layer of hot, partially molten material.

In this paper, we show how the migration of active seismic data can be used to identify and position a seismic discontinuity in a complex medium, and how the amplitude variations of the converted P to S waves can be interpreted to constrain the seismic velocities below the interface. For the application we turn our attention on Mt Vesuvius, an active volcano threatening a densely populated area. To better define its plumbing system, we investigate a mid-crust seismic discontinuity first identified by Zollo et al. (1996, Science, 274, 592-594) and assumed to be the top of a layer containing low-velocity material. We deduce a reference velocity model from previous works on first arrival times, and use it to migrate PP reflected and PS converted waves. In the migration image, the interface extends at least 20 000 m NE and 20 000 m SW of the volcano, at the depth of 8000 m, and with a mean dip less than 3 per cent. The migration of finite-difference synthetics exclude the fact that the migrated phases interpreted as 8000 m deep reflections or conversions might be multiples in the shallow layers. The PS and first arrival amplitudes are compared at a fixed station and for all the shots to determine the variations of the P-to-S conversion coefficient with the angle of incidence. It appears to vary very slowly over the whole range of incidence angle at disposal (50degrees-80degrees). This implies a dramatic drop of S-wave velocity from approximately 3600 m s(-1) above to less than 1000 m s(-1) below the interface. The smoothness of the variations indicate that the P-wave velocity also diminishes across the interface. The very low S-wave velocity, other geophysical observations at Vesuvius and in other volcanic systems, lead to the conclusion that below the 8000 m discontinuity there is a very extended layer of hot, partially molten material.

An investigation of the teleseismic P -wave coda is performed using the single-scattering approximation. The method allows one to image short-wavelength scale (less than or equal to2 km) velocity and density heterogeneities and structures that are barely detected by traveltime tomography. Source effects are removed by using receiver functions for data interpretation, but the amplitude (especially the sign of the signal) is synthesized. Both broad-band seismological stations in the southwestern Alps (France) and Campanian plain (Italy) are used for the illustration of the proposed method. Because of the large aperture of our arrays, laterally small-scale heterogeneities are difficult to image and we must assume lateral continuity of the detectable structure. If so, we show that this depth migration based on a single-scattering approach can recover both the depth and the geometry of the main discontinuities below the studied areas. In the southwestern Alps, we underline a complex crustal structure and Moho dipping topography. The Moho depth increases from 20 to 30 km between the coastline and the Mercantour range. In the Campanian plain (surrounding Mount Vesuvius) we also find a southeastwards-dipping Moho. Furthermore, the three main discontinuities are imaged, showing a relatively shallow mantle-crust discontinuity and a deeper one near the coastline.

An investigation of the teleseismic P -wave coda is performed using the single-scattering approximation. The method allows one to image short-wavelength scale (less than or equal to2 km) velocity and density heterogeneities and structures that are barely detected by traveltime tomography. Source effects are removed by using receiver functions for data interpretation, but the amplitude (especially the sign of the signal) is synthesized. Both broad-band seismological stations in the southwestern Alps (France) and Campanian plain (Italy) are used for the illustration of the proposed method. Because of the large aperture of our arrays, laterally small-scale heterogeneities are difficult to image and we must assume lateral continuity of the detectable structure. If so, we show that this depth migration based on a single-scattering approach can recover both the depth and the geometry of the main discontinuities below the studied areas. In the southwestern Alps, we underline a complex crustal structure and Moho dipping topography. The Moho depth increases from 20 to 30 km between the coastline and the Mercantour range. In the Campanian plain (surrounding Mount Vesuvius) we also find a southeastwards-dipping Moho. Furthermore, the three main discontinuities are imaged, showing a relatively shallow mantle-crust discontinuity and a deeper one near the coastline.

An investigation of the teleseismic P -wave coda is performed using the single-scattering approximation. The method allows one to image short-wavelength scale (less than or equal to2 km) velocity and density heterogeneities and structures that are barely detected by traveltime tomography. Source effects are removed by using receiver functions for data interpretation, but the amplitude (especially the sign of the signal) is synthesized. Both broad-band seismological stations in the southwestern Alps (France) and Campanian plain (Italy) are used for the illustration of the proposed method. Because of the large aperture of our arrays, laterally small-scale heterogeneities are difficult to image and we must assume lateral continuity of the detectable structure. If so, we show that this depth migration based on a single-scattering approach can recover both the depth and the geometry of the main discontinuities below the studied areas. In the southwestern Alps, we underline a complex crustal structure and Moho dipping topography. The Moho depth increases from 20 to 30 km between the coastline and the Mercantour range. In the Campanian plain (surrounding Mount Vesuvius) we also find a southeastwards-dipping Moho. Furthermore, the three main discontinuities are imaged, showing a relatively shallow mantle-crust discontinuity and a deeper one near the coastline.

This paper presents an application of 2.5-D asymptotic viscoacoustic diffraction tomography to ultrasonic data recorded during a physically scaled laboratory experiment. This scaled experiment was used to test the reliability of our method when applied to a real data set to estimate the attenuation factor Q. Diffraction tomography relies on ray theory to compute the Green functions in a smooth background medium and on the Born approximation to linearize the relation between the scattered wavefield and the velocity and Q perturbations. The perturbations are inferred from the data by an iterative (linear) quasi-Newtonian algorithm. The inversion formula was specifically developed to account for the acquisition geometry designed in this study. The derivation of the Hessian operator shows that, for this acquisition, the velocity and Q perturbations are theoretically decoupled. The processing of the data was split into two steps: first, we applied the tomography to the data without deconvolving them. Second, we designed a post-processing procedure for the tomographic images to remove the source signature and to estimate the absolute Values of the velocity and the attenuation factor Q. At the conclusion of the first step, both the velocity and the Q tomographic images allowed one to delineate the gross contour of the target. We obtained an excellent match between the observed data and the viscoacoustic ray-Born synthetics. The match obtained with the viscoacoustic rheology was significantly better than for a purely acoustic one. In the second step, the post-processing allowed us to recover the shape of the target. We estimated the absolute values of the velocity and Q, although we had no quality control with regard to these results (the rheological properties of the material used in this study were unknown). The results suggest that the uncertainty of the velocity measurement is lower than that for Q. The application presented in this study suggests that the procedure tha

This paper presents an application of 2.5-D asymptotic viscoacoustic diffraction tomography to ultrasonic data recorded during a physically scaled laboratory experiment. This scaled experiment was used to test the reliability of our method when applied to a real data set to estimate the attenuation factor Q. Diffraction tomography relies on ray theory to compute the Green functions in a smooth background medium and on the Born approximation to linearize the relation between the scattered wavefield and the velocity and Q perturbations. The perturbations are inferred from the data by an iterative (linear) quasi-Newtonian algorithm. The inversion formula was specifically developed to account for the acquisition geometry designed in this study. The derivation of the Hessian operator shows that, for this acquisition, the velocity and Q perturbations are theoretically decoupled. The processing of the data was split into two steps: first, we applied the tomography to the data without deconvolving them. Second, we designed a post-processing procedure for the tomographic images to remove the source signature and to estimate the absolute Values of the velocity and the attenuation factor Q. At the conclusion of the first step, both the velocity and the Q tomographic images allowed one to delineate the gross contour of the target. We obtained an excellent match between the observed data and the viscoacoustic ray-Born synthetics. The match obtained with the viscoacoustic rheology was significantly better than for a purely acoustic one. In the second step, the post-processing allowed us to recover the shape of the target. We estimated the absolute values of the velocity and Q, although we had no quality control with regard to these results (the rheological properties of the material used in this study were unknown). The results suggest that the uncertainty of the velocity measurement is lower than that for Q. The application presented in this study suggests that the procedure tha

This paper presents an application of 2.5-D asymptotic viscoacoustic diffraction tomography to ultrasonic data recorded during a physically scaled laboratory experiment. This scaled experiment was used to test the reliability of our method when applied to a real data set to estimate the attenuation factor Q. Diffraction tomography relies on ray theory to compute the Green functions in a smooth background medium and on the Born approximation to linearize the relation between the scattered wavefield and the velocity and Q perturbations. The perturbations are inferred from the data by an iterative (linear) quasi-Newtonian algorithm. The inversion formula was specifically developed to account for the acquisition geometry designed in this study. The derivation of the Hessian operator shows that, for this acquisition, the velocity and Q perturbations are theoretically decoupled. The processing of the data was split into two steps: first, we applied the tomography to the data without deconvolving them. Second, we designed a post-processing procedure for the tomographic images to remove the source signature and to estimate the absolute Values of the velocity and the attenuation factor Q. At the conclusion of the first step, both the velocity and the Q tomographic images allowed one to delineate the gross contour of the target. We obtained an excellent match between the observed data and the viscoacoustic ray-Born synthetics. The match obtained with the viscoacoustic rheology was significantly better than for a purely acoustic one. In the second step, the post-processing allowed us to recover the shape of the target. We estimated the absolute values of the velocity and Q, although we had no quality control with regard to these results (the rheological properties of the material used in this study were unknown). The results suggest that the uncertainty of the velocity measurement is lower than that for Q. The application presented in this study suggests that the procedure tha

International audience

International audience

International audience

International audience

International audience