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This paper presents and discusses the measurement of permeability of Neapolitan Yellow Tuff (NYT) samples obtained in the framework of a study concerning the phenomenon of bradyseism, i.e. the slow vertical movement of soil, in the Campi Flegrei caldera (Campania—Italy). Measurements have been performed under isothermal, non-isothermal and transient non-isothermal conditions using a specifically designed apparatus. Results of measurements of porosity of different samples are also reported. Experimental results in isothermal conditions show that the volume flux through the samples changes linearly with applied pressure. The values of permeability obtained turn out to be independent of the temperature and pressure gradients applied to the samples. This result is consistent with the fact that the permeability is a characteristic of the porous medium, and as such is not affected by temperature and pressure variation, at least in the range examined. The permeability values measured in our laboratories agree quite well with the ones measured in situ by the Agenzia Generale Italiana Petroli (AGIP) during a geothermal exploration of the Campi Flegrei area in 1980. An interesting, still unexplained phenomenon has been detected during transient phases when both pressure and temperature gradients were applied to the samples. The phenomenon consists in an enhancement of volume flux due to heat flux in the transient phase. The extra volume-flux disappears once the steady temperature gradient is reached.

A finite difference method has been developed to simulate water flow in a 2D porous medium with boundary conditions specified in terms of pressure and temperature. The method computes steady-state solutions for temperature, pressure and fluid velocities. With an appropriate choice for boundary conditions it is possible to simulate the behaviour of a geothermal system with an incremental pressure and/or temperature applied to a finite part of the base. Pressure and temperature changes produce a change in the fluid flow regime leading to different final temperatures, pressures and fluid velocities (steady state). The method is particularly suitable for simulating the coupled mechanical and thermalfluid-dynamical effects at calderas, when increases in pressure and heat flow from a magma chamber perturb the conditions of the uppermost aquifer system. Several tests performed with realistic values for the parameters of the porous medium show that increases of pressure at the base are efficiently propagated in the upper medium by water flow, so that the effective centre of overpressure is shifted to considerably shallower levels. This effect can strongly modify the shape of ground deformations at calderas, and strongly amplify the peak values. The proposed method has been applied to the explanation of the very peculiar ground deformation observed at Campi Flegrei during 1982–1984, which showed very high uplifts (1.8 m) and a strong concentration in a small area (3×3 km 2 ). An integrated mechanical–thermalfluid-dynamical model for this caldera is proposed, which could be generalised to other areas.

Mechanical and thermal-fluid-dynamical contributions to unrest phenomena at active calderas are quantitatively evaluated, and a mixed mechanical-thermal-fluid-dynamical model is developed, to explain the unrest phenomena at Campi Flegrei caldera. Mechanical modelling involves the use of analytical and finite element formulations. Thermal-fluid-dynamical effects are considered in the framework of a one-dimensional approximation, solved by an analytical method. The results shed new light about the coupled effect of mechanically and thermally induced perturbations and variations in the geothermal fluid circulation regime. Unrest episodes at Campi Flegrei are interpreted in terms of the joint effect of a first, essentially mechanical, phase due to overpressure in a magma chamber and to a progressive amplification and final decay due to the circulating water. The elastic response of the rocks to the migrating front of overpressure is shown to be strongly conditioned by the caldera structure, which produces a concentration of strain in the inner part, and a stress concentration, causing the local seismicity, around the caldera borders. Thermal-fluid-dynamical effects are also shown to have important implications on the evolution of the volcanic system towards critical phenomena, which must be considered for a meaningful evaluation of volcanic hazard.

Summary This study evaluated the immunogenic and protective effects of plasma-derived and DNA recombinant anti-hepatitis B virus vaccines administered to infants at various ages and with different vaccination schedules: 3 monthly doses in the first 3 months of life, 3 doses (at 3, 5 and 11 months) or 2 doses (at 1 and 3 months) or 2 doses (at 3 and 5 months). Anti-HBs (hepatitis B surface) and anti-HBc (hepatitis B core) markers were investigated twice: one month and ten years after vaccination in 261 children immunized with plasma-derived vaccine, and one month and five years after vaccination in 449 children immunized with DNA recombinant vaccine. In all groups, the appearance of anti-HBs protective levels one month after vaccination and their persistence in the following years were found in a larger number of subjects when the vaccine doses had been administered after the third month of life rather than in the first three months. Moreover, our results show that the reappearance of surface antibodies a week after the booster, in vaccinated children who became anti-HBs − in the years following vaccination, occurred in a larger number of subjects when the primary vaccination with 3 doses had been performed in the first quarter or with 2 or 3 doses in the second quarter. In contrast, protective levels of anti-HBs were found in a small number of children belonging to the group vaccinated with 2 doses in the first three months, and among them the majority seroconverted only one month after the booster. Anti-HBc was found 10 years after vaccination in only one child immunized with 2 doses of plasma-derived vaccine, and 5 years after vaccination in two children immunized with 2 doses of DNA recombinant vaccine. All these children were found to lose anti-HBs, and none of them had signs of disease or became a carrier. Based on these results, the disappearance, in some children, of protective levels of anti-HBs in the years following vaccination does not mean the loss of anti-HBV protection. In fact, the trial showed that they reacted immediately to booster stimulation, demonstrating a solid immunologic memory. Therefore, there may be no reason for giving booster injections when the vaccination of infants is carried out correctly.

Isothermal matter transport due to gradients of chemical potential was studied with different electrolytic solutions and an hydrophobic porous membrane. Countercurrent fluxes of solute and solution in bulk were observed in each case. Impeding volume flux, transmembrane pressures are developed proportional to concentration gradients and much smaller than Van't Hoff osmotic pressures, indicating a poor membrane reflectivity. Starting from our experimental observations we propose a simple model of an idealized membrane, whose pores interact only hydrodynamically with bulk flow, while countercurrent diffusion of solvent and solute occurs in each channel along the respective gradients of chemical potential. This model is shown to work in every respect just as real-life membranes.