Fidecaro, Francesco

The Einstein Telescope (ET) is a proposed next-generation, underground gravitational-wave detector to be based in Europe. It will provide about an order of magnitude sensitivity increase with respect to the currently operating detectors and, also extend the observation band targeting frequencies as low as 3 Hz. One of the first decisions that needs to be made is about the future ET site following an in-depth site characterization. Site evaluation and selection is a complicated process, which takes into account science, financial, political, and socio-economic criteria. In this paper, we provide an overview of the site-selection criteria for ET, provide a formalism to evaluate the direct impact of environmental noise on ET sensitivity, and outline the necessary elements of a site-characterization campaign.

The site of the European Gravitational Observatory (EGO) located in the countryside near Pisa (Tuscany, Italy) was investigated by a microgravity vertical gradient (MVG) survey. The EGO site houses the VIRGO interferometric antenna for gravitational waves detection. The microgravity survey aims to highlight the gravity anomalies of high-frequency related to more superficial geological sources in order to obtain a detailed model of the lithologic setting of the VIRGO site, that will allow an estimate of the noise induced by seismic waves and by Newtonian interference. This paper presents the results of the gradiometric survey of 2006 in the area of the interferometric antenna. MVG measurements allow us to enhance the high frequency signal strongly associated with the shallow structures. The gradient gravity map shows a main negative pattern that seems related to the trending of the high density layer of gravel that was evidenced in geotechnical drillings executed along the orthogonal arms during the construction of the VIRGO complex. Calibrating the relationship between the vertical gradient and the depth of the gravel interface we have computed a model of gravity gradient for the whole VIRGO site, defining the 3D distribution of the top surface of this layer. This latter shows a NE-SW negative pattern that may represent a palaeo-bed alluvial of the Serchio from the Bientina River system.

A gradiometric microgravity survey was performed in the site of the EGO (European Gravitational Observatory) located in the countryside near Pisa (Tuscany, Italy). The main purpose of the current study is to delineate high-frequency gravity anomalies and to correlate them with the shallow geological sources in order to obtain a detailed model of the lithological setting. The survey network is compose by 59 gradiometric stations located in the 4X4 Km2 target area. The highly detailed positioning of the station points can be carried out by means of a GPS (Global Positioning System). The measurements were made using a portable tower frame and the LaCoste&Romberg D-model gravity meter with Aliod 100 linear electronic feedback system, with a 1 µgal sensitivity. The study area is located in the Neogene-Quaternary basin of the lower Arno River Valley. The density values calculated from the 2.5D gravity model relative to the gravel (2.1 g/cm3) and the clay (1.5 -1.6 g/cm3). An anomaly negative pattern are shown in the gradient gravity map: the negative gravity anomaly seem related to the increase in depth of the layer of gravel - high layer density. The NE-SW negative pattern should be to represent a palaeo-bed alluvial of the Lower Arno and Serchio of Bientina River system, related to the Late Pleistocene extensional tectonics in the Pisa Plain.

Gradiometric gravimetry is a survey technique widely used in geological structure investigation. This work demonstrates the feasibility of a new class of low frequency accelerometers for geodynamics studies and space applications. We present the design features of a new low noise single-axis differential accelerometer; the sensor is suitable to be used in a Gravity Gradiometer (GG) system for land geophysical survey and gravity gradient measurements. A resolution of 1 Eötvös (1 Eö=10−9s−2) at one sample per second is achievable in a compact, lightweight (less than 2 kg) portable instrument, operating at room temperature. The basic components of the sensor are two identical rigidly connected accelerometers separated by a 15-cm baseline vector and the useful signal is extracted as the subtraction of the two outputs, by means of an interferometric microwave readout system. The structure will be engraved in a monocrystal of sapphire by means of Computer-Numerically-Controlled (CNC) ultrasonic machining: the material was chosen because of its unique mix of outstanding mechanical and dielectric properties.