Gradiometric microgravity survey in the area of VIRGO interferometric antenna (Pisa plain, Italy)

Abstract

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.