Gravity survey of the urban area of Milan (Italy)

Abstract

In the summer of 1988 a new gravity survey of the city of Milan was carried out. Within the Municipality's territory (182 km²) gravity was measured at 320 stations, utilizing the city levelling network that is checked every two years. The stations in the inner area were tied to the Agip^∗ gravity network of the Po valley, surrounding the city, in order to evaluate the regional field. In 1950 a first survey of the central urban area had been made on 220 stations; a residual, circular negative anomaly was found, occupying approximately the area of the municipality; at that time the source of this anomaly was not investigated.

The results show that the anomaly can be attributed mainly (50–60%) to the combined geological effects of the shallow formation (within a depth of about 150 m). Also the shorter wavelength anomalies can be attributed to lateral variations of the very shallow formations. Temporal variations due to the modifications of the aquifers or to subsidence were not observed on comparing the results of the two surveys, owing to the insufficient accuracy of the first measurements, but also because they are small in comparison with the effects of permanent sources.