Fractal Analysis of Data from Seismometer Array Monitoring Virgo Interferometer

The local Hurst exponent H(t) has been computed for an array of 38 seismometers, deployed at the Virgo West End Building for Newtonian Noise characterisation purposes. The analysed period is from January 31st, 2018 to February 5th, 2018. The Hurst exponent H is a fractal index quantifying the persistent behaviour of a time series, higher H corresponding to higher persistency. The adopted methodology makes use of the local Hurst exponent computed using small sliding windows, in order to characterise the properties of the seismometers. Hourly averages and averages of H(t) have been computed over the whole analysed period. Results show that seismometers placed on a concrete slab closer to the centre of the room systematically exhibit higher persistency than the ones that are not placed on it. Seismometers placed next to the outer walls also exhibit higher persistency. The seismometer placed on a thin metal plate exhibits instead very low values of persistency during the analysed period, compared to the rest of the array.