The geodetic laser interferometers at Gran Sasso, Italy: Recent modifications and correction for local effects

Two laser interferometers for geophysical researches are operating under the Gran Sasso massif, Central Italy, since several years. Their electro-optical set-up has been recently modified with respect to the original version to reduce costs and improve reliability. Output of each interferometer now consists of two quadrature signals, whose Lissajous figure is inverted for changes in length of the 90-m-long baselines. These top-rank instruments give high-quality strain data, but siting effects produce coupling among the different strain components, so that measured strain is not equal to the large-scale Earth strain. We estimate local effects using reference tidal strains and a matrix representation of the relationship between instrument and remote strains. The three coupling coefficient of each interferometer (obtained by best-fitting) are able to reduce differences between observed and predicted tides down to negligible levels.