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

Abstract

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.

Free Core Resonance parameters from strain data: sensitivity analysis and results from the Gran Sasso (Italy) extensometers

“…Thus we use a local model for the Mediterranean Sea at 1/30° resolution (http://volkov.oce.orst.edu/tides/med.html), including four diurnal (Q1, O1, P1, K1) and four semi‐diurnal (N2, M2, S2, K2) constituents. If this local model is used for the Mediterranean Sea, all tested global models give very similar results as regards total (solid Earth and ocean loading) computed tides (Amoruso & Crescentini 2009a), the maximum discrepancy being less than 10 −3 in amplitude ratio and 0.1° in phase; the only exception relates to Q1 as predicted using CSR4.0, whose phase differs by about 0.3° from the values computed using the other models. In what follows we combine the global ocean model TPXO7.2 with the local model for the Mediterranean Sea.…”

“…The shorter break in 2001 is due to laser tube replacement (erroneously indicated as happened in fall 2002 in fig. 5 of Amoruso & Crescentini 2009a). Upper plots: records decimated at 2 cycles per hour; the strain level after each long break is arbitrarily set to zero.…”

“…The instruments are based on the classical unequal‐arm Michelson set‐up and compare the optical length of a longer measurement arm (about 90 m in length) and a shorter fixed reference arm that is about 20 cm in length (Crescentini & Renzella 1991; Crescentini et al 1997; Amoruso & Crescentini 2009a). They are characterized by very high sensitivity (picostrain level, Δ l / l = 10 −12 ), wide frequency band, and large dynamic range, limited only by the capability of maintaining optical alignment.…”

Nonlinear and minor ocean tides in the Bay of Biscay from the strain tides observed by two geodetic laser strainmeters at Canfranc (Spain)

“…Although it is mainly devoted to Particle Physics and Astrophysics, it also hosts a geodynamical facility [ Díaz et al ., ] capable of recording continuously the whole geodynamic spectrum, from near‐field seismicity to tectonic deformation, including Earth tides. The heart of the geodynamical facility are two laser strainmeters (interferometers), similar to those operating in Central Italy for about 20 years [ Crescentini et al ., ; Amoruso and Crescentini , ]. These latter produced valuable results on slow earthquakes [ Crescentini et al ., ; Amoruso and Crescentini , ], long‐period toroidal Earth free oscillations [ Park et al ., ], earthquake nucleation [ Amoruso and Crescentini , ], the free nutation of the Earth core [ Amoruso et al ., ], and the correlation between groundwater flow and deformation in a fractured carbonate aquifer [ Amoruso et al ., ].…”

Features of the Earth surface deformations in the Kamchatka peninsula and their relation to geoacoustic emission

“…The advantage of a laser strainmeter against a mechanical one is the absence of a mechanically sensitive element (Agnew and Wyatt, 2003;Amoruso and Crescentini, 2009;Dolgikh et al, 2012). The effect of meteorological parameter variations on the instrument is mainly the change in the laser beam optical path.…”