GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 1.2 159 these spurious effects, second generation sensors should be placed deeper underground, at depth not less than 25-30 meters. Results and data analysis. On the long period (months, years) the strain values show the expected annual thermoelastic modulation together with a characteristic trend. To understand the components of this behaviour is crucial in working with every borehole instrument. The seasonal thermoelastic traction term is accounted for by a sine wave (one year period), while the remaining trend is of a different origin. In fact it is well known (UNAVCO Workshop, 2012) that there is a combined long-term effect due to two terms: the tendency of the soil to recover the unperturbed situation (that is before the well was drilled), and the curing of the grout used to keep the sensor in close contact with the surrounding rock. On the bases of the experience acquired by the borehole strainmeter community, these last two contributions can be considered by fitting raw data with a function formed by a linear term and an exponential term. In this way the “true” strain is obtained as the fit residual. We used the data from the nearest GPS stations to validate the strainmeter behaviour during a period of about fifteen months (fromMay 1, 2016 toAugust 5, 2017). First of all, with a basic approach, we considered the data from two couples of GPS stations, approximately positioned on the North-South and on the East-West directions and with their linking axes crossing the sensor site. In this way, knowing the respective differential movement and the reciprocal distance, it is possible to get an evaluation of the strain along the axes on the horizontal plane. In a second time we have utilised an open-source GPS strain calculator (UNAVCO, 2012) to obtain N-S and E-W strain values of the axes of an ellipse inside a triangle having three GPS stations at its vertexes and containing the installation site position. The centre of this ellipse is the geometrical centroid of the triangle. Because of the nearby GPS stations location, it has been possible to get the centroid position just few hundreds meters a part from our site, and therefore this kind of evaluation is quite representative of the real situation. This calculator requires the geographical coordinates of three GPS stations and their annual average drift velocities along N-S and E-W directions. The principal outputs are the velocity vector of the centroid (described by azimuth angle and modulus), the rotation angle around this point, and, above all, the bi-dimensional strain of the axes of the strain ellipse. In this way we have a complete and reliable description that is very useful to make a comparison with our data. GPS strain results indicate, for the considered time, an expansion in the E-W direction of about 2.1 microstrain (+2.1 με) and a contraction of 2.5 microstrain (-2.5 με) along NS. Considering the relation (DeWolf et al. , 2015): Δε V = [ - ν / (1 – ν) ](Δε E + Δε N ) correlating the strain along the axes (the Poisson’s ratio ν may be set equal to 0.25 for this estimation), we obtain also an evaluation of the vertical strain that in this case is of the order of +0.1 με. Fig. 2 shows Vertical, E-W, and N-S raw data. Looking at our detrended an low-pass filtered data (Fig. 3) we conclude that the EW direction really shows a weak positive trend, while the trend of the NS component is not in accordance with Fig. 2 - Strain raw data.