An investigation into shallow borehole tiltmeters

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Abstract
Shallow borehole tiltmeters are frequently used for ground deformation monitoring in volcanic areas, where they are usually installed within a few meters of the surface. A major problem concerns the noise affecting the installation at shallow depth where large tilt and strain are caused by temperature effects. The long term stability of these instruments and the best shallow depth of installation, in terms of best cost/benefit, are the crucial points. We conducted an experiment in order to compare the signals of three shallow borehole electronic tiltmeters installed in the same place at different depths. We verified the signal reliability in revealing the long term slow deformation and looked into the reduction of the temperature effects with depth. We present three years of data and discuss the limits and the advantages of the different installation depths.