GNGTS 2015 - Atti del 34° Convegno Nazionale

Misalignment angle correction of borehole seismic sensor. Despite all precautions, rotation may occur during installation of the sensors in the boreholes and therefore the orientation of borehole stations is unknown. An attempt was made in order to evaluate the misalignment angle correction of borehole seismic sensors of ED01, IMAA, MIRB, and MSN stations. The goal is to calculate the relative misalignment angles between sensors installed at depth, taking as a reference the sensor installed on surface, and considering that this instrument can be oriented by traditional methods, i.e. with compass. To determine the correction angle sensor in these stations we used a method developed and described by Grigoli et al. (2012). As mandatory requirement, the distance between sensors must be much smaller than the dominant wavelength of the signal used. Taking into account this condition, we selected the seismograms used for the determination of the rotation angles on the base of the following conditions: 1) teleseismic events with epicentral distances of at least 5000 km from the station; 2) regional events with epicentral distances up to 1000 km from the station; 3) magnitude of at least 6 degrees (Mw) and enough energy to be recorded simultaneously by all sensors. In all cases, the signal was pre-processed in the following way. First of all, it was down-sampled by decimation to 1 Hz sampling; secondly, static offset and linear trend were removed, and 5% tapering was applied. Third, the signal was pass-band filtered in the band 0.08-0.15 Hz by a Butterworth 4th order band-pass filter. Fig. 3 shows, as an example, the correction of the azimuthal rotation of the borehole sensor at Marsico Nuovo (MSN). The top left panel shows the traces of the Mw=6.4, 2013-10-12 13:11:54, Crete event recorded from MSN surface and MSN borehole sensors, respectively. The borehole sensor appears quite misaligned, with the respective influence on the quality of the recorded data. Essentially the effect of this error is that the amplitudes of the traces recorded by borehole sensor are strongly modified in relation with trace recorded at the surface (Fig. 3, top right). The same effect is observed in the particle motion before and after correction (Fig. 3, bottom left). Results show that misalignment angles range from some tens up to hundred Fig. 3 – Correction of the azimuthal rotation of the borehole sensor at Marsico Nuovo (MSN). Top left: example of traces used for the horizontal alignment calculation. Seismograms refer to Mw 6.4 Greece earthquake recorded by the station (MSN surface and MSN borehole sensors, respectively). The black box indicates the part of the waves used to represent the seismograms and hodograms shown in the following panels. Right: Comparison of the seismograms before (top) and after (bottom) misalignment correction. The blue traces always correspond to the reference surface sensor, while the green and red colours correspond to the original traces and those obtained after misalignment correction. Bottom left panel: particle motion (hodograms) related to station pairs before (left) and after (right) misalignment correction. The reference MSN surface sensor is represented in blue, while the MSN borehole sensor is represented in red. 140 GNGTS 2015 S essione 2.2