GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 3.1 549 around their own axis resulting in an unknown instrument location on the seafloor and unknown directions of the horizontal components, while the Z-component will be oriented vertical due to a gimballed suspension. The vertical alignment of the Z-component was assessed every 30 days by a tiltmeter. The trillium seismometer that was used is also equipped with a magnetic compass. Due to magnetic material on the flotation frame and the steel anchor of the instrument it is difficult to calculate the absolute directions of the horizontal components based on compass measurements. However, we could prove for all instruments that the horizontal components did not change their orientation during deployment time. The same is true for the vertical alignment of the Z-component. To provide absolute values of the direction of the horizontal seismometer components (H1 and H2) we used the active shots of the wide-angle profile P01, recorded at the end of the deployment period. Seismic shot sections for P01 were generated. The data were high pass filtered to remove the refraction phases, leaving the direct arrivals and their multiples (Fig. 2a). The data were subsequently plotted in a hodogram and orientation angles were calculated based on the shot azimuth (Fig. 2b). The rotation angles could be calculated with an accuracy Fig. 2 - (a) The three seismometer components and the hydrophone component for shot 2900 at OBS404 are displayed after a high pass filter of 10 Hz to minimise refracted phases and impose the direct arrival through the water. (b) Left panel shows a seismic shot section of the two horizontal components H1 (east) and H2 (north) after rotation. Right panel displays the geometry of single shots (red circle) and OBS (red triangle). The blue line represents all active source shots.