GNGTS 2018 - 37° Convegno Nazionale

GNGTS 2018 S essione 3.2 705 Trigger tests for the tomographic survey . Detailed velocity information is a basic requirement for reliable location of earthquakes in heterogeneous media, and seismic tomography is a helpful tool to estimate the velocity distribution. In our case-study, a well- designed tomographic survey necessitates a suitable source to be employed in order to trigger all the five three-component geophones installed on the unstable rock slope. For this reason, we planned three field tests on the top of the rock face and a hammer, firework charges and a seismic gun were selected to compare their capability to trigger the geophones. The first two tests using the hammer and firework charges have been carried out. The last test is supposed to be performed in near future. The results of the first two tests are discussed below. In the test performed with the hammer, 16 channels (1 for triggering and 15 for recording the three component signals from the 5 geophones) were used to record the data with 1 kHz sampling frequency. The minimum duration of recording was set to 5 s, including a 2 s pre- trigger window. Five hits were performed at the same hit point located close to the hole in which geophone 1 has been installed. Collected data were filtered using Reflexw software, and the processing involved DC removal, band-pass Butterworth filtering with band 5-100 Hz and notch filtering with frequency of 25 Hz, 50 Hz, 75 Hz and 100 Hz. Fig. 1 shows the recordings before and after filtering for one hammer hit. Tab. 1 summarizes the results for different channels; ‘Y’ indicates that the channel has sensed the signal and ‘N’ indicates that the signal has not been sensed by the channel. Fig. 1 - Recordings of each channel before and after filtering. Tab. 1 - Performance of each channel to sense the signal for different hits performed with the hammer. Trace no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Channel Trig 1X 1Y 1Z 2X 2Y 2Z 3X 3Y 3Z 4X 4Y 4Z 5X 5Y 5Z Hammer1 Y Y Y Y Y Y Y N Y Y Y Y N Y N N Hammer2 Y Y Y Y Y Y Y Y Y Y Y Y N Y Y N Hammer3 Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Hammer4 Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Hammer5 Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y