A lab-fabricated ocean bottom seismometer was modified and deployed terrestrially to detect low-frequency (<10 Hz) ground vibrations produced by debris flows. A frequency–response test of the new seismometer revealed that it can detect seismic signals at frequencies of 0.3–120 Hz. Its seismic ground motion detection ability was investigated by comparing its measurements of seismic signals produced by rockfalls with those of a geophone. Two new seismometers were deployed at the Aiyuzi Stream, Nantou County, Taiwan, in September 2012. Seismic signals produced by two local earthquakes, two teleseisms, and three debris flows detected by the seismometer in 2013 and 2014 were discussed. The seismic signal frequencies of the local earthquakes and teleseisms (both approximately 1800 km apart) were 0.3–30 and <1 Hz, respectively. Moreover, seismometer measurements revealed that seismic signals generated by debris flows can have minimum frequencies as low as 2 Hz. Time-matched CCD camera images revealed that debris flow surge fronts with larger rocks have lower minimum frequencies. Finally, because the seismometer can detect low-frequency seismic waves with low spatial decay rates, it was able to detect one debris flow approximately 3 min and 40 s before it arrived.
Normal incidence echo data (bottom reflection) can provide acoustic reflectivity estimates used to predict sediment properties with using seabed sediment models. Accuracy of normal reflection coefficient measurement thus become very significant to the bottom inversion result. A deep-towed camera platform with acoustical recording system, developed by the Institution of Undersea Technology, National Sun Yat-sen University, Taiwan, is capable of photographically surveying the seafloor in near scope and acquiring sound data. The real time data transference, including photography (optics) and reflection measurement (acoustics) can be implemented in the same site simultaneously. The deep-towed camera near the bottom was used in several experiments in the southwestern sea off Taiwan in 2014 to acquire acoustic LFM signal sent by surface shipboard source as incident signal as well as the seafloor reflections at frequency bands within 4–6 kHz. The error produced by compensating the roll-off of altitude of vehicle (propagation loss) can be eliminated, which is considered as near bottom self-calibrated measurement for normal reflection coefficient. The collected reflection coefficients were used to inverting the sediment properties with using the Effective Density Fluid model (EDFM), manifested by the coring and camera images. [This work is sponsored by the Ministry of Science and Technology of Taiwan.]
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