This project is aimed to characterize the noise levels of the broadband ocean bottom seismograph (BBOBS) in different channels and then calculate the coherences signals between each channels to estimate the tilt and compliance noises which leak into the vertical components. This is deeply essential while dealing with the vertical data for longer period data to do surface-waves imaging. The mainly dominant types of seismic noises observed in seismic data are primary, secondly microseism and Earth's hum. These seismic noises can be observed at broadband seismic sensors on land. The data recorded at broad-band ocean bottom seismograph (BBOBS) often contains significant other noise sources that are not present in land-based seismic records mainly due to the phenomenon happens in the water column. The peaks of primary and secondary microseisms in the spectrum reveal different patterns in different ocean environments or instruments. It is essential to understand of seismic noise of the data recording in any OBS experiment and eventually remove the significant noise signals from the raw OBS data. We applied spectrum analysis using probability density functions to characterize the background noise signals for 3-components ground motion data and pressure recoding at different water depth from several passive BBOBS experiments offshore Taiwan including TAIGER project. The results show that the temporal and spatial variations result from different types of ocean waves including microseismic signals, tilt and compliance noises. We observed clearly seasonal variations of the secondary microseismic peaks in all BBOBS offshore Taiwan. In addition, we compared the ambient noise characteristic of 59 BBOBSs offshore Taiwan with 22 BBOBSs in the central pacific (NoMelt project) and 30 BBOBSs in north Okinawa Trough (NOT project). The noise levels of BBOBSs offshore Taiwan show prominent secondary microseism peaks at a higher frequency compared to the global models which were averaged the noise levels of land stations. The secondary microseism shows double peaks and clear primary microseism shows up only in NoMelt BBOBSs. It mainly due to the relative quieter deep ocean environment. Furthermore, recent studies have clarified that there are two additional noises which are clearly recorded in BBOBS. One is tilt noise which mainly due to the coupling problems and the ocean bottom current. The other one is compliance noise which mainly due to long-period ( > 30 s ) ocean waves (known as the infra-gravity waves). We estimated the tilt and compliance noise by constructing the transfer functions and calculating the spectra and coherence of all pairs between vertical and 2 horizontal-components seismic data, and differential- pressure data. We first applied the techniques to the BBOBS in NOT project and found that the temporal variations of these two noises vary dramatically. This work is essential in order to tease out the tilt and compliance noises from raw vertical records. The results from this work will benefit for future studies such as broadband Rayleigh waves tomographic imaging.
|Effective start/end date||2019/01/01 → 2020/07/31|
- broadband ocean bottom seismograph (BBOBS)
- microseismic signals
- tilt noise and compliance noise
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