a. Seafloor geodetic surveys
This study was initiated by Prof. Masataka Ando, a NSC Chair Professor at IES from 2007 to 2013. In 2008, Dr. Shui-Beih Yu and I jointed the project and fulfilled a few tests off eastern Taiwan and the longest testing range reached 200 km away, and acquired the positioning accuracy similar with the former results. After 2013, Dr. Cheng-Horng Lin and Dr. Ya-Ju Hsu supported the manpower and funds for offshore field works. We are dealing with the analysis and collection data from the scheduled experiments of two ocean bottom networks which are located off-Ilan and off-Hualien in eastern Taiwan. In the off-Ilan arrary (depth at 1500 m), six seafloor geodetic surveys have been conducted on July 2012, April, July, September 2013, September 2014, June, September 2015, May and August 2016. The positioning results have been acquired on root-mean-square (rms) in 0.06 and 0.10 msec (i.e. 7 and 12 cm) of positioning accuracy. Compare the accuracy with previous studies in Chile and Japan, the results are slightly worse in 2-3 cm level. The velocity with respect to S01R estimated from the experiments from July 2012 to August 2016 is 2.5±1.9 cm/yr to the south 6.8±1.8 cm/yr to the west and 5.9±2 cm/yr subsidence. The movement behavior of this result is consistent with the onshore GPS results. The uncertainties of the velocities are still slight large to determine the positions precisely, However, the accuracy will be improved by prolonging the observation periods. In the off-Hualien array (depth at 4500 m), we installed 3 new ocean bottom units in 2014 which have a life-time reaching 7 years with new developed Li-batteries, to replace the old one that installed in 2009 and no response due to low battery power in 2013. The field campaigns were conducted in September 2015, July and August 2016. After a few more years of measurements, the relative velocity between the Philippine sea plate and Ryukyu island arc can be determined in a few centimeters accuracy near the Hualien offshore area. According to the further field works, two strategies have been executed to improve the positioning accuracy in later GPS/Acoustic surveys. Firstly, to reduce the noise level that affected the acoustic data from vessel (boat), a buoy has been invented and tested on 2012 and the quality of signal can be improved in 30% level. Secondly, to improve the positioning accuracy, the amount of received data will be increased.
b. GPSLAB development at IES
After the original designer L.C. Kuo’s retirement, a PhD student, Hsing Tung, and I took the efforts to realize and maintain the website operation well. Two strategies will be executed to improve the function and user friendly of the website. Firstly, we use the GIPSY OASIS free scientific GPS software to process the data all of over 300 CORS in Taiwan. Secondly, to improve the accuracy of the inter-seismic velocity in Taiwan area, a mathematic function to reduce the co-seismic and post-seismic displacements, seasonal, half-yearly and yearly effects to the time series provided by Dr. Ya-Ju Hsu, will be applied to determine the station velocity.
c. Monitoring the near-fault deformation
A PhD student, Hsing Tung, and I focus on applying Global Positioning System (GPS) and persistent scatters InSAR (PSInSAR) techniques to estimatet the velocity field for crustal deformation studies in the southernmost Longitudinal Valley and Taipei Basin areas. Especially, the study in the Taipei area, we will use high-resolution X-band Radar images from recently launched satellites with PSInSAR technique combining with LiDAR-derived DEM, continuous GPS array and precise leveling to monitor the land subsidence, active Shanjiao fault and Tatun volcano group in the Taipei area. This study will significantly contribute to better understanding of the geologic hazard processes and hazard mitigation in the metropolitan Taipei city (Paper has been accepted by Tectonophysics in 2016).
d. Enhancing the precision of low-cost single frequency GPS in geodetic applications
Geodetic group of IES and I deployed and maintained more than 50 low-cost single-frequency GPS receivers to densify the Continuously Operating Reference Station (CORS) along the Longitudinal Valley for crustal deformation monitoring in the real-time data transmission mode. Due to the turbulence of the ionospheric delay varies with latitude of the receiver position, the season, time of day and level of solar activity, the single frequency receivers cannot eliminate the effect of the ionospheric delay. Hence, we demonstrated a probable algorithm to mitigate the ionospheric effect for low-cost single-frequency receiver to apply in geodetic applications in low-latitude area, Taiwan, and the results show the co-seismic displacements can be obtained and the accuracy is almost the same as the CORS results.
e. Landslide monitoring
To better understand the behaviors and activities of deep-slip in the mountain area, Dr. Ya-Ju Hsu and I joint a 3-years’ integrated project (Establishment of a comprehensive landslide monitoring system and study of monitoring data and landsliding mechanisms: An example of Tai-Ping Shan Lan Tai area) supported by the Ministry of Science and Technology. In the first stage, 10 sets of low-cost single-frequency GPS receivers have been setup in Mt. Taiping after careful selection the potential monitoring stations from Dr. Roufei Chen by applying the TCPinSar (Temporarily Coherent Point InSAR) technique. All GPS data are real-time transmission to IES and computing in every hourly, 3-hours and daily, three types solution for different usages. During the time period when Typhoon Soudelor attacked Ilan area on 08 August 2015 and provided accumulated rainfall of 1900 mm in two days, obvious surface displacements have been detected. The purpose of this study is to realize the slip mechanism and triggering threshold of the large scaled landslide. For large scaled landslides in different geologic and geomorphologic terrains, different combinations of monitoring techniques will be proposed.
