A DENSE MICROTREMOR SURVEY FOR THE STRATIGRAPHIC CHARACTERIZATION OF THE GUANZILING GEOTHERMAL SITE IN SOUTHWESTERN TAIWAN

We conducted a dense microtremor survey for the geothermal exploration on the Guanziling hot spring area in Tainan City, Taiwan. We deployed each time 26 sets of DATA-CUBE³ recorders equipped with three-component receivers to collect >300 microtremor data in total along three two-dimensional profiles using both two circular arrays and dense single-station measurements. We employed various methods including horizontal-to-vertical spectral ratio (HVSR), frequency-wavenumber (F-K) array analysis, and spatial autocorrelation (SPAC) for data analysis. These techniques allowed us to estimate velocity values for different stratigraphic layers and determine the underlying interface depths. Based on HVSR calculations from 228 stations in the Guanziling area, it was observed that most stations within the surveyed area exhibited predominant frequencies ranging between 2-4 Hz. However, at the Nanliao anticline, a higher predominant frequency of up to 8 Hz was observed near the anticline axis compared to the surrounding areas. Conversely, at the Chientapu syncline, frequencies decreased to below 2 Hz. This suggests that near the anticline axis, stratigraphic depth is relatively shallow while it becomes deeper near the axis of the syncline. Array analysis results revealed that when frequencies exceeded 1 Hz, two concentrated phase velocities were vaguely observed extending from low to high frequencies, which indicating an interaction between fundamental mode and higher mode Rayleigh waves within this frequency range. Furthermore, discontinuities may be detected in the fundamental mode Rayleigh wave around 1-2 Hz which implies significant variations in velocity model within the Guanziling area with a possibility of presence of a low-velocity layer. Geothermal regions often exhibit anomalies characterized by reduced S-wave velocities in velocity profiles, and discontinuities in subsurface layers can provide insights into fracture zones or fault locations. By identifying these characteristics, we can infer buried pathways for hydrothermal fluids in the Guanziling geothermal target area.