| dc.description.abstract | TAIGER T6 transects is one of the four crustal-scale Wide Angle Refraction/Reflection (WARR) survey lines under TAiwan Integrated GEodynamics Research (TAIGER) project conducted in 2008 in Taiwan. The north main array consist of four shot points (N1, N2, N3, and N4) across northern Taiwan from west to east. A total of 456 geophones deployed with receiver interval of ~200m on average. However, the recorded data suffers from crooked survey line geometry, uneven shot energy, noise level. Near-surface elevation effects caused by the irregular source and receiver spacing, strong lateral velocity variations and rapid topography changes produce apparent 3D wave propagation so that the standard seismic data processing could not be fully applied. To compensate above-mentioned drawbacks, pre-conditioning of the data including trace editing and enhancements, deconvolution, frequency filtering, and Localized Slant Stack Transformation (LSST) was applied.
To study the inversion behaviors and features, feasibility studies of three crustal scale synthetic models containing: (1) flat layers without topography, (2) flat layers with topographic, (3) laterally varying topographic velocity models. Synthetic data is produced from Specfem2D computations. Effects of source and/or receiver elevation, velocity variation, and the combination of all were studied. Wavefield inversion is robust and stable for all test models. Inverted 1D velocity becomes less smooth when elevation effects are included with and without elevation corrections. Implementation of full-offset and offset-dependent wavefield inversion approach on the real data able to provide reasonable quasi-2D velocity models constructed based on seven 1D velocity profiles from four shots. Together with the inversion results from elevation corrected data through Kirchhoff Wave Equation Datuming (KWED), an adjusted model can be extracted to accommodate the best features among three different models. Quality check of the velocity models through wave simulations and travel time residual distributions show good fit and minimum error compared to the real data. Furthermore, travel time inversion were implemented based on the derived model from wavefield inversion to convert quasi-2D to pure 2D model as well as improve the resolution. Travel time inversion able to improve the resolution related to the near surface of the model as well as the high velocity anomaly below the Hsuehshan Range and Central Range. Also, travel time residual distributions shows that, the travel time inversion significantly improve the error, which range from ±0.5s to ±0.2s for N1-N3 and ±1.5s to ±0.5s for N4. Comparisons and validations between proposed model and several reference model shows improvement at shallow structure shallow part with sufficient accuracy and resolution through limited datasets and consistencies on the large scale.
Based on proposed velocity model and depth migrated image, several structure feature can be outlined. Existing sedimentary basin with layer thickness of ~3 km and relative low velocity from ~2.6 to 4 km/s in the Western Foothill. Thinning eastward feature judging from seismic velocity between 5.0 km/s – 5.5 km/s and highlighted interfaces from migrated image in the upper part within few kilometers presumably can be linked to the low-grade greenschist facies metamorphic basement. Possible indication of their synrift sediments and basement are being uplifted and exhumed. High velocity anomaly linked to the Northern Taiwan Conductor below Hsuehshan Range. Datong Bright Spot (DBS) reflector at ~20 km depth within ~61Km to 85Km of distance, can be linked to ultramafic intrusion. High velocity anomaly below Central Range represent the Tanano Schist outcrop presumably consisted of ultramafic rocks and marble. Due to the limitation of data aperture, Moho discontinuity is not resolved. | en_US |