Body- and Surface-waves Wavefield Inversion in Southern Taiwan: Application to TAIGER T4b Wide Angle Refraction/Reflection Data

• Main Authors: Roma Widiyansari, 維蒂利
• Other Authors: How-Wei Chen
• Format: Others
• Language: en_US
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/3x93q2

碩士 === 國立中央大學 === 地球科學學系 === 106 === Surface and body waves generated by near-surface explosions and recorded along T4b line is investigated for imaging subsurface velocity structure in Southern Taiwan. Surface waves propagate only at the Earth’s surface meanwhile body waves propagate deep through the body/interior of the Earth. For crustal-scale velocity structure investigation, wave-field processing and inversion of both body and surface waves records can be very useful to image Earth’s subsurface structures without picking travel-time. Seismic dataset used in this research is a crustal-scale Wide Angle Refraction/Reflection (WARR) survey under TAiwan Integrated GEodynamics Research (TAIGER) project conducted in 2008 in Taiwan. The south main array consist of four shot points (S1, S2, S3, S4) across southern Taiwan from west to east. A total of 609 geophones was deployed with receiver interval of about 200m. We use two approaches to estimate separate Vp and Vs velocity models along T4b line. The first method is through implementation of Multichannel Analysis of Surface Wave (MASW) technique to determine near-surface velocity structure. Shear wave velocity is the best indicator of material stiffness for engineering related applications. The near-surface soil or formation shear wave velocity (Vs) characteristics can be retrieved by analyzing the dispersion curve associated with the fundamental mode of surface waves in heterogeneous media. MASW method is only applicable for shot number 1 (S1) and shot number 2(S1) that possess significant amount of ground-rolls records. The second method is based on tau-p wave-field inversion. Through wave-field transformation and downward continuation approach, application of long-offset (or local short-offset) seismic data to estimate compressional wave velocity (Vp) structures can be achieved via direct global (or localized) search algorithm. Through wave-field processing strategy, independent Vp and Vs velocity models can be obtained. From inverted shear and body wave velocity structure, we can see obvious velocity gradient exist in southwestern Taiwan Coastal Plain with the bedrock boundary is slightly west-dipping. Assume the shear wave velocity of Pliocene to Miocene bedrock is 1.5 km/s, the thickness of alluvial sediments at the west-end of western Coastal Plain is ~1.8 km and become thinner toward the east with depth ~0.9 km. The Vs30 site classification for shot S1 is Class D1 and for shot S2 is Class D3 which all corresponds to “stiff soil”. From body wave wave-field inversion, subsurface structure can be imaged up to depth of approximately 20 km. In the Western Coastal Plain, a thick sediment layer (~ 3 km) with fairly low velocity (Vp= 1.5-4 km/s and Vs=250-1500 km/s) and high gradient changes produce significant seismic reverberations (refracted free-surface multiples) shown in data S1 and S2. Evidence of strong lateral velocity changes marked the transition between Coastal Plain and Western Foothills near east side of shot point S2. Strong lateral velocity transition also can be observed between Central Range and Coastal Range at shot point S4. The contour lines for Vp values between 5.0 to 6.5 km/s highlight the general feature of crust layer which thicken in the west (depth=8 and 16 km respectively) and thinned toward the east (depth =4 and 9 km respectively). By examining the contour value of 6.0 and 6.5 km/s, a relatively flat structure feature extend from Coastal Plain to Western Foothills; thickening at the distance between 40-78 km and thinning toward Central Range and Coastal Range provide a vivid view of crustal thickening and thinning feature associate with tectonic structure along the T4b line. The 6 km/s contour shown at the depth of 13 km from Coastal Plain and reaches the depth of ~7 km at Coastal Range may highlight the potential boundary between upper and lower crust (Conrad discontinuity?). The extreme thinning (necking) of the continental crust can be occurred by a typical rift zone at continental margin. The 6.5 km/s and a high velocity lower crust layer with velocity ranging from 6.9 (or 7.0) to 7.5 km/s may show similar features and may highlight the existing boundary between lower crust and upper mantle (Moho discontinuity?) which can be identified from wavefield processing and velocity inversion studies. With more careful investigate near-surface velocity distribution within 2.5-5 km, crust thickness can be thinner than the result from tomography studies. Picking the first arrival time with trial-and-error processes are somehow tedious, not efficient and time-consuming may lead to overly smoothed model. The advantages of the proposed method are no need to manually pick travel time and automatically and stable update of velocity model. The quality of the derived model is effectively checked by the consistency between computed travel-time with seismic records. Keywords: Tau-p Transform, Tau-p Inversion, MASW, Surface Waves, Ground-roll, TAIGER Project