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Reducing multiple contaminations in reflection seismic data remains one of the primary challenges in marine seismic data processing. Besides geological settings, its effectiveness is also dependent on the multiple removal methods. In this study, we undertook two legacy 2D multi-channel seismic data crossing the accretionary wedge off SW Taiwan to test the efficiency of various multiple-attenuation scenarios. The tectonic domain has resulted from the incipient arc-continent collision between the northern rifted margin of the South China Sea and the Luzon volcanic arc. The wedge extends from shallow water to deep water bathymetries, hence promoting both short-period and long-period multiples within the seismic records. A cascade of de-multiple methods was tested to attenuate multiple energy under various seafloor bathymetry and tectonic areas. The first step relies on the periodicity nature of multiples. Spatial dependent predictive deconvolution in the x-t domain was performed to attenuate reverberations and improve temporal resolution in the time domain. Wave-equation multiple attenuation (WEMA) was applied to suppress the water layer multiples based on a combination of numerical wave extrapolation in the shot domain through water layer and water bottom reflectivity. Surface-related multiple elimination (SRME) aimed to attenuate the residual water bottom multiple and peg-leg multiple by assuming surface-related multiples can be kinematically predicted via convolution of pre-stack seismic traces at possible surface multiple reflection locations. The second step exploits the spatial move-out difference behavior between primaries and multiples. Parabolic Radon transforms far-offset multiples by subtracting noise energy in the τ-p domain, whereas the frequency-wave number (F-K) filter aimed to eliminate any residual multiples energy in the F-K domain. Predictive deconvolution improved seismic resolution and suppressed sea-bottom reverberation energy in the continental and lower wedge slopes, but not in the upper wedge slope. WEMA, Radon filter, and F-K filter reduced multiples energy both at the continental slope and wedge slope; whereas SRME made minimal impact on both areas. Since the reflection seismic datasets stretch diverse tectonic environments and water depth, there was no single multiple attenuation method capable to suppress multiples in all tectonic environments and bathymetry. [ABSTRACT FROM AUTHOR]

Abstract: Off southwest Taiwan, a west-advancing orogenic wedge has obliquely impinged on the northern continental slope of the South China Sea (SCS) margin. We analyzed a dense grid of multi-channel seismic profiles to reveal the tectonic features in this oblique collision setting. In the upper SCS slope and adjacent to the accretionary wedge, the rifted continental margin is characterized by a deep-seated, oceanward-dipping, listric and active normal fault with rotated hangingwall strata beneath the slope. To the west of this slope segment, the mid-Oligocene to Recent post-breakup sediments show prograding and aggrading shelf-margin clinoforms that cover a series of small, possibly Paleogene, rift basins. In the submarine accretionary wedge, a series of west-vergent, NNW-striking fold-and-thrust structures characterizes the lower wedge, which can be further divided into frontal and rear segments with distinct structural features. The frontal segment is characterized by four west-vergent blind thrusts with gently folded limbs. In contrast, the rear, arcward segment is characterized by west-vergent, emergent and imbricate thrusts with tilted beds truncated at the hangingwalls. Strata within slope basins are tilted arcward, with dips that increase with depth, indicating continued relative uplift along thrust planes during sedimentation. Pulsed thrust activity is further evidenced by an array of arcward-dipping unconformable surfaces with westward onlapping strata in the basins. Longitudinal sedimentary tapering of pre-orogenic sediments correlates strongly with curvature of the submarine frontal accretionary belt, suggesting that pre-orogenic sediment thickness is the major control on the geometry of frontal structures. The preexisting SCS slope that lies obliquely in front of the advancing accretionary wedge has impeded the advancing of frontal folds resulting in a successive termination of folds against and along strike of the SCS slope. The existence of the SCS slope also leads the strike of impinging folds with NNW-trend to turn more sharply to a NE-strike, parallel to strike of the SCS slope. Our analysis shows that the pre-orogenic mechanical/crustal heterogeneities and seafloor morphology exert strong controls on the thrust-belt development in the incipient Taiwan arc-continent collision zone. [Copyright &y& Elsevier]

Abstract: Southern Taiwan and its offshore area lie in the region where the Luzon volcanic arc initially collides with the rifted China continental margin. Because of the incipient arc-continent collision, the structures vary markedly along-strike the collision zone so as the patterns of seismicity. We use new seismic reflection profiles and integrate existing data to reveal major tectonic features and potential seismogenic faults of the study area. The accretionary wedge in the incipient arc-continent zone can be divided into the lower slope, upper slope, and backthrust domains, respectively. These structural domains reflect different aspects of wedge deformation, and exhibit significant structural variations along-strike. Reflection seismic data show that the prominent seismogenic structures in the Taiwan incipient collisional wedge include: (1) frontal decollement beneath the lower-slope domain, (2) out-of-sequence thrusts bordering the lower-slope and upper-slope domains, (3) megathrust that cuts into the oceanic (?) basement beneath the upper-slope domain, and (4) the Chaochou-Hengchun faults in the onshore upper-slope domain. Thermal regime for those structures indicates that the megathrust and part of frontal decollement are seismogenic. The geometry of the frontal decollement, out-of-sequence thrusts and megathrust is analogous to those observed along the Nankai prism of Japan, so that they are possibly capable of generating great earthquakes as shown in the Nankai Trough. Beneath the lower and upper-slope domains off SW Taiwan, the seismicity is characterized by mantle earthquakes with the accretionary wedge being largely aseismic. We interpret the lack of prominent seismicity within the accreted wedge to result from excess fluid pressure that has significantly weakened the wedge materials and fault zones and therefore results in less seismicity. The predominant mantle earthquakes beneath the accretionary wedge, however, may result from water-enriched mantle materials infiltrated during previous Mesozoic subduction event and later rift events. The volatile contents may have significantly reduced the rigidity of the mantle, leading to the mantle being more susceptible for brittle deformation and hence anomalously high seismicity. [Copyright &y& Elsevier]