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An analysis of chirp sonar signals backscattered from ocean sediments is presented in this paper to estimate sound attenuation in ocean sediments. The theory of applying a matched filter to linear frequency modulation chirp signals for generating time compressed pulses (CPs) is described and implemented. After the matched filter technique was applied, the signal-to-noise ratio (SNR) of the CP was enhanced. However, a smearing effect resulting from the convolution operation diminished the advantage of the improved SNR. The chirp signal processing algorithm is described and applied to a sample data set collected from an area with a variety of different sediments offshore northeastern Taiwan. For the exploration of signal features, time–frequency analysis is employed to reveal the characteristics of chirp signals presented in the joint time–frequency domain. Based on the spectral ratio method, ocean sediment attenuation properties are compared to those obtained from core samples analyzed in the laboratory. The observed attenuation coefficient was lower than those of some of the previous studies, but it was still within the reported data range. Factors that could contribute to the uncertainty are discussed. [ABSTRACT FROM AUTHOR]

Newly compiled bathymetry, sub-bottom profiling and seismic data are used to illustrate seafloor morphology, submarine canyon networks and stratigraphic framework in Huatung Basin (HB) and an adjacent sub-basin, named as East Luzon Arc Basin offshore east Taiwan. Based on seismic facies characteristics and isopach maps, three major stratigraphic sequences are recognized and we suggest that they could be resulted from pre- and syn -Taiwan arc-continent collision. Based on the sediment thickness and sedimentation rates, ages of the HB strata could be estimated. Pelagic sediments formed the lower HB sequence which follows the HB basement reliefs since 50–102 Ma. Then, the initiation of Taiwan orogen and Luzon Arc volcanic activities changed the deposition pattern after 5 Ma, and mass transport deposits developed in middle HB sequence. After rapid uplift of the Taiwan mountain belt since about 1.5 Ma, complex canyon-levee systems and thick strata formed the upper sequence in the northern HB. In contrast, sediment thickness is thin and no submarine canyon developed in southern HB. The seafloor in South Huatung Basin is flat, except in its eastern part where couple basement ridges appeared. In North Huatung Basin, canyon-levee complex and sediment waves are the most distinct morphological features in the drainages of two submarine canyons. We propose a three-stage process for sediment wave development: (1) sediment gravity flow can initiate a base of sediment wave; (2) wavy geometry begins to develop; (3) sediment waves continuously aggrade due to overbank sediments in the canyon-levee system. This study presents the evolution of sedimentary processes in a deep-sea basin and how the deposition patterns were affected by the convergent tectonics. • We report stratigraphic framework and sediment waves of Huatung Basin firstly. • Major stratigraphic sequences above oceanic basement of Huatung Basin are mapped. • Geological evolution of sedimentary processes & sediment wave fields in the basin are presented. • The age of Huatung Basin is estimated based on calculated sedimentation rates & sediment thickness. [ABSTRACT FROM AUTHOR]

We analyzed original and published seismic data to investigate the distribution and characteristics of subsurface fluid flow features in the gas hydrate provinces along the northeastern South China Sea (SCS) continental slope, including Jiulong Ridge, Horseshoe Ridge, Pointer Ridge, and Formosa Ridge. Numerous features indicating the presence/migration of hydrocarbons, such as bottom simulating reflections (BSRs), bright spots, gas chimneys, and fluid flow through faults, are identified. The results reveal that the hydrofracturing-induced gas chimneys act as the primary conduits for the overall focused fluid flow in the study area, though fluid flow along permeable faults is also observed at Pointer Ridge. Thirty-three gas chimneys identified are categorized into two types depending on their capability for focused fluid flow migrating into the gas hydrate stability zone (GHSZ). The type-I chimneys that transport fluids into the GHSZ contribute to gas hydrate formation and may even lead to seafloor seepage. Buried by a considerable thickness of sediments, the type-II chimneys cannot directly contribute to generating gas hydrate and surface seepage. Our results suggest that the sedimentary processes and fluid accumulation significantly control the development of gas chimneys in the study area. Since the focused fluid flow conduits that indicate overpressured fluids are critical pathways that feed gas into the GHSZ and form gas hydrates, utmost attention should be paid to them during hydrocarbon exploration. [ABSTRACT FROM AUTHOR]

High-resolution bathymetry collected with an autonomous underwater vehicle (AUV) along the flanks of three ridges of the accretionary prism offshore southwestern (SW) Taiwan revealed more than 650 elongated depressions in water depths ranging from 1155 to 1420 m. The depressions are between 12 and 129 m long, 5 to 70 m wide, and up 9 m deep at their center and shallowing downslope to about 1-m depth. Due to their shape in downslope cross section, they are termed comet-shaped depressions (CSD). The CSD occur in patches of more than 100 with densities of 53 to 98 CSD/km2. In addition, seven topographic mounds were mapped and interpreted as pingos, which remotely operate vehicle (ROV) observations and sampling show to be covered with authigenic carbonate. These features overlie areas where multichannel seismic reflection (MCS) profiles show bottom simulating reflectors (BSR) and dipping strata extending from below the BSR to near the seafloor. We consider comet-shaped depression, a new type of pockmark, forms on a sloping seafloor where fluids expulsion occurred. We also suggest that the two types of distinctive geomorphic features are attributed to fluid venting which occurs at different rates, with the mounds developing slowly over time, but the CSD forming in discrete events perhaps associated with large earthquakes. [ABSTRACT FROM AUTHOR]

Strong ocean current influences a marine seismic survey and forces the streamer off-course from the survey line. The sideway drift of the streamer results in that the reflection data are no longer distributed in common midpoint gathers along the survey line but become swath distribution on one side of the ship track. This effect is known as "streamer feathering" which degrades the profile image of the 2D processed seismic data. However, if we have long streamer or closely spaced parallel 2D seismic survey lines, we may turn this deleterious effect into a good opportunity to generate 3D seismic volumes with swath distributed reflection data. We present two case studies in which 2D seismic data were collected offshore eastern Taiwan where the strong Kuroshio Current heavily influenced the ship speed and caused large streamer feathering. The first case is a large-offset 2D seismic profiling data collected using a 6-km long streamer. We processed the swath part of the reflection data in 3D that not only avoids the inappropriate smearing effect in 2D data processing but also generates a 3D seismic volume to help the seismic interpretation. In the second case, we adjusted our 2D survey strategy when realizing that strong Kuroshio Current was causing significant streamer feathering, and collected a set of closely spaced parallel 2D seismic lines. This multi-swath dataset covers a broad area which enables us to generate a 3D seismic volume. Since our datasets are not real 3D seismic data, we have tailored our processing flows to deal with different data configurations and limitations of each dataset. Our results show that not only we have enhanced 2D seismic images of the originally-interested survey lines, but also provide information on 3D geometry of the geological features imaged. The benefits and limitations of utilizing the streamer feathering effect to generate 3D seismic volumes from 2D seismic profile data are reported. Overall, this approach is a considerable way to handle 2D seismic data with large streamer feathering for both avoiding unreliable 2D seismic images and obtaining information on 3D geometry of the geological features imaged. [ABSTRACT FROM AUTHOR]

Pointer Ridge is a gas hydrate prospect on the South China Sea continental slope offshore SW Taiwan. It is characterized by densely distributed bottom simulating reflections (BSRs), active gas seepage, and potential sandy gas hydrate reservoirs. To understand how the fluids have migrated toward the seafloor, and the role of geological processes in the gas hydrate system, we have collected and analyzed high-quality 2D and 3D reflection seismic data. We first mapped the spatial distribution of the BSRs, and interpreted a major normal fault, Pointer Ridge Fault (PR Fault). The NE-SW trending fault dips to the east, and separates the erosional regime to the west from the depositional regime to the east. One active vent site was identified directly above the PR Fault, while another is located on a topographic high to the west of the fault. On the hanging block of the fault we found at least one major unconformity. The seismic data indicate refilled channels with coarser-grained sediments in the hanging wall of the normal fault. Seismic attribute analysis shows subsurface fluid conduits and potential gas hydrate reservoirs. We propose two types of gas chimneys, which are separated by the fault. Gas plumes derived from hydroacoustic data are mostly from the footwall block of the fault. We infer that fluid flow is more active in the erosional environment compared to the depositional one, and that this is the result of reduced overburden. The methane-bearing fluids migrate upward along the PR Fault and chimneys and form hydrates above the base of the gas hydrate stability zone. Based on seismic interpretation and seismic attribute analysis, we postulate that the channel infill constitutes the most promising hydrate reservoirs in this geological setting. In the surveyed area of Pointer Ridge these channels occur mainly below the gas hydrate stability zone. • Present original 2D/3D seismic data to reveal the geologic setting of a potential gas hydrate prospect off SW Taiwan. • Active fluid flow processes are studied by analyzing water column and seismic data. • A conceptual model is proposed for the gas hydrate system of Pointer Ridge by detailed seismic attribute analysis. • Potential gas hydrate reservoirs that might be targets for future exploration are identified. [ABSTRACT FROM AUTHOR]