Heavy rainfall, frequent earthquakes, and fragile geological conditions in Taiwan result in landslides and debris flows, especially during the heavy rainfall and typhoon seasons. Debris flow hazards and the associated prevention countermeasures have now become a major concern in Taiwan''s efforts to remedy these disasters. This paper uses resistivity image profiling methods to investigate the mechanism by which accumulated sediments contributes to debris flows in gullies. Three disaster areas of debris flow are selected for this study: Shuei-mo-keng Stream and Jhong-Ho Village in northern Taiwan, and Chen-You-Lan Stream in central Taiwan. This paper will present detailed on-site geological, geomorphological, and geoelectrical observations of the path of the debris flow in the study areas. The results of this study allow a better understanding of the features of the debris flow, as well as identify the actual sources and causes of the hazards.
Seismic data acquired from the North West Shelf (NWS) of Australia contains strong multiples. Water bottom peg-legs generated from the shallow water bottom have similar moveout with primaries from deeper reflectors. Move-out driven demultiple methods such as Radon de-multiple often fail in this situation. Alternative demultiple solutions are the wave-equation methods of Surface Related Multiple Elimination (SRME), Iterative SRME (ISRME), and Interbed Multiple Elimination (IME). This paper addresses the problem of peg-leg multiples that heavily contaminate the shallow water seismic data from the NWS. To understand these multiples full-wavefield and ray-traced synthetics were generated. Radon demultiple and SRME were both tested on the synthetics. We conclude that SRME succeeded whilst Radon failed in peg-leg multiple dominant data. We demonstrate the multiple attenuation success on real data using the SRME method. The final image is an outstanding result compared with processing using other demultiple methods.
Whether you can use a sparse set of source angles in delayed-shot migration, or not, depends on how well your actual set of sources approximates (or can be made to approximate) an unaliased “antenna”. If you have a well sampled source antenna, then sparse angles can be used to create high-quality images since you can transform your data into the equivalent delayed-line-source experiment. If your collection of sources does not satisfy this requirement, then you may have to use a large number of angles to construct artifact-free images.
This paper proposes a modified conjugate-gradient (CG) method, called the conjugate-guided-gradient (CGG) method, as an alternative iterative inversion method that is robust and produces a parsimonious model estimation. The CG method for solving least-squares (LS) problems can be modified to solve for a different norm or different minimization criteria by guiding the gradient direction appropriately. The guiding can be achieved by iteratively reweighting either the residual vector or the gradient vector during iteration steps like IRLS (Iteratively Reweighted LS) method does. Unlike the IRLS method, however, the CGG method doesn''t change the corresponding forward operator of the problem.
Carazzone, J.J. (ExxonMobil Upstream Research Company) | Burtz, O.M. (ExxonMobil Upstream Research Company) | Green, K.E. (ExxonMobil Upstream Research Company) | Pavlov, D.A. (ExxonMobil Upstream Research Company) | Xia, C. (ExxonMobil Exploration Company)
PetroChina conducted a multichannel large-offset 2-D seismic survey in the Yumen Oil Field, Northwest China, in September, 2004. The objective is to delineate the complex, imbricate structure associated with the Yumen reservoir beneath the high-velocity Kulong Shan allocthonous rocks so as to accurately position production wells in the future. The data were acquired using a common-spread recording geometry whereby the receiver spread was fixed for all shots. A total of 1,401 receiver groups was placed along a 28,000-m line traverse in the SSW-NNE dominant structural dip direction at a 20-m interval. A total of 211 shots was fired at a 200-m interval along the line traverse, beginning at a location outside the spread and 7 km away from the first receiver group in the SSW end of the line. The distance between the first and last shot locations is 42,000 m.
We analyzed the Yumen large-offset data for earth modeling and imaging in depth. By a nonlinear first-arrival traveltime tomography, a velocity-depth model was estimated for the near-surface. Then, a subsurface velocitydepth model was estimated based on rms velocities derived from prestack time migration of shot gathers combined with half-space velocity analysis to improve the accuracy of velocity estimation below the complex overburden structure associated wth the high-velocity Kulong Shan rocks. An attempt also was made to model not just the near-surface but also the subsurface by the application of nonlinear traveltime tomography to first-arrival times picked from all offsets. Finally, prestack depth migration of shot gathers from a floating datum that is a close representation of the topography was performed to generate the subsurface image in depth.
The pressure sensitivity of rock elastic properties and seismic velocities is dependent upon pore space, grain size, grain sorting and cementation. An increasing effective pressure gradually reduces the throats between pore. It forces closure of compliant pores with low aspect ratio and reduces porosity. It also changes contact configuration that brings more cement in load bearing network. Present study models stress dependent velocity changes through this mechanical rearrangement of pore and contact system.
Although a granular rock is likely to have pore with a spectrum of aspect ratio, we put forward the concept of an ‘effective aspect ratio’ that simulates the rock elastic behavior. Such ratio can be inverted through effective medium solution. It is observed that the effective aspect ratio increases with increasing pressure due to gradual closure of compliant pores having low aspect ratio. This inturn reflects into stress sensitivity of velocities. Using dry core measurements of shear and compression velocities on variety of sandstone under multi-pressure conditions, the validity of proposal is demonstrated.
The present work also suggests a stress dependent matrix shear modulus. It accounts for the changing inter-granular frictional force and grain slippage/rotation tendencies, and varying cement contacts in matrix network. It is concluded that matrix shear modulus is significantly affected under low to moderate stress conditions, while changes are small in well cemented granular rocks. The varying aspect ratio and matrix shear modulus can be represented by uniform power laws and govern the changing velocities. It provides a rock intrinsic view to explain the stress dependent elastic behavior and is found applicable to sandstone with different cementation and porosity.