| The energy demand of the world is increasing every day, forcing the oil industry to explore for oil hydrocarbon traps in locations where the problem complexity requires special imaging techniques. Dealing with complex near-surface structures is the most challenging problem for land seismic exploration. The seismic waves are often distorted and dispersed by extremely low and rapidly varying velocities, as well as poor or inconsistent coupling amongst shots and receivers. It is not uncommon when the near-surface problem degrades the surface reflection data to a useless level. To treat the problem, I employ a first-arrival seismic tomography to model the near-surface velocity structures. The removal of the induced near-surface distortions on seismic data is based on two principles: layer stripping and surface consistency.; Though this approach has been taken by previous workers, our method has several unique features. First, the subsurface is parameterized by a grid of irregularly shaped blocks that mimic the topography and help to reduce the number of model variables. Second, a multi-scale inversion is employed that has been shown to give superior results than the conventional single-scale tomography. Third, the initial reference model is established based on the actual travel-time data, hence greatly reduces the concern for picking a wrong initial model. The accuracy of the first-arrival tomography is verified using various synthetic models simulating the complexity of real near-surface conditions. The method is applied to three field 2-D data sets in very different regions: a land data set from a mountainous thrust area in South America, a marine data set from the Gulf of Mexico, and a long-offset data set from the Tarim Basin in China.; In addition, I attempt to implement reflection tomography as an alternative to conventional velocity analysis. Using reflection seismic tomography, we invert for velocity-depth models, which can be used for imaging or time processing. Finally, major limitations of first-arrival tomography and reflection seismic tomography are addressed. |
