3-D seismic travel-time tomography validation of a detailed subsurface model: a case study of the Záncara river basin (Cuenca, Spain)

摘要

A high-resolutionseismic tomography survey was acquired to obtain a full 3-D P-wave seismicvelocity image in the Záncara riverbasin (eastern Spain). The study area consists of lutites and gypsum from aNeogene sedimentary sequence. A regular and dense grid of 676 shots and 1200receivers was used to image a 500?m×500m area of theshallow subsurface. A 240-channel system and a seismic source, consisting ofan accelerated weight drop, were used in the acquisition. Half a milliontravel-time picks were inverted to provide the 3-D seismic velocitydistribution up to 120m depth. The project also targeted the geometry ofthe underground structure with emphasis on defining the lithological contactsbut also the presence of cavities and fault or fractures. An extensivedrilling campaign provided uniquely tight constraints on the lithology; theseincluded core samples and wireline geophysical measurements. The analysis ofthe well log data enabled the accurate definition of the lithologicalboundaries and provided an estimate of the seismic velocity ranges associatedwith each lithology. The final joint interpreted image reveals a wedge-shapedstructure consisting of four different lithological units. This studyfeatures the necessary key elements to test the travel time tomographicinversion approach for the high-resolution characterization of the shallowsubsurface. In this methodological validation test, travel-time tomographydemonstrated to be a powerful tool with a relatively high capacity forimaging in detail the lithological contrasts of evaporitic sequences locatedat very shallow depths, when integrated with additional geological andgeophysical data.