3D Seismic Traveltime Tomography Validation of a Detailed Subsurface Model: The case study of the Zancara River Basin (Cuenca, Spain)

Abstract

Abstract. A high-resolution seismic tomography survey was acquired to obtain a full 3D P-wave seismic velocity image in the Zancara River Basin (east of Spain). The study area consists of lutites and gypsum from a Neogene sedimentary sequence. A regular and dense grid of 676 shots and 1200 receivers was used to image a 500 x 500 m area of the shallow subsurface. A 240-channel system and a seismic source consisting of an accelerated weight drop, were used in the acquisition. Half million traveltime picks were inverted to provide the 3D velocity model that allowed to resolve the structure up to 120 m depth. The project targeted the geometry of the underground structure with emphasis in defining the lithological contacts but also the presence of cavities and fault/fractures. An extensive drilling campaign provided uniquely tight constraints on the lithology; these included core samples and wireline-log geophysical measurements. The analysis of the well-log data enabled the accurate definition of the lithological boundaries and provided an estimate of the seismic velocity ranges associated to each lithology. The final joint interpreted image reveals a wedge shaped structure consisting of four different lithological units. This study features the necessary key elements to test the traveltime tomographic inversion approach in the high-resolution characterization of the shallow subsurface. In this methodological validation test, traveltime tomography demonstrates to be a powerful tool with a relatively high capacity for imaging in detail the lithological contrasts of evaporitic sequences located at very shallow depths, when integrated with additional geological and geophysical data.