A high-resolution seismic tomography survey was acquired to obtain a full 3-D P-wave seismic velocity image of the Záncara river basin (eastern Spain). The study area consists of lutites and gypsum from a Neogene sedimentary sequence. The project also targeted the geometry of the underground structure with emphasis on defining the lithological contacts but also the presence of cavities and faults or fractures. An extensive drilling campaign provided uniquely tight constraints on the lithology; these included core samples and wireline 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 with each lithology. The final joint interpreted image reveals a wedge-shaped structure consisting of four different lithological units. The study features the necessary key elements to test the travel time tomographic inversion approach for the high-resolution characterization of the shallow surface. In this methodological validation test, travel-time tomography demonstrated 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.
The data acquisition contract was awarded to ENRESA and took place in January 2014. Seismic data was successfully collected in the Záncara river basin (Cuenca, Spain). This is a high-resolution seismic tomography survey to obtain a full 3-D P-wave seismic velocity image of the studied area. A regular and dense grid of 676 shots and 1200 receivers was used to image a 500 m x 500 m area of the shallow surface. A 240-channel system and a seismic source, consisting of an accelerated weight drop, were used in the acquisition. Half a million travel-time picks were inverted to provide the 3-D seismic velocity distribution up to 120 m depth. The 3-D survey was acquired in five swaths, each one consisting on five receiver lines, resulting in a total of 3380 shot gathers. Technical specifications of the profiles: Receiver number: 240, Receiver interval: 2 m, Source accelerated weightdrop: 250 kg, Source interval: 6 m, Sample rate: 1 ms, Record time: 4 s.
In this repository, in addition to the seismic 3DSurvey, a resistivity model (VICANAS_3D_Res_UTM30), a lithological model (VICANAS_3D_Vp_Res_Lito_UTM30), and a training set (VICANAS_Training_set) are available. In order to improve the geological interpretation of the seismic tomography, we integrated it with the resistivity model to build a 3D lithological model. To this aim, we created a new bi-parameter grid with Vp and Res values at each node. Then, we lithologically classified the nodes using supervised learning according to a training set extracted from the wells.
Convenio Colaboración CSIC-ENRESA. Código CSIC: 20133830