Analysis of seismic diffractions from carbonate nodules in clay formations

We study seismic detectability, response and location of carbonate nodules in the Boom Clay Formation, one of the potential plastic clays in Belgium to be used as a host rock for radioactive waste disposal. This formation typically contains distinct horizons of carbonate-rich concretions, named septaria, which are characterized by the presence of internal cracks that are normally filled with water. High-resolution identification of the septaria dimensions and distribution is crucial for excavation planning and safety assessment. We perform diffraction analysis with seismic data of two distinct surveys, using high-frequency reverse vertical seismic profiles (RVSP) in a shallow well and crosswell geometry, respectively. To support the interpretation of diffractions after wavefield separation, we calculate full-wave propagation in synthetic models. These are based on the borehole seismic velocity profiles, filled with different distributions of septaria nodules at relevant depths. We present preliminary coherency results obtained by signal focusing and MUltiple SIgnal Classification (MUSIC). The approach is based on singular value decomposition to better detect and resolve the septaria location. Depending on S/N conditions, the use of semblance may be also appropriate. After calibration of the coherency approach with synthetic data, we test the 2D application with field-recorded signals, to image selected diffraction points.