3D pseudo-seismic imaging of transient electromagnetic data – a feasibility study

We investigate a pseudo-seismic approach based on the so-called inverse Q-transform as an alternative way of processing transient electromagnetic (TEM) data. This technique transforms the diffusive TEM response into that of propagating waves obeying the standard wave-equation. These transformed data can be input into standard seismic migration schemes with the potential of giving higher resolution subsurface images. Such images contain geometrical and qualitative information about the medium but no quantitative results are obtained as in model-based inversion techniques. These reconstructed images can be used directly for geological interpretation or in further constraining possible inversions. We extend the original Q-transform based on an electrical-source formulation to the case of a large-loop TEM source. Moreover, an efficient discrete version of the inverse of this modified Q-transform is presented using a regularization method. Application of this inverse transform to the measured TEM responses gives the corresponding pseudo-seismic data, which are input into a 3D migration scheme. We then use a 3D boundary element type of Kirchhoff migration to ensure high computational efficiency. This proposed method was applied to both synthetic data as well as field measurements taken from an engineering geology survey. The results indicate that the resolution of the TEM data is significantly improved when compared with standard apparent-resistivity plots, demonstrating that higher resolution 3D transient electromagnetic imaging is feasible using this method.