Structurally coupled inversion of ERT and refraction seismic data combined with cluster-based model integration

Abstract

Electrical resistivity tomography (ERT) and refraction seismics are among the most frequently used geophysical methods for site-investigations and the combined results can be very helpful to fill in the gaps between the point measurements made by traditional geotechnical methods such as Cone Penetration Test (CPT), core-drilling and geophysical borehole logging. The interpretation of the results from a geophysical investigation constituting a single method often yields ambiguous results. Hence, an approach utilizing multiple techniques is often necessary. To facilitate interpretation of such a combined dataset, we propose a more controlled and objective approach and present a method for a structurally coupled inversion of 2D electrical resistivity and refraction seismic data using unstructured meshes. Mean shift clustering is used to combine the two images and to compare the separate and coupled inversion methodologies. Two synthetic examples are used to demonstrate the method, and a field-data example is included as a proof of concept. In all cases a significant improvement by the coupling is visible. The methodology can be used as a tool for improved data interpretation and for obtaining a more comprehensive and complete picture of the subsurface by combining geophysical methods.

Geophysical pre‐investigation for a Stockholm tunnel project: joint inversion and interpretation of geoelectric and seismic refraction data in an urban environment

“…The influence of the coupling parameters (a, b, and c) is shown in detail by Hellman et al . (). The roughness with its entries contains the structural information (gradients) of a model.…”

“…The coverage is calculated as the summation of all sensitivity values assigned to a model cell and is closely connected to the model resolution, which is also based on the sensitivity distribution as shown by Ronczka et al . (). For ERT, coverage‐based thresholds are defined to separate regions of high, medium, and low reliability, which are not, partially or completely faded out when presenting inversion results.…”

“…Flow‐chart of the joint inversion (after Hellman et al ) with the starting models of the individual inversions and , their models after n iterations and , the starting model of the joint inversion and , and the final models and . Hereby, denotes the resistivity distribution and, v, the velocity distribution.…”

“…A more detailed description of the algorithm and extensive tests on synthetic cases were finally given by Hellman et al . (). They also investigated the influence of the different parameters in the coupling equation, which is used for calculating the elements of the weighting matrix.…”

“…A successful application of this approach for an underwater survey was presented by Ronczka et al . (). They show that a combination of geophysical methods increases the reliability and reduces the ambiguity of the results and, thus, eases the geological interpretation.…”