This work presents the evaluation effectiveness of the GPR tomography for characterization of square voids in engineering structures. One of the columns of the iconic main building of the Moscow State University was selected as a target object with known configuration. Given that the heterogeneity of the column complicates the interpretation of GPR data acquired in reflection mode, transmission tomographic ray coverage was employed. Mathematical modelling and tomographic inversion to locate real objects were carried out. The radar tomography and the common-offset GPR geometry techniques are compared with respect to the acquired information about location, shape and dimensions of the void inside the column. The tomography results are characterized by high precision and are more reliable compared to the results of single-fold GPR survey.