Is VIRT an Efficacious Strategy for Refraction Inversion?

Abstract

Visual interactive ray trace (VIRT) inversion is a manual approach to refraction tomography. VIRT tomograms do not detect a major 50 m wide shear zone with a low seismic velocity at Mt Bulga. This failure is attributed to the probable use of a low resolution starting model, specifically the smooth velocity gradient wavepath eikonal traveltime (WET) tomogram, for the VIRT inversion. In this case, the low resolution of the VIRT tomogram is another demonstration of non-uniqueness. Alternatively, a starting model has been generated with the conventional reciprocal method, in which the existence of the low velocity region is unequivocal. In this case, confirmation bias has been employed to remove any expression of the low velocity region. VIRT tomography generates complex velocity models of the weathering from relatively small numbers of traveltimes, indicating that VIRT is overfitting those data. The extensive use of vertical interfaces across which there are large contrasts in seismic velocities is not consistent with standard models of normal weathering profiles. By contrast, VIRT generates simple velocity models in the sub-weathering from many traveltimes, indicating that VIRT is underfitting those traveltimes. Refraction tomograms produced with WET tomography using smoothed and detailed starting models generated with the generalized reciprocal method (GRM) have smaller errors than tomograms obtained with VIRT tomography and with WET tomograms generated with VIRT starting models. VIRT neither improves the accuracy nor the geological verisimilitude of refraction tomograms. Furthermore, VIRT is time consuming and subjective. Although technically, VIRT is efficacious, the alternatives of automatic refraction tomography are more practical, more accurate, and generate more useful tomograms. In the final analysis however, VIRT is simply outdated.