Is visual interactive ray trace an efficacious strategy for refraction inversion?
Derecke PalmerSchool of Biological, Earth and Environmental Sciences, The University of New South Wales, Sydney 2052, Australia. Email: d.palmer@unsw.edu.au
Exploration Geophysics 41(4) 260-267 https://doi.org/10.1071/EG09028
Submitted: 10 June 2009 Accepted: 27 October 2010 Published: 15 December 2010
Abstract
Visual interactive ray trace (VIRT) inversion is a manual approach to refraction tomography. VIRT tomograms neither detect nor define a major 50 m wide 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 the ubiquity of non-uniqueness.
Alternatively, the conventional reciprocal method has been used to generate a starting model, 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 in the VIRT tomogram.
By contrast, WET refraction tomograms produced with smooth and detailed starting models generated with the generalised reciprocal method (GRM) clearly define the 50 m wide zone with the low seismic velocities. The low velocity zone is confirmed with a priori information, specifically the inverted head wave amplitudes and a spectral analysis of the refraction convolution section, and by a posteriori information, specifically numerous closely spaced orthogonal refraction profiles. Furthermore, the GRM tomograms have smaller misfit errors than the tomograms obtained with VIRT tomography and with WET tomograms generated with VIRT starting models.
VIRT tomography generates complex velocity models of the weathering from relatively small numbers of traveltimes, indicating that VIRT is overfitting the 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, nor is it indicated in the traveltime graphs. By contrast, VIRT generates simple velocity models in the sub-weathering from many traveltimes, indicating that VIRT is underfitting those traveltimes.
VIRT neither improves the accuracy nor the geological verisimilitude of refraction tomography. Furthermore, VIRT is time consuming, subjective, and in the final analysis, simply outdated. Although technically, VIRT is efficacious, the alternatives of automatic refraction tomography are more practical, more accurate, and generate more useful tomograms.
Key words: GRM, inversion, non-uniqueness, refraction, tomography, VIRT.
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