oa Non-Uniqueness with Refraction Inversion – The Mt Bulga Shear Zone

Standard implementations of refraction tomography routinely use low resolution starting models. Tomograms obtained from these starting models can frequently fail to detect even major lateral variations in seismic velocities, such as a 50 m wide low velocity shear zone at Mt Bulga.

By contrast, the successful detection of the shear zone is unequivocal in tomograms generated with the generalized reciprocal method (GRM). Further improvements in resolution, which facilitate the definition of additional zones with moderate reductions in seismic velocity, are achieved with a novel application of the Hilbert transform to the GRM refractor velocity analysis algorithm. However, the improved resolution also requires the use of a lower average vertical seismic velocity which accommodates a velocity reversal in the weathering. The lower seismic velocity is derived with the GRM, whereas most refraction tomography programs assume vertical velocity gradients as the default.

Although all of the tomograms are consistent with the traveltime data, the resolution of each tomogram is comparable only with that of the starting model. Refraction tomography rarely, if ever, extracts additional detail which is not apparent in the starting model: it essentially smoothes unrealistic or inconsistent starting models. Therefore, it can be concluded that the major effect of refraction tomography is largely cosmetic.

A comparison of the errors of tomographic inversion does not “prove” that a given result is either “correct” or even geologically reasonable. This study proposes that the three tomograms generated with detailed GRM time and velocity models for the optimum XY value and ± half the station spacing, are a more useful measure of the uncertainties with refraction inversion. Non-uniqueness is most conveniently resolved with refraction attributes derived from the digital field data, before the implementation of more expensive drilling programs.