Shallow seismic refraction interpretation with visual interactive ray trace (VIRT) modelling

Abstract

A comparison of first-arrival times obtained by both finite-difference and ray trace modelling demonstrates the applicability of ray tracing to shallow refraction interpretation. Most shallow seismic refraction interpretations are still achieved with conventional time-term methods and those, such as the GRM, that are derived from these methods. In complex conditions, these methods are inaccurate, as many first-arrivals originate from diffracted waves and waves that have laterally penetrated irregular refractors. These processes also affect first-arrival amplitudes, as is demonstrated with an irregular finite-difference model; nevertheless, first-arrival times are still primarily used in shallow refraction interpretation. An interactive ray-trace modelling method, based on two-point ray tracing through a discrete, two-dimensional model with constant velocity compartments and incorporating buried diffraction sources, is used to compute accurate first-arrival-times. This method is designed for checking and refining interpretations in pre-selected areas, rather than as a production tool for routine processing of large refraction data sets. Implementation of the method on a PC using screen displays of first-arrival ray paths, and field and synthetic arrival time data, allows inappropriate arrivals caused by model deficiencies to be recognised and eliminated. Model adjustments are made directly on the screen and are guided by their effect on computed times and the agreement with field times. The improved algorithms and interactive steps of the visual interactive ray trace (VIRT) modelling eliminate instabilities that occur with many automated, ray-based inversion methods and inaccuracies arising from use of oversimplified or inaccurate initial seismic models. This interpretation method is a powerful teaching tool that readily highlights model and data deficiencies and can generate robust models for automated inversion. The VIRT method is applied to re-interpret shallow refraction data from the London-Victoria shallow gold deposit occurring in a major fault zone. The final model obtained with VIRT agreed closely with known geology and showed that the gold mineralisation occurs in a higher-velocity region within the weathered rock above a lower-velocity region in the underlying bedrock.