Comparison of shallow seismic refraction interpretation methods for regolith mapping

Abstract

A number of shallow refraction interpretation methods are compared in variable regolith conditions using synthetic and published field data. The synthetic model contained a low velocity zone in a depression at the base of the regolith. Independent interpretation with the Reciprocal Method was in reasonable with this model. The Generalised Reciprocal Method performed poorly on this model, both smoothing and considerably underestimating the depth to the simulated regolith base and greatly narrowing the low velocity zone. However, neither the Reciprocal nor Generalised Reciprocal Methods produced valid velocity analyses over the low velocity zone, as diffracted and non-critically refracted wave arrivals are used. Wavefront Eikonal Traveltime Tomography identified the rapid thickening of the regolith over the depression, but introduced an artefact near the low velocity zone, and the regolith base was not easily located. Low wave path densities over the depression identified by this method also indicated that the interpretation should be treated with caution. The field example was over a variable regolith with a faulted contact between rocks of differing weathering characteristics. Visual Interactive Ray Tracing and Wavefront Eikonal Traveltime Tomographic interpretations were in good general agreement for this example. These interpretations differed considerably from the original Generalised Reciprocal Method interpretation that contained a wide low velocity zone at the contact. This is likely to be an artefact of the Generalised Reciprocal Method interpretation process. While our comparisons are not definitive and all the interpretation methods that were compared have deficiencies and limitations they do offer some guidance to improving shallow refraction interpretation for regolith mapping. This is achieved by combining some of the methods and involves the Reciprocal Method, Wavepath Eikonal Traveltime Tomography, and Visual Interactive Ray Tracing. Interesting subsurface features and limitations are highlighted by joint use of ray path displays and wave path density diagrams together with various statistical goodness-of-fit measures to the field data. While this interpretation approach should be more robust, it does not eliminate personal bias nor overcome inherent limitations in the shallow seismic refraction method for regolith mapping, such as the delineation of laterally hidden low velocity zones.