Understanding near-surface velocity effects using physical models

Abstract

One of the problematic areas when using seismic methods is to image geological structure beneath near-surface geophysical anomalies. Scaled physical modelling of geological structure involves the building of elastic 3-D models which are constructed of materials having the physical properties typical of the geological structure they are simulating (Evans et al., 2007). Seismic physical modelling uses materials which can simulate the seismic response through simulation of seismic velocities within an ultrasonic range which scales to the geological structure to be imaged. The density of the material may not scale to that of the rock, but for the most part seismic in general and data processing in particular, uses velocity as the main parameter for imaging structure. In choosing the appropriate simulation parameters, distance to target, time and velocity, are the main parameters to be scaled. Anomalies may be caused by the presence of sand dunes, irregular topography, wet or dry surfaces or sub-surface creeks and wash-outs, surface platforms having different velocities from their base layer and near-surface high/low velocity layering. The surface or near-surface irregularities are caused by all forms of incoherent seismic disturbances such as scattering, diffraction, refractions, seismic interference effects, and energy absorption. Irregular velocities can result in pull-ups and push-downs, and a high-low-high velocity near-surface layering can result in guided waves which swamp any reasonable reflected energy trying to pass through such irregular velocity layering. We provide examples of issues which are difficult to resolve using seismic methods and show their typical seismic signature.