SEISMIC REFRACTION AND SCREENING BY THIN HIGH‐VELOCITY LAYERS *

Abstract

AbstractThe screening effect of thin, relatively shallow high‐velocity layers often presents considerable problems in seismic exploration. Such layers prevent the greater part of the seismic energy from travelling to greater depths and introduce additional refraction arrivals, confusing the seismogram still further.In order to investigate both the screening and refractive properties of high‐velocity layers, scale‐model experiments have been made over a wide range of layer‐thickness/ wavelength ratios (0.05 < d/λ < 2) for suitably chosen material contrasts. The results may be summarised as follows.Refraction arrivals from thin layers in the field may be recognised by their relatively rapid amplitude decay. Furthermore, the “echeloning”‐effect observed for refraction first arrivals may be due to the presence of a (thin) layered structure. Since the apparent refraction velocity varies with d/λ when d/λ < 1, differences between vertical well‐log velocities and velocities observed along the surface may be expected, making time/depth conversion using surface velocity data inaccurate.Transmission of elastic energy may be expected, if anywhere, only near the shotpoint, at small geophone offset, and for relatively thin screens (d/λ < 0.1). The transmitted signal shape is then independent of the layer thickness. This transmitted energy may be registered either in a reflection set‐up with geophones near the shotpoint, or in long‐distance refraction work.Three possibilities are offered for overcoming the screening effect of thin high‐velocity layers: Use longer‐wavelength signalsApply short‐spread reflection shootingApply long‐distance refraction shootingThe experimental results obtained in scale‐model arrangements of such set‐ups confirm the potentialities of these methods.