Reversed Multi-offset VSP Traveltime Tomography Employing Direct-push Technology for Near-surface Seismic Velocity Imaging

For a wide variety of environmental and engineering geophysical applications reliable knowledge of the seismic velocity distribution in near-surface sediments is essential. We evaluate the suitability of the minimally invasive direct-push technology for rapid 2D traveltime tomography imaging P-wave velocity variations in near-surface unconsolidated sediments. We acquire reversed multi-offset vertical seismic profiles (VSP) employing geophones placed on the earth’s surface and a seismic source temporarily installed in the subsurface using direct-push technology. The 2D seismic velocity distribution is tomographically reconstructed from first arrival traveltimes obtained from a number of closely spaced reversed multi-offset VSP measurements. First, we test this approach in a synthetic study that utilises realistic geometries. This is followed by a field data example measured over alluvial deposits in a former river flood plain. The P-wave velocity model finally obtained contains information about high and low velocity anomalies and structurally matches the results of cone penetration tests (CPT) and gamma radiation data acquired along the profile. Additionally, the method enabled a significantly larger penetration depth compared to a conventional refraction tomography survey.