A comprehensive seismic velocity model for the Netherlands based on lithostratigraphic layers

Abstract

Abstract A seismic velocity model is necessary to map depth and thickness of subsurface layers interpreted from seismic reflection images. We have built a seismic velocity model (VELMOD-1) for the entire Netherlands area, both onshore and offshore, using non-confidential data (sonic logs, time-depth pairs, lithostratigraphic marker depths and downhole position data) of 720 boreholes in DINO - National Geoscientific Portal, and a preliminary isochore map (in seismic traveltime representation) of the layer of the Zechstein Group. The model is based on the V int -z mid method applied to the following lithostratigraphic layers: Lower, Middle and Upper North Sea groups; Chalk Group; Rijnland Group; Schieland, Scruff and Niedersachsen groups; Altena Group; Lower and Upper Germanic Trias groups; Upper Rotliegend Group; and Limburg Group. Per layer, the linear least squares approximation, applied to V int as a function of z mid , provides parameters V 0 and K for a linear velocity function V(z) = V 0 + K · z. In VELMOD-1, K is constant, at least at the scale of structural elements, whereas V 0 varies with location. At borehole locations, V 0 is calibrated such that traveltime through the layer according to the linear velocity model equals the traveltime according to the borehole data. A kriging procedure is applied to the calibrated V 0 (x, y)-values resulting in an estimated V 0 -value at any other location. The model V 0 -values were determined on an areal grid with cells of 1 km × 1 km. On the same grid, kriged interval velocities constitute the model for the Zechstein Group. These interval velocities stem directly from interval velocities at borehole locations; at other positions they are also dependent on the thickness (in terms of seismic traveltime isochores) of the layer of the Zechstein Group. Maps are presented of the distributions of both V 0 and its standard deviation for two layers: that of the Chalk Group and that of the Lower and Upper Germanic Trias groups.