Velocity computation from checkshot surveys in deviated wells

Abstract

When checkshot surveys are done in deviated wells, two source‐receiver geometries are used. In one, the wave source is moved on the earth’s surface in unison with the horizontal movement of the receiver in the well so that the raypath from the source to the receiver is always vertical. This is the source‐over‐receiver approach. In the other approach, called source‐at‐rig, the source is fixed, usually staying in the vicinity of the drilling rig. The source‐over‐receiver method is usually preferred because the source‐at‐ rig method does not measure vertical traveltimes. If velocity varies with depth, the seismic rays from a source at the rig will bend, and a correction to vertical using the common straight‐ray assumption will not produce accurate vertical traveltimes or interval velocity measurements.