Depth imaging of elastic wavefields - where P meets S
Brian H. Hoffe, Laurence R. Lines
The multicomponent recording of reflected elastic wavefields allows for an analysis of compressional and shear wave velocities. For seismic arrivals initiated by dynamite sources and reflected from a layered sedimentary system, it is natural to consider both P and P-S reflections as recorded on vertical and in-line components of the geophones. Since P-wave and S-wave velocities will differ (roughly by a factor of 2), it is an interpretative art to correlate the P-wave reflections with P-S reflections. In an attempt to correlate or "tie" these reflections, time sections are often stretched and squeezed to match reflection positions. However, the domain in which these reflectors should naturally tie is the depth domain. Depth images for the P-P and P-S reflections can be obtained using depth migration. We illustrate this method of reflector imaging via poststack depth migration "where P meets S" on a data set from the Blackfoot field.
The processed P-P and P-S structure stacks for the 20 m receiver interval data were depth migrated via a reverse-time algorithm using P and S velocities measured in a well directly adjacent to the seismic profile. These depth migrated sections show good correlation over the well interval where velocities were measured. Thus migration of both vertical and converted wave stacked data to the depth domain may provide a much better means of correlation than the current practice of correlating in time which is rather cumbersome and degrades the quality of the data being compared.