SEAM Update

Abstract

A special challenge of land seismic exploration is dealing with elastic surface waves at the Earth-air interface. These waves (commonly called “ground roll”) are induced by free-surface boundary conditions that hold when an elastic solid (Earth) is in contact with free space (air, effectively) and are generally the slowest decaying and slowest moving events on shot gathers. The synthetic seismograms in Figure 1 nicely illustrate these features of surface waves. Taken from a finite-difference computation using the SEAM geologic model of a midcontinent basin containing shale-gas reservoirs—the “unconventional” model (see the June 2012 issue of TLE)—the traces show the vertical particle velocity recorded by a symmetrical spread of receivers flanking a vertical point-force source acting on the surface.