20.1 Creating the synthetic seismogram

A seismic source initiates a wavefield that interacts with the earth. The waves are partially transmitted and will never return to be measured by receivers at the earth's surface. But part of the wavefield is reflected and returns to the surface. The measured field at the surface is the seismic response of the earth at that location. The fundamental problem addressed in seismic modeling, or simulation, is calculation of the seismic traveltime and amplitude response for a given earth model and recording geometry.

We know from earlier chapters that the earth model consists of those physical properties that influence seismic wave propagation. For an elastic earth, these include compressional wavespeed, shear wavespeed, and mass density. For an acoustic earth model, we only need a sound speed and mass density.

All 1D seismic concepts are still present and important in 2D seismic work, which in turn underlay 3D seismic. At the heart of 1D seismic are well logs and the creation of a synthetic seismogram (or simply synthetic) for seismic event identification and depth conversion ^[146]. If a vertical seismic profile is available for a particular well, then a synthetic seismogram is not needed. The VSP gives direct measurement time, depth, and waveform. An alternative to a full synthetic is to convert a sonic log to time and velocity then directly overlay this on the seismic data. A third possibility is to overlay impedance in this way.

The purpose of 1D simulation is to create...