The usual seismic sound fields we record are due to what we frequently consider to be point sources. As they propagate into the Earth, they radiate in all directions. The normal to the propagating wave front at any given subsurface location points in the direction of what you can think of as a ray. Since the propagation is normally not constrained with regard to direction, this normal can point in any direction consistent with the sound speed in the medium through which the field is propagating. If the normal points upward, it is an upward traveling wave; if the normal points downward, it is a downward traveling wave. Clearly such fields change directions at 45 degrees, and they become purely horizontal waves at 90 degrees.
If we arbitrarily assume that what was recorded only propagated upward, much of the true wavefield will not be properly imaged unless the assumption is true, but it should be clear that this kind of assumption cannot possibly be true. Nevertheless, the assumption that wavefields travel only in the upward direction is a major part of the migration/inversion/imaging algorithm set.
Note that while the Earth permits wave motion in all directions, migration algorithms may or may not be able to handle these motions. Equation 1 summarizes the type of algorithms that are currently used in practice. We will consider each of these types of waves and how they impact the imaging process in the rest of the book.
The measurement of seismic waves is accomplished using a variety of receivers or phones. These devices can measure the velocity of particle motion (accelerometers), pressure changes (marine geophones), and even the two shear waves. Modern OBC data is usually acquired using all four of these devices. Note that, although each class of phone records only data of that type, it also records all waves that converted from one form to another. Thus, unraveling, migrating, or imaging these data is only possible if they are all handled together. This is still a daunting task, even with today's massive computer power. The following list summarizes the type of microphones in use today.
