The second relation consists of the Zoeppritz equations, a set of equations that describes how the incident wave energy encountering an interface becomes partitioned, or divided, between reflected and transmitted waves. This partitioning is governed by the reflection and transmission coefficients, which depend on the impedance contrast across the interface and the incidence angle of the incident wave. The reflection coefficient is the ratio of the amplitude of the reflected wave to that of the incident wave. When the coefficient is zero, there is no reflection, and all incident energy is transmitted across the interface. The positive or negative sign of the reflection coefficient indicates whether the incident wave encounters a lithology of higher or lower impedance across the interface. Nonzero reflection coefficients result in partial reflection and transmission.
Reflection seismic data may be acquired on land or at sea. During land acquisition, seismic data are collected from a grid of seismic receivers deployed on the ground. The seismic source is moved and triggered in a pattern that interweaves with the grid of receivers.
For marine acquisition, sources and streamers, which are arrays of receivers attached to a cable, are deployed off the back of a slowly moving ship; seismic sources are usually in front of the streamers. As the ship moves, the sources fire at regular intervals, and the receivers record the signals. The ship typically traverses a grid pattern covering the survey area.
Both on land and at sea, each receiver records a trace, which represents the amplitude of seismic signal and noise received during the recording time. Because multiple recording devices are activated when the source is triggered, multiple traces are produced.
A seismic record is the collection of traces recorded from a single source point. A record is a section, or cube, of data with distance or geographic location along the horizontal axis, or axes, and recording time along the vertical axis. Each trace is plotted at its receiver location, and its positive and negative deviations from zero indicate amplitude variation. Usually, time, rather than depth, is plotted along the vertical axis. The recording time is two-way traveltime (TWT) because the signal must travel from the surface to the reflector and back up to the receiver on the surface.
The high-frequency content of seismic signals that travel through the Earth experiences natural attenuation. For imaging features in the deep subsurface, geophysicists want to record the lowest possible frequencies, which are the least attenuated and have deep penetration power. To attain accurate vertical resolution of underground features, the survey must also record the highest possible frequencies, which become progressively weaker with distance. Consequently, geophysicists design seismic sources, sensors and surveys to be able to generate and record broadband signals that contain the widest possible range of frequencies.
Sorting and Gathering
The acquired traces may be gathered into several datasets. A common source gather is the collection of seismic traces that have the same source location. Another useful gather is the common midpoint (CMP) gather (Figure 2). A midpoint is the surface location halfway between a source and a receiver location. The number of traces in a CMP gather is the fold. The distance between source and receiver is the offset.