Abstract So far, the complexities of the seismic source and receiver have been discussed. Once a seismic signal is transmitted and received, it must be recorded. The different types of signals discussed in this chapter are defined as follows: (1) Source signal —The pressure field created by the seismic source. (2) Reflectivity signal —The earth's reflection sequence convolved with the source wavelet. (3) Seismic signal —Everything received as a result of the source firing. The seismic signal includes the reflectivity signal as well as ground roll, refractions, diffractions, sideswipe, channel waves, etc. (4) Received signal —The electrical output of the receiver group. This is the seismic signal plus all environmental noise. (5) Recorded signal —The data, that is the instrument-filtered signal plus any additive instrument noise, which go on tape. The information contained in a signal can be characterized by three quantities: signal-to-noise ratio, bandwidth, and duration. Signal-to-noise ratio can have different meanings depending on the circumstances. For example, diffractions from out of the reflection plane are noise on 2-D data but are part of the signal in 3-D surveys. In seismic exploration, the recorded signal bandwidth is usually 0–250 Hz or lower. Often, data are processed in a narrower band, say 5–80 Hz, even though they may be recorded in a broader band. The duration of recorded signals depends on the nature of the source and target depth. Impulsive sources, such as land dynamite or marine air guns, create a source signal with a duration of a few

Abstract Information gathered in the course of seismic exploration and stored on magnetic tape is made up of a mixture of signal and noise in varying proportions. The aim of every geophysicist is to reconstitute records with an increased signal-to-noise ration to simply interpretation. During replay of the magnetic tape, selection is made of the useful signal from other less useful signals and from the noise. This selection is evaluated in the frequency domain. Recordings carried out in marine seismic work illustrate this phenomenon; in order for them to be usable, the spurious effect arising from reasonance of the water layer must be removed. For this purpose the geophusicist resorts to two main procedures at the analog playback center: electrical filtering in the frequency domain and delay-line filering in the time domain. Unlike the former which allow him only passbands, the delay-line filters provide broader possibilities. This kind of equipment can, in addition to the passbands, achieve band-reject filters of different wideth and slopes going from suppression of a fixed frequency, as for example the 60-hz industrial power, to attenuation of a wide frequency band as is the case in deconvolution procedures. Narrow rejection bands can still be regularly spaced over the spectrum allowing for removal of multiples that may exist in land or marine seismic.