Published:January 01, 1997
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:
Source signal—The pressure field created by the seismic source.
Reflectivity signal—The earth's reflection sequence convolved with the source wavelet.
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.
Received signal—The electrical output of the receiver group. This is the seismic signal plus all environmental noise.
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
Figures & Tables
A Handbook for Seismic Data Acquisition in Exploration
The science of seismology began with the study of naturally occurring earthquakes. Seismologists at first were motivated by the desire to undetand the destructive nature of large earthquakes. They soon learned, however, that the seismic waves produced by an earthquake contained valuable information about the large-scale structure of the Earth’s interior.
Today, much of our understanding of the Eart’s mantle, crust, and core is based on the analysis of the seismic waves produced by earthquakes. Thus, seismology became an important branch of geophysics, the physics of the Earth.
Seismologists and geologists also discovered that similar, but much weaker, man-made seismic waves had a more practical use: They could probe the very shallow structure of the Earth to help locate its mineral, water, and hydrocarbon resources. Thus, the seismic exploration industry was born, and the seismologists working in that industry came to be called exploration geo-physicists. Today seismic exploration encompasses more than just the search for resources. Seismic technology is used in the search for waste-disposal sites, in determining the stability of the ground under proposed industrial facilities, and even in archaeological investigations. Nevertheless, since hydrocarbon exploration is still the reason for the existence of the seismic exploration industry, the methods and terminology explained in this book are those commonly used in the oil and natural gas exploration industry.
The underlying concept of seismic exploration is simple. Man-made seismic waves are just sound waves (also called acoustic waves) with frequencies typically ranging from about 5 Hz to just over 100 Hz. (The lowest sound frequency audible to the human ear is about 30 Hz.) As these sound waves leave the seismic source and travel downward into the Earth, they encounter changes in the Earth’s geological layering, which cause echoes (or reflections) to travel upward to the surface. Electromechanical transducers (geophones or hydrophones) detect the echoes arriving at the surface and convert them into electrical signals, which are then amplified, filtered, digitized, and recorded. The recorded seismic data usually undergo elaborate processing by digital computers to produce images of the earth’s shallow structure. An experienced geologist or geophysicist can interpret those images to determine what type of rocks they represent and whether those rocks might contain valuable resources.