Seismic Energy Sources
Published:January 01, 1997
For many years a single explosive charge was the most often used source of seismic energy. A single charge is an impulsive point source; that is, all of its energy is generated at one time in one location. For small charges, the amount of seismic energy produced per shot can be increased simply by increasing the charge size. However, as years of experimentation have shown, there are diminishing returns of seismic energy as large charges are made even larger. Single impulsive point sources cannot efficiently produce the amount of seismic energy needed to image deeper targets well. Three strategies evolved for overcoming the limitations of an impulsive point source: (1) distribute the source energy in space, (2) distribute the source energy in time, and (3) distribute the source energy in space and time.
A source can be distributed in space by dividing the single large charge into smaller point charges and firing them together in spatial patterns. The resulting source array produces more seismic energy than does its single-charge counterpart. As an additional benefit, the array pattern can be designed to reduce noise problems, just as can be done with a receiver array (see Chapter 2). Primacord is an example of a spatially distributed explosive source.
There are two ways a source can be distributed in time. In the discrete method, a single massive charge is replaced by many small charges that are fired sequentially from a single shotpoint. The resulting data records are stacked to simulate the shot of
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.