Receiver Design and Characteristics
Published:January 01, 1997
In geophysical exploration, seismic data are acquired by firing an energy source on or near the Earth’s surface and recording the energy reflected back to the surface from the geologic substrata. This chapter discusses the methods by which the geophysicist detects reflected energy on land and offshore. Geo-phones are used by land explorers, and hydrophones are used in the marine exploration industry. The energy detected at the surface contains useful signal but also unwanted noise.
This chapter explains the operation of the receiver phone, how it converts acoustic energy into an electrical signal, and how this signal is passed along cables to recording instruments. Because much of the useful reflected energy is weak compared with geologically generated coherent surface noise (see Chapter 1), the geophysicist often must attempt to enhance the signal level and reduce the coherent noise level. We exploit the fact that the useful signal is arriving almost vertically when compared with the horizontally arriving noise. The signal is enhanced, when compared to the noise, by using many receivers in a geometric pattern that attenuates energy traveling horizontally. Such arrangements are known as arrays.
A complete discussion of array theory requires the reader to have a good knowledge of mathematics; since that is not the intention of this book, the mathematical treatise has been included as Appendix B. In contrast, this chapter describes a simple, practical approach to array design and noise attenuation that the reader can apply to real field problems with the aid of a simple
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.