GeoScienceWorld
Volume

A Handbook for Seismic Data Acquisition in Exploration

By Brian J. Evans

Abstract

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.

  1. Page 1
    Abstract

    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 100Hz.

  2. Page 47
    Abstract

    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

  3. Page 99
    Abstract

    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

  4. Page 159
    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

  5. Page 187
    Abstract

    Accurate positioning of a seismic line is as crucial as having the best possible data quality. Positioning is important for three reasons: (1) many data processing steps require accurate relative source and receiver positions; (2) tying several seismic lines together requires knowledge of where they are relative to one another; and (3) when drilling sites are selected from seismic data they have to be referenced back to an actual location on the Earth’s surface. Of these reasons, perhaps the last is most important: No exploration company wants to spend millions of dollars drilling, only to miss the target because the seismic data were mispositioned.

    Accurate positioning is not a trivial task, especially for marine surveys. Once a seismic vessel has sailed along an intended line, no permanent evidence remains behind to show where the ship actually sailed. Furthermore, at sea, intended shot and receiver positions cannot be identified by markers prior to shooting. Finally, during shooting, both the ship and the trailing equipment are somewhat at the mercy of the wind, currents, and wave action; the position of the shots and receivers cannot, therefore, be controlled accurately. For these reasons, positioning in marine surveys is a so-called real-time activity; that is, position measurements have to be made, recorded, and processed as a line is shot.

    For land seismic surveys, positioning does not have the real-time urgency that it does in marine surveys. The shot and receiver positions can be marked on the ground either before or during the shooting of

  6. Page 221
    Abstract

    Before a survey begins in a new area, there should be a review of the local geology, past seismic surveys and well-log data, and seismic interpretation objectives. A knowledge of such data allows forward planning of the geophysical operations to ensure objectives are met. The local geology will provide information on geologic parameters that will affect the survey, such as the target depth, geologic features of interest, the maximum dip to be expected, and the level of resolution needed to image the target adequately. Past seismic survey data can act as a guide for the success of proposed geophysical parameters. For example, if one proposes to use a source at least as strong as that used in earlier work, and the earlier source successfully illuminated the target, then the proposed source also should succeed. Past survey data also can show where problems existed and how they were overcome and if economical data-quality improvements are feasible. Once the geophysical parameters have been established for a survey, they must not be changed without contractor approval from the client company. This chapter discusses how to determine the field parameters needed to meet the goal of recording the highest quality 2-D field data. The special requirements of 3-D surveys are considered in Chapter 7.

  7. Page 243
    Abstract

    The goal of seismic data acquisition and processing is to provide a realistic image of the subsurface structure of the Earth. Since that structure is three-dimensional, one might suspect that 2-D data acquisition and processing can be inadequate. That is indeed the case: 2-D seismic data can provide a misleading image of the earth’s subsurface, even for the simplest 3-D structures. Figure 161, for example, shows a reflector that has a dip direction perpendicular to the shooting direction of a 2-D seismic line. When the data are processed, the cross dip causes two problems in the image. First, the part of the reflector being imaged is not the part that actually lies vertically under the 2-D line, as an interpreter unaware of the cross dip would assume. Second, the reflector’s depth is incorrect. Both of these problems occur because the actual reflection points do not lie in the vertical plane below the line.

    The solution to these problems is to record 3-D seismic data and to process them using 3-D rather than 2-D imaging algorithms. In 3-D acquisition, the data are collected over a 2-D surface area instead of along 1-D lines. After 3-D processing, reflection events appear as 2-D surfaces rather than as 1-D events. A dipping, reflecting plane such as that in Figure 161 is then correctly imaged.

    French (1974) illustrated the benefits of 3-D versus 2-D seismic methods in a classical modeling study performed at Gulf Research Laboratories. One of French’s models (unpublished) had a normal fault

  8. Page 273
  9. Page 276
  10. Page 279
  11. Page 282

Purchase Chapters

Recommended Reading