An Introduction to Geophysical Exploration E-Book

Philip Kearey

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 591

Veröffentlichungsjahr: 2013

Beschreibung

This new edition of the well-established Kearey and Brooks text is fully updated to reflect the important developments in geophysical methods since the production of the previous edition. The broad scope of previous editions is maintained, with even greater clarity of explanations from the revised text and extensively revised figures. Each of the major geophysical methods is treated systematically developing the theory behind the method and detailing the instrumentation, field data acquisition techniques, data processing and interpretation methods. The practical application of each method to such diverse exploration applications as petroleum, groundwater, engineering, environmental and forensic is shown by case histories.

The mathematics required in order to understand the text is purposely kept to a minimum, so the book is suitable for courses taken in geophysics by all undergraduate students. It will also be of use to postgraduate students who might wish to include geophysics in their studies and to all professional geologists who wish to discover the breadth of the subject in connection with their own work.

Leseprobe

Contents

Preface

1 The principles and limitations of geophysical exploration methods

1.1 Introduction

1.2 The survey methods

1.3 The problem of ambiguity in geophysical interpretation

1.4 The structure of the book

2 Geophysical data processing

2.1 Introduction

2.2 Digitization of geophysical data

2.3 Spectral analysis

2.4 Waveform processing

2.5 Digital filtering

2.6 Imaging and modelling

Further reading

3 Elements of seismic surveying

3.1 Introduction

3.2 Stress and strain

3.3 Seismic waves

3.4 Seismic wave velocities of rocks

3.5 Attenuation of seismic energy along ray paths

3.6 Ray paths in layered media

3.7 Reflection and refraction surveying

3.8 Seismic data acquisition systems

Further reading

4 Seismic reflection surveying

4.1 Introduction

4.2 Geometry of reflected ray paths

4.3 The reflection seismogram

4.4 Multichannel reflection survey design

4.5 Time corrections applied to seismic traces

4.6 Static correction

4.7 Velocity analysis

4.8 Filtering of seismic data

4.9 Migration of reflection data

4.10 3D seismic reflection surveys

4.11 Three component (3C) seismic reflection surveys

4.12 4D seismic reflection surveys

4.13 Vertical seismic profiling

4.14 Interpretation of seismic reflection data

4.15 Single-channel marine reflection profiling

4.16 Applications of seismic reflection surveying

Further reading

5 Seismic refraction surveying

5.1 Introduction

5.2 Geometry of refracted ray paths: planar interfaces

5.3 Profile geometries for studying planar layer problems

5.4 Geometry of refracted ray paths: irregular (non-planar) interfaces

5.5 Construction of wavefronts and ray-tracing

5.6 The hidden and blind layer problems

5.7 Refraction in layers of continuous velocity change

5.8 Methodology of refraction profiling

5.9 Other methods of refraction surveying

5.10 Seismic tomography

5.11 Applications of seismic refraction surveying

Further reading

6 Gravity surveying

6.1 Introduction

6.2 Basic theory

6.3 Units of gravity

6.4 Measurement of gravity

6.5 Gravity anomalies

6.6 Gravity anomalies of simple-shaped bodies

6.7 Gravity surveying

6.8 Gravity reduction

6.9 Rock densities

6.10 Interpretation of gravity anomalies

6.11 Elementary potential theory and potential field manipulation

6.12 Applications of gravity surveying

Further reading

7 Magnetic surveying

7.1 Introduction

7.2 Basic concepts

7.3 Rock magnetism

7.4 The geomagnetic field

7.5 Magnetic anomalies

7.6 Magnetic surveying instruments

7.7 Ground magnetic surveys

7.8 Aeromagnetic and marine surveys

7.9 Reduction of magnetic observations

7.10 Interpretation of magnetic anomalies

7.11 Potential field transformations

7.12 Applications of magnetic surveying

Further reading

8 Electrical surveying

8.1 Introduction

8.2 Resistivity method

8.3 Induced polarization (IP) method

8.4 Self-potential (SP) method

Further reading

9 Electromagnetic surveying

9.1 Introduction

9.2 Depth of penetration of electromagnetic fields

9.3 Detection of electromagnetic fields

9.4 Tilt-angle methods

9.5 Phase measuring systems

9.6 Time-domain electromagnetic surveying

9.7 Non-contacting conductivity measurement

9.8 Airborne electromagnetic surveying

9.9 Interpretation of electromagnetic data

9.10 Limitations of the electromagnetic method

9.11 Telluric and magnetotelluric field methods

9.12 Ground-penetrating radar

9.13 Applications of electromagnetic surveying

Further reading

10 Radiometric surveying

10.1 Introduction

10.2 Radioactive decay

10.3 Radioactive minerals

10.4 Instruments for measuring radioactivity

10.5 Field surveys

10.6 Example of radiometric surveying

Further reading

11 Geophysical borehole logging

11.1 Introduction to drilling

11.2 Principles of well logging

11.3 Formation evaluation

11.4 Resistivity logging

11.5 Induction logging

11.6 Self-potential logging

11.7 Radiometric logging

11.8 Sonic logging

11.9 Temperature logging

11.10 Magnetic logging

11.11 Gravity logging

Further reading

Appendix: SI, c.g.s. and Imperial (customary USA) units and conversion factors

References

Supplemental images

Index

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Preface

This book provides a general introduction to the most important methods of geophysical exploration. These methods represent a primary tool for investigation of the subsurface and are applicable to a very wide range of problems. Although their main application is in prospecting for natural resources, the methods are also used, for example, as an aid to geological surveying, as a means of deriving information on the Earth’s internal physical properties, and in engineering or archaeological site investigations. Consequently, geophysical exploration is of importance not only to geophysicists but also to geologists, physicists, engineers and archaeologists. The book covers the physical principles, methodology, interpretational procedures and fields of application of the various survey methods. The main emphasis has been placed on seismic methods because these represent the most extensively used techniques, being routinely and widely employed by the oil industry in prospecting for hydrocarbons. Since this is an introductory text we have not attempted to be completely comprehensive in our coverage of the subject. Readers seeking further information on any of the survey methods described should refer to the more advanced texts listed at the end of each chapter.

We hope that the book will serve as an introductory course text for students in the above-mentioned disciplines and also as a useful guide for specialists who wish to be aware of the value of geophysical surveying to their own disciplines. In preparing a book for such a wide possible readership it is inevitable that problems arise concerning the level of mathematical treatment to be adopted. Geophysics is a highly mathematical subject and, although we have attempted to show that no great mathematical expertise is necessary for a broad understanding of geophysical surveying, a full appreciation of the more advanced data processing and interpretational techniques does require a reasonable mathematical ability. Our approach to this problem has been to keep the mathematics as simple as possible and to restrict full mathematical analysis to relatively simple cases. We consider it important, however, that any user of geophysical surveying should be aware of the more advanced techniques of analysing and interpreting geophysical data since these can greatly increase the amount of useful information obtained from the data. In discussing such techniques we have adopted a semiquantitative or qualitative approach which allows the reader to assess their scope and importance, without going into the details of their implementation.

Earlier editions of this book have come to be accepted as the standard geophysical exploration textbook by numerous higher educational institutions in Britain, North America, and many other countries. In the third edition, we have brought the content up to date by taking account of recent developments in all the main areas of geophysical exploration. We have extended the scope of the seismic chapters by including new material on three-component and 4D reflection seismology, and by providing a new section on seismic tomography. We have also widened the range of applications of refraction seismology considered, to include an account of engineering site investigation.

1 The principles and limitations of geophysical exploration methods

1.1 Introduction

This chapter is provided for readers with no prior knowledge of geophysical exploration methods and is pitched at an elementary level. It may be passed over by readers already familiar with the basic principles and limitations of geophysical surveying.

The science of geophysics applies the principles of physics to the study of the Earth. Geophysical investigations of the interior of the Earth involve taking measurements at or near the Earth’s surface that are influenced by the internal distribution of physical properties. Analysis of these measurements can reveal how the physical properties of the Earth’s interior vary vertically and laterally.

By working at different scales, geophysical methods may be applied to a wide range of investigations from studies of the entire Earth (global geophysics; e.g. Kearey & Vine 1996) to exploration of a localized region of the upper crust for engineering or other purposes (e.g. Vogelsang 1995, McCann et al. 1997). In the geophysical exploration methods (also referred to as geophysical surveying) discussed in this book, measurements within geographically restricted areas are used to determine the distributions of physical properties at depths that reflect the local subsurface geology.

An alternative method of investigating subsurface geology is, of course, by drilling boreholes, but these are expensive and provide information only at discrete locations. Geophysical surveying, although sometimes prone to major ambiguities or uncertainties of interpretation, provides a relatively rapid and cost-effective means of deriving areally distributed information on subsurface geology. In the exploration for subsurface resources the methods are capable of detecting and delineating local features of potential interest that could not be discovered by any realistic drilling programme. Geophysical surveying does not dispense with the need for drilling but, properly applied, it can optimize exploration programmes by maximizing the rate of ground coverage and minimizing the drilling requirement. The importance of geophysical exploration as a means of deriving subsurface geological information is so great that the basic principles and scope of the methods and their main fields of application should be appreciated by any practising Earth scientist. This book provides a general introduction to the main geophysical methods in widespread use.

1.2 The survey methods

There is a broad division of geophysical surveying methods into those that make use of natural fields of the Earth and those that require the input into the ground of artificially generated energy. The natural field methods utilize the gravitational, magnetic, electrical and electromagnetic fields of the Earth, searching for local perturbations in these naturally occurring fields that may be caused by concealed geological features of economic or other interest. Artificial source methods involve the generation of local electrical or electromagnetic fields that may be used analogously to natural fields, or, in the most important single group of geophysical surveying methods, the generation of seismic waves whose propagation velocities and transmission paths through the subsurface are mapped to provide information on the distribution of geological boundaries at depth. Generally, natural field methods can provide information on Earth properties to significantly greater depths and are logistically more simple to carry out than artificial source methods. The latter, however, are capable of producing a more detailed and better resolved picture of the subsurface geology.

Several geophysical surveying methods can be used at sea or in the air. The higher capital and operating costs associated with marine or airborne work are offset by the increased speed of operation and the benefit of being able to survey areas where ground access is difficult or impossible.

Table 1.1 Geophysical methods.

Method

Measured parameter

Operative physical property

Seismic

Travel times of reflected/refracted seismic waves

Density and elastic moduli, which determine the propagation velocity of seismic waves

Gravity

Spatial variations in the strength of the gravitational field of the Earth

Density

Magnetic

Spatial variations in the strength of the geomagnetic field

Magnetic susceptibility and remanence

Electrical

   Resistivity

Earth resistance

Electrical conductivity

   Induced polarization

Polarization voltages or frequency-dependent ground resistance

Electrical capacitance

   Self-potential

Electrical potentials

Electrical conductivity

   Electromagnetic

Response to electromagnetic radiation

Electrical conductivity and inductance

   Radar

Travel times of reflected radar pulses

Dielectric constant

A wide range of geophysical surveying methods exists, for each of which there is an ‘operative’ physical property to which the method is sensitive. The methods are listed in Table 1.1.

The type of physical property to which a method responds clearly determines its range of applications. Thus, for example, the magnetic method is very suitable for locating buried magnetite ore bodies because of their high magnetic susceptibility. Similarly, seismic or electrical methods are suitable for the location of a buried water table because saturated rock may be distinguished from dry rock by its higher seismic velocity and higher electrical conductivity.

Other considerations also determine the type of methods employed in a geophysical exploration programme. For example, reconnaissance surveys are often carried out from the air because of the high speed of operation. In such cases the electrical or seismic methods are not applicable, since these require physical contact with the ground for the direct input of energy.

Geophysical methods are often used in combination. Thus, the initial search for metalliferous mineral deposits often utilizes airborne magnetic and electromagnetic surveying. Similarly, routine reconnaissance of continental shelf areas often includes simultaneous gravity, magnetic and seismic surveying. At the interpretation stage, ambiguity arising from the results of one survey method may often be removed by consideration of results from a second survey method.

Geophysical exploration commonly takes place in a number of stages. For example, in the offshore search for oil and gas, an initial gravity reconnaissance survey may reveal the presence of a large sedimentary basin that is subsequently explored using seismic methods. A first round of seismic exploration may highlight areas of particular interest where further detailed seismic work needs to be carried out.

The main fields of application of geophysical surveying, together with an indication of the most appropriate surveying methods for each application, are listed in Table 1.2.

Exploration for hydrocarbons, for metalliferous minerals and environmental applications represents the main uses of geophysical surveying. In terms of the amount of money expended annually, seismic methods are the most important techniques because of their routine and widespread use in the exploration for hydrocarbons. Seismic methods are particularly well suited to the investigation of the layered sequences in sedimentary basins that are the primary targets for oil or gas. On the other hand, seismic methods are quite unsuited to the exploration of igneous and metamorphic terrains for the near-surface, irregular ore bodies that represent the main source of metalliferous minerals. Exploration for ore bodies is mainly carried out using electromagnetic and magnetic surveying methods.

In several geophysical survey methods it is the local variation in a measured parameter, relative to some normal background value, that is of primary interest. Such variation is attributable to a localized subsurface zone of distinctive physical property and possible geological importance. A local variation of this type is known as a geophysical anomaly. For example, the Earth’s gravitational field, after the application of certain corrections, would everywhere be constant if the subsurface were of uniform density. Any lateral density variation associated with a change of subsurface geology results in a local deviation in the gravitational field. This local deviation from the otherwise constant gravitational field is referred to as a gravity anomaly.

Table 1.2 Geophysical surveying applications.

Application

Appropriate survey methods*

Exploration for fossil fuels (oil, gas, coal)

S, G, M, (EM)

Exploration for metalliferous mineral deposits

M, EM, E, SP, IP, R

Exploration for bulk mineral deposits (sand and gravel)

S, (E), (G)

Exploration for underground water supplies

E, S, (G), (Rd)

Engineering/construction site investigation

E, S, Rd. (G), (M)

Archaeological investigations

Rd, E, EM, M, (S)

* G, gravity; M, magnetic; S, seismic; E, electrical resistivity; SP, self-potential; IP, induced polarization; EM, electromagnetic; R, radiometric; Rd, ground-penetrating radar. Subsidiary methods in brackets.

Although many of the geophysical methods require complex methodology and relatively advanced mathematical treatment in interpretation, much information may be derived from a simple assessment of the survey data. This is illustrated in the following paragraphs where a number of geophysical surveying methods are applied to the problem of detecting and delineating a specific geological feature, namely a salt dome. No terms or units are defined here, but the examples serve to illustrate the way in which geophysical surveys can be applied to the solution of a particular geological problem.

Salt domes are emplaced when a buried salt layer, because of its low density and ability to flow, rises through overlying denser strata in a series of approximately cylindrical bodies. The rising columns of salt pierce the overlying strata or arch them into a domed form. A salt dome has physical properties that are different from the surrounding sediments and which enable its detection by geophysical methods. These properties are: (1) a relatively low density; (2) a negative magnetic susceptibility; (3) a relatively high propagation velocity for seismic waves; and (4) a high electrical resistivity (specific resistance).

1. The relatively low density of salt with respect to its surroundings renders the salt dome a zone of anomalously low mass. The Earth’s gravitational field is perturbed by subsurface mass distributions and the salt dome therefore gives rise to a gravity anomaly that is negative with respect to surrounding areas. Figure 1.1 presents a contour map of gravity anomalies measured over the Grand Saline Salt Dome in east Texas, USA. The gravitational readings have been corrected for effects which result from the Earth’s rotation, irregular surface relief and regional geology so that the contours reflect only variations in the shallow density structure of the area resulting from the local geology. The location of the salt dome is known from both drilling and mining operations and its subcrop is indicated. It is readily apparent that there is a well-defined negative gravity anomaly centred over the salt dome and the circular gravity contours reflect the circular outline of the dome. Clearly, gravity surveys provide a powerful method for the location of features of this type.

2. A less familiar characteristic of salt is its negative magnetic susceptibility, full details of which must be deferred to Chapter 7. This property of salt causes a local decrease in the strength of the Earth’s magnetic field in the vicinity of a salt dome. Figure 1.2 presents a contour map of the strength of the magnetic field over the Grand Saline Salt Dome covering the same area as Fig. 1.1. Readings have been corrected for the large-scale variations of the magnetic field with latitude, longitude and time so that, again, the contours reflect only those variations resulting from variations in the magnetic properties of the subsurface. As expected, the salt dome is associated with a negative magnetic anomaly, although the magnetic low is displaced slightly from the centre of the dome. This example illustrates that salt domes may be located by magnetic surveying but the technique is not widely used as the associated anomalies are usually very small and therefore difficult to detect.

3. Seismic rays normally propagate through salt at a higher velocity than through the surrounding sediments. A consequence of this velocity difference is that any seismic energy incident on the boundary of a salt body is partitioned into a refracted phase that is transmitted through the salt and a reflected phase that travels back through the surrounding sediments (Chapter 3). These two seismic phases provide alternative means of locating a concealed salt body.

Fig. 1.1 The gravity anomaly over the Grand Saline Salt Dome, Texas, USA (contours in gravity units — see Chapter 6). The stippled area represents the subcrop of the dome. (Redrawn from Peters & Dugan 1945.)

Fig. 1.2 Magnetic anomalies over the Grand Saline Salt Dome, Texas, USA (contours in nT — see Chapter 7). The stippled area represents the subcrop of the dome. (Redrawn from Peters & Dugan 1945.)

For a series of seismic rays travelling from a single shot point into a fan of seismic detectors (see Fig. 5.21), rays transmitted through any intervening salt dome will travel at a higher average velocity than in the surrounding medium and, hence, will arrive relatively early at the recording site. By means of this ‘fan-shooting’ it is possible to delineate sections of ground which are associated with anomalously short travel times and which may therefore be underlain by a salt body.

An alternative, and more effective, approach to the seismic location of salt domes utilizes energy reflected off the salt, as shown schematically in Fig. 1.3. A survey configuration of closely-spaced shots and detectors is moved systematically along a profile line and the travel times of rays reflected back from any subsurface geological interfaces are measured. If a salt dome is encountered, rays reflected off its top surface will delineate the shape of the concealed body.

Fig. 1.3 (a) Seismic reflection section across a buried salt dome (courtesy Prakla-Seismos GmbH). (b) Simple structural interpretation of the seismic section, illustrating some possible ray paths for reflected rays.

Fig. 1.4 Perturbation of telluric currents over the Haynesville Salt Dome, Texas, USA (for explanation of units see Chapter 9). The stippled area represents the subcrop of the dome. (Redrawn from Boissonas & Leonardon 1948.)

4. Earth materials with anomalous electrical resistivity may be located using either electrical or electromagnetic geophysical techniques. Shallow features are normally investigated using artificial field methods in which an electrical current is introduced into the ground and potential differences between points on the surface are measured to reveal anomalous material in the subsurface (Chapter 8). However, this method is restricted in its depth of penetration by the limited power that can be introduced into the ground. Much greater penetration can be achieved by making use of the natural Earth currents (telluric currents) generated by the motions of charged particles in the ionosphere. These currents extend to great depths within the Earth and, in the absence of any electrically anomalous material, flow parallel to the surface. A salt dome, however, possesses an anomalously high electrical resistivity and electric currents preferentially flow around and over the top of such a structure rather than through it. This pattern of flow causes distortion of the constant potential gradient at the surface that would be associated with a homogeneous subsurface and indicates the presence of the high-resistivity salt. Figure 1.4 presents the results of a telluric current survey of the Haynesville Salt Dome, Texas, USA. The contour values represent quantities describing the extent to which the telluric currents are distorted by subsurface phenomena and their configuration reflects the shape of the subsurface salt dome with some accuracy.

1.3 The problem of ambiguity in geophysical interpretation

If the internal structure and physical properties of the Earth were precisely known, the magnitude of any particular geophysical measurement taken at the Earth’s surface could be predicted uniquely. Thus, for example, it would be possible to predict the travel time of a seismic wave reflected off any buried layer or to determine the value of the gravity or magnetic field at any surface location. In geophysical surveying the problem is the opposite of the above, namely, to deduce some aspect of the Earth’s internal structure on the basis of geophysical measurements taken at (or near to) the Earth’s surface. The former type of problem is known as a direct problem, the latter as an inverse problem. Whereas direct problems are theoretically capable of unambiguous solution, inverse problems suffer from an inherent ambiguity, or non-uniqueness, in the conclusions that can be drawn.

Using the same principle, a simple seismic survey may be used to determine the depth of a buried geological interface (e.g. the top of a limestone layer). This would involve generating a seismic pulse at the Earth’s surface and measuring the travel time of a pulse reflected back to the surface from the top of the limestone. However, the conversion of this travel time into a depth requires knowledge of the velocity with which the pulse travelled along the reflection path and, unlike the velocity of sound in water, this information is generally not known. If a velocity is assumed, a depth estimate can be derived but it represents only one of many possible solutions. And since rocks differ significantly in the velocity with which they propagate seismic waves, it is by no means a straightforward matter to translate the travel time of a seismic pulse into an accurate depth to the geological interface from which it was reflected.

The solution to this particular problem, as discussed in Chapter 4, is to measure the travel times of reflected pulses at several offset distances from a seismic source because the variation of travel time as a function of range provides information on the velocity distribution with depth. However, although the degree of uncertainty in geophysical interpretation can often be reduced to an acceptable level by the general expedient of taking additional (and in some cases different kinds of) field measurements, the problem of inherent ambiguity cannot be circumvented.

The general problem is that significant differences from an actual subsurface geological situation may give rise to insignificant, or immeasurably small, differences in the quantities actually measured during a geophysical survey. Thus, ambiguity arises because many different geological configurations could reproduce the observed measurements. This basic limitation results from the unavoidable fact that geophysical surveying attempts to solve a difficult inverse problem. It should also be noted that experimentally-derived quantities are never exactly determined and experimental error adds a further degree of indeterminacy to that caused by the incompleteness of the field data and the ambiguity associated with the inverse problem. Since a unique solution cannot, in general, be recovered from a set of field measurements, geophysical interpretation is concerned either to determine properties of the subsurface that all possible solutions share, or to introduce assumptions to restrict the number of admissible solutions (Parker 1977). In spite of these inherent problems, however, geophysical surveying is an invaluable tool for the investigation of subsurface geology and occupies a key role in exploration programmes for geological resources.

1.4 The structure of the book

The above introductory sections illustrate in a simple way the very wide range of approaches to the geophysical investigation of the subsurface and warn of inherent limitations in geophysical interpretations.

Chapter 2 provides a short account of the more important data processing techniques of general applicability to geophysics. In Chapters 3 to 10 the individual survey methods are treated systematically in terms of their basic principles, survey procedures, interpretation techniques and major applications. Chapter 11 describes the application of these methods to specialized surveys undertaken in boreholes. All these chapters contain suggestions for further reading which provide a more extensive treatment of the material covered in this book. A set of problems is given for all the major geophysical methods.

2 Geophysical data processing

2.1 Introduction

Geophysical surveys measure the variation of some physical quantity, with respect either to position or to time. The quantity may, for example, be the strength of the Earth’s magnetic field along a profile across an igneous intrusion. It may be the motion of the ground surface as a function of time associated with the passage of seismic waves. In either case, the simplest way to present the data is to plot a graph (Fig. 2.1) showing the variation of the measured quantity with respect to distance or time as appropriate. The graph will show some more or less complex waveform shape, which will reflect physical variations in the underlying geology, superimposed on unwanted variations from non-geological features (such as the effect of electrical power cables in the magnetic example, or vibration from passing traffic for the seismic case), instrumental inaccuracy and data collection errors. The detailed shape of the waveform may be uncertain due to the difficulty in interpolating the curve between widely spaced stations. The geophysicist’s task is to separate the ‘signal’from the ‘noise’and interpret the signal in terms of ground structure.

Analysis of waveforms such as these represents an essential aspect of geophysical data processing and interpretation. The fundamental physics and mathematics of such analysis is not novel, most having been discovered in the 19th or early 20th centuries. The use of these ideas is also widespread in other technological areas such as radio, television, sound and video recording, radioastronomy, meteorology and medical imaging, as well as military applications such as radar, sonar and satellite imaging. Before the general availability of digital computing, the quantity of data and the complexity of the processing severely restricted the use of the known techniques. This no longer applies and nearly all the techniques described in this chapter may be implemented in standard computer spreadsheet programs.