This book began as a revision of the classic text, Eve and Keys – Applied Geophysics in the Search for Minerals. However, it soon became obvious that the great advances in exploration geophysics during the last two decades have so altered not only the field equipment and practise but also the interpretation techniques that revision was impractical and that a completely new textbook was required.

Readers of textbooks in applied geophysics will often have a background which is strong either in physics or geology but not in both. This book has been written with this in mind so that the physicist may find to his annoyance a detailed explanation of simple physical concepts (for example, energy density) and step-by-step mathematical derivations; on the other hand the geologist may be amused by the over-simplified geological examples and the detailed descriptions of elementary concepts.

The textbook by Eve and Keys was unique in that it furnished a selection of problems for use in the classroom. This feature has been retained in the present book.

INTRODUCTION

General

Gravity prospecting involves measurements of variations in the gravitational field of the earth. One hopes to locate local masses of greater or lesser density than the surrounding formations and learn something about them from the irregularities in the earth's field. It is not possible, however, to determine a unique source for an observed anomaly. Observations normally are made at the earth's surface, but underground surveys also are carried out occasionally.

Gravity prospecting is used as a reconnaissance tool in oil exploration; although expensive, it is still considerably cheaper than seismic prospecting. Gravity data are also used to provide constraints in seismic interpretation. In mineral exploration, gravity prospecting usually has been employed as a secondary method, although it is used for detailed follow-up of magnetic and electromagnetic anomalies during integrated base-metal surveys. Gravity surveys are sometimes used in engineering (Arzi, 1975) and archaeological studies.

Like magnetics, radioactivity, and some electrical techniques, gravity is a natural-source method. Local variations in the densities of rocks near the surface cause minute changes in the gravity field. Gravity and magnetics techniques often are grouped together as the potential methods, but there are basic differences between them. Gravity is an inherent property of mass, whereas the magnetic state of matter depends on other factors, such as the inducing fields and/or the orientations of magnetic domains.

INTRODUCTION

The application of several disciplines – geology, geochemistry, geophysics – constitutes an integrated exploration program. In a more restricted sense we may consider the integrated geophysics program as the use of several geophysical techniques in the same area. The fact that this type of operation is so commonplace is because the exploration geophysicist, by a suitable selection of, say, four methods, may obtain much more than four times the information he would get from any one of them alone.

Before elaborating on this topic it is necessary to point out again the paramount importance of geology in exploration work. Every geological feature, from tectonic blocks of subcontinental size to the smallest rock fracture, may provide a clue in the search for economic minerals. Thus geologic information exerts a most significant influence on the whole exploration program, the choice of area, geophysical techniques, and, above all, the interpretation of results. Without this control the geophysicist figuratively is working in the dark.

The subject of integrated geophysical surveys has received considerable attention in the technical literature since about 1960. In petroleum exploration the combination of gravity and magnetic reconnaissance, plus seismic for both reconnaissance and detail (and, of course, various well-logging techniques during the course of drilling) is well established.

The best combination for an integrated mineral exploration program is not so definite because of the great variety of targets and detection methods available. Base-metal search is a case in point.

INTRODUCTION

Importance of Seismic Work

The seismic method is by far the most important geophysical technique in terms of expenditures (see Table 1.1) and number of geophysicists involved. Its predominance is due to high accuracy, high resolution, and great penetration. The widespread use of seismic methods is principally in exploring for petroleum: the locations for exploratory wells rarely are made without seismic information. Seismic methods are also important in groundwater searches and in civil engineering, especially to measure the depth to bedrock in connection with the construction of large buildings, dams, highways, and harbor surveys. Seismic techniques have found little application in direct exploration for minerals where interfaces between different rock types are highly irregular. However, they are useful in locating features, such as buried channels, in which heavy minerals may be accumulated.

Exploration seismology is an offspring of earth-quake seismology. When an earthquake occurs, the earth is fractured and the rocks on opposite sides of the fracture move relative to one another. Such a rupture generates seismic waves that travel outward from the fracture surface and are recorded at various sites using seismographs. Seismologists use the data to deduce information about the nature of the rocks through which the earthquake waves traveled.

Exploration seismic methods involve basically the same type of measurements as earthquake seismology. However, the energy sources are controlled and movable, and the distances between the source and the recording points are relatively small.

INTRODUCTION

General

Magnetic and gravity methods have much in common, but magnetics is generally more complex and variations in the magnetic field are more erratic and localized. This is partly due to the difference between the dipolar magnetic field and the monopolar gravity field, partly due to the variable direction of the magnetic field, whereas the gravity field is always in the vertical direction, and partly due to the time-dependence of the magnetic field, whereas the gravity field is time-invariant (ignoring small tidal variations). Whereas a gravity map usually is dominated by regional effects, a magnetic map generally shows a multitude of local anomalies. Magnetic measurements are made more easily and cheaply than most geophysical measurements and corrections are practically unnecessary. Magnetic field variations are often diagnostic of mineral structures as well as regional structures, and the magnetic method is the most versatile of geophysical prospecting techniques. However, like all potential methods, magnetic methods lack uniqueness of interpretation.

History of Magnetic Methods

The study of the earth's magnetism is the oldest branch of geophysics. It has been known for more than three centuries that the Earth behaves as a large and somewhat irregular magnet. Sir William Gilbert (1540–1603) made the first scientific investigation of terrestrial magnetism. He recorded in de Magnete that knowledge of the north-seeking property of a magnetite splinter (a lodestone or leading stone) was brought to Europe from China by Marco Polo.

Many years ago when I was an undergraduate, the late Arthur F. Buddington of Princeton University pointed out to a group of us students in his petrology class that, at least once in every lifetime, such revolutionary new ideas are introduced into each field of science that well-established ideas must be abandoned and the whole view of the field thought out afresh. Such revolutions have affected seismology three times, each conveniently marked by a very large earthquake.

Previous to the Lisbon, Portugal earthquake of 1755, earthquakes were viewed largely as “acts of God” Imposed on mankind in retribution for misbehavior; afterward, they were studied more as natural phenomena, and knowledge of them grew gradually but steadily as a result of careful observations. This change in the way natural phenomena were viewed was not unique to seismology but had been developing in all aspects of science at least since the time of such men as Rene Descartes (1596–1650) in France and Gottfried Wilhelm Leibnitz (1646–1716) in Germany. For nearly a century and a half after 1755, understanding of earthquakes was limited to what could be learned by visual observation. This was due to lack of an adequate means of measuring ground motion. It was not until the development of sensitive seismographs toward the end of the nineteenth century that seismograms became good enough to recognize the various types of pulses that are propagated.

Direct observation

As long as the medieval belief persisted that earthquakes were a punishment imposed upon man by God in retribution for sin, there was no reason to suspect that any one region should be more subject to earthquakes than another. After all, to err is human; and, consequently, sin must be universal. As worldwide communications improved, however, it became apparent either that the people of certain areas had fallen further from grace than others or that some other explanation for earthquakes was necessary.

Before the invention of the telegraph in 1840, news traveled slowly, and knowledge of even serious natural disasters often was not widely disseminated. The earliest extant compendium of seismicity is a list of 91 earthquakes between 34 and 1687 a.d., prepared by D. Vincenzo Magnati and first published in 1688 (Heck and Davis, 1946). There seems to have been no systematic attempt to gather information on the occurrence of earthquakes until Karl Ernst Adolf von Hoff began publishing annual lists of worldwide earthquakes in 1826 (Davison, 1927). Local lists had been prepared previously. For example, Zachary Grey in England published a pamphlet describing 61 earthquakes in 1750. Elie Bertrand's 1757 memoir on Swiss earthquakes included a list of 155 shocks. Domenico Pignataro listed 1186 shocks in Italy for the period of 1783–6; and Francisco Antonia Grimaldi in 1784 published a list of Calabrian earthquakes for the period 1181–1756. Louis Cotte in 1807 prepared a catalog of earthquakes of northern Europe.

SELF-POTENTIAL METHOD

Origin of Potentials

Various spontaneous ground potentials were discussed in Section 5.2.1. Only two of these have been considered seriously in surface exploration, although the self-potential method is used in a variety of ways in well logging (§11.3). Mineralization potentials produced mainly by sulfides have long been the main target of interest, although recently exploration for geothermal sources has included self-potential surveys as well. The remainder of these spontaneous potentials may be classified as background or noise. This also means that geothermal anomalies become noise if they occur in the vicinity of a sulfide survey and vice versa (§6.1.4). A more detailed description of these sources follows.

Background potentials are created by fluid streaming, bioelectric activity in vegetation, varying electrolytic concentrations in ground water, and other geochemical action. Their amplitudes vary greatly but generally are less than 100 mV. On the average, over intervals of several thousand feet, the potentials usually add up to zero, because they are as likely to be positive as negative.

In addition there are several characteristic regional background potentials. One is a gradient of the order of 30 mV/km, which sometimes extends over several kilometers and may be either positive or negative. It is probably due to gradual changes in diffusion and electrolytic potentials in ground water. Sometimes a more abrupt change will result in a baseline shift of background potential. Another regional gradient of similar magnitude seems to be associated with topography.

The 14 or so years that have elapsed since the writing of Applied Geophysics have seen many changes as results of better instrumentation, the extensive application of computer techniques, and more complete understanding of the factors that influence mineral accumulations. Changes have not been uniform within the various areas of applied geophysics, however. In gravity field work there have been few changes except for the use of helicopters and inertial navigation, but the ability to calculate the gravity field of a complicated model, use the differences from the measured field to modify the model, and iterate the calculations has significantly changed gravity interpretation. The greatly improved sensitivities of proton-precession and optically pumped magnetometers and the use of gradiometers have considerably increased the number of meaningful magnetic anomalies extractable from magnetic data; iterative interpretation has also had a significant impact, as in gravity interpretation. No major individual innovations have affected seismic exploration but a combination of minor improvements has produced probably the greatest improvement in seismic data quality in any comparable period of time. The improved data quality has resulted in new types of interpretation, such as seismic stratigraphy; now, interactive capabilities promise major interpretational advances. Whereas there has been little change in self-potential methods, magnetotellurics has blossomed from a research tool to a practical exploration method. Resistivity methods have changed only a little, but perhaps the greatest changes in any area result from the development of a number of new electromagnetic exploration methods.

The vibrations produced by earthquakes stood out on the earliest seismograms because of their larger amplitudes over a continuous background of weaker motions. Macelwane (1953) states that by 1887 John Milne had concluded that microseisms provided a background of unrest everywhere on earth. Some of this background noise could be explained by local disturbances such as traffic-generated vibrations, building (and other) vibrations due to human activities, and ground movements associated with wind and falling water. It was quickly realized that background noise was reduced when the seismograph was isolated from obvious sources of disturbance and placed on a foundation firmly attached to solid rock. However, there remained characteristic ground motions for which no local source was apparent. The most prominent of these were vibrations in the range of 2–10 s which rose and fell in amplitude with time and were stronger in winter than in summer (see, e.g., Iyer, 1964; Brune and Oliver, 1959).

Timoteo Bertelli in 1872 appears to have been the first to recognize that these disturbances correlated with barometric depressions (Gutenberg, 1958B). Bertelli proposed the term microseismi to describe the small movements, which he observed by watching the free end of a pendulum through a microscope. In 1900, Father Josi Algue correlated such microseisms with storms crossing the Philippine Islands (Carder and Eppley, 1959).

The mechanism by which storms produce microseisms was at first thought to be surf breaking along coasts (Wiechert, 1904).

Identification of different seismic pulses

Although Poisson worked out the theory of transmission of elastic waves in solids in 1831, the relation of this theory to seismic waves was not realized for a long time. This is not surprising considering that the first practical seismograph was not invented by Cecchi until 1875 and that John Milne began recording earthquakes only in 1880. In 1847, William Hopkins pointed out that earthquakes must consist of elastic waves. Robert Mallet (1852, 1861) accepted this idea and even tried to measure their velocity of transmission. His instrumentation, however, was inadequate, and he obtained velocities that were much too low to be other than some phase of what we now know to be surface waves.

The earliest seismograms showed that earthquakes consist of a principal series of oscillations preceded by weaker motions and followed by a gradual dying out of the waves. Little attention was at first given to the preliminary oscillations (called vorlaufer, meaning forerunners) though it was suspected that they might be compressional waves. The principal oscillations were at first thought to be shear waves. Richard D. Oldham (1899, 1900) in studying seismograms of the great 1897 Indian earthquake identified the earliest arrivals as compressional and showed that a later vorlaufer pulse was a shear wave (Figure 5.1). The two vorlaufer pulses were given the names primary and secondary pulses, now abbreviated P and S.

Mythology

To most ancient and medieval people (and to some people even today), an earthquake is an act of God, or some other supernatural power, visited on mankind as punishment for misbehavior. John Milne (1886) summarized various mythological causes such as the squirming of a giant catfish beneath Japan, a frog (Mongolia), a hog (Celebes), or a tortoise (American Indians).

More mechanistic explanations were preferred by Greek writers. Thales of Miletas (around 580 b.c.) suggested that the earth floated on a universal ocean whose storms shook the land (Sachs, 1979). Winds in subterranean caves also were commonly postulated as the cause. Aristotle (340 b.c.) credits Anaxagoras as the originator of this idea. Aristotle recognized an association between some earthquakes and volcanic eruptions. Because volcanic eruptions often included great and violent exhalations of gas, it was logical to suppose that earthquakes preceding the main eruption resulted from the progression of such gases from one underground cavern to another. The next concept was the possibility that collapse of cavern roofs after an eruption generated earthquake vibrations. René Descartes postulated that subterranean gases exploded to cause earthquakes (Geike, 1905).

By the seventeenth century, many descriptions of the effects of earthquakes had been published, probably with exaggeration and other distortions (Deresiewicz, 1982). Displacements of the ground surface were recognized as an effect of earthquakes but were not yet associated with the source of the vibrations.

Introduction

Tsunami (sometimes spelled tunami) is a Japanese word that is used for large sea waves generated by earthquakes and other sudden mechanisms. The word means literally “long harbor wave” (Imamura, 1937, p. 123; Milne and Lee, 1939, p. 34; Darbyshire and Ishiguro, 1957), but because in Japan the largest destructive waves in harbors are those caused by earthquakes, the name has become attached to seismic sea waves. There is a long history of such waves. Nicholas H. Heck (1947) has published a list of 270 tsunamis starting with one at Potidea, Greece in 479 b.c. William H. Hobbs (1907) summarizes the report of the Greek historian Herodotus describing how this tsunami saved the Greek city from invaders. A Persian army was besieging Potidea when the sea retreated, exposing the seaside border of the city. When the Persians moved to attack across the exposed seabed, the subsequent inundation drowned many of them, setting up a Greek victory.

Ambraseys (1962) lists 141 tsunamis in the eastern Mediterranean starting in the second millenium b.c. Possibly the strongest of these was produced by the explosion of Santorin volcano in the Aegean Sea in the fifteenth Century b.c. The site is the present location of the island of Thera. This tsunami is believed to have brought about the collapse of the Minoan culture on the nearby island of Crete, whose north shore was extensively flooded (Marinatos, 1939). Thera has also been postulated as the site of ancient Atlantis (Galanopoulos, 1960).

DIRECTION DETERMINATION

The three degrees of freedom, latitude, longitude, and elevation (or equivalents), constitute a rectangular coordinate system (neglecting earth curvature effects), and clearly determination of the orientations of the axes is important. The vertical direction is usually defined by means of a level or plumb line. Horizontal orientation can be determined with respect to the position of a celestial body, the Earth's rotation, magnetic north, or a previously known orientation.

Although the most obvious celestial orientation is by sighting on Polaris (which is not exactly at the celestial pole), any star can be used. Sighting on the Sun (sun shots) permits orientation during daylight. Nightime observations can establish orientation to a fraction of an arc second (arcsec).

Gyrocompass orientation is extensively used in marine and airborne surveys. A gyrocompass gives orientation with respect to true north within about 20 arcsec.

A magnetic compass is often used to determine direction with respect to magnetic north, correction for the magnetic declination giving true north. Diurnal variations may create 10 min of error, more during magnetic storms. Care has to be exercised that no local iron or electric currents distort the magnetic field.

Benchmarks (identified points of known location) often have nearby reference points (azimuth bars) that can be used to establish orientation.

DISTANCE MEASUREMENT

Direct measurement of distance is done by seeing how many times a standard measure will fit between points whose separation is to be determined, a technique called chaining, the unit of measure usually being a steel tape (a chain) of precise length.