Overview

Although simple in principle, the measurement of gravity for geologic purposes to an accuracy of the order of 10-8 to 10-9 of the Earth's gravitational field requires highly sophisticated instrumentation and rigorous survey procedures. Fundamentally because of the nature of the measurement, all gravity instrumentation must be mechanical. Most land gravity measurements are made with relative-measuring instruments using the zero-length spring principle to achieve high sensitivity in portable instrumentation. Although extensive governmental and commercial databases containing millions of observations exist, additional gravity surveying continues to achieve greater detail and accuracy in the data. Random errors are minimized in modern instrumentation by performing the observations automatically. However, residual errors remain, owing both to inherent instrumentation problems and non-geologic acceleration components that need to be considered while conducting surveys.

Increasingly accurate observations and improved anomaly resolution are being achieved especially in marine and airborne observations of gravity. A variety of instrumentation is used for measurements on mobile platforms, including modified zero-length spring gravimeters and electromagnetic accelerometers mounted on gyrostabilized platforms to minimize short-period horizontal and vertical accelerations due to movements of the ship or aircraft. In addition, highly accurate absolute gravity measurements are being made with stable, portable free-fall instruments, and rotating-disk gravity gradiometers are being used to observe the gravitational tensor components. These improvements are useful not only in mapping components of the gravity field related to variations in subsurface geology, but also in monitoring time-variable processes within the Earth associated with mass changes.

Introduction

The nature of terrestrial gravity and magnetic fields, the underlying principles of applying these fields to exploring the Earth, and an overview of the current practices for using these methods form the subject of the previous chapters. In addition to these principles and practices, a review of the applications of these methods of exploration is important to understanding them and their role in determining the nature, composition, and structure of the Earth. In this chapter, a brief introduction is given of the application of gravity and magnetic methods to subsurface exploration. It serves as an introduction to a website that accompanies this book, which can be accessed at http://www.cambridge.org/gravmag. The website presents an expanded explanation of the applications of the methods and examples of their application largely from published case histories. Providing the case histories on a website decreases the length of the book, allows the inexpensive use of color in illustrations, and makes it easier to update the material with new applications. The reader is urged to visit the website and study the representative examples of the application of gravity and magnetics as a useful adjunct to understanding the methods.

General view of applications

Geophysical methods have had a seminal role in determining the nature of the Earth, leading to our current state of knowledge about the origin and evolution of our planet and the processes involved in the Earth system.

The long march begins

The AWE scientists who were assigned to the forensic seismology programme in the late 1950s found that although there was much theoretical work on seismic-wave propagation, experimental and observational seismology were poorly developed. The main practical interest in seismology was in the destructive effects of large earthquakes. The only useful information routinely obtained from seismograms was arrival times of the most significant seismic phases, from which epicentres and focal depths could be estimated and Earth's structure derived. Most if not all stations used drum recorders operating at low magnification with compressed time bases (maximum speed around 1 mm s−1).

The continuous background of seismic noise has little effect on observations of the waves from large earthquakes as their amplitude is much greater than that of the noise. If seismology is to provide an effective way of detecting (and ideally identifying) explosions, the recording systems in use in the 1950s were inadequate for the detection of the weak signals from tests of a few kilotons (ground motions in the 30–90° range of around 10 nm at 1Hz). Many of the seismometers did not have the required sensitivity, and responded to unwanted non-seismic disturbances such as changes of air temperature and atmospheric pressure; seismometers that had the required sensitivity were heavy and bulky and so were unsuitable for deployment as arrays.

In September 1996 the United Nations Information Directorate (UNIDIR) held a press conference in Geneva to announce the successful negotiation of a treaty, the Comprehensive Nuclear Test Ban Treaty (CTBT), that when it enters into force will ban all nuclear explosions. At the request of the UNIDIR, Peter Marshall of the Blacknest Seismology Group of the Atomic Weapons Establishment (AWE) joined the panel to answer technical questions from the press (Figure 1). Why was Peter amongst all the experts who had attended the negotiations chosen to join the panel? Because all the delegates recognized that through his personal qualities and his expertise Peter contributed more than anyone else to the success of the technical negotiations. Professor Dr Peter Wille (German Delegation) expressed what many delegates felt:

Peter was able to make such a significant technical contribution to the negotiations because he had worked on the application of seismology to the verification of arms-control treaties, that is, forensic seismology, from its beginnings.

Overview

Gravity observations include the combined effects of instrumental, surface, terrain, and planetary sources in addition to the subsurface mass variations that are the objective of an exploration gravity survey. To isolate the effects of subsurface sources, extraneous effects which include both temporal and spatial variations are removed from the data using theoretical considerations, geological information, and empirical observations. Some are considered universally, but others only in specific geological, surface, and observational conditions. There is an increasing need to eliminate a broader range of extraneous effects more precisely as the objectives of gravity surveying require higher precision and accuracy and are focused on both long- and short-wavelength anomalies. Unwanted effects are removed by calculating the gravity anomaly, which is the arithmetic difference between the observed vertical acceleration of gravity and the predicted or theoretical acceleration at the observation site. Theoretical gravity is based on a conceptual model of the sources of gravity variations. This model varies depending on the intended use of the gravity anomaly and the conditions of the survey.

Three classes of anomalies are recognized. The primary class, planetary anomalies, incorporates only analytically determined planetary considerations in the theoretical model, e.g. the Bouguer gravity anomaly. A second type, geological anomalies, applies additional effects from known or postulated subsurface geological conditions in the model, e.g. the isostatic residual gravity anomaly. The third type, filtered anomalies, is calculated by removal of arbitrary gravity effects caused by unknown sources, empirically or analytically determined by filtering that enhances particular attributes of the spatial pattern of the anomalies, e.g. wavelength-filtered gravity anomalies. The latter type seeks to isolate or enhance those of interest at the expense of other anomalies.

Introduction

It is unlikely that many seismologists given a choice between seismic magnitude and some physical measure of seismic-source size would choose magnitude. As Thirlaway (1973) notes, some of the founders of seismology regarded seismic amplitudes as not worth serious study and since then others have argued that magnitudes should no longer be used (Jones, 2000). The main disadvantage of the magnitude scale is that it is not based on the physics of the source. It is an arbitrary scale; there is no wrong answer. Seismologists are therefore free to propose modifications to the scales. Scales defined on one type of recording system or component of ground motion, for example, are simply applied without change to new systems or components, which often then leads to attempts to relate the different scales.

Further, how A and T should be measured is often only loosely defined. Consequently, what is measured depends on analyst interpretation of the rules, and rarely are corrections applied to allow for: departures in the propagation path from some average structure; station effects; or the effects of earthquake radiation patterns. So it is easy to make the argument for abandoning magnitude as a measure of the size of a seismic disturbance and turning to physical measures such as seismic moment, which is based on the source physics and in theory at least, is independent of radiation patterns and Earth structure.

Overview

The gravity method of geophysical exploration is based on the measurement of variations in the gravity field caused by horizontal variations of density within the subsurface. It is an important technique for many problems that involve subsurface mapping, and it is the principal method in a number of specific types of geological studies. The method, which has its roots in geodetic studies from the seventeenth to the twentieth centuries, has developed rapidly over the past century as a result of significant technological advances: primarily high-accuracy gravity measuring instruments for use on land, sea, and air, and in space, as well as the increasing computational and graphics power of digital computers. These advances are backed up by continuing improvements in data processing, interpretational schemes, practical experience, and surveying methodologies.

Traditionally the gravity method has been used primarily in regional characterization of the Earth for determining the architecture of the crust, identifying potentially favorable regions for resource exploration, and developing conceptual exploration models. This is made possible by the millions of gravity observations now available worldwide in both public and commercial data sets. The accuracy now available in gravity observations and the relative ease of measurement have made the method viable for exploration objectives such as assessing subsurface changes over time and combined interpretation with seismic reflection mapping.

Sometime in the early 1970s, Dr Ronald (Ron) Girdler, University of Newcastle, UK, made a plea for someone in the seismology group at the Atomic Weapons Establishment (AWE), UK, to write a book on earthquake seismology based on the AWE's research in forensic seismology, that is, seismology applied to the verification of nuclear test bans. The principal purpose of the research was to provide comprehensive advice on forensic seismology on which the UK government could base its policy for the control of underground nuclear tests. The work at the AWE, however, has applications to seismology in general which was what interested Ron Girdler.

The UK's forensic seismology programme began at the AWE in the late 1950s. Work was carried out at both AWE Aldermaston (principally on the design and manufacture of seismological recording systems) and at the outstation AWE Foulness (on decoupling, seismogram analysis and seismometry). In 1961, much of the forensic seismology work moved to an unclassified site at Blacknest, a country house near AWE Aldermaston. Over the next ten years forensic seismology research at the AWE was increasingly concentrated at Blacknest.

I and my colleagues at AWE Blacknest, Peter Marshall and Frederick (Fred) Key, produced a rough draft of a possible book but it was clear from the draft that: some of the subjects covered, such as the application of seismometer arrays in seismology were then still undeveloped; the analogue recording and processing systems then used were obsolescent; and overall, the range of subjects covered was too limited.