Introduction

In a 1D earth, given a sufficient network of stations, times and epicentres are orthogonal so that unbiased epicentres can be estimated independently of the travel times; this is clear because if the P arrival times at a circle of stations are constant, the epicentre must be in the centre of the circle whatever the travel time. Networks of stations are not so well distributed in practice but epicentre estimates are still insensitive to travel-time tables, so that given the origin time of at least one earthquake, obtained from observations very close to the epicentre, corrections to travel times can be found by averaging the O–C residuals in a series of small distance ranges. Epicentres can then be revised using the updated travel-time tables. This method of ‘successive approximation’, where the tables and then the epicentres are repeatedly revised until corrections become small enough to neglect, is that used by Jeffreys and Bullen (1940).

The travel-time analysis carried out by Herrin et al. (1968) to derive the 1968 Tables and that of Kennett and Engdahl (1991) to derive the iasp91 Tables also use successive approximation. Herrin et al. (1968) estimate station effects but, just as distance corrections are found by grouping residuals in distance ranges and averaging, so station effects are found by grouping residuals by station and averaging. Kennett and Engdahl (1991) do not allow for station effects.

Introduction

At the beginning of the AWE research programme all recording was in analogue format – the continuously varying voltage out of the seismometer and the associated electronics were encoded as a continuously varying magnetic intensity on tape. Although analogue does not have the flexibility of digital recordings, much was achieved by AWE Blacknest with an analogue computer, supplemented by analogue devices including special-purpose tape loops, hardware filters and so on.

A consequence of the UK's early reliance on analogue recording is that AWE Blacknest has made little use of spectra. Spectra are most easily estimated from digital seismograms. Until the mid-1970s the group had no easy access to digital seismograms, so the seismologists learnt to interpret seismograms in the time domain and even when AWE Blacknest switched to digital recording, spectra were rarely used. Elsewhere, and particularly in the USA where digital seismograms were available from the mid-1960s, amplitude spectra have been much used.

One weakness of the reliance on the amplitude spectrum is that half the information in the original seismogram – the phase spectrum – is ignored, at least for body waves. This is perhaps understandable as interpreting the two spectra – amplitude and phase – together, is difficult. There is, of course, a way of taking account of the two spectra simultaneously and that is by interpreting the original seismogram.

Introduction

When the arrays came into routine operation the focus of research shifted from the design of the arrays and the recording systems to the analysis and interpretation of the SP body-wave (principally P) seismograms. With such high-quality recordings (on which local effects are suppressed) with the flexibility to change the playout speeds and magnification, routine analysis (the measurement of times, polarities, A and T) could be carried out with less error than on conventional paper and photographic recordings. The development of ways of increasing the bandwidth of SP recordings and correcting for the effects of attenuation further improved methods of analysis. But, whereas such improvements are valuable, there had to be more that could be learnt from array seismograms than this; and this proved to be so, for with the aid of the programs to synthesize seismograms, progress began to be made with the detailed interpretation of array recordings. An hypothesis about some feature of a seismogram could be tested by setting up the appropriate model and comparing the synthetic seismogram with the observed one. Synthesizing seismograms can also be used to explore the effects of variation in source and Earth models to answer the question ‘What would be the effect of…?’.

Overview

Gravity and magnetic anomaly data are commonly expressed in standard formats for electronic analysis and archiving in digital data bases. A voluminous literature full of application-specialized jargon describes numerous analytical procedures for processing and interpreting anomaly data. However, when considered from the electronic computing perspective, these procedures simplify into the core problem of manipulating a digital forward model of the data to achieve the data analysis objectives. The forward model consists of a set of coefficients specified by the investigator and a set of unknown coefficients that must be determined by inversion from the input data set and the specified forward model coefficients. The inversion typically establishes a least-squares solution, as well as errors on the estimated coefficients and predictions of the solution in terms of the data and specified model coefficients. The inversion solution is never unique because of the errors in the data and specified model coefficients, the truncated calculation errors, and the source ambiguity of potential fields. Thus, a sensitivity analysis is commonly required to establish an “optimal” set or range of solutions that conforms to the error constraints. Sensitivity analysis assesses solution performance in achieving data analysis objectives including the determination of the range of geologically reasonable parameters that satisfy the observed data.

Introduction

At an early USA–UK meeting to discuss test ban verification, the participants were asked what year they thought a treaty would be agreed. There was apparently a wide range of answers, but no one came close to 1996; even the most pessimistic prediction has turned out to be overly optimistic.

Whether there was really a missed opportunity in the early 1960s to agree a treaty is for others to decide. There seems to have been the political will in the USA and UK for a treaty, but with hindsight it would have had to be based on mutual trust because the verification regime that could have been put in place in those early days would have been weak and would have needed constant revision as unknown unknowns became known unknowns. The size and complexity of the Comprehensive Nuclear Test Ban Treaty Organization (CTBTO) shows just what is required for a truly global verification system.

The past 50 years has been a period of learning – and the learning continues – for those seismologists who have spent their careers trying to solve the problems of verification.

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.