Looking into the Earth

Geophysics is essential to understanding the solidEarth, particularly on a global scale. Modern ideasof the structure and evolution of continents andoceans, or of the formation of mountain chains onland and below the oceans, for instance, are basedextensively on discoveries made using geophysics.But geophysics can contribute to geological knowledgon all scales, from the global, through thmedium-scale such as regional mapping or the searchfor oil and minerals, down to the small-scale, such ascivil engineering, archaeology, and groundwater pollution,as well as detailed geological mappingGeophysics differs from other methods forstudying the Earth because it can ‘look into thEarth’, for its measurements are mostly maderemotely from the target, usually at the surface. It iable to do this because it measures differences in thephysical properties of the subsurface rocks or structures,which are revealed by their effects at the surfacesuch as the magnetic field of some rocks. But itdescribes the subsurface in physical terms - densityelectrical resistivity, magnetism, and so on, not interms of compositions, minerals, grain-sizes, and soon, which are familiar to the geologist. Becausegeologists are often unfamiliar with physics (and theassociated mathematics), there is a tendency eitherto ignore geophysics or to accept what a geophysicistsays without understanding the qualifications. This last is simply to treat the geophysicist assome sort of a magician. All subjects have their assumptions, often unstated but known to practitioners. Anyone who has used a geological mapknows that just because some area has the samecolour is no guarantee that the same rock will befound everywhere within it, or that faults occur exactly where marked and nowhere else. Similarly,there are reservations and limitations on what geophysicstells you, which need to be understood.The intention of this book is not to turn you intoa geophysicist (you may be glad to know), but simply to provide a basic grasp of the subject, so that, forinstance, if a seismic section is described as ‘unmigrated’you know whether it matters (it well might!).Or you may need to know whether ageophysical survecould help solve some geological problem of yours, so when you call on the services of a geophysicist(and worse things may happen!) you need to be able to explain your problem and understand his orher advice. You would not buy a car without specifyingthe number of seats, engine size, whether a sedanor sports model, and so on that you would like, anthe price you are prepared to pay; and you would listento claims about its fuel consumption, insurance group, and the choice of radios. Similarly, you needan understanding of geophysics.The purpose of this book is to explain how geophysicshelps us understand the solid Earth beneathour feet, and how - combined with the traditional methods of geology - gre