Palaeomagnetism utilises the fossil magnetism in rocks. One major application is to measure movements of rocks since they were magnetised, which can be due to plate movements or tectonic tilting. Another application is to measure the thermal history of a rock, such as reheating or the temperature of emplacement of a pyroclastic deposit. Successful applications require understanding the magnetic field of the Earth and how rocks become magnetised, and detecting whether this magnetisation has changed subsequently.

Mineral magnetism utilises the variation of the magnetic properties of rocks to study processes such as erosion and deposition. Magnetic fabric – when magnetic properties vary with direction in a rock – is used to deduce fluid flow when the rock formed, or subsequent deformation.

All the applications described in this chapter require access to the rocks, usually by collection of oriented material in the field.

These two methods are mainly used to prospect for conductive ores. Self-potential (or spontaneous potential), SP, depends on small potentials or voltages being naturally produced mainly by some massive ores. Induced polarisation, IP, in contrast, depends upon a small amount of electric charge being stored in an ore when a current is passed through it, to be released and measured when the current is switched off. IP is significant only for disseminated ores but can often be used to locate massive ores as these are commonly surrounded by disseminated ore. For both methods, potentials can also arise in other, usually unwanted, ways.

Both methods require electrodes and wires to make contact with the ground.

Archaeology is the study of how humans lived in the past, so no direct observation is possible. For this reason, information about how – and when – people lived has to be deduced from what they have left, both intentionally – such as monuments and written records – and, just as important, incidentally, in traces of buildings, fortifications, field systems, and so on, now often buried. Unravelling the sometimes long and complicated history of an archaeological site often requires the skills of a detective applied to meticulous and painstaking excavations. To aid the investigation, the archaeologist can call upon various techniques (i) to help find or map a site, (ii) to help date the site and its artefacts, and (iii) to help characterise artefacts, such as analysis of their materials to help find their source. Geophysics has a large contribution to make to the first two of these, and a lesser one to the third.

Dating is obviously important to understanding how cultures develop or relate to one another. For example, it was once thought that the builders of Stonehenge derived their culture from the ancient cultures of the eastern Mediterranean, such as ancient Greece, until carbon dating showed Stonehenge to pre-date them. The dating methods most used in archaeology are those suitable for younger materials (described in Section 15.12), but the potassium–argon method is the main method for dating early hominids, mainly in Africa where they extend back over 4 Ma.

This chapter is primarily about the role of geophysics in finding and extracting petroleum, for geophysics has a crucial role to play in the successive stages of hydrocarbon exploration and then its extraction. After looking at energy sources and demands in general, and the particular importance of hydrocarbons, we outline how petroleum originates in sedimentary basins and the conditions needed for it to collect into accumulations that are worth extracting. Gravity, aeromagnetic, seismic refraction, and reflection surveys are used to help locate basins. More detailed surveys, particularly using seismic reflection, are used to identify structures within them, such as anticlines, faults, or salt domes, that are likely to contain hydrocarbon traps, and to determine whether these contain pools of oil. This is followed by exploratory drilling, with well logging used to measure their vertical extent and the permeability, porosity, and hydrocarbon content of the reservoir rocks. Drilling is very expensive, so siting the wells has to be done with great care and relies primarily on detailed seismic reflection surveys. Later, surveys may be carried out to monitor extraction. The methodology of exploration and extraction of hydrocarbons is illustrated with two case studies from the North Sea, the first of gas, the second of oil.

Geophysical methods employed: Mainly seismic reflection and well logging, with some gravity and magnetics.

Radiometric dating provides a numerical date, in years, for a sample. This contrasts with geological dating, which provides a relative date, by assigning a sample to a position in a timescale, mainly using stratigraphic order and palaeontological correlations. Radiometric dating is chiefly applicable to igneous and metamorphic rocks; therefore it complements, rather than replaces, geological dating, which is mainly applicable to sedimentary rocks.

There are a number of radioactive elements that are found in rocks and hence a considerable number of radiometric dating methods. These have advantages and disadvantages, particularly in the types of rock and ‘event’ that they can date. Several dating methods used in combination may provide significantly more information than any single method. The calculation of dates depends upon a number of assumptions, but these can be checked to some extent, depending on the method. A particular application of radiometric dating is to measure cooling histories of rocks, particularly following thermal metamorphism, and is termed thermochronometry.

The geophysical methods described so far in this book have nearly all investigated the subsurface using measurements made at the surface. This chapter describes measurements made underground, mainly in boreholes but also in mines and tunnels. The main advantages compared to surface measurements are much increased detail and close correlation with geological observations at precisely known depths; the disadvantages are the cost of boreholes and often the limited volume surveyed.

The most important application is in the exploration, evaluation, and production of oil and gas, by providing information on porosity, permeability, fluid content, and saturation of the formations penetrated by a borehole. Other applications are in mineral exploration and evaluation, and in hydrogeology. Subsurface measurements between holes or between holes and the surface may be combined to deduce the intervening geology.

Most of the geophysical principles used are the same as those used to ‘look down’ from the surface, described in the preceding chapters, but instruments and measurements have to be adapted to ‘look sideways or upwards’ in the special conditions of the subsurface, particularly the confined space of a borehole, where they also have to overcome the alterations produced in the surrounding formation by the drilling. Well logging differs from most other geophyscal methods, not only in being carried out down a borehole, but in relating the physical quantities measured more specifically to geological properties of interest, such as lithology, porosity, and fluid composition.