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.

Gravity surveying, or prospecting, is a method for investigating subsurface bodies or structures that have an associated lateral density variation. These include ore bodies and intrusions whose density differs from that of the surrounding rocks, basins infilled with less dense rocks, faults if they offset rocks so that there are lateral density differences, and cavities.

Gravity surveys measure minute differences in the downward pull of gravity and so require extremely sensitive instruments. Gravity readings also depend on factors that have nothing to do with the subsurface rocks, and corrections for their effects are very important.

The resistivity of rocks usually depends upon the amount of groundwater present and on the amount of salts dissolved in it, but it is also decreased by the presence of many ore minerals and by high temperatures

The main uses of resistivity surveying are therefore for mapping the presence of rocks of differing porosities, particularly in connection with hydrogeology for detecting aquifers and contamination, and for mineral prospecting, but other uses include investigating saline and other types of pollution, archaeological surveying, and detecting hot rocks.

Resistivity surveying investigates the subsurface by passing electrical current through it by means of electrodes pushed into the ground. Traditionally, techniques have either been designed to determine the vertical structure of a layered earth, as vertical electrical sounding, VES, or lateral variation, as electrical profiling; however, more sophisticated electrical imaging methods are being increasingly used when there are both lateral and vertical variations.

We are increasingly aware that we should not take our environment for granted. Part of the concern is about damage caused by human activity; part is the realisation that nature is not always benign and may produce catastrophes to which we are increasingly vulnerable as populations grow and modern civilisation with its industry and transport systems becomes more complex. Some of these catastrophes can be investigated by solid earth geophysics, notably earthquakes (discussed briefly in Section 5.10) and volcanoes.

Volcanoes can inflict great catastrophes on mankind. Historically, they have caused far fewer deaths than earthquakes (or storms or flooding), but this may not be true in the future, for the geological record reveals vastly greater eruptions in the past. For example, the 1980 eruption of Mt. St. Helens, cause of the greatest volcanic damage in the United States in the 20th century, erupted 1 km3 of material, but in 1783–1784 Laki, Iceland, poured out 15 km3 of basalts, and Krakatoa in 1883 blew out a similar volume, while in 1815 Tambora, Indonesia, erupted about 50 km3 of magma. But 700,000 years ago, the Long Valley Caldera of California (not yet extinct!) discharged 500 km3 of ash, and even larger eruptions have occurred. In contrast, it is unlikely that tectonic earthquakes much larger than those experienced this century can occur.

On the brighter side, eruptions rarely occur without being preceded by increased activity long enough in advance to allow time for evasive action, one reason for their relatively low death count. …

Civil engineering often needs detailed information about the subsurface before starting construction of dams, bridges, roads, airports, buildings, tunnels, and so on. In the past, site investigation relied heavily on drilling, but though drilling can provide essential information it can miss important features between boreholes, and it does not give information about hazards such as earthquakes. A better strategy is first to carry out a combined geological and geophysical survey, and then concentrate drilling where the survey shows it will be most useful. For successful results, geophysicists and geologists need to be clear what information the engineer needs, while the engineer needs to understand what kinds of information the geophysicist and geologist can offer, and their limitations.

Applications where geophysics can be of use to civil engineers include mapping earthquake probability and severity; measuring the depth of unconsolidated cover or weathering, or the extent of infilling; detecting fracture zones; finding pipes and buried objects; locating voids, caves, and old mine workings; and investigating contaminated ground. Some of these have already been considered briefly, such as earth quakes (Chapter 5) and contaminated ground (Section 26.7). This chapter considers only the investigation of cavities and voids, which offers straightforward applications of geophysics.

Temperature generally increases with depth in the Earth, and this is responsible for the maturation of hydrocarbons, many forms of mineralisation, thermal metamorphism and, of course, volcanism and other igneous activity. At sufficient depth, rocks are too hot to fracture and deform ductilely, while at even greater depths they are hot enough to flow like a very viscous liquid. It is this ability to flow that allows the Earth to be an active planet, with mountain building, earth quakes, and plate tectonics; without its hot interior the Earth would be a dead planet, like the Moon.

To understand these processes we need to know the temperatures at different depths within the Earth. Unfortunately, there is no direct way of measuring temperature below the few kilometres that boreholes reach, and temperatures at greater depths are mainly inferred from measurements of the temperatures and thermal properties of rocks near the surface.

Radioactivity surveying measures the natural radioactivity due to potassium, thorium, and uranium in near-surface rocks, which has applications in geological and geochemical mapping, and is used to find ores of uranium and thorium or other types of ore that have associated radioactivity. It also has environmental applications, mapping radon, a hazard to health, in surface rocks and waters.

The most common surveying method detects γ rays, which can be used to identify the source element as well as detect the presence of radioactivity, and can be employed in ground or airborne surveys, but radon measurement often requires sampling below the surface.