482 ENGINEERING GEOLOGY/Geophysics Geophysics J K Gascoyne and A S Eriksen, Zetica, Witney, UK ß 2005, Elsevier Ltd All Rights Reserved Introduction Geophysics can be defined as the study of the Earth through the measurement of its physical properties Use of the discipline dates back to ancient times, but only since the advent of modern-day instrumentation has its application become widespread The development of modern geophysical techniques and equipment was initially driven by oil and mineral exploration during the early to middle parts of the twentieth century, and many of the instruments used today in engineering geophysics owe their evolution to the field of exploration geophysics Engineering geophysics involves using geophysical techniques to investigate subsurface structures and materials that may be of significance to the design and safety of an engineered structure Unlike the deeper investigations associated with exploration geophysics (up to 2–3 km), engineering surveys are usually concerned with investigation of the near-surface, at depths in the range of 1–100 m The key advantages of geophysics over intrusive site-investigation techniques, such as digging trial pits or drilling boreholes, are that geophysical methods are comprehensive and non-invasive Large areas can be evaluated rapidly without direct access to the subsurface One class of engineering geophysics, borehole geophysics, is an exception in that it makes use of boreholes already drilled to sample the local area around the borehole When combined with intrusive methods, geophysics provides a cost-effective means of analysing the undisturbed subsurface to aid selection of, and interpolation between, widely spaced sampling locations Engineering geophysics can be applied throughout the life cycle of an engineered structure, starting with the initial ground investigation to determine the suitability of a particular site and provide design-sensitive and critical safety information This may be followed by materials testing during the various stages of construction, monitoring the impact of construction on surrounding structures, on-going monitoring of the integrity of structures after completion, and helping to determine when to schedule essential maintenance tasks, such as pavement or ballast renewal on a road or railway, respectively The success of all geophysical methods relies on there being a measurable contrast between the physical properties of the target and those of the surrounding medium The properties used are typically density, elasticity, magnetic susceptibility, electrical conductivity, and radioactivity Knowledge of the material properties likely to be associated with a target is thus essential to guide the selection of the correct method to be used and to interpret the results obtained Often a combination of methods provides the best means of solving complex problems It is sometimes the case that, if a target does not provide a measurable physical contrast, the association of the target with other measurable conditions may indirectly lead to detection Methods Engineering geophysical methods can be split into two main categories – passive and active With passive methods, naturally occurring sources, such as the Earth’s magnetic field, over which the observer has no control, are used to detect abnormal variations in background caused by the presence of the target Interpretation of this data is non-unique and relies heavily on the knowledge of the interpreter Active methods involve generating signals in order to induce a measurable response associated with a target The observer can control the level of energy input to the ground and measure variations in energy transmissibility over distance and time Interpretation of this data can be more quantitative with improved depth control compared with passive methods, but ease of interpretation is not guaranteed Table lists of some of the techniques most commonly used in engineering geophysics Measurements are commonly taken at the surface and from boreholes, underground mineworkings, over or under water, or from aircraft platforms The advent of powerful computer-aided modelling has led to the development of a number of sophisticated imaging techniques, such as cross-hole seismic and resistivity tomography and reflective tomography, which are capable of imaging the properties of the ground in three dimensions between the surface and two or more boreholes or beyond the face of a tunnel Armed with a knowledge of the physical properties of a target (see Table 2), its burial setting, and the requirements of the survey, a feasibility assessment is carried out by a geophysicist to determine the likely deliverables of a geophysical survey Based on the results of this assessment, an appropriate geophysical