Objects of Geophysical Surveys

The objects of geophysical survey are to locate subsurface geological structures or bodies and where possible to measure their dimensions and relevant physical properties. In oil prospectingstructural information is sought because of the association of oil with particular features such as anticlines in sedimentary rocks. In mining geophysics the emphasis is on detection and determination of physical properties. Though mineral ore bodies give distinctive and measurable geophysical indications they are often of irregular shape and occur in rocks of complex structure, making precise quantitative interpretation difficult or impossible. In site investigation engineers may be interested in both structure and physical properties. Variations in bedrock depth are often needed on major construction sites and the mechanical properties of the overburden may be important when heavy loads have to be sustained.

A geophysical survey consists of a set of measurements, usually collected to a systematic pattern over the earth's surface by land, sea or air, or vertically in a borehole. In choosing the geophysical technique to be used to study a problem the contrasting properties of the subsurface rocks and their homogeneity within a particular formation are important factors to be considered.

The properties of rocks of which most use is made in geophysical prospecting are elasticity, electrical conductivity, density, magnetic susceptibility and remanence and electrical polarizability. To a lesser extent other properties such as degree of radioactivity are also utilized.

Gravitational and magnetic surveys make use of natural fields of force. Most seismic and electrical (including electromagnetic) methods, which involve the elastic and electrical properties of rocks, necessitate introducing energy into ground. Since the source is under control the source to detector distance can be varied. This makes it possible when gravity and magnetic fields are being utilized.

Many factors - geological, economic, logistic and what we might call geophysical govern the choice of method for particular survey. In many instances more than one method will be used to survey the same ground. The search for oil may start with gravity and airborne magnetic work as a preliminary to seismic shooting in localities determined by interpretation of the earlier surveys. Combining electromagnetic, magnetic and gravity data may make it possible to decide whether certain indications are of valuable metallic ores or merely of concentrations of uneconomic minerals.

Remote sensing aircraft

The final decision to be taken in prospecting and site investigation is whether or when to drill. In civil engineering where depths of investigation are small and high accuracy is required it may pay to dispense with geophysics and drill from the outset. As depths or distances to be covered increase, particularly if the geology is simple, geophysics will be increasingly used. In prospecting for oil the structures to be discovered lie at great depth, making exploratory drilling on a large scale prior geophysical survey out of question.

Generally, both economic and scientific factors have to be considered in deciding a drilling programme. The cost of drilling to map accurately the undulating surface of the bedrock on a site may be very high relative to the cost of using geophysics but if the economic penalties of inaccuracy are substantial it may be cheaper to drill. On the other hand, where a large area must be covered in detail but high accuracy is not essential, geophysics can be the obvious answer.

(D.H. Griffiths, R.F. King Applied Geophysics for Geologists and Engineers. The Elements of Geophysical Prospecting. Oxford, 1981)

4. Form adjectives from the following words.