Electrical Methods in Geophysical Exploration of Deep Sedimentary Basins

Abstract

The applications of deep probing electrical exploration methods are extensive. Primarily they are used in the general exploration of deep or shallow sedimentary basins, the exploration of the Earth’s deep crust, and in prospecting for oil and gas, geothermal and groundwater reservoirs. Some of the methods are also used in earthquake prediction research and in deep and shallow environmental studies and geotechnical work. This wide application highlights the importance of the electrical methods in practical geophysics and has led to the development of many different techniques tailored to these varying types of geological goal. The stated objective of this book is to provide a working knowledge of these deep-penetrating electrical methods, with specific reference to sedimentary basins. It is aimed at the practising geologist or geophysicist, especially those in the petroleum industry, and aims to serve as a guide, particularly in survey planning and data interpretation. The text concentrates on concepts, methods, exploration strategy and interpretation. Accompanying topics of mathematical equation derivation, instrumentation, field procedures and data processing are all covered but in a more limited way. The text opens with two introductory chapters, the first discussing the role of electrical methods in a selection of common fields of exploration, highlighting the importance and central role they play. Chapter 2 covers the electrical properties of sedimentary rocks briefly-the material is familiar but may be covered unsatisfactorily for a general reader. The main body of the book is divided into three chapters, discussing separately direct current resistivity methods, natural-source electromagnetic methods and controlled-source electromagnetic methods. Each chapter introduces its method with an overview of how it is applied, the data collected, the assumptions inherent to it, the limitations of its application and the associated concepts and terminologies appropriate to that method, for example the skin-depth concept in electromagnetic studies. The background theory to the methods is discussed, little attempt is made to derive the mathematical equations from first principles, and the discussion concentrates mainly on the practical aspects of modelling and data analysis. Variations of each method are discussed to highlight their advantages and disadvantages for particular survey objectives in terms of cost, data quality and sensitivity to a particular target. This gives a good practical guide to survey design. The final chapter reviews specifically the possibilities for the direct detection of petroleum. The direct measurement of the resistivity of oil and gas reservoirs, electromagnetic reflection methods and induced polarization methods, amongst others, are all discussed historically in the context of their success in directly determining the existence of hydrocarbons. This is given as a review of past work and no new contribution is attempted. This is a very readable and refreshing introductory book for some of the more practical aspects of the electrical methods used in geophysics. Often its brief treatment of some of the underlying theory is frustrating-this could be covered in a way more satisfying to the general reader by the addition of