Petroleum Geophysical Survey

Earth’s natural or passive physical properties such as gravity, magnetism, electromagnetism and electrical field and artificial or active properties such as seismic, induced electric fields and radar are utilized for geophysical surveying of the earth. This chapter describes the earth’s gravitational and geomagnetic forces. There are clear distinctions and obvious similarities between the two. The theory shows that both obey the ‘inverse squared law’, but their details, instrumentation, measurements and applications are quite different. Newton’s universal law of earth’s gravitational force is explained. Gravity determination by spring, pendulum and La Cost Romberg balances are included. Absolute gravity is not required; relative gravity or gravity anomaly is determined on the principle that the elongation of a spring or twist of a pendulum is proportional to the earth’s downward gravitational force. A gravity anomaly is not only affected by underground targeted rock but it is also produced by other several factors mentioned in the chapter that must be subtracted from the observed gravity anomaly. The gravity survey is carried out by a grid network of several measuring stations at the surface. Underground geology affects gravity measurements at the surface. The gravity anomaly data is used to interpret the subsurface rock geology, mineral distribution, types of minerals, depth, mass, symmetry, and shape etc. The theory of magnetism is complicated, so only the basics of magnetism are described. A lot of variation, both in direction and magnitude (vector), occur in the geomagnetic field which is explained by simple figures. The total geomagnetic field (F) is resolved into a horizontal component (H) and a vertical component (f z ). Terms like magnetic susceptibility, magnetic induction, magnetic permeability, magnetic elements, magnetic dipole moment, residual magnetism, Curie’s temperature, demagnetization and magnetic materials are discussed. The theory and working of four common magnetometers, fluxgate, proton, and alkali metal vapor and magnetic gradiometer, are given in the chapter. A geomagnetic anomaly is not only created by the subsurface targeted rock but by other factors mentioned in the chapter. Later anomalies are interfering and must be subtracted from the observed anomaly to get the net geomagnetic anomaly. Finally genuine magnetic anomaly data are interpreted in terms of minerals, depth, shape, density and three types of rocks.