This combination of textbook and reference manual provides a comprehensive account of gravity and magnetic methods for exploring the subsurface using surface, marine, airborne and satellite measurements. It describes key current topics and techniques, physical properties of rocks and other earth materials, and digital data analysis methods used to process and interpret anomalies for subsurface information. Each chapter starts with an overview and concludes by listing key concepts to consolidate new learning. An accompanying website presents problem sets and interactive computer-based exercises, providing hands-on experience of processing, modeling and interpreting data. A comprehensive online suite of full-color case histories illustrates the practical utility of modern gravity and magnetic surveys. This is an ideal text for advanced undergraduate and graduate courses and reference text for research academics and professional geophysicists. It is a valuable resource for all those interested in petroleum, engineering, mineral, environmental, geological and archeological exploration of the lithosphere.

A representative ultramafic intrusion at Red Mountain southeast of San Francisco, California was sampled for the purpose of studying the remanent magnetism and susceptibility of ultramafic rocks as functions of serpentinization. Due to the dependence of grain density upon the degree of serpentinization, the former was taken as a measure of the latter. Grain densities were accurately determined from bulk densities by taking into account the porosity and amount of adsorbed water contained in these rocks. In general, the porosity was found to increase with serpentinization. The magnetic data suggest that the remanent magnetization is mainly a chemical remanent magnetization acquired during the process of serpentinization whereby iron atoms, released from the silicate structure of the paramagnetic olivine and pyroxene, are oxidized to form ferrimagnetic magnetite. The intensity of both remanent and induced magnetization increases exponentially with serpentinization. However, the remanent magnetization becomes highly unstable as the rock becomes intensely serpentinized. This is due to the growth of magnetite grains to multi‐domain size and their oxidation, in some cases,to maghemite. The intensity of magnetization also depends on mode of occurrence and state of oxidation of the magnetite, and on original rock composition. Serpentinized dunites were found tobe less magnetic than equally serpentinized peridotites, because the olivine in the dunites has a lower iron content than the olivine and pyroxene in the peridotites. Therefore, it is expected that some serpentinized ultramafic bodies will have highly variable magnetic anomalies associated with them, while others of dunitic composition will have low magnetic anomalies.

The use of gravity gradient (GG) data in exploration is becoming more common. However, interpretation of gravity gradient data is not as easy as the familiar vertical gravity data. For a given source, regardless of its simplicity, gravity gradients often produce a complex pattern of anomalies (single, doublet, triplet, or quadruplet) as compared to the simple single (monopolar) gravity anomalies. This paper is a minitutorial on gravity gradients and is designed to provide a simple explanation of the complex pattern of GG anomalies and suggest some guidelines for the interpretation of measured surface GG data.To demonstrate the complex pattern of anomalies associated with gravity gradients, I will compute the gravity gradient components of the full gradient tensor starting with the basic building block, the gravitational potential. This will be followed by computing and examining: Corresponding author: afifhsaad@netscape.net AUGUST 2006 THE LEADING EDGE 949 Figure 15. Borehole vertical gravity and gravity gradient (apparent density) profiles due to a sphere of radius R, density contrast ∆ρ. Borehole distance X = 2R from the center of the sphere.

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