X CONTENTS 



CHAPTSR PAGB 



6. Applications of Geophysics in Engineering (Cont.) 



C. Corrosion Surveys 63 



D. Pipe and Metal Location 63 



E. Sound Ranging and Other Acoustic Detection Methods 63 



F. Gas Detection 64 



G. Thermal Detection 64 



PART II 



7. Gravitational Methods 67 



I. Introduction 67 



II. Rock Densities 70 



A. Determination of Rock Densities 70 



B. Factors Affecting Formation Densities 72 



C. Tabulations of Mineral and Rock Densities 77 



III. Gravitational Constant; Gravity Compensator; Gravity Multi- 



plicator 85 



IV. Principles of Gravitation as Applied in Gravity Measurements 88 



V. Pendulum and Gravimeter Methods 97 



A. Theory of the Pendulum on Fixed and Moving Support 97 



B. Observation and Recording Methods; Pendulum Apparatus — 103 



C. Time-Determination and Time-Signal-Transmission Methods. . . 113 



D. Instrument Corrections in Pendulum Observations 116 



E. Gravimeters 123 



F. Corrections on Observed Gravity Values 135 



G. Theory of Subsurface Effects; Methods of Interpretation 143 



H. Results of Pendulum and Gravimeter Surveys 157 



VI. Time Variations of the Gravitational Field 162 



A. Planetary (Lunar) Variations 163 



B. Secular (Geologic) Variations 165 



C. Changes in Water Level (Tides and the Like) 165 



D. Artificial Mass Displacements (Mining Operations) and the 



Like 167 



VII. Determination of the Deflections of the Vertical 167 



VIII. Torsion-Balance Methods 170 



A. Quantities Measured; Space Geometry of Equipotential 



Surfaces 170 



B. Theory of Torsion Balance 175 



C. Instrument Types, Instrument Constants 192 



D. Corrections 210 



E. Graphical Representation of Torsion Balance Data 244 



F. Theory of Subsurface Effects, Interpretation Methods 250 



G. Discussion of Torsion Balance Results 270 



8. Magnetic Method 293 



I. Introduction 293 



II. Magnetic Rock Properties 297 



A. Definition 297 



B. Methods of Determining Rock Magnetization 299 



C. Numerical Data on Magnetic Properties of Minerals and Rocks. . 309 



D. Factors Affecting Rock Magnetization 314 



