Electromagnetic Depth Soundings

10.------------------------, 10.------------------------. N ::IE ~ w ~ '" "'0.1 ~ 0 M z SIDE VIEW PLAN VIEW 0.01 LI --'---'---LLL-'---LLLlI ~-L---"---LLL..ll..L.LJ'-':';;' 0.01 1 100 10 100 fr Fig. 1. The behavior of the magnetic and electric fields from a vertical magnetic dipole source at the surface of the earth (after Wait [1951,1955]). earth, the experimental curve so plotted should have the same shape as a portion of the ap­ propriate theoretical curve, but with the ordinates and abscissas shifted by an amount dependent on the resistivity. The conductivity can be determined from the amount of shift between the field data and the theoretical curve. A detailed description of the curve matching procedure is found in a text by Keller and Frischknecht (1966). The curve matching procedure, though used, has several disadvantages. Measurements must be made over a diagnostic portion of the theoretical curve, one in which there is some curvature, so that the amount of shift required to make a match can be determined uniquely. This means that the approximate conductivity of the earth must be known when measurements are made. Secondly, determination of a single value of conductivity requires measurements made over a wide range of frequencies. This would appear to be wasteful of data, inasmuch as the equations indicate that a single measurement at a single frequency should be enough to de­ termine conductivity.

Contenu

Electromagnetic Sounding Methods -Introduction and History.- History of Electromagnetic Methods.- Electrical Prospecting with the Transient Magnetic Field Method.- I. Physical and Mathematical Foundation for the Transient Magnetic Field Method.- 1. Time-Domain and Frequency-Domain Methods in the Theory for Transient Magnetic Fields.- 2. Primary Magnetic Field of a Dipole.- 3. Use of the Principle of Reciprocity for Determining Transient Magnetic Fields.- II. Calculation and Analysis of Theoretical Curves.- 1. Numerical Evaluation of Transient Magnetic Fields.- 2. Asymptotic Behavior of the Vertical Component of the Transient Magnetic Field.- 3. Apparent Resistivity in the Transient Magnetic Field Method.- 4. Computation of Wave-limit Curves for Transient Magnetic Fields.- 5. Computation of Theoretical Curves for the Late Stage of the Transient Magnetic Field.- 6. Master Curves for Transient Magnetic Fields.- 7. Construction of Curves for Transient Magnetic Fields in the Far Zone.- 8. Analysis of Theoretical Curves for Transient Magnetic Fields.- 9. Maximum Resolution for Transient Magnetic Fields.- 10. Equivalent Curves for Transient Magnetic Fields.- III. Equipment.- 1. Recording Transient Magnetic Fields.- 2. Signal-to-Noise Ratio.- 3. Block Diagram for Equipment Used in the Magnetic Transient Method.- 4. Generating Equipment.- 5. Receiving Equipment.- 6. Particular Methods for Conversion.- 7. Alignment of an Amplifier.- 8. Equipment for Generating Synchronous Time Marks.- 9. Summary.- IV. Field Methods and Interpretation.- 1. Field Methods.- 2. Construction of Apparent Resistivity Curves.- 3. Topographic and Survey Control.- 4. Reduction and Presentation of Survey Results.- 5. Logistic Considerations.- 6. Basic Interpretation of the Final Stage of the Magnetic Transient.- 7. Use of Electric Log Data.- 8. Distortions of Magnetic Transient Curves.- 9. Possible Uses for the Magnetic Transient Method in Studying Structural Geology.- Appendix 1. Table of Hyperbolic and Inverse Hyperbolic Functions of Complex Arguments for Computing Wave Curves for a Transient Field.- Appendix 2. Typical Three-Layer and Four-Layer Wave Curves for Transient Fields.- Appendix 3. Nomogram for Determining the Correction for Finite Length of Source Dipole.- Appendix 4. Forms for a Field Log.- Fundamentals of Electromagnetic Sounding.- I. Geological Basis for Electromagnetic Sounding.- Field Sources and Models of the Geoelectric Section.- Maxwell's Equations and the Vector Potential.- Vector Potential in a Homogeneous Medium.- The Electromagnetic Field in a Layered Anisotropic Medium.- Electromagnetic Fields at the Surface of a Uniform Anisotropic Half- Space.- Calculation of the Quasistatic Electromagnetic Field at the Surfaces of a Layered Anisotropic Medium.- II. Principles of Quasistatic Electromagnetic Sounding.- Determining the Resistivity of a Homogeneous Anisotropic Half-Space.- Two Principles of Electromagnetic Sounding.- Induction Sounding.- Geometric Soundings.- Equivalence for Thin Layers.- Electromagnetic Sounding in the Presence of an Insulating Screening Layer..- Two Forms of Anisotropy and Their Effect on Electromagnetic Sounding..- Conclusions.- Concerning Some Causes for the Distortion of Transient Sounding Curves.- Measuring Array Located on a Sloping Surface.- Location of Source and Receiver at Different Heights.- Insulating Hemisphere near the Observation Point.- Concerning the Factors Distorting Frequency Sounding Curves.- Four-Layer Master Curves for Frequency Electromagnetic Sounding.