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.

Texte du rabat

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.- II. Calculation and Analysis of Theoretical Curves.- III. Equipment.- IV. Field Methods and Interpretation.- 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.- II. Principles of Quasistatic 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.