Analogue Model Measurements for Electromagnetic Variations Near Vertical Faults and Dykes

Abstract

An analogue model for studying the behavior of the natural geomagnetic and telluric field variations for various geological structures is described. Measurements of amplitudes and phases for the horizontal electric, the horizontal magnetic, and the vertical magnetic field components are obtained and discussed for various conducting vertical faults and dykes for both the H a n d the E polariza- tions. INTRODUCTIONA comparison of the amplitudes and phases of the natural geomagnetic and telluric field variations provides a method for obtaining information on the earth's crustal structure. The behavior of the naturally occurring electromagnetic variations observed a t the earth's surface is in general determined by the nature of the source field and by the distribution of electrical conductivity within the earth. Although much work has been done since the suggestion by Schuster (1889) that the conductivity distribution could be studied by measurements of the magnetic variations a t the surface of the earth, many interesting problems still remain. One problem of continuing interest is the study of the behavior of the electromagnetic field components in the neighborhood of discontinuities in conductivity, such as are found a t coastlines and faults in the earth's crust.The magnetotelluric method of analysis was suggested by Kato and Kikuchi (1950) and by Tikhonov (1950), and outlined by Cagniard (1953). The analysis assumes plane waves incident on a horizontally stratified conductor, and requires, as discussed by Wait (1954), that the fields be essentially constant from the point of observation to a horizontal distance of the order of a "skin depth" in a uniform conductor of infinite depth. Since the electromagnetic field is not uniform in the neighborhood of discontinuities in conductivity in the horizontal direction, the magnetotelluric method has limited validity in coastline and fault problems.Another method of analysis in common use a t present is based on the three conlponents of the natural magnetic field variations. The ratios of the amplitudes, together with the relative phases, are diagnostic of the subsurface structure.The problem of conductivity discontinuities has received considerable attention in recent years. The case of the magnetic field parallel to the trace of a vertical fault was studied by d' Erceville and Kunetz (1962). I t was extended to the case of a dyke discontinuity by Rankin (1962). Weaver (1962) treated the case of the magnetic field normal to the trace of the fault. His results indicate that the amplitude of the vertical magnetic field component increases sharply