A technique using an ionospheric modification instrument to produce controllable and steerable low frequency ionospheric arrays for remote sensing applications

Remote sensing of the Earth's crust using low frequency radio waves has been understood and applied for several years. The paper describes an application in which the generation of these low frequency probing waves are more favorably under the control of the experimenter and provides much more flexibility in carrying out probing of the Earth's crust. High power ionospheric modification facilities or "ionospheric heaters" as they are sometimes called can be used to generate ELF and VLF by modulating the strong electrojet current system which occurs naturally at high latitudes in the ionosphere. A new facility is now under construction in Alaska that has rather unique capabilities to generate a wide range of low frequencies. The authors, using a lower power heating facility in Alaska, have succeeded in not only generating these low frequencies but forming arrays of low frequency ionospheric dipole elements in the upper atmosphere; when this energy is injected into the Earth-ionospheric wave guide it can he steered over the Earth's crust in directions preferred by the experimenter performing the probing. The paper discusses the instrument, the generation of low frequency waves and ionospheric VLF arrays that can be effectively used for underground propagation experiments confined to a region of several kilometers centered upon the fixed heater location. Such a technique can be useful for the study of geophysics and detection of buried metal objects.