Geological applications of the VLF method

Abstract

Just like in the case of any electromagnetic (EM) method, the electrical properties of the ground affect the behaviour of radio waves as well. The first EM measurement using radio frequencies applied wave-tilt techniques and was made at relatively high frequencies with shallow penetration depth [1]. The earliest EM measurement with radio waves of 3–30 kHz was carried out in 1963 with the aim of ore prospecting [2]. In the late sixties commercially available ground very low frequency (VLF) instruments were introduced into near-surface exploration. These instruments can be used to observe either the magnetic field and/or to determine the terrain’s apparent resistivity. Over 1D half-space the magnetic field at the surface is linearly polarized. However in the presence of a lateral conductivity inhomogeneity – situated between the surface and skin depth – the total magnetic field at the surface will be elliptically polarized due to the induced magnetic field. Usually the induced vertical magnetic field component is small compared with the primary azimuthal magnetic field component. In this case the ellipticity of the magnetic polarization ellipse is approximately equal to the quadrature component of the ratio of the vertical and the azimuthal magnetic component, and the tilt angle of the ellipse approximately equals the real component of the same ratio [3]. If the radial electric field and the azimuthal magnetic field component are known the apparent resistivity at the VLF frequencies can be derived. Takacs was the first in Hungary to develop the radiokip method with instruments to apply the EM fields of distant LF transmitters for near-surface geological explorations [4]. The VLF method was introduced and intensively used in Transdanubian Central Range bauxite exploration by ELGI [5]. In the frame of this work Farkas developed the VLF invariant resistivity method based on the concept of the magnetotelluric impedance tensor [6]. The VLF method utilizes the frequency range of 10 kHz–30 kHz, providing poor depth resolution. To overcome this resolution problem this frequency range was extended and in addition to the VLF carrier waves the low frequency (LF) signals from civilian radiotransmitters are also utilized by the RMT (Radiomagnetotelluric) method. Takacs carried out and interpreted the first RMT soundings in Hungary in the range of 18.3 kHz and 630 kHz. He measured both the electric and magnetic field components and MT 1D inversion was applied [7]. The radiofrequency resistivity (RF-R) device measuring in the range of 12 kHz–240 kHz was successfully applied to delineate karst structures [8]. In 1973 Tilsley applied a portable VLF transmitter as a supplementary source to the regular VLF transmitters [9]. To cope with the interpretation problem arising from the mutual position between transmitter and structural strike direction or to overcome the poor coupling with the target the use of a portable transmitter can be recommended. For the determination of sufficient distance between the portable VLF transmitter and VLF profiles numerical modelling is also needed [10].