Very low frequency method
The very low frequency method (VLF method) is a passive electromagnetic method in geophysics . Using electromagnetic induction , conclusions can be drawn about the conductivity of the subsurface.
Procedure
The VLF method uses powerful transmitters from different countries that can be received worldwide and are used, for example, to communicate with submarines. Such long-wave transmitters are operated in a frequency range from 15 to 25 kHz. The transmitted horizontal component of the primary magnetic field induces eddy currents in the subsurface at a great distance from the transmitter (compare skin effect ). The secondary magnetic field (vertical component) generated in this way has an amplitude that is different from the primary magnetic field (normal field) and a phase shift, depending on the electrical conductivity of the subsurface. VLF measurements are used in particular to record changes in conductivity, caused by elongated vertical structures, in the field of geology and hydrology .
Working principle
The electromagnetic field of a VLF transmitter (vertical antenna or dipole) spreads out in the horizontal plane as concentric circles.
At a great distance from the transmitter (distance corresponding to a few skin depths of the transmitter frequency used), the primary field has the properties of a plane wave. A primary horizontal magnetic field (normal field, perpendicular to the direction of propagation) and a vertical electric field therefore exist above a poorly conductive subsurface.
If an electrically conductive disruptive body is present, eddy currents are induced in it, the secondary magnetic field of which is superimposed on the normal field of the transmitter. The VLF method measures the amplitude ratio between the vertical component of the secondary magnetic field and the horizontal component of the primary magnetic field (Hz / Hy). In addition to the amplitude difference, there is also a phase shift between the primary and secondary magnetic fields. The in-phase component (real part) is defined by the amplitude of the secondary magnetic field at which the primary magnetic field has its maximum. The quadrature component (imaginary part) is the amplitude of the secondary field with a phase shift of 90 ° compared to the amplitude maximum of the primary field.
The representations of the measurement results of the VLF method typically show percentage ratios of in-phase and quadrature along a profile and the indication of the frequency used, the direction of the transmitter, the direction of strike of the structure and the filters applied to the data. The contact between the well-conducting disruptive body and the surrounding poorly conducting material is indicated by the point of inflection in the in-phase values. Since the primary field in the VLF method originates from a specific transmission position, the measurement geometry in particular must be taken into account in the VLF method. The transmitter and the elongated disruptive body must meet the TE polarization (transversal electrical). I.e. the transmitter must be in line with the length of the structure. Any deviation from this measurement geometry creates so-called "mode mixing" and falsifies the measurement results. Therefore, when planning a VLF measurement, transmitter directions must be selected according to the expected structure geometry. The exploration depth of the method is determined by the penetration depth of the electromagnetic wave and can be estimated using the so-called skin depth.
Data evaluation and modeling
VLF measurements are mainly used to quickly map lateral conductivity changes, such as B. to determine the extent of an ore vein. From the turning points of the measurement data, the position of the maximum conductivity contrast and thus the position of the contact between z. B. the vein and the host rock can be determined. A Fraser filter is often applied to the data in order to display the position of the maximum conductivity contrast using maxima and minima of the in-phase instead of the turning points. The estimation of the depth and the resistance of the structure is generally done by two-dimensional forward modeling or inversion of the data.