Electromagnetic wave comprises of coupled alternating electrical and magnetic fields, directed at right angles to each other and to power vector defining direction of propagation. Electric field vectors will always align themselves at right angles to completely conductive surfaces and wave can thus be guided by enclosing conductors. Extent to which this is probable is managed by relationship between wavelength of radiation and dimensions of guide. Waves at VLF frequencies propagate very resourcefully over long distances in waveguide formed by ground surface and ionosphere.
Neither the Earth nor the ionosphere is the perfect conductor, and some VLF energy penetrates ground surface or is lost in space. Without this penetration, there would be neither military nor geophysical uses. Since it is, waves can be detected tens of metres below sea surface and are ideal for communicating with submarines. Amplitudes decrease exponentially with depth and secondary fields generated in subsurface conductors are likewise attenuated on their way to surface that is VLF surveys are skin-depth limited.
There are more than score of stations around world transmitting VLF signals continuously for military objectives. Message content is usually superimposed by frequency modulation on sinusoidal carrier wave, but occasionally transmission is chopped in dots and dashes resembling Morse code. Making geophysical use of the quenched-carrier signals is very hard. Transmission patterns and servicing schedules differ extensively but makers of VLF instruments are generally aware of current situation and give information on their websites.
Detecting VLF fields:
Geophysical user of VLF signal has control over neither amplitude nor phase of the signal. Readings of a single field component at single point are thus meaningless; one component should be selected as a reference with which strengths and phases of other components can be compared. Obvious choices are horizontal magnetic and vertical electric fields, as the approximate most closely to primary signals. VLF magnetic fields are detected by coils in which currents flow in proportion to number of turns in coil, the core permeability and magnetic field component along coil axis. No signal will be noticed if magnetic field is at right angles to this axis.
VLF electric field will induce alternating current in aerial comprising of straight conducting rod or wire. Signal strength is roughly proportional to amplitude of electric-field component parallel to aerial, and to aerial length.
Magnetic field effects:
Eddy currents induced by VLF magnetic field generate secondary magnetic fields with same frequency as primary but usually with different phase. Any vertical magnetic component is by definition anomalous, and most VLF instruments compare vertical with horizontal magnetic fields, either directly or by estimating tilt angles. Directions of changes in secondary magnetic fields are always in opposition to changes in primary field. Directly above steeply dipping sheet like conductor this secondary field may be powerful but will be horizontal and will not be noticed by most systems. On either side there will be detectable vertical fields, in opposite directions, stating anti-symmetric anomaly. Steeply dipping contacts also generate VLF anomalies that are positive or negative depending on sign convention. Classical anti-symmetric thin conductor anomaly can be looked on as being generated by two contacts very close together.
Two steeply dipping conductors close to each other generate a resultant anomaly which is usually like the sum of anomalies which would have been generated by each body singly. Where, though, one of the bodies is steeply dipping and other flat lying, results are harder to anticipate.
Electric field effects:
As Earth is not perfect conductor, VLF electric vectors near its surface are tilted, not vertical, containing horizontal components. Above homogeneous ground horizontal field would vary in phase from primary (vertical) field by 450, would lie in direction of propagation and would be proportional to square root of ground resistivity. Over the layered earth, magnitude of horizontal electric field (or tilt of total field) records average (apparent) resistivity, strongly biased towards resistivity of ground within about half a skin depth of surface. Phase angle will be greater than 450. if resistivity increases with depth in layered earth and less than 450 if it decreases. Sharp lateral resistivity changes distort this simple picture and very good (usually artificial) conductors generate secondary fields which invalidate assumptions on which resistivity calculations are based.
If horizontal primary and secondary fields which vary in phase are combined, resultant is also horizontal but varies from two components in both magnitude and phase. Secondary field which is vertical and in phase with primary produces a resultant that has same phase but is tilted and stronger. Vertical secondary field in phase quadrature with primary generates elliptically polarized wave. These are special cases. In general situation of inclined secondary field which is neither in phase nor in phase quadrature with primary, a tilted, elliptically polarized wave is generated. As secondary field has horizontal component, tangent of tilt angle is not similar to ratio of vertical secondary field to primary and, because of the tilt, quadrature component of vertical secondary field doesn't define length of minor axis of the ellipse.
Magnetic-component response of good conductor relies critically on orientation. This is also true in conventional EM surveys but EM traverses are generally laid out at right angles to probable geological strike, automatically making sure good coupling. In VLF work traverse direction is almost unrelated, critical parameter being relationship between strikes of conductor and bearing of transmitting station. A body which strikes towards transmitter is said to be well coupled, as it is at right angles to magnetic vector and eddy currents can flow freely. Current flow will or else be limited, decreasing strength of the secondary field. If probable strike of conductors in given area is either variable or unknown, two transmitters, bearing roughly at right angles to each other, must be utilized to generate separate VLF maps. Mercator projection map is of only restricted use in finding true bearings of VLF transmitters. Special Great Circle paths can be found using the computer program or globe and piece of string.
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