VLF Instruments, Physics tutorial

The EM-16:

The EM-16 comprises of housing having the electronics, to which is joined a conventional sighting clinometer, and T-shaped handle having two coils at right angles. Controls comprise two-position station selector switch, calibrated quadrature control and knob which amplifies audio tone which, though frequently very irritating, can be almost inaudible in areas like forests on windy days, where other noises compete. With phase control at zero, strength of tone is determined by setting of volume control and by component of VLF magnetic field parallel to axis of main coil. Measurements are made by finding orientations of this coil which produce nulls (minima). This is simplest if volume control is set so that at null the tone is only just audible. Before reading, direction of minimum horizontal component should be determined. Unless there is important secondary field, this also provides bearing of transmitter. Instrument is held with both coils horizontal, most suitably with short coil at right angles to stomach. Observer turns until null is found, at which stage magnetic field is at right angles to main coil and parallel to short coil. It is rarely essential to adjust quadrature control during this procedure; it must be reset to zero before trying to observe vertical field. There is no way of telling, and no significance in knowing, whether transmitter is to left or right of observer. Without changing position, observer then rotates instrument about short coil as axis in upright position and then tilts it in plane of clinometers (that must now be at eye level). Signal minimum happens when long coil is at right angles to major axis of polarization ellipse. Null will be inadequately defined if quadrature component (minor axis field) is large or if plane of polarization ellipse is not vertical. Definition can be enhanced by using quadrature control to subtract measured percentage of phase-shifted major-axis field, noticed by short coil, from quadrature field detected by long coil. At null, with instrument held in tilted position, quadrature reading provides ratio of ellipse axes and tangent of the tilt angle states in-phase anomaly.

EM-16 sign conventions:

At the null, long handle of the EM-16 points towards region of higher conductivity. The observer with conductor to front will have to lean backwards to get null and will see positive reading on clinometer. Observed from opposite direction, reading would be negative. To avoid confusion, all readings must be taken facing same direction and this must be recorded in field notes even if, as is suggested, a standard range of directions (like N and E rather than S or W) is adopted on all surveys. Quadrature anomalies generally show same polarity as in-phase anomalies but may be reversed by conductive overburden. Reversed in phase anomalies can be occurred by steeply dipping insulators surrounded in conductive country rock, that are rare, or by active sources like live power lines.

The EM-16R:

With additional circuitry contained in EM-16R plug-in module and 2 m length of shielded cable acting as the aerial, EM-16 can be utilized to compute horizontal electric fields. Cable is stretched out towards transmitter and two ends are pegged down. Long coil should point towards transmitter and, for convenience instrument is generally laid on ground. Short coil then detects maximum magnetic-field component. Null is attained by rotating 16R control, giving reading directly in ohm-metres. Phase shifts are also observed. EM-16R resistivities, that use horizontal magnetic field as phase reference, suppose a fixed ratio between horizontal magnetic and vertical electric components. This is not the situation if important secondary magnetic fields are there, and use of more stable vertical electric field as reference is to be preferred in instruments which provide this option.

Other VLF instruments:

Most of the alternatives to EM-16 also record magnetic field variations but estimate field components and their ratios rather than dip angles. Main advances comprise direct recording of data, frequently into memory and to front panel display, and elimination of use of the audible tone. Few instruments can evaluate natural magnetic and two-transmitter VLF fields concurrently, and few have been made self-orientating to enhance speed of coverage. Amplitudes may also be estimated but the base instrument is then required to correct for amplitude variations caused by meteorological changes along long transmission paths. Horizontal magnetic field directions are infrequently recorded but are usually less sensitive and less diagnostic than changes in tilt angle and need directional reference. Several instruments rely, as does EM-16, on crystal-controlled tuning to lock to desired station but others use high-Q tuning circuits. ABEM Wadi scans entire VLF band and presents user with the plot of signal strength against frequency, permitting informed choice of station.

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