electromagnetic methods in applied geophysics Varying currents are associated with varying magnetic fields, and varying magnetic fields induce currents in conductive materials. These currents in turn produce
secondary alternating magnetic fields which can be detected at the ground surface. These facts form the basis of electromagnetic methods in applied geophysics. Broadly, the methods fall into two main categories. In
continuous wave (
CW) methods, the inducing current has a sinusoidal form and a fixed frequency, which is generally in the range from a few hundred to a few thousand Herz (Hz). Some use is also made of communications transmissions (particularly military ‘VLF’ transmissions in the 15–25 kHz range) and of natural fields (magnetotellurics), which cover a very wide range of frequencies. In
transient e.m. (
TEM), current is induced by the collapse in magnetic field which occurs when current flow in a circuit is terminated abruptly. The TEM method is fundamentally multi-frequency and provides information which can be obtained from CW systems only by operating at a number of discrete frequencies. Another advantage of TEM is that measurements are made at times when no current flows in the source circuits, whereas CW measurements of secondary magnetic fields are made against primary-field backgrounds which are generally much stronger.
Various configurations of local electromagnetic source have been used, including very long grounded wires, large rectangular loops, (sometimes enclosing the area being surveyed and sometimes offset to one side of it), and small portable loops with diameters of about 10 cm. Small loops, which produce radiated fields approximating to those produced by alternating magnetic dipoles, are usually ferrite-cored to enhance the magnetic effects. Receiver coils are almost always small loops. Measurements may be made, very crudely, by measuring the dips of the resultants of the primary and secondary magnetic fields or in more sophisticated ways by measuring amplitudes and phases of various field components. Measurements of field directions and phase shifts make it possible to distinguish primary from secondary fields, and to estimate conductivity parameters for subsurface conductors.
One great advantage of electromagnetic methods is that they can be used from aircraft, making it possible to cover large areas rapidly. Most airborne CW systems now use multiple receiving coils with different orientations, and some use multiple transmitting coils. Coils may be mounted in the nose and at the tail of the survey aircraft, or at the wingtips, or in tubular fibreglass ‘birds’ towed beneath helicopters. Whatever the arrangement, corrections must be made for changes in separation and, more importantly, relative orientation of the receiver and transmitter coils caused by vibration and flexure. The importance of these corrections can be appreciated from the fact that, whereas a secondary field measured on the ground may exceed 10 per cent of the primary field, significant airborne secondary fields may amount to no more than a few parts per million.
Although electromagnetic methods were primarily developed for mineral search, relying on the very high conductivities of massive sulphide deposits, they are increasingly being applied to regional conductivity mapping for hydrogeological and environmental purposes. The use of repeat airborne surveys to monitor salinization of irrigated agricultural lands has been pioneered in Australia using both CW and TEM methods.
John Milsom
