Electromagnetic Techniques
Controlled source AudiomagnetoTellurics (CSAMT)
Detail
Temporal variations in the Earth's magnetosphere and ionosphere, caused by factors such as the solar wind and diurnal variation of the Earth's magnetic field, result in natural low-frequency magneto-telluric fields across the globe which induce alternating telluric currents within the ground. Higher frequency signals resulting from worldwide electrical storms are superimposed on these low-frequency fields. Conventional magneto-telluric survey techniques, such as natural-source MT and audio frequency MT, utilise the magnetic and electric components of the MT fields and currents in order to map variations in subsurface resistivity to depths of up to several hundred kilometres. However, the erratic nature of the source in terms of strength and direction mean that the signal has to be stacked for long periods of time at each station.
CSAMT is a specific derivation of conventional natural-source and audio frequency magneto-telluric methods, that utilises an artificial source (typically in the range 0.1Hz to 10kHz) in order to speed up data acquisition and provide a more reliable and stronger signal. The source normally comprises either a loop or long grounded dipole of up to several kilometres length. The dipole may be combined with a second orthogonal transmitter in order to provide two source polarisations. Simultaneous measurements of five separate parameters are taken at each location; the two components of the electric field and the three components of the magnetic field. Electric field measurements are acquired using orthogonal dipoles whilst the magnetic field vectors are measured using multi-turn high permeability coils. Modern CSAMT instruments also enable measurement of natural and audio-frequency MT signals in order to provide an extended exploration depth range (the lower the frequency the greater the depth of investigation).
Measurement of the change in the electric and magnetic fields over a range of frequencies enables an apparent resistivity sounding curve to be constructed. Apparent resistivity is combined with a measure of the phase difference between the electric and magnetic components. Over isotropic homogeneous ground the magnetic component will lag behind the electric component by Pi/4. However, if the resistivity varies with depth the measured phase difference will be different. Joint inversion of the data using both phase and apparent resistivity provides a more robust interpretation. The data are normally displayed as apparent resistivity versus frequency and phase difference versus frequency plots.
