Scale Model Studies of a Transient Electromagnetic Prospecting System Using an Interactive Minicomputer
Scale models have been used for over thirty years in studying the electrical and electromagnetic techniques of geophysical prospecting, and are particularly useful when mathematical or numerical methods are unreasonably difficult. A model study facility has been set up at Macquarie University, Sydney, Australia, to study the transient electromagnetic method of geophysical prospecting. The basic unit in the facility consists of a 35Kword INTERDATA minicomputer which controls the timing and waveform of the transmitted pulse as well as subsequent data collection and processing. A wide-band amplifiler provides an interface between the computer and the small multiturn loops used for model studies. Input and output data are fed via the computer to a teletypewriter. The use of an interactive technique between the computer and operator enables optimum timing and amplifier settings to be easily determined for each model. A model of a uniformly conductive half-space was used to validate the experimental arrangement, and excellent agreement was found between theoretical and model results. A model study of an orebody enabled estimates of bulk conductivity to be made.
Broadband Electromagnetic Methods
Recent applications of broadband electromagnetic methods used in exploration geology are covered in this chapter. Historically, electromagnetic detection systems used in mineral exploration efforts were designed to utilise a single frequency or a limited number of discrete frequencies. Application of electromagnetic detection methods in geologically complex environments has created a need for a more complete method of discerning the total geological environment. The objective is to be able to map the total geoelectric section, including resistive as well as conductive units. ‘Broadband’ electromagnetic methods have been developed to meet these needs. Electromagnetic detection systems capable of spanning up to five decades of spectral range are in current use. Recent developments in hardware technology and also in in-field data collection, processing, and sophisticated interpretational analysis are discussed in the chapter. These recent advances have contributed to our ability to resolve details of the three-dimensional distribution of resistivity in the subsurface. Results of both frequency-domain and time-domain surveys are presented.
The Frequency Dependence of the Perturbation Fields due to Conductivity Inhomogeneities in the Earth
The geomagnetic perturbation fields due to conductivity discontinuities in the Earth are investigated. Two models, one in which the discontinuity extends to infinite depth, and a second one which consists of a dike of finite depth are considered. The perturbation fields are studied for several different frequencies of the alternating inducing field for each model. Both the H-polarization and E-polarization cases are considered and the perturbation field component profiles as a function of height above the surface of the conducting region are studied. The perturbation fields are strongly dependent on frequency, and significant differences are exhibited between the H-polarization and E-polarization cases.