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.
Equipment for carrying out quantitative geoelectromagnetic model experiments
SummaryThe paper describes equipment designed to study the effect of a conducting hostrock/overburden on the electromagnetic anomaly of sulphide ore bodies embedded therein. The model was constructed strictly according to the theory of electromagnetic similitude so as to constitute a direct reading reproduction of the field vectors. The experiments were conducted at a fixed, crystal-controlled frequency of 100 kHz using mainly a graphite sheet immersed in a dilute hydrochloric acid solution of predetermined conductivity to simulate vein and manto type of ore deposits surrounded by a partially conducting zone. Both the inphase and quadrature components of the anomaly were measured in terms of the primary field after elimination of the regional anomaly by means of a measuring bridge and a compensator. A comparison of the anomaly profiles obtained over the ore model when it is held in air with those obtained for corresponding situations inside a conducting solution shows a general enhancement of response [1–3].
The Perturbation of Alternating Geomagnetic Fields by Three‐dimensional Island Structures
Summary The perturbation of alternating geomagnetic fields by island-type structures in three dimensions is considered. Two different structures are studied for sinusoidally time-varying source fields of three different frequencies. The general model is that of a semi-infinite conducting half-space with regions of different conductivities. A single-island model and a two-island model are considered. The equations and boundary conditions are solved by a numerical method and the results are compared between models. Contour plots of the amplitudes of the E and H components over the surface of the conducting region are compared near the islands. Profiles of the amplitudes of the E and H components are also given so that the values of the contours may be specified. Profiles of phase are given so that the sign of the observed E and H components at any particular instant in time may be compared. The distortion of the current lines and associated fields by island structures is illustrated in the results. Also, phase changes of the components due to the island structures are evident. The results indicate that a second smaller island near a larger island considerably modifies the fields associated with the larger island.