6: Electromagnetic Physical Scale Modeling
Physical scale modeling is an important technique for obtaining the electromagnetic response of a large variety of geologic models. In physical or analog scale modeling the geometry of the model is reproduced in the laboratory at a scale usually between 1:100 and 1:106. Generally, the electrical properties of the laboratory model also must be scaled. Measurements are made using miniaturized loops or electrode arrays. By use of appropriate scale factors the scale model measurements can be translated to be equivalent to those that would be obtained for idealized full-size models. As indicated elsewhere in this volume, analytical solutions for electromagnetic response exist only for a relatively small number of conductive structures. Numerical methods for solving two- and three-dimensional problems have been available only for a short time and their use is limited because of computational difficulties and the extensive computer time required. For many years albums of scale model results for finite structures have been one of the chief aids in interpretation of electromagnetic profiling data. However, scale model results are seldom used directly in interpretation of soundings because data that cannot be fit with layered earth models are not interpreted quantitatively and because develpment of analytic solutions and computational techniques for determining the response of a layered earth have kept up with the development of suitable sounding equipment.
Figures & Tables
Over the last two decades there have been significant advances in electromagnetic (EM) methods of exploration, as evidenced by the extensive research carried out at various companies, universities, and government research organizations; by the large number of papers published on the subject; and by the numerous workshops on various EM topics held in conjunction with the SEG Annual Meetings.
Early EM methods were largely designed by the Scandinavians and the Canadians for exploration under glaciated Precambrian shield conditions, where the resistivities of the host rock and overburden are generally high. They did not work well in areas with conductive overburden or host rock. The lack of sophistication in data gathering and processing severely limited their exploration depth. Moreover, early EM systems were relatively heavy, cumbersome, and slow in operation.