8: Inversion of Controlled-Source Electromagnetic Data
Given a set of geophysical measurements, we want to determine all information possible about the geologic structure that gave rise to the data. There are two aspects. First, because we use electrical and electromagnetic (EM) methods of exploration we want to determine the geoelectric structure of the earth; i.e., the conductivity and/or the permittivity as a function of depth and lateral distance. If we can do this, then the second aspect will relate the geoelectric structure to the geologic structure. In some cases, a one-to-one relationship exists between the two; in others, there may be only a partial but useful overlap. This chapter addresses the first aspect–determining the geoelectrical structure based on controlled-source electromagnetic (CSEM) data.
The earliest inversion methods can be described as trial-and-error exercises. Starting with a general model, such as a layered earth or a thin dike, and then adjusting the parameters of the model we achieve a best-fit to the data. The parameters in this case are the physical dimensions and conductivities of the structures. Basically the exercise consists of running a number of forward models, and systematically adjusting the important parameters until the response matches the observed data within the noise level of the measurements.
Model-fitting by trial-and-error is still the most common interpretation technique for multidimensional models, but there are problems with the approach:
1. There is an a priori restriction to classes of models for which a relatively fast forward solution exists.
2. Because relatively simple models can have several parameters, some of which may be strongly correlated, the trial-and-error process may involve a prohibitive amount of time.
3. Fitting an assumed physical model to observed data involves a high degree of subjectivity. It is important to estimate confidence intervals for the various parameters and for other statistical measures that provide insight into how appropriate the model is. The trial-and- error approach does not provide this information.
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Electromagnetic Methods in Applied Geophysics: Volume 1, Theory
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.