7: Detection of Repetitive Electromagnetic Signals
The principle of increasing the accuracy in the measurement of a given quantity by averaging repeated determinations of its magnitude is probably as old as experimental science itself. Considerable advances in electronics were required, however, before this simple principle could be directly incorporated into signal detectors used in physics and chemistry. With the advent of solid state devices, builders of geophysical equipment for electrical and electromagnetic measurements quickly saw the advantages of analog averaging detectors and included them in their designs. Thus virtually all geophysical equipment built after 1970 uses a detector that coherently samples and averages the observed repetitive electrical signal. Since 1980, the availability of reliable, compact, digital microprocessors has resulted in an upgrading of the analog equipment to full or partial digital status. In modern equipment the analog to digital conversion takes place right at the sensor output and all the subsequent signal processing is done digitally.
A qualitative discussion of electronic signal averaging techniques is given in Hieftje (1972). More recently Neelakantan and Dattagupta (1978, 1980) did a quantitative analysis of signal enhancement by a boxcar integrator and a lock-in amplifier, two of the most commonly used signal averaging detectors. In spite of the popular use of such detectors in geophysical equipment only two articles on the subject [San Filipo and Hohmann (1983) and Macnae et al. (1984)] have appeared in scientific journals dedicated to geophysics. While each article concerns a specific subject, namely low frequency EM data acquisition by San Filipo and Hohmann and noise processing techniques for the UTEM3 equipment by Macnae et al., both articles contribute much to our understanding of specialized detectors for the sensing of electrical signals used in geophysical exploration.
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.