The choice of equipment used in a particular survey should depend on the depth range under consideration: in crustal studies, induction coil magnetometers are used frequently, the sampling is quick and the ‘processing’ (described in Chapter 4) is usually performed in the field. Fluxgate magnetometers provide a response at longer periods than induction coils, and are used if larger penetration depths are under consideration. In many cases, data from very short to very long periods are desirable, and two different sensors are combined at each site. It is vital that anybody writing or modifying processing software has access to all information regarding the analogue electronics of the system (e.g., calibration coefficients for filters) that is to be used in conjunction with the software.

We suggest a rule of site spacing: not too close and not too sparse. The question whether we should deploy magnetotelluric sites along a profile, or as a 2-D array is discussed in the context of the geological complexity of the target area, the available hardware and the financial resources. In many cases, a trade-off has to be found between the desire to have many sites and hence a good spatial resolution and the wish to achieve high-quality data by occupying sites for a long time.

Target depths and choosing the right sensors and equipment

Considering the period range

From the definition of penetration depth (Equation (2.20)), we can estimate a period range associated with a particular depth range of interest, provided that an estimate of the subterranean conductivity is known.