Karsten Bahr

On the combination of the magnetotelluric and the geomagnetic depthsounding method for resolving an electrical conductivity increase at 400 km depth

The simultaneous use of two different passive electromagnetic (EM) sounding techniques for resolving the upper mantle electrical conductivity structure is emphasized: The geomagnetic (GM) method uses magnetic field variations at periods between 3 hours and 600 hours, yielding penetration depths between 300 km and 1000 km. The magnetotelluric (MT) method is applied to field variations at periods between a few seconds and 16 hours and yields penetration depths between a few km and 600 km. This is an extension of both methods, using additional D[sub St]-data for MT and additional S[sub q]-data for GM. In application to field data from Western Europe the resulting overlap of the target depths includes an electrical conductivity increase from 10[sup [minus]2] S/m at 300 km to 1 S/m at 700 km depth while a single conductivity jump at either 400, 500 or 700 km cannot explain the broadband data. Although the olivine to spinel transition is a possible explanation of the conductivity enhancement at 400 km depth, another mineralogy could also occur. This conductivity increase has not been resolved in other sub-continental profiles and lateral heterogeneity is probable in this depth range. 27 refs., 3 figs., 2 tabs.

Percolation in the crust derived from distortion of electric fields

Fractal crack distributions in the crust have been discussed in the context of earthquake cycles, fluid percolation, and the electrical conduction mechanism. With the impedance matrix rotation analysis in magnetotellurics (MT), a special type of apparent amplification of variational electric fields in one horizontal direction was detected. Here a possible link between crustal fracture networks and MT distortion is suggested. It is shown that the strong electric field amplification can be explained with a very heterogeneous crack network which in one direction stays in the vicinity of the percolation threshold. The apparent similarity to fluid percolation models does not automatically support the fluids paradigm, but rather allows for a choice between present fluid percolation and ‘paleopercolation’ to be the origin of the conductivity structure. In the latter model, the fluid caused the precipitation of graphite which now forms a partly interconnected network.

Ulrich Schmucker (1930–2008)

Ulrich Schmucker passed away in a Beijing, China, hospital in the early morning of 27 October 2008. Ulrich had been in China to attend the 19th International Workshop on Electromagnetic Induction in the Earth, a biannual workshop he had helped found and had attended for more than 30 years without fail.

Practical Magnetotellurics: Planning a field campaign

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.