The conductivity distribution in the North German sedimentary basin derived from widely spaced areal magnetotelluric measurements

Abstract

For the first time, a systematic magnetotelluric survey has been made of the North German sedimentary basin. It was intended to obtain details concerning the thickness, extent and structure of the sedimentary basin. A model of the basin was derived from data collected at about 50 sites. The model contains the distribution of the integrated conductivity of the strata above and below the Zechstein base, and depth values for the magnetotelluric (MT) basement ranging from 9 km ( in the south) to 18 km (in the north). The integrated conductivity of the strata above the Zechstein base decreases from 4000 mho in the north to 5 mho in the south. In the pre-Zechstein this trend is reversed, with 8000 mho in the south decreasing to 2000 mho in the north. In the southern part unexpectedly low resistivities (approximately 1 ohm .m) occur in the pre-Zechsteiti. The model calculations suggest that the source of the North German conductivity anomaly lies in the upper crust. The anomaly is caused by the contact of poorly conducting material with very thick (approximately 10 km) well-conducting sediments, which additionally show lateral conductivity variations outside the contact area.

Reducing magnetic field induced noise in broad-band seismic recordings

“…This is the case for stations HLG and RGN, in particular, because the anomaly in the vertical component of the magnetic field changes its polarity between WNG (Wingst, Fig. 3) and NGK (Niemegk) due to the North German conductivity anomaly (Losecke et al 1979). The sensitivities given below are likely to overestimate the true effect since the magnetic intensity of the storm increases with latitude within Germany.…”

Increase of electrical conductivity with pressure as an indicator of conduction through a solid phase in midcrustal rocks

“…alteration as the rock was uplifted to the surface [Duba, 1992;Katsube and Mareschal, 1993] enhanced interest in the potential of carbon to influence crustal conductivity. As can be seen in Table 1 [Duba et al, 1988] from the north German conductivity anomaly [Losecke et al, 1979]. The latter, however, do not speak to the broader issue of conductivities of crystalline rocks.…”

“…As can be seen in Table 1, the exceptionally high conductivity of graphite or amorphous carbon means that only small amounts are necessary; further, several geochemical means for widespread deposition of such films had been hypothesized [Mathez and Delaney, 1981;Mathez, 1987;Sanders, 1991;Tingle et al, 1991;Walther and Althaus, 1993;Mareschal et al, 1992;Mathez et al, 1995]. A persistent problem has been that samples have rarely been available from the few documented regions having conductivities attributed to carbon [e.g., Alabi et al, 1975;Sternberg, 1979;Haak et al, 1991;JOdicke, 1992], with the exception of black shales [Duba et al, 1988] from the north German conductivity anomaly [Losecke et al, 1979]. The latter, however, do not speak to the broader issue of conductivities of crystalline rocks.…”

The North German Conductivity Anomaly revisited

“…After first magnetotelluric (MT) measurements at the geomagnetic observatory Niemegk (Wiese 1962a), on Rügen island (Porstendorfer 1975), and on a profile in western Germany, Vozoff & Swift (1968) and Losecke et al (1979) proved in addition, that electrical conductivity of the young sedimentary layers with around 1 S m −1 was significantly higher than expected. Similar investigations were also conducted in the northern and southern parts of East Germany (Porstendorfer 1975; Göthe 1990; Ritter 1990).…”