On the detectability of a highly conductive layer in the upper mantle beneath the Black Forest crystalline using magnetotelluric methods

Abstract

S U M M A R Y Simultaneous geomagnetic and telluric variations (600 s-4 hr) were observed at two stations on the Rhine-Graben sediments and on the neighbouring Black Forest crystalline in order to extend the pulsation observations (10-600 s) of former magnetotelluric (MT) surveys to longer periods. These observations completely describe the anomalous magnetic and telluric fields in terms of a set of transfer functions using the Black Forest station as 'normal'. In the period range from 30 s to 4 hr the horizontal and vertical magnetic transfer functions of the Rhine-Graben anomaly are characterized by continuously decreasing amplitudes. The local Schmucker induction arrows point uniformly 45" south-east for all long-period geomagnetic transfer functions, and the NW-SE-striking direction in the upper mantle derived from the induction arrows is also consistent with the preferred telluric direction in the Black Forest. In both areas large offsets of apparent resistivity and phase curves for the E-and B-polarization directions are observed, except that the phase curves converge above 400s in the long-period range. The interpretation of the period dependences of the observed magnetic and telluric transfer functions from 600 s to 4 hr by a 2-D conductivity model requires a highly conductive layer in the upper mantle at a depth of 120 km beneath the Black Forest. This confirms Bahr's (1992) model for a highly conductive layer in the upper mantle that has also been found south of the Alps (Braitenberg et al. 1994) and which is interrupted by low conductive alpine roots. However, no information about such a layer beneath the Rhine Graben can be derived. 2-D model studies have shown that an improved depth resolution of highly conductive layers in the upper mantle is possible using telluric phases, but the transfer functions for the vertical magnetic fields are not sensitive enough for the determination of the depth of an upper mantle conductive layer.

2-D Magnetotellurics at the geothermal site at Soultz-sous-Forêts: Resistivity distribution to about 3000 m depth

“…At the longest periods the phase angle approaches 458 and the apparent resistivity about 1 Vm. These are common features at long periods in MT measurements in the Upper Rhine Graben (Tezkan, 1994). In this period range the swift angles drop to 30-208 and at longer periods skew increases to nearly 1.…”

“…Two positive conductivity anomalies were interpreted as the Couche rouge layer at a depth of approximately 500 m and a brine bearing fault at approximately 1500 m depth, which is in agreement with the results from 2-D seismic interpretation. Earlier electromagnetic studies refer to the Rhine graben as a conductive regional structure (Albouy and Fabriol, 1981;Babour and Mossier, 1980;Menvielle and Tarits, 1986;Teufel, 1986;Tezkan, 1994). Some of these studies focus on the deep structure of the Rhine graben using long periods with respect to periods used for geothermal prospection (Babour and Mossier, 1980;Tezkan, 1994).…”

“…Earlier electromagnetic studies refer to the Rhine graben as a conductive regional structure (Albouy and Fabriol, 1981;Babour and Mossier, 1980;Menvielle and Tarits, 1986;Teufel, 1986;Tezkan, 1994). Some of these studies focus on the deep structure of the Rhine graben using long periods with respect to periods used for geothermal prospection (Babour and Mossier, 1980;Tezkan, 1994). Others account for the scale of induction space at defined period ranges and its implication on interpretation in this area (Menvielle and Tarits, 1986).…”

How is the European Lithosphere Imaged by Magnetotellurics?

“…Tezkan (1994) studied Rheingraben whereas Bahr et al (1993) and Tarits et al (2004) studied the roots of Alps.…”