Low electrical-resistivity anomalies (Central Europe)

ERCEUGT GROUP* (v. HAAK)

The map of geomagnetic induction arrows (Atlas Map 12) displays the lateral resistivity distribution of the crust in the central segment of the EGT. This complements electrical resistivity models as calculated from magnetotelluric measurements in the past 10 years compiled along the central segment ofthe EGT presented by the ERCEUGT group (1992).

The almost 1000 km-Iong transect (see Figs. 11.1 and 11.2) shows low-resistivity structures in the middle and lower crust. These structures can be related to different tectonic units along the profile. Prominent features are the two-dimensionallow-resistivity structures at the transition from the Moldanubian to the Saxothuringian and from the Saxothuringian to the Rhenohercynian unit of the Variscan Belt. Further obvious features are the depth correlation of the resistivity horizon in the Rhenish Massif with a seismic low-reflectivity band, the very high conductance of an upper to middle crustallayer in the North German Lowlands and its cessation at the river EIbe towards the Baltic Sea.

The question remains how representative these resistivity distributions along the transect may be for the Central European area of the Variscan belt. We can answer this question with respect to prominent lateral variation of low-resistivity anomalies. Such lateral resistivity changes within the crust and upper mantle can be mapped by geomagnetic induction vectors, which can be constructed almost directly from corresponding geomagnetic recordings. Since the construction and interpretation of geomagnetic inductions vectors is one of the earliest electromagnetic methods in geophysics, most of the central part of the EGT has been covered in the last 35 years with measurements of induction vectors. During this timespan the method and the instrumentation itself were developed, therefore the quality and reliability of individual results were inhomogeneous. Berktold (1991) critically collected all available vectors in the western part of Germany. With the political unification of Germany this collection of induction vectors was completed by Ritter (1990).

GEOMAGNETIC DEPTH SOUNDING OR MAGNETOVARIATIONAL METHOD

Although the basic framework of the magnetovariational method has been introduced in the previous chapter, our data handling and interpretation are slightly different. Risking repetition for the sake of clarity we give here the specific details of our method. We use the

*ERCEUGT=Electrical Resistivity along the Central segment of the EUropean GeoTraverse.

ERCEUGT is: K. Bahr, A. Berktold, H. J. Brink, V. Haak, St. Hofer, H. Jödicke, A. Junge, K.

Knödel, W. Losecke, E. Ritter, and R. Volbers.

I

LOW-RESJSTIVITY ANOMALlES 45

three components of the time-varying geomagnetic field to derive a quantity called the 'induction arrow' or 'tipper' (Vozoff 1972). Graphical representations ofthe tipper in a map are often referred to as induction arrows.

Fig. 11.1 The loeation ofthe eleetrieal resistivity seetion. The map and the two boundaries between the three teetonie units Moldanubian region MN, Saxothuringian region ST and Rhenohercynian region RH were adaptedfj'om Franke et al. (1990b). Northern boundary ofthe Varisean zone after Murawski et al. (1983). NGLL: North German Lowlands. D . .K: shotpoints ofthe seismic EGT fine (Aiehroth and ProdehI1990). Thin fine (km 0-910): magnetotellurie profile. The numbers are distancesfrom the most southernly sounding site. The profile is in the range 0-500 km identical with the profilesofDEKORP 2-S andDEKORP 2-N. Full circles with labels ZAR, HAUS,DACK, OBER, GRE refer to five sites for whieh the original data are discussed by ERCEUGT (1992). Triangles with labels BSG, RAB, LAU refer to three ultral-low-frequency MT sitesforwhich a very deep (upper mantle) resistivity profile has been caleulated (see ERCEUGT 1992).

N

~

, -

"

NORTH GERMAN LOWLANDS

A CONTINENT REVEALED

I

Tipper Ratio

-- ~

⁴⁰

.r:

-

^0.. _CD

o

80

o

^{100 200 300}

I

400 500 600 700 800 900

Distance (km)

Fig. 11.2. The electrical resistivity section from the south (km 0) to the Baftic Sea (km 900 ).The tipper ratio (determinedfor 1000 s period) is associated with major variations ofthe electrical conductance of the low-resistivity layers in the crust. The conducting layer at 85 km depth which is shown at km 380 refers to the local determination of the asthenosphere at site RAB (see Fig. 6 in ERCEUGT 1992). No information on the conductivity ofthis depth range is available so far for other parts ofthe profile.

In the application of the tipper, a 2-D or 3-D earth and a homogenous source field is assumed. In an elongated very low-resistivity anomaly the increased currents produce a stronger inhomogeneous magnetic field at the surface. This increased magnetic field may immediately be recognised by the increased vertical to horizontal magnetic field ratio. This ratio may be positive or negative, the change of sign occurs over the centre of the elongated conductor.

TUE GEOMAGNETIC INDUCTION ARROW

Induction arrows (Schmucker 1985) are derived from the linear relationship (1) B ·(D = zH(D B (D + zD(D B (D

Z,l x,n y,n

where T

=

²

n/I

(j

=

frequency) the period

B , B ,B = magnetic north (index x), east (index y) and vertical (index z) x y z

components

index n

=

normal, unaffected by an electrical conductivity anomaly index i

=

in the electrical conductivity anomaly internally induced magnetic field

For data from a single station as they are presented here in the map equation (1) may be replaced by

(2) B (D

=

^{a(D B (D} + b(D B (D

z x y

where B ,B ,and Bare the total field amplitude measured at the same site.

x y z

I

LOW-RESISTIVITY ANOMALIES

Here often the following approximations are valid (Jones 1981):

(3) Bz,i "" B z' Bx,n "" Bx' By,n"'" By

Z H "'" a and Z D "'" b

with a(I) and b(I) as eomplex-valued funetions of the period.

47

I

These two funetions may be represented for aseleeted eonstant period graphieally on a geographie map by arrows or veetors:

(4) P

= -

^x a -y b the real arrow,

r r

Q = x a. + y b. the imaginary arrow

1 1

where x and y are the unit veetors in north and south direetion.

The Parkinson eonvention has been ehosen here: the arrows point towards high-eonduetivity zones. Two other kinds of induetion arrows are in use, see Jones (1981) and Gregori and Lanzerotti (1980) for their definition. The arrows supplied by Ritter (1990) in the eastem part of Germany are 'Wiese' arrows rotated by 180⁰ to agree with the Parkinson eonvention.

Im Dokument Atlas of Compiled Data (Seite 50-53)