Greinar (Vísindafélag Íslendinga) - 01.01.1976, Page 17

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the dipole curves. According to these models the 20 Qm layer,
which begins at 1200 m depth, reaches down to 20-30 km depth,
where it is replaced by a layer with 70-100 Qm. The lower part
of Figure 5 shows the apparent resistivity at Thingvellir as a func-
tion of period, from Grillot (10). Based on his results Hermance
and Grillot (6) have calculated models showing resistivities be-
tween 1 and 20 Qm at depths from 2 to 10 km. This is in good
agreement with the results from Mývatnsöraefi.
Finally dipole and Schlumberger measurements from Hérad in
the Tertiary flood basalt region of Eastern Iceland, are shown in
Figure 6. The resistivity here is much higher than on the older
regions west of the active zone. It is of the order of 1000 Qm at
the depth corresponding to layer 3 and there is a minimum, around
300 Qm, at ahout 1000 m depth. A magnetotelluric measurement
at this site shows a strong tensor separation of the resistivities for
periods greater than 50 s, but for 10 s the Cagniard estimates are
scattered around 1000 Qm in good agreement with the dipole
results.
In smnmary the resistivity in layer 3 is highly variable in the
Icelandic crust. At 5 sites in the geologically older flood basalt
zone west of the volcanic zone the values are, at least in the upper
part of layer 3, between 150 and 300 Qm, but in the active zone
it is around 10-20 Qm. At one site in the eastern flood basalt zone
Table 1.
dipole resistivity depth to gradient
Place name in layer 3 layer 3
see Fig. 1 Qm km °C/km
Vatnsdalur HUD 3 300 1 70
Kjalames DDE 2 300 2 170
Mosfellsheidi DE 4 300 2 170
Thorskaf j ardarheidi VED 1 200 4-6 70
Hérad HED 1 1000 3.5 40
Audkúluheidi HUD 1 300 3 70
Thingvellir DE 5 10-20 3-3.5 180
Mývatnsöra efi MVD 1 10-20 2-3.5 180