Subpart 4A: Geophysical loggings

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Contractor

Frank Roth

Subcontractor

Valgardur Stefánsson

In the framework of the EC project, Earthquake Prediction Research in a Natural Laboratory, a pilot study has started to obtain a time series of logs in the South Iceland seismic zone (SISZ). An 1100 m deep borehole (LL-03, Nefsholt) inside the zone (63.92$^{\circ }$N, 20.41$^{\circ }$W, 7 km south of the seismic station SAU) is used and provides the unique opportunity to perform measurements much nearer to earthquake sources than usual - the hypocenter depths at that location range between 6 and 9 km. Moreover, data can be obtained for a depth interval of more than 1000 m, uninfluenced by the sedimentary cover and less disturbed by surface noise.

In the preparational phase of an earthquake, stress accumulation is expected to be connected with the creation of borehole breakouts, changes in the number and size of cracks, a possible variation of the stress direction, etc. Therefore, the following set of geoparameters is monitored:

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P-wave travel time.

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Electrical conductivity.

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Water content and porosity.

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Stress information from borehole breakouts (orientation and size).

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Crack density, crack opening.

This is achieved by repeated logging with tools as:

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Sonic log (BCS).

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Dual induction/latero log (DIL).

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Neutron log.

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Four-arm-dipmeter (FED).

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Borehole televiewer (BHTV).

The neutron log is run with the logging equipment of OS, the rest with the Halliburton logging truck of GFZ.DR.DBL.

Temporal changes visible in these logs will be correlated with data obtained by other methods used in the whole project, as there are: seismicity, anisotropy observed in S-waves, crustal deformation, gravity, etc.