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\fbox{4} Borehole monitoring of fluid-rock interaction

In the framework of the PRENLAB projects, repeated loggings are carried out to obtain a time series of logs in the South Iceland seismic zone (SISZ)(Figure 11). 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 preparatory phase of an earthquake, stress accumulation is expected to be connected with the creation of borehole breakouts (BOs), changes in the number and size of cracks, a possible variation of the stress direction, etc. Therefore, the following set of geoparameters is monitored:

$\bullet$ P-wave travel time.
$\bullet$ Electrical conductivity.
$\bullet$ Stress information from borehole breakouts (orientation and size).
$\bullet$ Crack density.

This is achieved by repeated logging with tools as:

$\bullet$ Sonic log (BCS).
$\bullet$ Gamma-ray (GR).
$\bullet$ Spectral gamma-ray (SGR).
$\bullet$ Dual induction/latero log (DIL).
$\bullet$ Neutron-neutron log.
$\bullet$ 16"- and 64"-resistivity log.
$\bullet$ Spontaneous potential log (SP).
$\bullet$ Four-arm-dipmeter (FED).
$\bullet$ Borehole televiewer (BHTV).

The neutron-neutron log, the 16"- and 64"-resistivity log and the SP log are run with the logging equipment of OS, the rest with the Halliburton logging truck of GFZ.


Investigations on the stress field in the SISZ:

Besides the repetition of logs in borehole LL-03 Nefsholt, we performed single logging campaigns at other boreholes to check the state of the regional stress field. This is important for two reasons:


  
Figure 11: Map of Iceland showing the location of the site of repeated logging (Nefsholt) and of the other boreholes, where measurements have been performed. The box in SW Iceland indicates the orientation and the length og the South Iceland Seismic Zone (SISZ).


  
Figure 12: Example for the borehole breakouts found in Žykkvibær (THB 13) with two cross sections. The two panels show the amplitude of the reflected signal (left) and the radius calculated from the travel time (right), both unwrapped from N over E, S, W to N. Vertical axis: depth in meters. Breakouts appear as vertical bands of low reflection amplitudes. Due to low reflection amplitude, the values for the radius are often missing in these parts, resulting in black bands. In the two cross sections, the black lines indicate the range in azimuth of the picked breakouts.


  
Figure 13: Map of South Iceland showing the stress orientations found at Nefsholt, Žykkvibær and Böšmóšsstašir.


 
Table 2: Stress orientations found at SISZ from borehole televiewer logs.
Well: Logged Interval with Total length Orientation Std.
  interval: BOs/ vert. of BOs/ vert. of $\sigma_H$: deviation:
    fractures: fractures:    
BS-11 703-1090 m 713-934 m 45.0 m fract. N45$^\circ $E-N90$^\circ $E --
LL-03 80-1100 m 780-983 m 5.0 m BOs N30$^\circ $E 12$^\circ $
THB-13 466-1225 m 925-941 m 3.5 m BOs N21$^\circ $E 10$^\circ $
 


Results:

The results can be summarized as follows:


next up previous
Next: Active deformation determined from Up: Methods and resaults Previous: Shear-wave splitting monitoring pre-fracturing
Margret Asgeirsdottir
1999-12-21