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:

• From the San Andreas fault we know that fault zones may be in a low stress state between earthquakes, which gets visible through stress orientations perpendicular to the strike-slip fault instead of pointing at it under an angle of 30$^\circ$ to 45$^\circ$. To determine the present state of stress in the SISZ, it is important to see if there are stress components, that are not perpendicular to existing faults and favour earthquakes on them.
• The SISZ is no typical transform zone. Looking at the orientation of opening at the adjacent rifts, one would expect a left-lateral strike-slip zone in roughly E-W direction (N103$^\circ$E) to connect the Reykjanes ridge and the eastern volcanic zone of Iceland. Instead earthquakes occur on en-echelon N-S striking right-lateral faults. Assuming an angle of 45$^\circ$ between the maximum horizontal compressive stress and the fault (as it is done constructing fault plane solutions) both planes are equivalent. From a rock mechanics point of view, expecting an angle of about 30$^\circ$ between fault and maximum horizontal principal stress, the stress orientation at N-S striking faults should be N30$^\circ$E, compared to N60$^\circ$E at an E-W striking transform.

  

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 Þykkvibær (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, Þykkvibær and Böðmóðsstaðir.

 

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:

• The repeated measurements of sonic P-wave velocity and the latero log resistivity show good repeatability.
• The repeated resistivity measurements with the dual induction log (deep and medium penetration) show a change between the logging campaigns in September and December 1997. The values of voltage of the induction log with deep penetration generally increased and the values of the induction log with medium penetration generally decreased. No correlation to any seismic activity or other observations was found, so far. Investigations on possible reasons are still going on.
• The stress orientations found at all three locations are similar and agree with a left-lateral strike-slip regime. They are not perpendicular to existing ruptures found for large earthquakes in the SISZ and, therefore indicate, that the SISZ is not a weak fault, as postulated for the San Andreas fault. Borehole breakouts observed in Nefsholt and Þykkvibær show the maximum horizontal principal stress at an azimuth of approximately NNE (N21$^\circ$E to N36$^\circ$E), see Figure 13 and Table 2. The data obtained in Böðmóðsstaðir show an average direction of maximum principal horizontal stress ENE, see Figure 13 and Table 2. Thus, the direction of maximum principal horizontal stress, as found by the borehole televiewer data, varies from NNE (north of the SISZ, BS-11) to ENE (south of the SISZ, THB-13). This NNE variation is slightly more than the standard deviation. From a rock mechanics view, the stress directions found at Nefsholt (LL-03) and Þykkvibær (THB-13) fit to N-S striking faults, as they are found in the SISZ. On the other hand, the orientation of maximum horizontal principal stress found at Böðmóðsstaðir fits to the model of an E-W striking transform fault zone. Similar stress orientations were also found by Stefánsson et al. 1993 ( [52]) from fault plane solutions. This was confirmed by Angelier et al. 1999 ( [2]), who found a mean orientation of $\sigma_H$ of N56$^\circ$E derived from 1916 fault plane solutions quality selected from 4413 earthquakes in the SISZ during the years 1995 to 1997. A NE-SW orientation of $\sigma_H$ was also the result of investigations of Crampin et al. 1999 on shear-wave splitting due to crustal stress anisotropy at four of six seismic stations in the SISZ.