Tasks 3 and 4: Evaluation of measuring results

Start: July 1996 (month 5)
End: February 1998 (month 24)
Responsible partner: GFZ.DR.DBL

Borehole-televiewer measurements
The televiewer-data collected in the well at Nefsholt have been presented in the previous reports. These previous presentations have to be corrected for an error in orientation! Attempts were made in 1996 to correct for instrumental problems with the orientation of the data. Unfortunately, these efforts now turned out to have failed. To give the data the right orientation, it is necessary to change the coordinate system from clockwise to counterclockwise rotation. General breakout-orientations and fracture statistics from the measurements in Nefsholt with the correct orientation are presented in words below. Also are discussed breakouts in the borehole BS-11 (Bödmódsstadir), which was logged down to 1090 m, and the borehole THB-13 (Thykkvibær), down to 1245 m.
Nefsholt: The previously found orientation of the borehole breakouts of about N35$^{\circ }$E have to be corrected in 120.5$^{\circ }$ to 126$^{\circ }$, giving a direction of the maximum horizontal principal stress of N30.5$^{\circ }$E to N36$^{\circ }$E. The circular standard deviation (1$\sigma$) is of the order of 10$^{\circ }$. The stress direction is in agreement with the expected stress direction from large scale tectonics (left-lateral strike slip). The length of picked breakouts sums up to approximately 5.5 m.

308 fractures have been picked in the depth interval between 300 m and 1090 m. Determination of dip-angle and -direction are presented in Figure 26.

  

Figure: Strike (upper row) and dip (lower row) of fractures observed in Nefsholt. Blue diagrams: dip-angles greater 60$^{\circ }$; red diagrams: dip angles less than 60$^{\circ }$; green diagrams: all fractures.

They show that most of the steep dipping fractures (dip angle greater than 60$^{\circ }$) are dipping ESE, a slightly smaller set of steep fractures dipping WNW. Considering all fractures, this trend is much less pronounced, but there is still a preferred dip-direction of W to WNW with a smaller dataset of dip-angles ESE. The averaged strike of these fractures is thus NNE, like the strike of the fractures observable at the surface.
Thykkvibær: Borehole breakouts have been found in the depth intervals 925 m to 927 m and 937 m to 941 m. The breakouts in these two depth intervals sum up to approximately 3.5 m. Data quality is rather poor due to weak reflection amplitudes. Figure 27 shows a data example. The average breakout azimuth is between

  

Figure 27: Example for the borehole breakouts found in Thykkvibær with two cross sections. The two panels show the amplitude of the reflected signal (left) and the radius calculated from the travel time (right) unwrapped from North over East, South, West to North. 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 missing in these parts, resulting in black bands. In the two cross sections, the black lines indicate the picked breakouts.

N105$^{\circ }$E (upper depth-interval) and N121$^{\circ }$E (lower depth-interval). Statistic analysis over the whole depth range gives a breakout-orientation of N111$^{\circ }$E with a circular standard deviation (1$\sigma$) of about 10$^{\circ }$. This would mean that the larger principal horizontal stress is in average orientated N21$^{\circ }$E. This is in agreement with the stress directions expected from the overall tectonics as well as with the results from Nefsholt.
Bödmódsstadir: No breakouts have been observed in this borehole, but there are vertical fractures visible between 713 m and 934 m depth. The length of the vertical fractures sums up to 45 m. Vertical fractures are expected to occur in the direction of the maximum horizontal principal stress because of tensile failure of the borehole wall. They are supposed to be drilling induced and not of natural origin. These fractures occur at an azimuth of N45$^{\circ }$E to N90$^{\circ }$E. One data example is presented in Figure 28.

  

Figure 28: Example for drilling induced vertical fractures observed in the borehole at Bödmódsstadir. Same principle of displaying the data as described in Figure 26. The fractures appear as vertical stripes of low reflection amplitude. Values for the radius calculated from travel time are missing for these stripes due to low amplitudes.

Additional to the drilling induced fractures, 52 natural fractures have been picked in the depth interval between 820 m and 1060 m. The steep dipping fractures (dip angles greater 60$^{\circ }$) show a dominant strike ENE. Regarding all natural vertical fractures, there are two datasets: One striking nearly N and the other striking ENE to E.

The results can be summarized as follows:

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The repeated measurements of sonic P-wave velocity, resistivity and gamma-ray show good repeatability. Neither changes in logs nor changes in seismic activity have been observed yet.

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Borehole breakouts observed in Nefsholt and Thykkvibær correlate with the expected large-scale stress direction in the SISZ: they show the maximum horizontal principle stress at an azimuth of approximately NNE (N21$^{\circ }$E to N36$^{\circ }$E). The data obtained in Bödmódsstadir show an average direction of maximum principle horizontal stress ENE. Thus, the direction of maximum principal horizontal stress as found by the televiewer data varies from NNE (north of the SISZ, BS-11) to ENE (south of the SISZ, THB-13). This variation is of the same order as the standard deviation.

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Fractures found down to nearly 1100 m depth show the same dominant strike as those observed at the surface. A dependency of fracture orientation with depth or with geographical latitude could not be found so far. Fracture picking in the data collected in Thykkvibær is not finished yet.

Outlook $\\$It is planned to continue the logging activities in 1998, depending on seismic activity. After that, the work will be focussed on theoretical investigations.

Besides K. Henneberg, the proposer and the subcontractor, the following scientists and technicians have worked in the subproject, especially the field campaigns: G. Axelsson (OS), H. Bäßler (Karlsruhe), C. Carnein (GFZ), E.T. Elíasson (OS), H.-J. Fischer (GFZ), S.T. Gudlaugsson (OS), G. Hermannsson (OS), M. Hönig (GFZ), S. Mielitz (GFZ), J. Palmer (GFZ), H. Sigvaldason (OS), Ó. Sigurdsson (OS), B. Steingrímsson (OS), V. Stefánsson and M. Thoms (GFZ).