Task 2: Check for changes - results from repeated logging

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

Repeated measurements in Nefsholt showed good repeatability, especially of the sonic log. The standard deviation is in average between 2.5% and 3.5% of the average travel time measured.

A good interpretation can be made in depth intervals with high sonic amplitude. In these intervals the standard deviation usually is lower than 2.5%. Depth intervals where the amplitude of the registrated signal is very low show significant variation in the (automatically) picked travel-times and thus cannot be considered for an analysis of changes. Small differences between repeated logs in the high amplitude intervals seem to be caused by inaccurate depth matching due to slightly different curve shapes (Figure 23). Thus, no changes in the sonic velocity have been observed so

  

Figure 23: Data example showing from left to right panel: an average curve of sonic amplitude, a superposition of the average compensated travel times measured in October 1996 and in December 1997 (each averaged over four runs performed immediately one after the other), the difference between these two average travel time curves (absolute value, green curve) and the standard deviation (absolute value) derived from the four runs performed in December 1997. Compensated travel times are given in microseconds per foot. The difference between the average travel times exceeds the standard deviation only in low amplitude intervals (not shown here) or at depths with inaccurate depth-matching (for example around 740 m).

far. The same is true for the resistivity measurements, as can be seen from Figure 24.

  

Figure 24: Data example showing from left to right panel: gamma-ray log in API, compensated travel time in microseconds per foot and the sonic amplitude, and three repeated measurements of latero log (LL3), medium induction log (ILM) and deep induction log (ILD). Month an year of campaign are given.

On the other hand, no relevant seismic activity has been observed in the surrounding of this borehole since the beginning of the measurements.

We are thankful that Orkustofnun provided a lithology log of the well at Nefsholt. This log shows a nearly continuous sequence of altered basalt, interrupted only by a few thin sedimentary layers and some thin layers of dolorite, hyaloclastites and fresh basalt. Corresponding to a porosity change within each basaltic layer, the sections of altered basalt show very different characteristics in sonic velocity and resistivity as well as in the neutron log. These differences allow to distinguish between the lava-flows (Figure 25).

  

Figure 25: Data example showing the correlation between the performed logs and the lithology log provided by Orkustofnun. GR = gamma-ray in API, DT = compensated travel time in microseconds per foot, neutron = neutron log in API, ILD and ILM = induction log deep and medium in Ohmm, LL3 = laterolog in Ohmm.