ter antenna, emitting 60-MHz radar waves, manages to survey a rock volume rock with a
Ž .
radius of 30 to 40 m in reflection mode Fig. 5 . This means that near the surface, it would be
possible to record the same reflection from ei- ther of the holes if there was an adequately
oriented anomaly in the rock volume between the holes. At the bottom of the investigated
plane the distance is much too far for the reflec- tion mode investigation. As a result, it is not
always easy to correlate zones between the two holes. It is even more difficult to extrapolate a
zone accurately when the target is at the sur- face.
Radar waves for the cross-hole survey were generated with 22-MHz dipole antennas to cover
the somewhat large distance in the deeper parts Ž
. of the investigated plane Fig. 5 . In total, 4040
rays were recorded with a minimum length of 76.5 m and a maximum length of 222.0 m. Data
quality of recorded rays was excellent and very
Ž few rays had to be omitted 1.9 of the velocity
. rays and 2.1 of the amplitude rays .
5. Discussion of results
When plotted in the plane confined by the two holes, as in Fig. 6, it is difficult to extrapo-
late and connect reflectors between the holes since there is no dip information.
If the interpreted reflectors are plotted in 3D Ž
. Fig. 7 , there is a possibility to connect the
various features. Due to the quite large number of reflectors, this is still difficult. The distance
is not, unfortunately, the only problem as there is a limited accuracy in the determination.
The tomographic inversion of the data re- sulted in plots of the velocity and amplitude
distributions in the plane. In the plots, low velocity or high attenuation characterizes frac-
ture zones across the plane. The features are generally easier to locate in the velocity tomo-
Ž .
gram Fig. 8 . Six features were interpreted as significant
zones and subsequently compared to the zones found with the directional antenna. There are
possibly more structures within the tomogram,
Fig. 9. Comparison of features interpreted from the veloc- ity tomogram and interpreted reflectors that show how
overall structural interpretation can be improved. Labeled Ž
. lines A–G are features related to tomography. Other lines
are reflectors. Width of each line corresponds to dip out of the reflector, wide line dips more than thin line.
but they do not appear as clearly. The signifi- cant zones delineated by the velocity variations
generally correlate with interpreted reflectors, although only a subset of the reflections origi-
Ž .
nate from the low velocity zones Fig. 9 . The low velocity zone in the tomogram around 200
Ž .
m Figs. 5 and 8 , appears to cause several
reflections in both boreholes but the directions are contradictory. In fact, there are no reflectors
that accurately describe the low velocity zone. With the information from the tomogram, the
correct location of the zone can be established. Note that although the boreholes are quite close
near the surface, there are not many interpreted reflectors that are found in both holes.
There are a few possible reasons for the discrepancy between single-hole surveys and
cross-hole surveys. The most important reason in this case is probably the difference in resolu-
tion.
6. Conclusions
