Drilling, regardless of the method employed, involves a sizeable expense in the evaluation of
a site. Hence, it is important to extract as much information as possible from the boreholes.
This paper deals with the difficulties in ex- trapolating geologic surface and subsurface in-
formation when investigating large, deep vol- umes of rock and how such difficulties can be
resolved, or at least decreased, with borehole radar measurements.
2. Deep underground repositories
The investigations prior to the siting of a repository for radioactive waste involve isolat-
ing or detecting a volume of rock that fulfils some basic requirements. According to the
Ž .
Swedish Nuclear Waste Management SKB ,
the main objectives of the initial site investiga- Ž
. tion are SKB, 1992 :
Ø To define the site-specific position of a rock
volume for a repository; Ø
To plan the surface and subsurface facilities; Ø
To provide a basis for a preliminary site- specific performance assessment; and
Ø To provide information to ensure safe and
efficient underground activities. The Swedish bedrock is usually highly com-
petent with very low porosity. Hence, the frac- tures and fracture zones are fundamental con-
cepts in the site characterization as the struc- tures control the groundwater transport and the
mechanical stability. In other words, most of the work is concerned with determining the geome-
try of these structures.
A Swedish repository will be located in Pre- cambrian crystalline rocks. It will be a multi-
barrier system intended to minimize the proba- bility of radioactivity escaping to the surface. At
a depth of approximately 500 m, the storage will be protected from natural and human dis-
turbance,
supposedly maintaining
favorable conditions for isolating waste without further
human aid. The waste is to be enclosed in steel-coated copper canisters deposited in verti-
cal holes about 1.5 m in diameter. A final full-scale repository might contain as many as
4500 canisters and thereby covering an area of about 1 km
2
. Fig. 1 shows a hypothetical reposi- Ž
. tory about 10 of full-size placed at 500 m
¨
depth below the island Aspo. The three-dimen-
¨
Ž .
sional 3D fracture zone model in the figure, which governs the layout of the facility, was
developed from surface-based investigations of
¨
Ž .
the Aspo Tiren et al., 1996 . A fracture has to
¨ ´
fulfil certain requirements to be included in the model, such as being possible to extrapolate and
identify in more than one location, borehole or at the surface. The extrapolation of features was
essentially performed with the directional bore- hole radar.
3. Extrapolation of geological data
According to the definition, fractures are dis- continuities with mechanical or tectonic origin.
In geology, the term is used to describe every- thing from microfractures to large faults. Fur-
ther, the fractures can have several different chemical and physical properties: thus, the geo-
physicist who wants to detect and classify frac- tures using borehole geophysics stands before a
very challenging task. One reason is that no geophysical method reacts directly to the frac-
ture. Instead, the geophysicist has to determine how each fracture affects the measurements
recorded with the different probes. To simplify investigations concerning fractures, it is often
assumed that the strike of the fracture is orthog- onal to the drillhole and filled with some fluid,
usually water. Furthermore, the fracture surfaces are assumed to be smooth and perfectly plane.
The 3D modeling of fractures and fracture zones is usually based on the correlation of
fractures in drillcores and surface structures. Remote analysis and surface geophysics help
determine the location of surface structures. The correlation of surface data and borehole struc-
¨
Ž Fig. 1. Layout of a potential repository located at 500 m depth governed by 3D structural model of Aspo, Sweden Tiren et
¨ ´
. al., 1996 . Fractures have to fulfill certain requirements to be included in the model, such as being possible to extrapolate
and identify in more than one location, borehole or at the surface. The extrapolation of features was essentially performed with the directional borehole radar.
tural information along the hole going down- wards is usually not very difficult near the
surface but becomes increasingly complicated Ž
as holes become deeper. At great depths c. 500 .
m , it is extremely difficult to determine which one of all the zones along the hole is the one
that correlates with the surface expression, Fig. 2. The strike of a structure is generally well
defined, based on the surface data, but the dip
Ž of the interpreted structure is at best
based .
solely on surface data determined with an accu- racy of 10
8. Assume that there is a zone with an inter-
preted dip of 60 10 8 westwards and that the
mean spacing between fractured sections is con- stant. The number of equally possible interpreta-
tions is a function of the orientation of the borehole and the distance between the borehole
and the structure at the surface, Fig. 2, where the average distance between minor zones is
assumed to be 15 m, while it is 30 m between major zones. When a borehole dipping 60
8 to- wards the structure is drilled 50 m away from
the structure there are two possible solutions in the most favorable case. The borehole will in-
tersect the zone at 40 m depth. If, however, this example is repeated for the same zone at 500 m
Ž .
depth, there are several 10 equally possible solutions within a borehole length interval from
Ž 470 to 708 m corresponding to vertical depths
. of 410 to 615 m . Although the zones of frac-
tured rock do not occur as uniformly in ‘‘real’’
Fig. 2. Potential error in the location of a zone at depth due to errors in borehole deviation measurements and dip Ž
. determinations of zones at the surface Tiren et al., 1996 .
´
rock, there will be several possible interpreta- tion of the location of a structure at great depths.
4. Borehole radar
