rock, there will be several possible interpreta- tion of the location of a structure at great depths.
4. Borehole radar
Ž .
Ground penetrating radar GPR is a term that covers a group of high-frequency electro-
Ž .
magnetic methods EM . All these methods in- volve transmitting very high frequency EM
waves and recording the resulting signals from reflections within the medium or from transmis-
sion passing through the media. The technique can be used to examine and locate interfaces in
solid or liquid media, which exhibit changes in electrical properties. The exact technique em-
ployed, transmission or reflection, being deter- mined on the basis of the object examined and
the medium involved.
The borehole radar is a special case of con- ventional GPR with several features that distin-
guish it from the surface tools. The most impor- tant is perhaps that the borehole itself enables
emplacement of the antennas quite close to the objects to be investigated, resulting in precise
target responses. Furthermore, the receiver and transmitter antennas may be lowered into differ-
ent holes, so called cross-hole measurements. The information obtained with this setup often
complements the single-hole reflection surveys. Two different properties of radar wave propaga-
tion normally determined are velocity and atten- uation.
The propagation of electromagnetic waves through rocks and soils is mainly a function of
the dielectric constant and the electrical conduc- tivity. The potential of a feature in the bedrock
to show as a distinct radar reflector or anomaly depends primarily on the contrast in the dielec-
tric constants of the feature and that of the
Ž .
bedrock Olsson et al., 1990 . Because of the large contrast in dielectric constant between
Ž .
bedrock about 5
and the water-filled pores Ž
. dielectric constant for water is 81 in the frac-
tured rock the fractures are well indicated.
Ž .
Fig. 3. Principle of results from two types of borehole radar antennas, conventional dipole left and directional antenna Ž
. Tiren et al., 1996 . The prior only gives the intersection angle while the latter gives both intersection angle and direction.
´
4.1. Directional single-hole surÕeys Most borehole antennas are axially oriented
dipoles. Consequently, the response of a survey includes information from the entire volume of
rock surrounding the hole. In our case, it is necessary to determine the orientation of single
features and we have therefore used a direc- tional receiver antenna. It is directional in the
sense that it allows for the absolute determina- tion of strike and dip of all planar reflectors. In
Fig. 3, the principle of the interpreted results from an omnidirectional dipole antenna is com-
pared to that of the directional antenna. The design of the antenna limits the practical
range of angles at which reflectors are deter- mined with the borehole radar. Fig. 4 shows an
example of how the spatial density of fractures interpreted depends upon the angle of intersec-
tion between the borehole and the fractures. There is no reason to assume that there is a bias
in the natural distribution of zones in this rock mass and therefore this distribution can be as-
sumed in other rocks as well.
Fig. 4. Distribution of fractures detected by borehole radar relative to the angle of intersection between borehole and Ž
. structures Tiren et al., 1996 .
´
Previous investigations show that the ability of the borehole radar to detect features within
the rock mass appears to have a maximum when intersecting structures at an angle of c. 30
8, while it diminishes rapidly for lower intersec-
tion angles and more slowly for higher intersec- Ž
. tion angles. At high intersection angles - 75
8 , the ability to detect structures is the lowest. This
geometrical relationship has to be considered when planning borehole radar surveys and espe-
cially when the target will be investigated from several holes drilled in different directions. The
target could be a specific object such as a fracture zone, but it could also be a rock vol-
ume. An orthogonal borehole configuration does not optimize the potential of the borehole radar,
as there will be orientations of structures poorly
Ž .
sampled by all boreholes Tiren, 1998 .
´
Since the reflection coefficient is highly de- pendent on the variation in water-content in the
rock mass the radar reflection surveys result in selective picking of features. Bedrock features
that give rise to reflectors need to be of a certain size and preferably, but not always, are the
effect of water-saturated porosity. The radar reflectors of concern in this study
Ž are generally planar and large in extent 20–40
. m or more structures. At the intersection of
radar reflectors and borehole, the core logs of- ten express increased fracturing. Locally the
fractures appear to be sealed.
4.2. Tomography The principle of cross-hole surveys is that the
transmitter and receiver are located in such a manner that direct rays traveling between them
pass through the medium. For each possible antenna configuration, a trace is recorded at the
receiver. Each trace corresponds to a raypath between the boreholes. Along each raypath, the
travel time and the amplitude of the first arriv- ing direct wave is determined. Both arrival times
and amplitudes can be analyzed with tomo- graphic inversion.
Fig. 5. Dipole radar component of single hole surveys in the two holes together with velocity tomogram from plane delineated by the two holes.
Fig. 6. Interpreted reflectors from directional antenna sur- veys in the two boreholes, plotted in the tomographic
plane. Numbers to the left in plot are reflectors found in Ž
. Ž .
the left borehole RO-KR4 Carlsten and Wanstedt, 1996 .
¨
Tomographic inversion involves dividing the interval between the boreholes into segments
and assigning approximate values of the dielec- tric constant or attenuation to each segment in
an iterative manner. The values are adjusted by means of the amplitudes and travel times of the
rays passing through each segment, calculations are continued until the travel times and attenua-
tions correspond to the measured values. The resolution of the method is a complex function
of the wavelength, the transmitter and receiver
Ž .
point spacing along the respective holes , as well as the distance between the boreholes. The
resolution is on the order of meters. Tomography and reflection surveys are af-
fected a bit differently by variations in the rock mass. For example, a wide, severely fractured
and water-bearing zone will probably cause a reflection at either of the boundaries. The mini-
mum velocity in the tomogram, on the other hand, will probably be located in the center of
Ž the zone where the porosity and water content
. reaches a maximum .
4.3. Description of measurement setup Borehole radar measurements were per-
formed in two co-planar holes 509 and 301 m deep, respectively, at the Romuvaara investiga-
tion site in Finland. Single-hole directional sur- veys were performed in each hole. Furthermore,
Fig. 7. Interpreted reflectors from directional antenna sur- Ž
. veys in the two boreholes, plotted in the volume 3D
surrounding the tomographic plane.
cross-hole measurements were performed be- tween the holes. The distance between the holes
is 73 m at the surface and about 201 m at depth. The use of a directional tool enables the
determination of location, strike and dip of re- flectors in the vicinity of the hole. The transmit-
Fig. 8. Velocity tomogram between the two boreholes with interpreted structures. Dashed lines are possible structures picked from the velocity tomogram.
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
