The horizontal tomography reveals a large dam- aged zone inside the section extending downwards
Ž .
see Fig. 4a . The rays tend to travel around this damaged area. Apart from a small zone in the upper
right-hand part, the pillar is quite damaged. Its mean Ž
y1
. velocity 3811 m s
is well below the average velocity of mechanically sound pillars at this level
Ž
y1
. around 4500 m s
.
4. Comparison and interpretation
As previously discussed, these two geophysical techniques provide complementary information. The
first classical means of combination therefore is to superimpose the cracks detected by GPR onto the
seismic tomography displaying the velocities. We would expect to be able to correlate the localization
of the main cracked areas by GPR with the damaged zones corresponding to low seismic velocities.
The major problem herein concerns the human factor, which influences the selection of certain
cracks over others, thereby implying that an absence of cracks signifies a homogeneous area. The choice
of which cracks to retain depends on the relative amplitude of each of their echoes. This logical com-
parison reveals its drawbacks either when numerous pillars or when different processing users are in-
volved.
4.1. Radar tomography In order to take into account all of the diffracted
signals, radar processing is conducted automatically. By virtue of the possibility to survey from all sides
of the pillars, coupled with the fact that the depth investigated is greater than the thickness, each pro-
file presents information on every area of the pillars. An accurate localization of the diffracting areas en-
ables mapping the pillar by adding this information by a classical imaging process.
This information, generated from the echoes, de- pends on the depth of the cracks, their target section
and their filling. However, since the pillars display a Ž
. high number of cracks many of which are visible ,
small discontinuities, voids or diffracting points, the amplitude and the number of echoes are proportional
to the level of damage in a given area. So, the principle of this radar tomography is to
design a square imaging section from each GPR profile, perfectly localized in a common coordinate
system. For that purpose, processed GPR profiles need to be extended. Indeed, they are 10 m deep, and
need some more scans on both sides in order to reach 10 m large. This process is available in the
software WinRad by copying and adding the first and the last scan until the GPR central section is
correctly positioned on the pillar location.
Since these profiles were already migrated and surface normalized, the four maps can be superim-
posed in order to represent the pillar by a radar image. The dark plots are then added, thus increasing
the darkness, with the assumption that the result is correlated with a high damage level.
This last step is accomplished by means of an image processing software for PCs called APIC-
TUREB, which has been designed and developed at the LCPC laboratory by J.M. Molliard. Analysis and
processing on gray-level pictures is possible through the use of its own library of filters, morphologies,
averages, operations and false colors. Moreover, macro-orders allow automating the radar imaging
Ž .
process see Fig. 5 . The borders of the pillars are drawn over the radar
tomographies in order to localize the damaged areas, to avoid taking into account the gray values beyond
the pillars, and to allow paying special attention to those areas located very close to the borders.
We consider that the dark plots have been roughly correctly added, due firstly to the fact that the sur-
face normalization gets corrected by the half-wave- length of the radar pulse, which allows positioning
the maximum reflected energy from each profile at the same place for each fracture. The second reason
is that the visual investigation showed only vertical, or sub-vertical, external cracks on the pillars, i.e. no
3D migration corrections are necessary on the pro- files.
4.2. Comparison Fig. 6 presents both tomographies simultaneously
for Pillar 1. The shape of the dark plots is completely
Ž .
Fig. 5. Radar tomography by image processing Pillar 1 .
Ž different due to the large number of data several
hundred seismic data points vs. several thousand .
radar data points , yet we are still tempted to corre- late both of these tomographic images.
GPR processing was carried out to focus the presentation not only on the cracks but also on the
diffracting areas. These areas can be considered as small discontinuities, voids or diffracting points,
which are correlated with a specific level of damage. However, we must take into account the EM energy
resulting from the main cracks, which can locally increase the apparent EM damage level. For this
reason, it is useful to include the information from the presentation of the cracks into the radar imaging.
Fig. 6. Radar and seismic imaging on Pillar 1.
Similarly, the damage zones are localized by low Ž
. seismic velocities plotted in dark . Pillar 1 therefore
appears to be a good example of a non-homogeneous pillar, in which most of the damaged zones are
detected either by GPR or by seismic imaging. Both the left and center parts of the pillar display lower
seismic velocities and higher densities of EM re- flected energy at the same locations.
This kind of correlation is confirmed in Pillar 2 by the sub-vertical narrow damaged area in the
center of the pillar, which has been detected by Ž
. either one of the two NDT approaches see Fig. 7 .
Fig. 7. Radar and seismic imaging on Pillar 2.
Fig. 8. Radar and seismic imaging on Pillar 3.
The overlap of the main cracks on the radar tomog- raphy is significant, as shown in the center part
where the dark plots are not caused only by the presence of a single major crack.
For all of the tomographies studied, a comment on the border effects is necessary. Due to the low
density of rays near the corners, the values of seis- mic velocities are not accurate, and in most instances
should be used with caution. Hence, both the seismic tomography and the ray curve density map must be
presented.
For GPR imaging, the localization of the bottom of the pillar is inaccurate for each profile, and espe-
cially for unevenly-shaped pillar. Moreover, the
Fig. 9. Radar and seismic imaging on Pillar 4.
shape of the pillar can disturb some parts of the tomography near the borders. Pillar 3, in which the
left and lower sides are not perpendicular, provides a good example. The last GPR scan, at the border of
both profile’s sides, has been copied and then re- peated in order to lengthen the profiles to the right
Ž dimension for image processing principle presented
. Fig. 5, on Side A . The information related to this
last part of the GPR profiles can interfere with the imaging. Thus, both the upper left-hand and lower
right-hand parts of Pillar 3 do present some inaccu- rate results.
With respect to the seismic tomographies, Pillars 3 and 4 are more homogeneous and display high
Ž .
velocities see Figs. 8 and 9 . The ray coverage and inversion convergence are similar to that of Pillar 1:
they are not shown here for purposes of conciseness. These seismic results demonstrate that the pillars are
Ž
y1
. mechanically sound velocities around 4400 m s
. In this context, the radar imaging does not seem to
be heterogeneous. The dark plot density is low and roughly constant in the maps, which suggests that the
radar and seismic imaging are in accordance.
We must nonetheless be careful to avoid linking the EM power reflection directly to low seismic
velocities. Even though we cannot distinguish seri- ously damaged zones on the radar tomographies, we
are still not in a position to assume that these pillars are mechanically sound. Confirmation can only come
from seismic investigation, which correlates high velocities with mechanical soundness.
5. Conclusion
