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Journal of Applied Geophysics 45 2000 157–169
www.elsevier.nlrlocaterjappgeo
GPR and seismic imaging in a gypsum quarry
Xavier Derobert , Odile Abraham
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LCPC — Centre de Nantes, Section Reconnaissance et Geophysique, Route de Bouaye, BP 4129-44341 Bouguenais Cedex, France Received 16 June 1999; accepted 30 June 2000
Abstract
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A combination of ground penetrating radar GPR and seismic imaging has been performed in a gypsum quarry in western Europe. The objective was to localize main cracks and damaged areas inside some of the pillars, which presented
indications of having reached stress limits. The GPR imaging was designed from classical profiles with GPR processes and a customized, PC-based image-processing software. The detection of energy reflection seems to be an efficient process for
localizing damaged areas. Seismic tomographic images have been obtained from travel time measurements, which were
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inverted using a simultaneous iterative reconstruction technique SIRT technique in order to provide a map of seismic velocities. The imaging and techniques employed are compared herein.
The two techniques are complementary; seismic tomography produces a map of velocities related to the state of the pillar’s internal stress, while radar data serve to localize the main cracks. Moreover, these imaging processes present
similarities with respect to the damaged zone detection. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Ground penetrating radar; Seismic; Data processing; Imaging; Fractures; Damaged zones
1. Introduction
A gypsum quarry in western Europe has revealed stability problems which require local reinforcement.
The galleries concerned have a section of approxi- mately 6 m in width and 7 m in height; the pillars
have a square section, with a minimum side length of 7 m. During mining operations at the quarry, no
special precautions had been implemented. The re- sult is manifested in the irregularity of the pillars’
shape and the many visible cracks on their sides. Laboratory experiments on numerous samples, in-
Corresponding author. Tel.: q33-2-40-84-59-11; fax: q33-2- 40-84-59-97.
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E-mail address: xavier.derobertlcpc.fr X. Derobert .
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cluding mineralogical, mechanical and ultra-sonic tests, have shown no significant seismic anisotropy.
In some areas, the high density of fracturing and the potential for cross-cracking, combined with the
damaged zones, has imposed the need to determine the distribution or continuity of the fractures. For this
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purpose, a non-destructive testing NDT campaign has been carried out to select certain pillars that
present damage characteristics. The objective herein was to localize the disaggregated areas inside these
pillars, which correspond to high levels of stress, along with the main cracks. Two complementary
techniques were employed: seismic tomography and radar investigation.
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Ground penetrating radar GPR is a very useful technique for carrying out geological NDT, which
detects dielectric contrasts at the boundary planes by
0926-9851r00r - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž
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the reflection of electromagnetic EM pulses. The degree of crack detection depends on various param-
eters, such as the equivalent target section and the filling of cracks by clay, water or air. In general, the
rock’s dielectric attenuation is very low, thereby suggesting several meters of radar investigation
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. Stevens et al., 1995; Toshioka et al., 1995 . The
literature does provide some results concerning the coefficient of reflection as a function of the dielectric
contrast and the incident angle of the target section, which can be modeled in order to predict the poten-
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tial expected resolution Olhoeft, 1998 . Although this technique is quick and easy to use, its major
limitation lies in its inability to yield information on the state of stress in the structure.
For this reason, a secondary campaign of seismic tomography is to produce a map of objects’ internal
mechanical properties in a non-invasive fashion. By measuring the travel times of the compression wave
between source and receiver points around the ob- ject, it is possible to calculate a map of the compres-
sion wave velocity. In the case of an a priori homo- geneous material, the appearance of a zone of lower
velocity indicates that the material has weathered locally.
Seismic transmission tomography using travel times is more sensitive to zones of micro-cracking
than to isolated cracks, especially if the micro-cracks are not closed and if the material is damaged. In the
case of a homogeneous medium, the difference in travel times, both with and without an isolated crack,
might very well be of the same order of magnitude as the level of accuracy in the times chosen. Spathis
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et al. 1983 showed that the rising time is often more sensitive to cracking than the travel time.
Consequently, radar and seismic tomography are fully complementary, by virtue of their ability to
provide different information in the geological diag- Ž
. nostic process MacCann et al., 1988 .
2. Radar investigation
