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Journal of Applied Geophysics 45 2000 141–156 www.elsevier.nl
rlocaterjappgeo
Evaluation of GPR techniques for civil-engineering applications: study on a test site
G. Grandjean , J.C. Gourry, A. Bitri
BRGM, Direction de la Recherche, Departement Geophysique et Imagerie Geologique, AÕe. Claude Guillemin, BP 6009, F-45060 Orleans Cedex 2, France
Received 29 October 1998; accepted 12 July 2000
Abstract
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Different ground-penetrating radar GPR techniques have been tested on the same site in order to establish the performance and reliability of this method when applied to civil-engineering problems. The Laboratoire Central des Ponts et
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Chaussees LCPC test site at Nantes, France, was selected because it includes most of the underground heterogeneities
´
commonly found in urban contexts, such as pipes, small voids, etc. The GPR survey consisted in recording measurements in Ž
. Ž
. Ž
tomographic surface to horizontal borehole measurements , monostatic 2D surface profiling and bistatic Common Mid w
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Point CMP modes above various buried heterogeneities. Different processing techniques were also performed, such as
tomographic inversion, 2D and 3D migration, velocity analysis, as well as numerical simulations, the results of which can be Ž .
summarized in three points. 1 Although the different filling materials of the site can be distinguished by velocity and attenuation tomography, the buried heterogeneities are more difficult to identify because of limited resolution related to
Ž . angular aperture and Fresnel zone. 2 2D surface profiling can detect the different shallow heterogeneities, such as pipes
and voids, down to a depth of several meters. Additional processing, such as forward modeling and attenuation curve analysis, provides more quantitative information related to the medium. A comparison between 2D and 3D migrated data
Ž . highlights the error introduced when the structures are considered to be perfectly cylindrical. 3 CMP analysis gives
relatively good estimations of vertical velocity contrasts when the medium is layered. A lithologic log can be derived assuming that the velocity changes are related to material variations. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Geotechnics; Test site; Ground-penetrating radar; Velocity and attenuation tomography; Modeling
1. Introduction
The detection of underground heterogeneities us- ing non-destructive methods is a crucial problem in
Corresponding author. Tel.: q33-2-38-64-34-75; fax: q33-2-
38-64-33-61. Ž
. E-mail address: g.grandjeanbrgm.fr G. Grandjean .
urban environments, especially for trenchless works. For example, unknown concrete house foundations
or sandstone blocks located along the path of a drilling machine can cause major equipment damage
and thus a loss of time and money. Geophysical methods, particularly the ground-penetrating radar
Ž
. GPR , can detect such superficial bodies with a
relative efficiency depending on the field context, the dielectric properties of the host material and the
0926-9851 r00r - see front matter q 2000 Elsevier Science B.V. All rights reserved.
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PII: S 0 9 2 6 - 9 8 5 1 0 0 0 0 0 2 1 - 5
nature and size of the bodies. The objective of this work, funded by the BRGM and the National Project
on Trenchless Works, is to review the capabilities of GPR in urban environments for civil-engineering
applications. We approached this problem by com- paring the results obtained at the same site using
different survey configurations and different process- ing techniques.
The success of such work is mainly conditioned by the features of the site where data acquisition is
carried out. Firstly, the site must be representative of the urban environment, which means that the proper-
ties of the host materials and buried heterogeneities must be consistent with those commonly found be-
neath cities. Furthermore, the selected site must be well known and calibrated so as to be able validate
the tested methods, i.e. the nature of the host mate- rial and the position of the buried heterogeneities
must be precisely known. Finally, to ensure optimum data acquisition, the site must be easily accessible
Ž and as free as possible from noise sources
e.g. .
electrical installations , trees, etc. By respecting these criteria, a good compromise should be reached be-
tween reality and an idealized underground model. The geotechnical test site of the Laboratoire Central
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des Ponts et Chaussees LCPC at Nantes, France
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Chazelas et al., 1997 was selected because it satis- fies all these requirements.
Numerous studies describe efficient GPR tech- niques for detecting and imaging underground pipes,
Ž voids, etc. Zeng and McMechan, 1997; Powers and
. Olhoeft, 1996; Tong, 1993; Annan et al., 1990 .
However, because each technique is generally con- sidered individually in a specific context, it is diffi-
cult to compare reliability when applied together under the same field conditions. In view of this, we
present here the results of three different experiments systematically tested and compared at the same test
site. The first experiment consisted in tomographic measurements from the surface to a horizontal bore-
hole, and was dedicated to estimating the velocity and attenuation fields across the site. The second was
a series of monostatic 2D surface profiling above each known buried heterogeneity and recorded using
different antenna frequencies, complemented by 3D coverage over a specific area. Finally, bistatic Com-
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mon Mid Point CMP measurements in the sub- horizontally layered part of the site constituted the
third experiment. To guarantee the quality of inter- pretations, we tested different processing techniques,
such as velocity and attenuation inversion, 2D r3D
migration plus forward modeling, and velocity analy- sis on the CMPs, which led to the definition of the
most appropriate technique for imaging each object and characterizing the dielectric behavior of each
host material.
After a description of the LCPC test site, an inventory of the different survey configurations used
and the associated results are presented, followed by a discussion concerning the contribution of each
acquisition and processing technique to imaging un- derground heterogeneities.
2. Field measurements and processing
