Rock Mech Rock Eng (2010) 43:105–116
DOI 10.1007/s00603-009-0028-2
REVIEW PAPER
Exploration of Tunnel Alignment using Geophysical Methods
to Increase Safety for Planning and Minimizing Risk
Bodo Lehmann Æ Dirk Orlowsky Æ Rüdiger Misiek
Received: 12 July 2007 / Accepted: 23 October 2008 / Published online: 6 February 2009
Springer-Verlag 2009
Abstract Engineering geophysics provides valuable and
continuous information for the planning and execution of
tunnel construction projects. For geotechnical purposes
special high-resolution geophysical methods have been
developed during the last decades. The importance of
applying geophysical methods in addition to usually used
geological and geotechnical exploration techniques is
increasing. The main goal is to achieve an accurate and
continuous model of the subsurface in a relative short
period of operation time. The routine application of engineering geophysical methods will increase in the coming
years. Due to the high acceptance of engineering geophysics at construction sites, much wider application of
geophysical investigations is expected. The combination of
different methods—geophysics, geology, and geotechnics
as well as the so-called joint interpretation techniques—
will be of essential importance. Engineering geophysics
will play an important role during the three phases: geological investigation, tunnel planning, and execution of
tunnel construction. If hazards are well known in advance
of a tunnel project the safety of workers will essentially be
increased and geological risks will be minimized by means
of successful and interdisciplinary cooperation.
B. Lehmann (&) D. Orlowsky R. Misiek
Exploration and Geosurvey Division, DMT GmbH & Co. KG,
Am Technologiepark 1, 45307 Essen, Germany
e-mail: Bodo.Lehmann@dmt.de
URL: http://www.dmt.de
D. Orlowsky
e-mail: Dirk.Orlowsky@dmt.de
R. Misiek
e-mail: Ruediger.Misiek@dmt.de
Keywords Line investigation Karst cavities
Geophysics Seismics Tomography Elastic parameters
Strain
1 Introduction
Tunnel construction depends especially on a qualitative and
quantitative description of the underground conditions.
Engineering geology and hydrogeology offer a number of
methods in order to investigate these conditions with the
desired precision. These methods (e.g., DIN 4020, DIN
4021, SIA 199, EC: ENV 1991) are generally recognized to
be state of the art and they are regulated in a wide range of
standards. Geological studies generally imply the adoption
of near-surface mapping along exposures or the use of deep
drillings with core recovery and analyses. However, even
within intensive and expensive investigations of the foundation soil, distances between drillings can easily range
between 50 and 200 m. Furthermore, in the case of detailed
explorations of deeper targets, i.e., crossing waters or deeply located tunnels, boreholes are drilled more than 1,000 m
apart. Therefore, to overcome uncertainties in the description of physical properties determined from a few drillings
only, the importance of geophysical investigations is
increasing due to the fact that these methods yield continuous information along profiles or about the complete area
under investigation of relevant physical subsoil properties.
2 Possibilities to Apply Engineering Geophysics
The investigation of subsoil conditions is always important
when buildings are created on the surface or built into the
soil. Especially for tunnel constructions, planners and
123
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B. Lehmann et al.
supervisors need very detailed and comprehensive information about the subsoil where they plan to drive the
tunnel. Comparable to a medical case a detailed examination with, e.g., radiographs, is essential for the preparation
of an operation. No responsible doctor would perform an
operation without having this urgently needed information
in advance. Otherwise, disregard of these precautionary
investigations would result in a kind of Russian roulette.
The same holds for engineering: a wrong diagnosis due to a
lack of information can cause immense damage.
The procedures of applied geophysics (engineering
geophysics) were developed during the last 20 years, based
on the classical applications in coal, oil, and natural-gas
exploration. Based on these developments today’s possibilities to apply geophysics in tunnel construction projects
are various and supply the engineer with a range of
information about the structure of the subsoil. Using geophysical measurements it is possible to obtain relatively
quickly a model of the structure and physical properties of
the subsurface along a line (profile) or—by the combination of several profiles or rather a planar assembly—a
three-dimensional picture of the underground. The application of geophysical methods is used to determine the
physical characteristics of the inaccessible underground
without touching the subsoil to a larger extent (Lenz et al.
1997; Althaus and Räkers 1998; Althaus et al. 2000).
The physical characteristics change according to the
lithological, structural, and mechanical changes in the
underground, so that they can be detected with geophysical
techniques if physical changes reach values of certain
significance. After applying filter and evaluation procedures the subsoil can be described by models. In order to
obtain a correct estimation of the possibilities of geophysics (Räkers et al. 1996) and also to prevent
overreliance on geophysical methods, the following statements can be made:
•
•
•
•
A complete transparent underground cannot be produced by geophysical investigations.
Geophysical experiments are indirect procedures and
should be combined with direct investigations (e.g.,
drillings) and calibrated with these.
Conceptual formulations and subsoil models have to be
discussed between the geotechnical consultant and the
geophysicist before any measurement starts.
It depends on the experience of the geophysicist to find
the most reasonable geophysical method or its combination with others in economic and technical terms with
appropriate consideration given to the desired
application.
Considering these statements, engineering geophysics
yields valuable additional information to the standard
subsoil investigations and helps to implement the planning
and investigation steps in an economical and technically
optimized way. Therefore, the geophysical investigation
program has to be designed for the problems to be investigated. Usually, geophysical investigations will consist of
a number of different techniques and procedures. A
selection of the most important engineering geophysical
procedures are presented in Table 1.
Table 1 Frequently applied engineering geophysical methods with measured variables and their targets
Methods
Targets
Parameter
Geophysical methods/techniques
Seismics
Layers, loosening, cavities, change of
material, faults, obstacles
Elastic parameter
Reflection seismics
Refraction seismics
Borehole seismics (tomography, VSP)
Surface wave seismics
Vibration monitoring
Gravity
Cavities (e.g., dolines, gallery)
and inclusions
Density
Measurement of the relative mass attraction
Magnetics
Iron, ores, minerals, masonry,
refuse dumps
Susceptibility
Measurement of the total magnetic Earth’s
field/measurement of the gradient
Geoelectrics
Layers, change of material, groundwater,
cavities, metal, sewers, contamination
Electric resistivity
Electric sounding/tomography
Electromagnetics Layers, change of material, groundwater,
cavities, metal, sewers, contamination
Electric resistivity
Electromagnetic induction transient
electromagnetics/magnetotellurics
Radar
Layers, change of material, inclusions,
obstacles, sewers
Electric resistivity
Electromagnetic reflection
Borehole
measurements
Layers, water saturation, porosity,
groundwater flow, cavities, loosening
Elastic parameter, density,
susceptibility, electric
resistivity
e.g. caliper, c- and n radiation, c-c-(density)log, full-wave-sonic-log, SP-log, televiewer
VSM vertical seismic profiling
123
Exploration of Tunnel Alignment using Geophysical Methods
Depending on the geotechnical and geological questions
and on the required resolution, data-acquisition parameters
of the used geophysical method need to be adapted. Due to
acquisition parameters large-scale measurements can be
carried out to investigate depth ranges of several kilometres
and small-scale measurements to investigate near-surface
conditions at depths down to a few meters only. Geophysical applications in tunnel projects are not only used
for planning purposes but also during the construction
phase. An overview of the two stages is given as follows:
•
Planning Stage
•
•
•
•
•
•
Investigation of the bedrock structure (e.g., boundary between different rock horizons, layer
thicknesses or geological anomalies)
Investigation of cavities (e.g., karst cavities, mining
structures)
Recognition of obstacles (e.g., buildings, pipelines)
Derivation of the geotechnical parameters in terms
of density and deformability of the rock mass
Investigation of the water level or extended waterbearing strata
Investigation of contaminated areas
The results of geophysical applications during the
planning state offer a cost-effective and purposeful
arrangement for further and more detailed investigations.
The recognition of obstacles is of crucial importance for
the planning of the building procedures. Additionally, the
following tasks have to be performed at the construction
stage:
•
107
application are described. The potential of geophysics
substantially increases by using a combination of different
methods. The following examples of successful and economic method combinations can be mentioned:
•
•
•
Seismic reflection and refraction methods in addition to
gravity for the investigation of alluvial fills and karst
cavities
Geoelectric and seismic reflection techniques for
hydrogeologic questions
Ground-penetrating radar (GPR), gravity, and seismic
reflection methods for the localization and characterization of fault zones
Furthermore, an important step for a successful investigation on tunnel alignment is the ability to create
geophysical models of the area of interest. This step is
often called calibration. Direct investigations (drillings,
etc.) always supply exact, but only very local, information.
However, geophysics supplies physical parameters, which
are regarded as integral values over a wider surface or a
larger volume. The first step for investigating a tunnel site
is the planning and performance of a geophysical program
with interpretation of the results. Accordingly, an optimized drilling program is needed. Finally, the drilling
results are used to calibrate the geophysical results and to
generate the desired geotechnical model.
4 Seismic Methods
4.1 General Information
Construction Stage
•
•
•
Measurements to look ahead of the tunnel face;
Quality assurance (e.g., when executing a concrete
slab below the water table, sealing membrane,
friction piles in water-filled excavations);
Vibration measurements to determine the effects on
human beings and buildings. With knowledge of the
influencing parameters the method for tunnel driving can be optimized.
The application of the first two steps in the construction
stage will avoid unexpected events during the building
execution (e.g., finding obstacles which cannot be detected
from the surface, or the occurrence of defects in a deep
excavation).
3 Combination of Methods and Involvement of Direct
Exposures
In previous chapter the most important geophysical methods are listed in Table 1 and the possible fields of
Seismic methods use information from elastic waves to
generate a model of complex underground structures.
The important parameters for wave propagation are the
wave velocities in the different layers and the densities
of the materials. The underground affects the propagation
of seismic (elastic) waves by mechanisms such as
reflection, refraction, diffraction, absorption, and dispersion (Fig. 1).
In engineering geophysics seismic waves are generated
artificially, e.g., by explosives, hammer blows, vibrators,
accelerated weight drops, implosions or other seismic
sources. In water, systems such as a sparker, boomer or
airguns are applied. For the recording of seismic wave’s
geophones, accelerometers or hydrophones are used. The
data-acquisition parameters are adapted to the special situation, respectively. Seismics is usually carried out along
profile lines, i.e., two dimensions (2D). For spatial investigation of the underground a laminar measuring setup of
the source positions and receiver locations is necessary,
which results in a three-dimensional (3D) insight into the
underground.
123
108
B. Lehmann et al.
Drop Weight
Geophone
surface
Reflection
Inhomogeneity
r¼
v
2
rffiffiffi
t
:
f
Under optimal conditions the radius of the Fresnel
zone reduces to one-quarter of the wavelength. Thus,
high frequencies (small wavelengths) are needed for a
high resolution of the underground. Since higher
frequencies are attenuated more strongly than lower
frequencies underground it is necessary to find an
optimal balance between signal energy and frequency
content.
The following seismic methods can be used successfully
for the exploration of tunnel alignments (Fig. 2) depending
on the exploration goals and accessibility:
Strata boundary
Refraction
Fig. 1 Seismic principle for high-resolution investigation of subsoil
conditions
The seismic display definition is subdivided into the
vertical and the lateral resolution. The vertical resolution,
e.g., the resolution of boundaries of the beds, especially
depends on the frequency content of the seismic signals
and of course on the signal-to-noise ratio of the registered
wave field.
Generally the following formula applies:
v ¼ kf
with:
v
k
f
wave velocity (m/s)
wavelength (m)
frequency (Hz).
Usually, the range between one-quarter and one-half of
the wavelength is defined as the vertical resolution. Hence,
with a seismic wave velocity of 300 m/s in the near-surface
underground and with a signal frequency of 300 Hz the
wavelength is calculated to 1 m. Thus, the maximal vertical resolution is about 0.25–0.50 m.
The lateral resolution is defined as the minimal lateral
extension of an object that can be resolved. The signal that
is emitted from a certain point and recorded at a receiver
originates from a constructional interference from a
broader zone along the reflecting boundary (Fresnel zone).
When the distance between two reflecting points is smaller
than the radius of the Fresnel zone, no differentiation
between the two points is possible.
The radius r of the Fresnel zone depends on the seismic
velocity v, the dominating frequency f (Hz), and the travel
time t (usually measured in seconds):
123
Fig. 2 Basic variants of applied seismics (seismic refraction, reflection method, borehole seismics)
Exploration of Tunnel Alignment using Geophysical Methods
•
•
•
Reflection seismic method
Refraction seismic method (standard, CMP, tomography)
Well shooting (VSP, tomography)
109
velocity
reflected
wave
Within the seismic data different waves types are measured, which are interpreted with respect to the given task:
•
•
•
•
Compression waves (P-wave)
Shear waves (S-wave, SH, SV)
Surface waves (Rayleigh wave, Love wave)
Disturbing waves/noise (air waves, electrical influences, traffic noise, etc.)
Apart from the effects described above (reflection,
refraction, diffraction, etc.), there is the need to consider
those seismic waves which have the ability to change their
modes at the boundaries between different media (e.g.,
from P- to S-wave). Further information can be found in
Baker (1999), Evans (1997) or Yilmaz (2001).
4.2 Reflection Seismics
The reflection seismic method is based on processing
reflected waves that have been generated at the surface and
recorded after traveling through the underground. At
boundaries or at inhomogeneities in the underground,
where the seismic impedance (i.e., the product of velocity
and density) of a material changes, a part of the energy of
these waves is reflected.
To determine seismic velocities and to optimize the
relation between signal and background noise, seismic
waves are recorded with a number of geophones, which are
arranged in constant distances along a line. The data
acquisition usually takes place according to the so-called
roll-along/end-on procedure. This means that the source
point is located at the beginning or end of an active geophone profile and moves the spread in front or behind. With
the help of the distances between the source location and
the geophones and with determined travel times of these
waves (i.e., the time the wave needs to travel from the
seismic source to the reflecting boundary of the bedrocks
and back to the geophone) the depth to the reflecting
boundary of the bedrocks (the distance to the reflecting
object) can be calculated. A resulting travel time curve is
shown in Fig. 3.
The portion of the energy reflected at a layer interface
(or at an object) depends on the ratio between the seismic
impedances on both sides of the interface. During seismic
reflection data processing the seismic traces are sorted
according to the common mid points (CMP) between the
source and receiver positions in the field. After applying
dynamic corrections (e.g., velocity corrections) the amplitudes of the traces are summed up (stacked) so that at each
direct
wave
offset
shot point
geophone line
surface
reflected
wave
reflector
Fig. 3 Shapes of the ray paths and travel time curves of reflected
seismic waves
CMP a stacked trace of all contributing records is generated. The resulting CMP stacked section corresponds to a
registration, with which both the source and geophone
apparently are positioned at the same location, namely the
CMP. Due to the fact that several seismic traces are
assigned to one CMP, so-called multiple coverage is
obtained. The advantage of a multiple coverage compared
with single trace measurements is an improved signal-tonoise ratio and thus higher quality and accuracy of the
results. The principle of sorting the traces in CMP gathers
is shown in Fig. 4.
The final result of reflection seismic data processing is a
so-called seismic reflection stacked section.
Figure 5 shows an example of such a section with
interpreted reflecting horizons along a line investigation
over a planned tunnel. Due to the fact that reflected
amplitudes are limited only to that range, where interfaces
are present an interrupted reflection can be interpreted as a
disturbance in that special layer interface. On the basis of
calculated seismic velocities the positions of reflecting
horizons (horizontal lines in Fig. 5) and of faults or
traveltime
reflected
waves
direct
waves
Offset
CMP
Geophone array
Source positions
Earth’s surface
reflected
waves
Reflector
Fig. 4 Shapes of the seismic ray paths and travel times in a CMP
gather
123
110
Fig. 5 Example of a seismic
stacked section with
interpretation of an exploration
for the Koralm tunnel between
Graz and Klagenfurt (Austria)
B. Lehmann et al.
250m
NW
SE
1200m ü. N.N.
-400
Depth [m]
~ tunnel
level
-1200
disturbances (vertical colored lines in Fig. 5) can exactly
be identified within the reflection seismic stacked section.
Together with drilling results each reflecting horizon can
be assigned to a geological layer interface, in which also
the individual disturbance (faults, leachings, interstratifications, etc.) can be identified.
Travel time
-1800
Direct
Wave
Shot point
If the underground is characterized by increasing seismic
velocities (e.g., change of elastic modulus) with increasing
depth, seismic waves travel along layer interfaces, if they
dip with a certain angle onto a boundary. In such a case,
parts of the seismic energy are permanently radiated back
(refracted) to the Earth’s surface. Thus, the refracted waves
are recorded by geophones located at the Earth’s surface.
The travel times of the refracted waves are determined and
the seismic velocities are calculated with respect to the
depth of each layer.
Figure 6 shows the principle of the refraction seismic
method, the ray paths of the seismic waves underground,
and their travel time curves. It can be recognized that at a
certain distance refracted waves appear as the first arrivals
because they are faster than the direct waves. Therefore,
refracted waves overlap the direct waves although their
travel distances are much higher. Calculating the reciprocal
gradients of the travel time curves the velocities of the
seismic waves (direct and refracted) are determined. The
extension of the travel time curve of refracted waves back
to the source position determines the so-called intercept
time (Ti). By knowing seismic velocities and intercept
times standard refraction seismic methods [generalized
reciprocal method (GRM), wavefront method, intercept
time method] provide a velocity-depth model of the
underground. Seismic velocities are assigned to different
123
Refracted Wave
Ti
4.3 Refraction Seismics
4.3.1 Standard Refraction Seismics
Reflected
Wave
Geophone spread
Earth surface
refracted
wave
Refractor
Fig. 6 Principle of the refraction seismic method: ray paths and
travel time curves
materials and thus, a velocity-depth model displays the
material properties of the underground structures.
Figure 7 shows the result of a standard refraction seismic processing. The calculated thicknesses of layers (areas
of constant velocities) are represented with reference to the
topography of the Earth’s surface above sea level. Each
layer displays a characteristic velocity which is related to
the elastic modules of underground materials. By means of
the standard refraction seismics the stratigraphy of the
underground can easily be determined.
4.3.2 CMP: Refraction Seismics
A method to improve the signal-to-noise ratio of refracted
waves is common-midpoint (CMP) refraction seismics
(Orlowsky et al. 1998). Applying this technique, the shallow underground is described using all information
(amplitude, frequency, phase characteristics) of the wavetrain, following the first break (first-break phase). Thus, the
layering can be determined, and additionally locations of
Fig. 7 Typical result of
standard refraction seismics for
the exploration of a tunnel; the
layered structure at depth
becomes obvious
sea level [m]
Exploration of Tunnel Alignment using Geophysical Methods
111
300
Deckschicht (V0)
weathering
layer (V0)
(V1)
1.
(V1)
1stRefraktor
Refractor
250
2. Refraktor
2nd Refractor
(V(V2)
2)
velocity [m/s]
200
V0
1000
V1
2000
V2
3000
0
500
sea level [m]
300
1000
1500
CMP refraction seismic data processing. The topography of
the Earth’s surface and the wave fields of two explored
refractors, in each case related to sea level, are shown in
Fig. 8a. In this section, many disturbances are identifiable
with perpendicular throws of 3 to approximately 20 m.
Figure 8b shows the interpreted structures of Fig. 8a. In
contrast to the results of the standard refraction seismic
method (Fig. 7), both the stratigraphy and the complete
tectonics of the near-surface underground are determined.
disturbances such as faults, weak zones, and clefts can be
detected. This information is of great importance when, for
example, investigating underground water migration paths,
which often follow faults or weak zones. In CMP refraction
seismics the roll-along method is used for data acquisition.
Thus, the recorded seismic data can be simultaneously
processed with reflection seismic techniques to describe
deeper underground structures.
CMP refraction seismics is usually applied in combination with the generalized reciprocal method (GRM;
Palmer 1986). This joint application is possible because of
the close relationship between the two methods in their
kinematical description. To maintain the advantages of the
roll-along technique in data acquisition for the combination
of both methods, the refraction seismic data must be treated
in such a way that it can be used simultaneously by CMP
refraction seismics and by GRM. GRM needs records of
forward and reverse data. Thus, the data must be sorted as
shot-receiver gathers and as receiver-shot gathers to obtain
the raw data for a GRM analysis. For CMP refraction
seismics, the data is sorted in CMP offset gathers. The
results of the GRM analysis are used to determine optimum
stacking velocities and integration boundaries for a partial
radon transformation in the inversion process of CMP
refraction seismics so that local irregularities of interest can
be detected and identified. Figure 8 shows the results of
4.4 Refraction Tomography
Refraction tomography is an extension of the standard
refraction seismic method. Applying refraction tomography
a continuous increase of seismic velocities with increasing
depth in each layer is considered which is caused by the
corresponding stress increase. Furthermore, differences of
seismic velocities within a single layer (and thus different
degrees of disturbances or changes of the elastic modulus)
are analyzed with the help of refraction tomography.
Refraction tomography is applicable to both compression
waves (P-waves) and to shear waves (S-waves).
Layers with constant seismic velocities are only an
approximation and display a first-estimate model which
usually differs from real underground conditions. Due to
different degrees of disturbances or the presence of
(a)
weathering layer (V0)
1st Refractor (V1)
250
2nd Refractor (V2)
200
0
500
1000
1500
distance [m]
sea level [m]
Fig. 8 The results of processed
CMP refraction seismic data
from preliminary exploration of
a tunnel alignment (a). The
refractor is represented as a
complete wave field. Locations
of disturbances (faults) can be
recognized on the basis of the
shape of the phases. An
interpretation of the results is
shown in (b). The tectonics of
the underground structure
becomes clear
distance [m]
300
250
(b)
S1
S3
S2
S4
2. Refraktor
(V2) (V )
2nd
Refractor
2
Deckschicht (V0)
weathering
layer (V0)
st
1. Refractor
Refraktor (V1) (V
S61)
S51
S7
S8
S9
200
123
112
B. Lehmann et al.
inclusions of foreign materials, the seismic velocities
within layers can vary significantly. In particular cavities,
weathered rock or erratic blocks show either a reduction or
an increase of seismic velocities in comparison with the
surrounding material.
Applying refraction tomography, special requirements
have to be met during the data-acquisition process. The
field geometry is comparable to parameters used in standard refraction seismics or CMP refraction seismics along
line profiles. However, a substantially higher measuring
effort has to be made. The geophone and source point
spacing have to be selected much smaller, in order to
achieve a significantly higher coverage of refracted waves
in the underground to calculate a continuous seismic
velocity distribution. In the case of standard refraction
seismics, travel time curves, which comprise a number of
determined travel times, are allocated to a single layer. In
contrast to this, refraction tomography associates to each
single travel time a velocity value in a certain depth and at
a certain lateral position. The determined velocity field
calculated from the travel time’s results in a continuous
velocity-depth model of the investigated subsurface.
Figure 9 shows a result of refraction tomography processing displayed as a continuous velocity-depth profile. In
contrast to the standard refraction seismic method no discrete layers need to be associated for the structure of the
subsurface. In Fig. 9 a relative reduction of the seismic
velocity can be recognized between distances of 50 and
70 m along the profile and at depths between 10 and 18 m.
This kind of low-velocity zones cannot be resolved by the
standard seismic refraction method. An allocation of
velocity values to rock parameters is possible on the basis
of calibration drillings.
4.5 Surface Wave Seismics
3000
0
-10
2000
-20
0
123
50
distance [m]
100
1000
p-velocity [m/s]
Fig. 9 Result of a refraction
tomography in the area of the
planned high-speed railway line.
A low-velocity zone can be
recognized at distances between
50 and 70 m at depths of
10–18 m
depth [m]
When exciting seismic waves at the Earth’s surface, in
addition to P- and S-body waves, surface waves with relative strong amplitudes are generated. Surface waves are
differentiated into Love and Rayleigh waves. Love waves
show a displacement which is horizontal and perpendicular
to the direction of wave propagation, whereas Rayleigh
waves show vertical particle motions. The energy of the
surface waves propagates mainly horizontal and is generally bounded to the Earth’s surface. Thus, surface waves
have relative strong amplitudes at the surface, which
decrease exponentially with increasing depth. Therefore,
surface waves are usually used for the investigation of
anomalies in the near surface.
The most important nature of surface waves is their
dispersion character, i.e., the propagation velocity described as a function of frequency. Oscillations with large
wavelengths (low frequencies) are influenced by a larger
depth range than oscillations with short wavelengths (high
frequencies). Due to the relative low propagation velocity
the wavelengths of surface waves are relative short, and
thus the resolution of structures in the near-surface
underground is relative high when applying surface wave
analyses. Therefore, for example, polluted areas and their
lateral demarcation disruptions in the near-surface underground as well as cavities, erratic blocks, manmade
hazards, etc. can be detected with relative high accuracy.
It is relatively simple to distinguish between surface
waves and body waves (P- and S-waves) in seismograms
due to the fact that surface waves are much slower than
body waves and they have relative strong amplitudes.
These characteristics allow exact velocity determination of
surface waves in distinguished frequency ranges. Knowing
the velocities in the selected frequency ranges, velocities
can be assigned to a certain depth range of the underground. This method is usually applied to maximum depths
of about 15 m depending on the geological situation in the
exploration area. During the analysis process of surface
wave propagation, maps of the velocity distribution for
different frequencies (and thus for different depth levels) of
the surface waves are generated.
For analyzing surface waves, single-shot seismograms
and so-called constant-offset (CO) sections are usually
used. CO sections are displays of seismic traces in which
the distances between the source locations and the geophones are kept constant. Inhomogeneities detected in CO
sections represent a first indicator of anomalies in the
Exploration of Tunnel Alignment using Geophysical Methods
4.6 Seismic Tomography
distance [m]
0
113
50
100
0
time [m/s]
reflection of the
surface wave
500
Fig. 10 Example of the analysis of surface waves to map anomalies
in the near-surface underground
near-surface underground and thus these locations are used
for intensified analysis of the surface waves of single-shot
sections. Single-shot analyses include an intensified
determination of the dispersion characteristics, the identification of locations where the dispersion changes
(inhomogeneities) in the underground, the determination of
amplitude changes, and a detection of the reflections of the
surface waves at near-surface anomalies. Figure 10 shows
an example of a single-shot analysis of surface waves with
respect to anomalies in the near-surface underground. The
surface waves are reflected at an obstacle at a depth of 3 m.
Applying seismic borehole tomography usually results in a
display of the P-wave velocity distribution in a plane of the
area between two boreholes (i.e., the source and the receiver
boreholes). With specially adapted seismic sources and
receivers high data quality can be received without damaging the boreholes during data acquisition. To explore the
plane between two boreholes in detail the data-acquisition
technique and the field layout have to provide the possibility
to analyze wave propagation in all directions (every possible wave path and ray angle). Each travel time of a seismic
wave which propagates from the source position to the
receiver location depends on the propagation velocity of the
seismic wave in the material between the boreholes.
Figure 11 shows a result of a seismic tomography
between two boreholes (B1 and B4) in an area dominated
by sand and clay. To interpret the results additional
information of geotechnical and geophysical borehole
measurements were used, so that velocity values could be
assigned to special soil types. Without the results of seismic tomography the interpretation of the existence and
distribution of sand lenses would not have been possible.
4.7 Vertical Seismic Profiling
Additional information for the calibration of seismic surface measurements can be gathered using the vertical
seismic profiling (VSP) technique. In VSP experiments,
usually source points are located at the Earth’s surface,
Fig. 11 Result of seismic
tomography showing the
distribution of sand lenses
between boreholes B1 and B4
after calibration of the velocities
based on information from
geotechnical and geophysical
borehole measurements
123
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B. Lehmann et al.
generating signals which are recorded in vertical drilling.
Source and receive positions can also be exchanged. Using
the VSP technique, an identification of seismic reflectors
and the corresponding velocity distribution of seismic
waves is possible. Thus, VSP measurements are especially
applied together with reflection seismic investigations for
the calibration of reflecting horizons and their corresponding depths. Furthermore, structures below the
borehole bottom can be explored in more detail compared
with reflection seismic measurements at the Earth’s surface
alone. VSP measurements support the results evaluated
from all other seismic measurements performed at the
Earth’s surface so that it is advisable always to use surface
seismic methods together with the VSP technique.
waves have been developed in past decades. Furthermore,
to record S-waves, receivers with three recording components (two horizontal and one vertical component) have to
be used. Applying the refraction tomography technique, it
is possible to generate velocity models for both P-waves
and S-waves. In connection with borehole measurements
seismic velocities can be directly correlated to measured
densities so that the distribution of the dynamic elastic
modulus can be calculated from the formulae above.
5 Case Studies
5.1 Seismic Investigations for a High-Speed Railway
Line in the Alps
4.8 Calculation of Geotechnical Parameters
Propagation velocities of different wave types together
with densities are correlated with elastic parameters of
underground structures. The correlation is described with
the following formula:
k ¼ q v2P 2v2S
l ¼ qv2S
¼ k þ 23 l
K ¼ qv2P 43 l
1 2
2
r ¼ vS 2 vP ðv2S v2P Þ
E ¼ 2v2S qð1 þ rÞ
with
k
l
K
r
E
q
vp
vs
Lamé constant
shear modulus (Lamé constant)
compression modulus
Poisson’s ratio
dynamic elastic modulus
density
P-wave velocity
S-wave velocity.
From these formulae it can be recognized that seismic
velocities are directly influenced by the elastic parameters of the rock mass. Material changes lead to a change
of the velocities and thus, vice versa, detected velocity
changes are a hint of changing properties in underground
structures.
However, to calculate elastic parameters of underground
materials it is necessary to determine both P- and S-wave
velocities. Since P-waves have the highest propagation
velocity, they appear as the first arrivals in seismograms,
and therefore their velocity can easily be determined.
However, S-waves with lower propagation velocities are
more difficult to identify. Therefore, special data-acquisition and processing techniques are needed for the
generation and identification of S-waves. Special sources
which generate especially S-waves and which prevent P-
123
In a region of a planned high-speed railway line in the
Alps, extensive seismic investigations were carried out.
Goal of the geophysical investigations was the geological
exploration of those areas which may influence the stability of the planned tunnel. High-resolution seismic
investigations gave information about the structures in
the near-surface underground as well as at greater depths.
The focus of the exploration was the detection of possible fault zones in the depth range of the planned tunnel
at a level of approximately 400 m above sea level. Since
the surface of the exploration area exceeded 1,600 m
above sea level the exploration depth for the geophysical
surface methods was about 1,200 m. Applying high-resolution seismic methods the geological conditions at the
depth of the tunnel were investigated appropriately. With
a combination of several seismic methods the geological
and tectonic conditions in the underground (see Fig. 5)
was obtained. In the interpreted seismic profile, reflection
horizons are specified and tracked along the section.
Disturbances can clearly be recognized. Applying the
CMP and the standard refraction seismic methods the
thickness of the surface layer as well as the depth and
the gradient of the changes from block debris to crystalline rock were identified. Together with the results of
geological mapping and borehole geophysics a general
model of near-surface and deep structures was developed
successfully.
5.2 Estimation of Rock Properties for a Line
Investigation in Austria
During the approval stage of a tunnel project, in connection
with the redevelopment of an old tunnel in Austria, geological and geotechnical exploration methods were used in
addition to refraction seismics and radar methods, in order
to explore the invert, the crown, and the sidewalls of an
exploration tunnel. By integrating geological, geotechnical,
Exploration of Tunnel Alignment using Geophysical Methods
115
distance [m]
0
5
10
15
170
160
3300
3000
155
sliding body
2700
2400
2100
150
1800
1500
145
1200
900
140
planned tunnel
velocity [m/s]
sea level [m]
165
600
300
100
Fig. 13 Result of seismic tomography for a road tunnel to map a
slide body
5.3 Line Investigation of a Tunnel Crossing a
Landslide
Fig. 12 Comparison between seismic velocities and the uniaxial
compressive strength of the mica slate, which was determined by
uniaxial laboratory measurements
and geophysical interpretation of all data, the structure and
disturbances of regions in the near field of the tunnel were
explored. The high-resolution radar method provided a
continuous structure profile along the invert, the crown,
and the sidewalls and gave an indication of disturbances.
By using the high-resolution refraction seismic method the
invert of the tunnel could be explored and the seismic
velocity of the rock mass below the ballast could be
measured. The quality of the rock mass and the degree of
disturbance around the tunnel was evaluated by correlating
seismic velocities with the surrounding rock properties.
The results are shown in Fig. 12.
Figure 13 displays the results of seismic tomography for
the line investigation of a road tunnel. Two adjacent
boreholes were drilled with depths of about 35 m having a
distance of 13 m. Locations marked with stars and points in
Fig. 13 display the position of sources and receivers in the
boreholes. It can be recognized that a Devonian landslide
body was precisely detected. The course of the planned
railway alignment contacted this unstable area so that,
based on the tomography results, the course of the tunnel
was moved approximately 30 m into the solid Devonian
rock mass in order not to be tangent to the landslide body.
5.4 Line Investigations for a New High-Speed Railway
Line in the Southern Part of Germany
For the investigation of the underground along a new highspeed railway line in the southern part of Germany a
combination of the three geophysical methods, seismic,
geoelectric, and microgravity (joint interpretation) was
applied. Seismic and geoelectric methods were used for
modeling the lithologic and tectonic structures as well as for
calculation of the corresponding depths of detected anomalies, which were filled and unfilled cavities, subsidence,
surface disintegrations, karstifications, and deep-ranging
123
116
B. Lehmann et al.
Fig. 14 Map of the anomaly
factor calculated by joint
interpretation of various
geophysical methods in an area
of a high-speed railway line
dolines. However, since unfilled cavities due to karstifications were the main problem for the stability of the highspeed railway line the microgravity method was applied to
detect those locations where a mass deficit (and thus a
cavity) in the underground was to be expected. A seismic
velocity reduction at special locations in the underground
together with high electric resistivities was an additional
hint of the existence of cavities in the underground. Thus,
the information of a mass deficit in the underground, lowvelocity areas, and high resistivity were collected together
(joint interpretation) to calculate the so-called anomaly
indication factor (anomaly factor), which took high values
at those locations with high likelihood of the occurrence of a
relevant cavity. The anomaly factor values were gathered
and displayed on a map of the investigated area. Locations
with a high anomaly factor (red areas) were opened with
drillings and if necessary the cavities were filled with
concrete suspension to stabilize the railway line. The result
of this procedure is presented in the map of Fig. 14. The
results of the geophysical explorations demonstrated the
advantages of the joint interpretation technique using the
information of all applied methods. A very good correlation
between the results of the joint interpretation and the corresponding drillings to fill the detected cavities was
recognized. Thus, not a single but a combination of several
geophysical methods was the only instrument that could
ensure a complete result. The geological characteristics of
the underground depend on various parameters. Anomalies
appear at those locations where one of these parameter
changes. It is impossible to determine all geological
parameters by applying only a single method. Using the
joint interpretation technique, even more abnormalities can
be described and characterized very precisely. With respect
to the given karst problem, the combination of three different independent geophysical methods in addition to
geotechnical drillings was the optimum procedure to stabilize the underground.
6 Conclusions
Engineering geophysics provides valuable and continuous
information for the planning and execution of projects. The
123
application of geophysical methods in addition to usually
applied geological and geotechnical exploration is necessary to achieve accurate and continuous models of the
subsurface in a short time schedule. To achieve optimum
benefit from geophysical investigations it is necessary to
apply these methods:
•
•
After intensive discussions between geologists engineers and geophysicist
At an early stage of the project.
Thus, engineering geophysical methods will become an
important part of planning and execution of an engineering
project, with safety planning being essentially increased
and risk being minimized by means of successful and
interdisciplinary cooperation.
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