Seismic surveying tools for the early detection of ... - GeoExpert AG

Seismic surveying tools for the early detection of rock instability zones
W. Frei
GeoExpert AG, Amlikon, Switzerland
ABSTRACT: Surface seismic surveying methods are recommended during the early phase of a geotechnical
site characterization programme, preferrably prior to drilling and excavation activities, with the objective of
detecting critical zones for more detailed invasive follow-up investigations, such as boreholes.
By combining the evaluation results of reflection seismic profiling and refraction tomographical inversion in a
joint representation, i.e. hybrid seismic surveying, many interpretational ambiguities are eliminated. Critical
zones of rock weakness and reduced stiffness, attributable to weathering deconsolidation and tectonic
faulting, are identified in a reliable manner. By using the appropriate recording equipment, the acquisition of
the data of the two above mentioned methods can now be combined into one single field operation, resulting
in a significant reduction in time and costs for the field crews. The significance of the key data acquisition
parameters with regard to the imaging resolution and to the desired investigation depth is discussed.
1 INTRODUCTION
The direct relationships between the compressional
p-wave and the shear s-wave velocities on one hand
and rock strength on the other are well documented
in literature (N. Barton, 2006). Although these
relationships are well known to the geomechanical
practitioner, the systematic and timely application of
egineering surface seismological surveys for the
detection of weakness zones still is not an integral
part in many geotechnical site characterization programmes.
The seismic techniques commonly in use for
geomechanical engineering are
1.1 Surface seismic methods
A1 Conventional high resolution reflection seismic
profiling.
A2 The seismic refraction method of p-wave diving
wave tomography.
A3 The seismic refraction method of s-wave diving
wave tomography.
A4 Multichannel Analysis of Surface Waves
(MASW) for the determination of the shear
wave velocity field (active, using => controlled
seismic sources, and passive methods).
A5 Spectral Analysis of Surface Waves (SASW);
dual channel recording of the ground unrest).
1.2 Seismic methods in boreholes
B1 Down hole p- & s-wave velocity function determinations.
B2 Cross hole p- & s-wave velocity function
determinations.
B3 Wireline sonic logging.
2 COMPARATIVE PERFORMANCE
VALIDATION OF EACH METHOD
Table 1 on the next page lists the merits and shortcomings
of each technique for the various survey
objectives in relation to all the other methods.
Performance ratings are defined on a scale from 0
(very poor) to 5 (very good). The attainable investigation
depths range from zero to high (> 500 m).
lim. : denotes method and equipment inherent depth
limitations;
bhd : denotes bore hole depth.

Table 1. Performance rating of the various seismic surveying methods in geomechanical engineering
INVESTIGATION REQUIREMENTS &
SURVEYING METHOD
OBJECTIVES A1 A2 A3 A4 A5 B1 B2 B3
High resolution at shallow depths (< 20m) 3 5 5 5 2 2 5 4
High resolution at greater depth (> 25 m) 5 4 4 3 3 2 5 5
Attainable depth of investigation high lim. low low lim. bhd 130 m bhd
Rock/soil quality & rippability indicator 3 5 5 5 3 4 5 4
Detection of velocity inversions 3 5 5 2 2 3 5 5
Fault indicator 5 3 4 2 1 1 3-4 5
General method reliability 4-5 4 4 3 2 3 4-5 4
Area wide information available? 5 5 5 5 5 0 0 0
Cost / Usefulness (C/U) ratio in the overall context
of a site characterization programme
5 5 3 3 3 3 2 3
In order to assess the geotechnical situation in 3D
at an early stage of a site characterization programme,
the surface methods of p-wave refraction diving
wave tomography (A2) and of conventional p-wave
reflection seismic profiling (A1) are the appropriate
choice, as can be concluded by validation of the
performance ratings in table 1.
By using the reflection seismic profiling method
(A1) the stratigraphic layering and tectonic details
are being probed by seismic signals being emitted
from the surface and reflected back from various
depth levels in a near vertical direction. The resulting
seismic section has the appearance of an X-ray
type image (Fig. 1) as it depicts the underground
structures in great detail even at larger depths.
The velocity information obtained in the near
surface range (

Figure 1. Reflection seismic section showing geological and tectonic structures along a 1 km long traject of a planned motorway
tunnel near Moutier, Western Switzerland. Note the structural complexity of the thrust faulted fold tectonic setting of Jurassic
formations overlain by Tertiary molasse material. The projected tunnel intersects several major faults.
Figure 2. P-wave seismic velocity field derived by refraction diving wave tomography along the reflection seismic section in
Figure 1. The colour encoded velocity scale illustrates the direct relationship between p-wave velocity and rock strength. The
2500 m/s contour lines in white mark the center of the transition zone between decomposed and poor quality rock material
and solid, good quality rock. The RQD values determined in the boreholes situated at various positions on the profile are
useful for calibrating the seismic velocities and confirm that poor to very poor rock quality prevails over a distance of more
than 90% of the length of the tunnel.
4 SHEAR WAVE VELOCITY DETERMINA-
TION
Hybrid seismic surveying, i.e. reflection seismic
profiling (A1) and refraction tomography (A2)
usually provide sufficiently meaningful results in
geomechanical site characterizations. The costs for
acquiring s-wave refraction tomography (A3) data
are about 3-4 times higher than for p-wave surveys.
In addition, the determination of the s-wave refraction
arrival times very often is not a straightforward
task because s-wave signals arrive later than the pwave
first breaks, which causes the s-wave signal to
be contaminated by all sorts of other signal events in
the acoustic wavetrain and by the effects of p-waves
converted to s-waves at at layer boundaries characterized
by acoustic impedance contrasts.
Another persistent difficulty is the generation of a
good quality s-wave source signal of adequate
strength.
The reliability of data processing results of the
surface wave dispersion analysis methods of MASW
(A4) and SASW (A5) is strongly dependent on a)
the quality of the raw data, b) of the experience and
the routine of the processing geophysicist, and c) of
the surface and subsurface complexities of the site to
be investigated.
Among the surface seismic methods, the s-wave
determination techniques (A3-A5) are not routinely
applied in practice. Their results are to be considered
supplementary to the p-wave methods of reflection
seismic profiling A1) and refraction tomography
(A2).

Figure 3. S-wave velocity field derived by MASW evaluation
(A4) with inserted curves of SPT values for correlation
purposes.
Figure 4. Seismic shear wave generation for s-wave refraction
tomography (A3) and for down-hole s-wave velocity measurements
(B1) by means of a 70 kg beam anchored to the ground
with the help of steel spikes.
Figure 5. P-wave velocity field between two bore holes by
cross hole tomography (B2). Note the good agreement of the
seismic velocity field with the stratigraphic column and the
SPT results.
A concise and comprehensive tutorial covering
theoretical and practical aspects of shear wave
determination methods can be found on the web site
www.masw.com. The author of this site also gives
an introduction to all seismic surveying techniques
applicable in geomechanical engineering on the site
www.parkseismic.com.
REFERENCES
Barton, N. 2006. Rock Quality, Seismic Velocity, Attenuation
and Anisotropy. 729p. Taylor & Francis, UK &
Netherlands. ISBN 9-78041-539-4413
Osypov, K., 2000, Robust refraction tomography, 70th Ann.
Internat. Mtg., Soc. Expl. Geophys., 2032-2035.
Sheriff, E. R., 2002, Encyclopedic Dictionary of Applied
Geophysics, Fourth Editition, Soc. Expl. Geophys., ISBN 1-
56080-118-2