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SEISMIC MEASUREMENTS FOR LANDSLIDES INVESTIGATIONS
C. Ungureanu1, V. Buzuleac1, R. Buzuleac1, A. Damian1, I. Panea1
University of Bucharest, Faculty of Geology and Geophysics,Romania
1
Summary
We performed seismic reflection measurements over an area affected by landslides
with open fractures at the surface. We processed the field records to obtain time/depth seismic
sections used, later, to build an image for the geological structure in the first tens of meters
below the soil surface. Two-dimensional velocity models were obtained from the inversion of
the first-arrival travel-times picked on common-source gathers. Information about the S-wave
velocity was obtained from the inversion of the dispersion curves computed for the surface
waves seen on the field records. Shallow wells were drilled, down to about 10 meters, to
verify the results of seismic data processing and interpretation.
Introduction
Detailed information about the geological structure on the first tens of meters below
the soil surface can be obtained from the analysis and processing of the seismic reflection
data. Elastic moduli can be computed using the P- and S-wave velocities determined from
seismic reflection and refraction data (Panea and Mocanu, 2013). The use of seismic and
electrical surveys in geotechnical projects performed to investigate landslides and to design a
subsea tunnel was well described by Ylmaz (2013).
In our study, we used seismic reflection measurements to investigate an important
landslide from a hilly area. We analyzed the samples taken from shallow wells drilled in
points along the seismic profiles to obtain information about the lithology and elastic moduli.
Description of seismic data acquisition
Seismic reflection measurements were performed on two profiles, P1 and P2, using
two Geodes (Geometrics) with 24 vertical-component geophones/Geoda (Figure 1). The
seismic energy was generated using a hammer of 10 kg in points spaced at 0.5 meters.
Geophone spacing was 0.5 meters. Time sampling interval was 1 msec. Record length was 2
seconds.
Analysis of seismic reflection data
Both seismic reflection datasets were processed following the same steps but using
various parameters in order to obtain the best signal-to-noise ratio possible for both, pre- and
post-stack data. Examples of raw records are displayed in Figure 2. The surface waves are
characterized by high amplitude and low apparent velocities. All surface waves are not
spatially aliased, due to the geophone spacing used for data acquisition (Figure 2). The
geometry was defined for linear profiles. The static corrections were computed at a final
datum of +100 meters. The surface wave attenuation was done using a band-pass frequency
filter applied for 22-102 Hz and the fk filtering. The remaining surface waves will be
attenuated after stacking of traces sorted after the common-depth-point (CDP). The interactive
velocity analysis was performed using filtered CDP gathers. The time and depth seismic
sections for P1 and P2 are displayed in Figures 3 and 4. Both types of seismic sections
provide useful information about the geological structure from the investigated area.
First-arrival travel-times were picked and used in time-term inversion (Geometrics) to
obtain P-wave velocity models. Figure 5 shows examples of field records with picked first
arrivals and the P-wave velocity model obtained after the inversion of the first-arrival traveltimes. Surface waves from the field records were used to compute the dispersion curves. S-
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wave velocity models were obtained from the inversion of the dispersion curves (Geometrics
package).
Figure 1: Investigated area with the location of seismic profiles P1 and P2 (left)
and fractures at surface (right)
Figure 2: Raw records displayed in the (t,x)- and (f, k)-domain selected from seismic profile P1 (up)
and P2 (down); geophone spacing of 0.5 meters; time sampling interval of 1 msec.
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Figure 3: Time (up) and depth (down) seismic section for seismic profile P1. Final datum at 100
meters
Figure 4: Time (up) and depth (down) seismic section for seismic profile P2. Final datum at 100
meters
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Figure 5: (a,b) Field records with picked first arrivals and (c) P-wave velocity model obtained from
the inversion of the first-arrival travel-times.
Acknowledgments
The seismic measurements were performed using equipment from Geometrics.
Seismic data procesing was performed using SeisSpace (Grant from Halliburton).
References
Panea, I., Mocanu, V. (2013) High-resolution estimation of elastic properties of the near
surface based on vertical-component seismic data data processing, Expanded
Abstracts, Ann. Internat. SEG Mtgs, Houston, USA.
Ylmaz, O. (2013) Applications of engineering seismology n urban areas, The Leading Edge,
32(3), 264-274.