12th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems

We present a number of new ways to use the seismic cone penetrometer (SCPTU). These
applications include the prediction of stratigraphic boundaries in advance of the cone as well as
the refinement of the first break shear wave velocities. These applications depend upon the
acquisition of vertical seismic profile (VSP) data using the cone. The VSP acquisition is
achieved using standard SCPTU accelerometers and is an extension of SCPTU shear wave
velocity surveys. The data consist of both an upgoing and downgoing seismic wavefield. These
wavefields are easily identifiable and can be separated with a series of simple processing
techniques. VSP data are most commonly associated with common depth point (CDP) reflection
seismic surveys. We have demonstrated how the data can also be used effectively to obtain
geotechnical information.

Roadway collapses have occurred along stretches of the Appalachian Highway in
southern Ohio due to ground subsidence associated with abandoned coal mine workings. The
authors joined a team led by faculty from Wright State University to investigate efficient
methods for detection of potential trouble areas before collapses occur. Seismic surface wave
measurements were made in both “sounding” and “profiling” modes. In the sounding mode, the
Spectral-Analysis-of Surface-Waves (SASW) method is employed to establish shear wave
velocity profiles at discrete locations. This approach is intended to delineate areas of reduced
overall stiffness, which might be caused by raveling of earth above mine workings, or, if not,
imply susceptibility for future instabilities. In the profiling mode, measurements are made at
constant offset and constant spacing in order to observe magnitudes and spectral content of body
wave interference. This approach is geared toward detection of discrete cavities or major fracture
zones caused by cavity collapse. Data were collected along two stretches of highway, one of
which has been drilled to establish ground truth. Results of the applications of sounding and
profiling methods for rapid assessment of sites with discrete subsurface anomalies are
encouraging. Additional analyses are planned to expand the capabilities of surface wave
applications through enhanced understanding of reflection and conversion phenomena caused by
subsurface anomalies.

During the course of a seismic survey across a region characterized by extremely high
Poisson’s ratios we observed an unusual phase. This phase arrives between the direct
P-wave and ground roll and has a significant amplitude at short offsets. Numerical
modelling indicates that it is a non-geometric shear wave generated by the interaction of
the explosive seismic source with the free surface. For the processing and interpretation
of very shallow seismic data, the presence of such non-geometric waves causes problems,
because the additional complexity in the seismic record may be interpreted in terms of
structural features in the subsurface. Another interesting phenomenon in high Poisson’s
ratio environments is the dramatic rotation of the P-wave polarization at the free surface.
This effect has to be taken into account when analysing multi-component recordings.

The dynamic surface wave prospecting technique was used in the non-intrusive evaluation
of the foundation quality of the Yongjiang River submarine tunnel. The prospecting results
show that the overall strength of the foundation is high enough to meet the designed
standard, however, there do exist low velocity heterogeneities. One possible reason for the
occurrence of low velocity zones is the disturbance of the river water and mud during the
process of mortar consolidation. The interface between the mortar layer and the
underlying gravel layer can not be well resolved. The results from the surface wave
technology have been confirmed by other methods such as the physical modeling
simulation and prospecting using body waves.

In order to identify shallow structures using fundamental-mode Rayleigh waves,
we use the multiple filter technique and measure group velocity across a study area.
First, we identify the mean velocity by stacking the power densities along constant
velocities for the complete set of filtered traces. Second, we identify peaks in the power
density consistent with the fundamental mode and eliminate those associated with higher
modes. Third, we perform a constrained inversion for optimal arrival times using
minimal deviations in travel time between stations corrected for differences in distance.
The resulting travel times are used in a tomographic inversion for group velocities in the
study area. The tomographic inversion is repeated for many frequencies giving a
dispersion curve for each point in the study area. Then, the vertical velocity structure at
any point in the study area can be interpreted from its dispersion curve. Velocity
anomalies of 10 percent are easily mapped by this technique. Rayleigh waves in the
frequency range of 2 to 50 Hz are sensitive to soil velocities at depths of 1 to 10 meters.
This depth range contains the near-surface structures that control dispersion of
underground contamination or which were introduced by the burial of wastes. The use of
tomography eliminates the need to make measurements directly above contamination.

This study aims at the characterization of two geotechnically different sites by an inversion
analysis of surface wave dispersion. Site #l was a construction site where loose soil layer
overlays weathered gneiss zone. Site #2 was reclaimed land where dredged ocean sediments
is underlain by original ocean sediments. The phase velocities of Rayleigh waves were
obtained by FFT of the Rayleigh waves of the sites. An initial velocity model was assumed
from the phase velocities. The final shear wave velocity profiles as well as the spatial
resolution of the profiles were obtained using the generalized inverse method. The shear wave
‘velocity of site #l increases gradually to a depth of 10 meter while that of site #2 shows a low
velocity zone at a depth of 1 meter. Then, it increases rapidly to a depth of 3 meter, and stays
almost constant up to a depth of 10 meter. The spatial resolution of the shear wave profiles is
better in the shallower layers. This study indicates that the inverse method of surface wave
dispersion can be used effectively to characterize geotechnical sites with reasonable spatial
resolution; lm of averaging thickness in the upper 3m, 2 m averaging thickness in 3-6 meter
depth, 3-4 m averaging thickness below 6 meter depth.

Two landfills were investigated aiming at finding out what advantages could come out of
using two geophysical methods providing each geometrical information or mechanical
information in combination. An old landfill previously characterized by 2D-resistivity
surveying was reinvestigated using surface wave seismics, SASW. Four points were chosen
for SASW tests located along the resistivity profiles. On a modern landfill an area used as
reference site, a specific cell with a volume of 40 x 40 x 9 m, a second survey was carried out
using true 3D-resistivity and SASW in five points. The objective of the project was mainly to
find out if a change in resistivity corresponded to a change in shear wave velocity. It was
concluded that the combination of the SASW method and resistivity surveying is an efficient
use of geophysical methods when the aim is to geotechnically characterize the subsurface.

We have investigated the structure of an abandoned landfill in Lancaster, PA, with
near-surface geophysical techniques, including magnetics, conductivity and magnetic
susceptibility. Our previous study used a wide range of data, including field observations,
historical aerial photographs and magnetic and conductivity measurements along 10
transects across the site to delineate the general boundaries of two landfills (de Wet et al., in
press). That low resolution data suggested that one of the landfills was heterogeneous. In
this study we investigated the compositional variations in this landfill by 1) collecting
higher resolution magnetic and susceptibility data along several of the transects, and 2)
completing several high resolution grids over selected areas of the landfill.
The total field magnetic intensity data and the magnetic susceptibility data showed
large anomalies over the landfill compared to background areas and clearly defined the
boundaries of the landfill. The magnetic susceptibility data for the landfill (collected using a
Bartington MS2D instrument) indicated numerous anomalies in the upper 10 to 15 cm of
the landfill. Specific magnetic susceptibility anomalies were unrelated to the large magnetic
intensity anomalies measured in this study, or the anomalies in the quadrature or in-phase
conductivity data collected in the earlier study. Data collected at different sampling intervals
along widely spaced transects were successful in delineating the landfill and identifying the
general heterogeneity of the landfill itself. Detailed grid maps provided additional
information about the subsurface that was not evident from the transect data. Sampling
scale is an important consideration in geophysical investigations of this type. High
resolution grid maps showing a variety of geophysical results provide maximum
information about subsurface conditions.

NGI has developed a module for the cone penetration test implementing the NGI step
frequency radar system. The instrument will allow nearly continuous measurements of the
dielectric constant and the bulk soil resisitivity throughout a vertical soil profile. The module
will be used in environmental investigations, where contaminants may alter these parameters
from the natural value in uncontaminated soils. This paper presents the technical details of the
design and some preliminary laboratory test results.

The addition of passing lanes has been a long standing need on Idaho Route 55. This
narrow, winding road is one of the few routes that links Boise, Idaho, with destinations to the
north. The roadway is bounded by a steep rising slope on the east, and a precipitous drop to the
Payette River on the west. Additionally, the area is known for landslide hazards. Given these
hazards and uncertainties in the subsurface geology, Idaho Transportation Department (ITD)
commissioned an engineering geophysical survey to aid in the design of the passing lanes just
south of Banks, Idaho.
The engineering design required that several issues be addressed. These included the
following:
1. Slope Stability
2. Amount of blasting required.
3. Need for retaining walls.
4. Volumes of soil and rock to be excavated.
To help design engineers address these questions, P-wave refraction mapping was used to
produce cross-sections perpendicular to the roadway. A novel approach was needed, since traditional
in-line shooting would have required offsets greater than were available. The solution was
to perform a 3-D, ground consistent, delay-time inversion. Soil and rock properties, and variations
in the thickness of overburden were determined. These results were found to be in
agreement with track-hoe trenching done at selected locations along the roadway.
The geophysical work revealed that the site differed significantly from assumptions used
in the initial design stages of the project. The potential for landslides was found to be significant.
Given the updated geologic picture and projected costs, ITD decided to abandon this site for the
passing lanes. The project was terminated, thus avoiding excessive costs that would have been _
experienced due to the differing site conditions.