environment

Vol. 11 No. 1 2004
the cone. Decreasing the confining pressure result in,
besides a decrease of the cone penetration resistance,
large differences in size of the area affected and degree
of density changes. A larger difference is observed by
lowering the sand density. The size of the area dilating
is significantly smaller and the degree of dilation is much
lower. The use of rigid boundaries only resulted in a
small decrease of the dilating area.
It was not possible to observe shear bands in the sand
sample with the CT- scanner setup present in the Faculty
of applied Earth science of the Technical University in
Delft. Reasons for this are the not perfect homogeneity
of the sand samples and the noise in the CT-scanner.
Abstract thesis Sabine
Backx
TU Delft
Investigation of practical use of synthetic and mixed
arrays in 2D resistivity surveys and the optimization
of acquisition configurations
The last decades shallow depth geophysics are
becoming more used for engineering geological
purposes. One of the used methods is the direct current
(DC) resistivity method. Lately, multielectrode systems
have become more user friendly thanks to increasing
computer power. But the fieldwork necessary for data
acquisition is still a very time consuming job. To minimize
this amount of work, synthetic electrode arrays can be
very useful.
The goals of this research are twofold: The first is to
consider the feasibility of synthetic electrode arrays
created out of the pole-pole configuration from the geoelectrical
method. The second goal is to find out if mixed
arrays with these synthetic data would lead to better
results with a better resolution and benefits from all the
different configurations compared to one single
measurement.
For most common arrays, with the aid of four different
models, that represent different possible geological
features, the conversion from pole-pole to synthetic
data is investigated. Every model has three different
options, resembling different resistivity distributions.
So boundaries going from high to low resistivity and
the other way around. It shows that the conversion
into other common arrays is useful, since an increase in
data points, and therefore an increase in resolution
appears. Even when the accuracy limits of the later used
apparatus were taken into account the results were still
very good.
For the same models mixed arrays were created. In this
research, mixed arrays are arrays that consist out of
both the pole-pole data set and the synthetic common
array data set. They are placed in one data file and
inverted simultaneously. The results give a good level
of detail for the shallow part, but also have the benefits
of the large depth that the pole-pole array can achieve.
The mixed arrays show at deeper parts even a lower
sensitivity to resistivity changes than the pole-pole
array. Apparently the software uses more information
when it is not being told what type of array is used.
Accuracy plots are produced by comparing the inversion
results of all the different arrays with the original models.
The relative difference between these points is plotted
in graphs. These show that the singular arrays have an
enormous oscillating pattern where the mixed arrays
show a more constant deviation. The accuracy graphs
show for every array an increase in deviation when
approaching the boundaries of the model. This is due
to the absence of data beyond the boundaries.
A field survey has been conducted to check the
modeling results with the real life data. Four different
lines were being measured. One to check the
repeatability of testing when the stakes remain in the
ground over night. A second line was used to see if a
limited distance of the far away electrodes would
influence the data. Both lines gave a positive response
in the sense that there were no influences visible from
both actions.
The last two lines were the actual lines used for the
research purpose. One line used 48 electrodes and was
measured with pole-pole, pole-dipole, dipole-dipole and
schlumberger. The 72-electrode line was only measured
with pole-pole, pole-dipole and schlumberger. Both lines
have been measured with different arrays while the
stakes remained in the ground, to ensure an exact
positioning. The pole-pole data was converted into
synthetic data and this synthetic data set could then be
compared to the original measured arrays. The synthetic
data sets showed an increase in data points and
therefore resolution. Even when taking into account
the limitations of the apparatus, and minimizing the
amount of significant numbers possible, the subsurface
image that was produced was still very useful. Of course,
the conversion only works with a good pole-pole data
set, so it is possible that this data set has to be edited
before it is ready for conversion.
Extra points for further research are the expansion into
3 dimensions, better software and an extended field
survey.
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