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ELECTRICAL PROPERTIES OF OIL-POLLUTED GROUNDS LABORATORY MEASUREMENTS
Sergey I. Volkov, Aleksandr A. Gorbunov, Vladimir A. Shevnin
Moscow State University, Geological Faculty, Dept. of Geophysics, 119899
Moscow, Russia
E-mail: gorb@geophys.geol.msu.ru




INTRODUCTION
Oil pollution of soils and grounds along with other kinds of
hydrocarbon pollution has become one of the greatest environmental hazards.
Thus locating the polluted sites and estimating pollution risks are the
important problems facing the specialists in a number of related
industries. resistivity and chargeability techniques of determining the
ground properties are often applied to solve these problems. The dependency
of electrical properties on the hydrocarbon pollution is to be investigated
to achieve a success. Laboratory sample measurements are long used
worldwide for this purpose.
The main targets of the investigation attempted was:
1) to test the computerized measurement system developed by the authors;
2) to collect data concerning the electrical properties vs. hydrocarbon
pollution dependency on typical surface grounds of Moscow urban area;
3) to find out time dependencies of electrical properties under various
conditions of hydrocarbon pollution for these grounds.
OBJECTS OF INVESTIGATION
A set of surface ground samples was collected at typical sites of
Moscow urban area. These grounds are mainly sands, partly ferrous with
certain contents of gravel and organic materials. Sample porosity was about
15 percent and gravel contents was about 10 per cent.
The samples were taken from non-polluted site and the pollutants were
added in them in the laboratory. All samples were saturated with fluid
(pollutant) completely before the first measurement. Further on the degree
of saturation was kept up by adding water to the samples daily. The water
emulsions of oil products were used as pollutants:
for Sample 1 - non-polluted (pure water);
for Sample 2 - the 5% emulsion of petrol;
for Sample 3 - the 20% emulsion of petrol;
for Sample 4 - the 5% emulsion of motor oil;
for Sample 5 - the 20% emulsion of motor oil.
The measurements were fulfilled daily during 4 weeks. The earthing
conditions were kept as similar as possible.
MEASUREMENT IMPLEMENTATION
Resistivity and chargeability of the sample substance were measured
using the scheme presented at Fig. 1. The current electrodes A and B were
made of metal plates coated from the outer side with dielectric.
Homogeneous electric field was created in the volume between them. The
potential difference was measured between the non-chargeable measurement
electrodes M and N. It is possible to estimate actual resistivity using
such an array. The resistivity was measured at frequencies of 0.5, 1, 4.88
and 10 Hz. The measurement took place in the course of applying electric
current of the corresponding frequency to the sample with the certain time
delays. The resistivity data were used to calculate per cent frequency
effect (PFE) representing chargeability.
The L-1250 DAC/ADC converter by the L-Card Ltd. (Russia) was used as
the electrical current source and voltage measurement device. This
converter is the IBM PC ISA extension card based on ADSP-2105 digital
signal processor (DSP) and by means of the properly written firmware for
the DSP it can be used as the I/O coprocessor for the host (IBM PC) system.
Original software has been developed for card control in the course of
measurement. The software includes the DSP firmware module (hard Real-Time
(RT) microkernel operating system), the L-1250 device driver (host
operating system extension) and the control application (User Interface).
Designed so control software can be used as a very flexible task-related
and RT-robust control/measurement tool for modern non-RT multitasking
operating systems such as Windows 95/98/NT.
Test measurements in circuits with known resistance values proved the
satisfactory operating of the system. The results of test measurements
coincide with those using standard resistivity survey equipment.
MEASUREMENT RESULTS
The resistivity and chargeability graphs for all samples are presented
at Figs. 3 and 4. The data presented are average values for all
frequencies.
It was supposed, that resistivity change rate had been controlled by
the processes of water and pollutant evaporation. It means that time
dependencies for resistivity and chargeability can be presented by [pic]
and h(t)~h(0)+C2 t expressions respectively. The data deviation from these
dependencies were regarded as measurement errors resulting from earthing
conditions and temperature variations. The average relative error value
thus estimated makes 7 per cent for resistivity and 5 per cent for
chargeability.
Trends derived from the measured data using the dependencies under
consideration are presented at Figs. 4 (resistivity) and 5 (chargeability).
The graphs represent data for Samples 2-5 compared with the data for Sample
1. Thus the difference between non-polluted Sample 1 and differently
polluted Samples 2-5 is demonstrated at these figures. The result show
obviously that the rate of resistivity and chargeability changes depends
directly on pollutant type and contents.
The chargeability and resistivity of the polluted samples occurred
decreased gradually in the course of the experiment. The resitivity change
made 20-50 per cent and the chargeability change made 20-70 per cent as
compared to the values at the beginning of the experiment.
The resistivity difference between non-polluted sample and other
samples grows constantly. The rate of this growth ifs generally less for
petrol-polluted Samples 2 and 3. It can be also mentioned that for petrol-
polluted samples the rate is greater for the less polluted sample, while
for motor-oil-polluted Samples 4 and 5 the irate is greater for more
polluted sample.
The chargeability difference between non-polluted sample and other
samples increases rapidly during the first week and then the rate of change
grows less becoming almost negligible in the end of the measurement time
span. Unlike resistivity, chargeability graphs can be grouped rather by the
type of pollutant than by its content. The rate and the stabilization level
is greater for petrol-polluted samples. The chargeability of oil-polluted
sample with greater pollutant contents is stabilized much earlier and at
the lowest level of all. The less polluted samples appear to have the
greatest rate regardless of the pollutant type.
DISCUSSION
The main process controlling the changes in resistivity and
chargeability is water evaporation and related changes of mineralization.
In the condition of the discussed experiments adding water to the samples
as it evaporates from them results in constant increase of mineralization
as the salts added with each portion of water remain in the sample after
evaporation. This means the decrease of resistivity as the mineralization
increases. The chargeability of conductive media decreases with the growth
of conductivity. This is the secondary effect of the minerlaization
increase. The rate of evaporation is influenced by the pollutant type and
contents. The results of the experiment may be commented from this point of
view.
The lighter pollutant (petrol) evaporates more intensively than the
heavier one. It means that more volume of the sample is freed daily and
replaced by the water bringing salts into the sample. The motor oil not
only evaporates slower but also prevents water evaporation by means of
sticking the pores. It means that the initial properties of the sample are
preserved longer in the sample with the greater content of the heavier
pollutant (motor oil). The effect of less concentrated and lighter
pollutants is less valuable. Due to faster petrol evaporation the
resistivity of petrol-polluted samples become closer to those of pure
sample and the difference between the samples increases slower. Due to
difference of the resistivity (mineralization) decrease rate the
stabilizing of the samples' chargeability does not occur simultaneously.
CONCLUSION
The targets posed at the beginning of the experiment have been
generally achieved. Computer-based laboratory measurement system has been
tested and occurred to produce data suitable for reasonable geophysical
interpretation. The electrical properties of wide-spread type of rocks has
been investigated.
The hydrocarbon pollution effect on the properties of the surface sands
has been demonstrated. Approximate time dependencies of resistivity and
chargeability for various kinds and intensities of hydrocarbon pollution
has been derived and the qualitative physical model of the controlling
processes has been suggested.
ACKNOWLEDGMENTS
This experiment was implemented with the support of the Russian
Foundation for Fundamental Research (RFFI), grant No. 98-05-65059.



[pic]
Fig. 1. Measurement scheme.
[pic]
Fig. 2. Sample resistivity graphs.
[pic]
Fig. 3. Sample chargeability graphs.
[pic]
Fig. 4. Sample resistivity trends compared with Sample 1.
[pic]
Fig. 5. Sample chargeability trends compared with Sample 1.