Application of Geophysical Methods in the study of [616328]

Application of Geophysical Methods in the study of
contaminated soils : A review
ANDREEA – GEORGIANA IRINEI
University of Bucharest, Faculty of Geology and Geophysics

Abstract . Soil contamination is caused by the
presence of different chemical components or
other alteration types in the natural soil
environment. The concern over soil
contamination is quite high because it affects
first human health, agricultural activities and
the entire environm ent. There are different types
of human contamination ( e.g. direct contact with
the contaminated soil, vapors from the
contaminants, and from secondary
contamination of water supplies within and
underlying the soil ) that can affect human life .
In this cas e geophysics plays an important
role in investigation and monitoring of
contaminated soils. Because geophysical method s
are non-destructive and it can give accurate
answers from underground, they are considered
attractive tool s for describing the subsurfac e
properties without digging. Geophysics has been
already applied with success in various case
studies .
Popular geophysical methods used in the
study of contaminated soils are electrical
methods, electromagnetic method, Ground
Penetrating Radar and magne tic method. These
methods can be used in a case study taking in
consideration different characteristics ( e.g, the
contaminated area, the pollution type). In this
review can be seen the methodology for each
geophysical method and some examples
regarding th e use of them.
Keywords —soil contamination, geophysical
methods, electrical methods, Ground Penetrating
Radar I. INTRODUCTION

Because every day new buildings and roads are
built it seems that we forgot about a vital part of the
Earth with a high importance for us , the soil. We
know that is has different names, such as dirt, mud
and ground. However, it is definitely a very
important thing to us. The plants that feed us grow
in soil and keeping it hea lthy is essential to maintain
a beautiful planet. Like all o ther forms of nature,
soil also suffers because of pollution . The pollution
or contamination of soil is something common in
these days, and almost all the time the cause is
something human -made.
The changes caused on soil comtamination are
variable in spac e and time [1]. As a consequence, a
continuous and precise spatially and temporal
investigation of the soil physical and chemical
properties is required. Geophysical methods have
been applied to soil sciences for a considerable
period [1] [2] . The general principle of geophysical
exploration is to collect data with higher accuracy in
a non -intrusive way in the medium under
investigation [2].
Soil contamination h as many causes and the main
one are industrial activity, agricultural activity using
modern pesti cides and fertilizers which are full of
chemicals , waste disposal, oil leaks and acid rains.
Soil contamination has been a serious problem
for the environment for many years. In order to
have a proper remediation, the characterization of
the contaminated zone should be performed by
different techniques [11], usually direct methods,
such as physical analysis ( i.e. odor, color, and
texture) and chemical analysis ( i.e. pollutant

concentration)[1]. In recent years, geophysical
techniques have become useful in the
characterization of contaminated soils [1], and can
make a good team with direct methods in order to
find the best solution in investigation of soil
contamination [7].

II. GEOPHY SICAL METHODS

Geophysical methods has different applications
in the stud y of contaminated soils. Based on a
variety of factors of contaminated area usually you
can choose the best geophysical method to use in
order to obtain the most accurate answers. H ere is
presented the methodology for each of geophysical
methods.

A. Electric al methods

Theory and basic principles

In this case t he purpose of electrical resistivity
surveys is to determine the resistivity distribution of
the soil volume [11]. In practice a rtificially
generated electric currents are generated to the soil
and after that are measured the resulting potential
differences [9]. Potential diffe rence patterns can
provide us information on the form of subsurface
heterogeneities and most important, their electrical
properties [2]. The greater the electrical contrast
betwe en the soil matrix and heterogen eity, the
easier is the detection. Because the soil is composed
from a variety of rock types the e lectrical resistivity
of the soil is similar with the variability of soil
physical properties. The current flow line
distribut ions used in electrical surveys depend on
the medium under investigation [17]; usually being
concentrated in conductive volumes [6].
For a simple body, the resistivity ρ (Ωm) is
defined as follows:
(1)
with R being the electrical resistance ( Ω), L the
length of the cylinder (m) and S is its cross –
sectional area (m2).
The electrical resistance of the cylindrical body R
(Ω), is defined by the Ohm’s law as follows:
(2)
with V being the potential (V) and I is the current
(A). Electrical characteristic is also commonly
described by the conductivity value s (Sm_1), equal
to the reciprocal of the soil resistivity [7] . Thus:
(3)

Measurement of electrical resistivity usually
requires four electrodes: two electrodes called A
and B that are used to inject the current (‘‘current
electrodes’’), and two other electro des called M and
N that are used to record the resulting potential
difference (‘‘potential electrodes’’). Using these
four electrodes can be built different geometries in
order to obtain the best data.

The electrical resistivity of soil is a function of a
number of different properties, including the nature
of the solid constituents ( e.g particle size
distribution, mineralogy), arrangement of voids ( e.g
porosity, pore size distribution, connectivity),
degree of water saturation (water content), electri cal
resistivity of the fluid (solute concentration) and
temperature [2]. The air medium is an insulator (i.e.
infinitively resistive), the water solution resistivity
is a function of the ionic concentration, and the
resistivity of the solid grains is relat ed to the
electrical charges density at the surface of the
constituents. These parameters affect the electrical
resistivity, but in different ways and to different
extents [6]. Electrical resistivity experiments have
been performed in the past to establish relationships
between the electrical resistivity and each of these
soil characteristics.

B. Magnetotelluric method

The radiomagnetotelluric method was used
successfully in the past for the exploration of waste
deposits and groundwater, for engineering
problems , for archaeological research, and for the
detection of underground caves [16].
The radiomagnetotelluric method uses carrier waves
from high -power civilian and military transmitters
operating in a frequency range between 10 kHz and
300 kHz. The rad iomagnetotelluric instrument is
really easy to use and it allows scalar measurements
[16]. At great distance s from the transmitters, local
electromagnetic fields can be assumed to be those
of a plane wave. The electromagnetic field consists
of a horizontal magnetic field, perpendicular to the
direction of propagation, and a horizontal electric
field in the direction of propagation [8]. The
presence of any anomalous conductivity structure in
the earth modifies the observed magnetic and
electric fields. The r atio of the orthogonal electric
and magnetic horizontal components for the
observed frequencies is related to an average
resistivity (apparent resistivity) of the subsurface.
The phase difference between them also contains
information about the conductivit y structure.
Analogous to magn etotellurics, the apparent
resistivity and phase values are calculated for
selected frequencies from the measured mag netic
and electric field values [10].

C. Ground Penetrating Radar

Ground penetrating radar (GPR) is a very u seful
geophysical method for a variety of surveys . It can
be used in general to study contaminants in
groundwater, subsurface faulting, and underground
cavities (natural or man -made), all of which pose
potentially a cause to soil contamination [12].

The G PR technique is similar in principle to
seismic reflection and sonar techniques. Ground
Penetrating Radar system send short pulses of high
frequency (10 -1000MHz) electromagnetic energy
into the gro und from a transmitting antenna [8]. The propagation of th e signal s depends on the
frequency -dependent electrical properties of the
ground [6]. Electrical conductivity of the soil along
the propagation are used to limit the depth of
penetration and to receive data about earth
formations. Electrical conductivity o f the soil is
primarily dependent on the moisture content and
mineralization present in formation .

When the radiated energy with an amplitude A
encounters an inhomogeneity in the dielectric
constant ( ε) of the subsurface, part of the incident
energy with an amplitude AR is reflected back to
the radar antenna and part or the energy with
amplitude AT is transmitted into and possibly
through the inhomogeneity [12]. Figure 1 shows the
basic components and functional operation of a
pulse -mode GPR system. The electrical properties
of geological materials are governed primarily by
the water content, dissolved minerals, clay and
heavy mineral content [14]. Reflected signals are
amplified, transformed to the aud io-frequency

range, recorded, processed, and displayed. From the
recorded display, subsurface features such as soil/
soil, soil/rock, and unsaturated/saturated interfaces
can be identified [13].

D. Magnetic method

Magnetic susceptibility is a measure of the iron
bearing components in the material and can be used
to identify the type of the material and the amount
of the iron -bearing minerals it contains [15]. Iron
and steel will also contribute to a susceptibility
reading. The magnetic techniques have been a pplied
in the past with demonstrable success in the
pollution studies The major advantages of
environmental magnetism are the high sensitivity
and speed of magnetic techniques. Even minute
quantities of magnetic particles in bulk samples can
be measured ra pidly. Many anthropogenic
emissions contain fine particles which are highly
magnetic. Therefore, K of polluted material can
give a general view of the degree of pollution.
The magnetic properties depend on the grain
size, concentration and type of the magnetic
minerals in the soil. Ferrimagnetic minerals (such
as magnetite) have the strongest influence on the
magnetic properties. Diamagnetic minerals have
low negative susceptibilities, whil e paramagnetic
minerals show positive values of the magn etic
susceptibili ty (K) [15].

III. CASE STUDY – OIL POLLUTION

As mentioned before g eophysical met hods are
very useful in investigating the geological
environment. Because this subject can be very large
in this part can be seen examples of soil
contamination b y hydrocarbons and applicability of
geophysics in investigating the contaminated area
[5].
In these days soil contamination by hydrocarbons
represents a severe environmen tal problem and is
considered a geological hazard [4]. For this reason
there were made a lot of study cases using
geophy sics for investigation and monitoring of
contamination. Frequently hydrocarbons contamination is with
crude oil that belong to light-non-aqueous -phase –
liquids class (LNAPL), in general having a density
lower than the water [5]. In cases of severe soil
pollution, three pollution phases can be observed:
– Separate Hydrocabons Phases – pollutant in a
liquid form or trapped in porous space of vadose
zone.
– Dissolved Phase Hydrocarbons (PDH) – resulted
from hydrocarbons soluble fractions.
– Volatile Phase Hydrocarbons (PVH) – formed by
oil volatile compounds [3].
The migration of contaminant in soil is
influenced by many factors. Most important are the
source activity (continuous, discontinuous,
accidentally), the volume of the contaminant
solution discharged into the medium, its properties
(soluble and volatile compounds, viscosity) but also
the medium characteristics such as soil texture, its
porosity and permeability, soil water -holding
capacity and terrain slope [3].
Taking in consideration all these factors and
geological environment can be decided which is the
best geophysical method to use.

a) Hydrocarbons contaminated soil investigated
using electromagnetic methods

Soil contamination caused by hydrocarbons
represents usually represents a problem for the
reorganization of closed factories, refineries and
tank farms. These areas have mainly been
characterized by the electrical properties of
hydrocarbons [5]. Successful detection of
hydrocarbon contaminants underg round using
geophysical methods, especially by electric and
electromagnetic techniques took place successfully
in the past.
In the study case presented in this chapter
radiomagnetotelluric measurements on a
contaminated area took place near the Brazi
Refinery in Romania to demonstrate the
applicability of this method for the exploration of
contaminated areas. The site proposed for this study
lies in the proximity of the Brazi Refinery close to
the city of Ploiesti, which is located about 60 km
north of Bucharest in Romania. There are several

high-production refineries within the Ploiesti
region. Due to oil products leaking from technical
installations and from the sewerage in the area of
these refineries, oil product films have developed
over the ground water table – in some places to a
considerable thickness. Due to the high
contamination caused by the Brazi Refinery, the
groundwater in the area cannot be used for drinking
purposes.
Scalar radioma gnetotelluric measurements were
carried out on a contamina ted test area close to the
Brazi Refinery in Romania in order to detect and to
monitor a 1 m thick oil layer expected at 5 m depth.
Radio transmitters broadcasting in a frequency
range from 10 kHz to 300 kHz were selected to
observe the apparent resistivit y [16]. They were
located parallel and perpendicular to the assumed
strike direction of the contamination plume. The
data were interpreted by a 2D inversion technique
from which the conductivity structure of the area
was deri ved. The 2D inversion models of all
profiles on the contaminated area show a poor –
conductivity zone above the groundwater table
which could be associated with the oil
contamination.
Radiomagnetotelluric measurements were made
in two phases: first on the contaminated area during
a wet pe riod in 1999 and the second one during a
dry period in 2000.
In this survey we re used four pairs of frequencies
perpendicular to each other. The apparent resistivity
and the phase data were collected on 16 profiles.
Assuming a 2D resistivity distribution in the survey
area, the response of the transmitters parallel to the
strike can be associated with the E -polarization
(TE-mode) and tho se perpendicular to it with the
Bpolarization (TM-mode) [16].

Figure 2. Spatial distribution of the apparent resistivit y
for the frequency 198 kHz (TE -mode). The markings
indicate the location of the radiomagnetotelluric stations.
The data were observed during a wet period in 1999 .

Figure 3. Spatial distribution of the phase for the
frequency 198 kHz (TE -mode). The mark ings indicate
the location of the radiomagnetotelluric stations. The
data were observed during a wet period in
1999.

Approximately one year later, the survey area
was overlapped with the measurements conducted
in 1999 and extended southwestwards. The
reference field was located far away from the
refinery .

Figure 4 . Apparent resistivity of TE -mode obtained in
1999 (left) and in 2000 (right).

Looking at the differences of apparent restitivity
sections it can be seen that the frequencies obtained
after investigation in 1999 are lower than those
obtained in 2000 with the exception o f the lowest
frequency . The apparent resistivities obtained in
1999 are, in general, lower than 100 Ωm; those
measured in the western part of the survey area
proved to be h igher than those in the eastern part
where they are almost all lower than 50 Ωm. All
apparent resistivities obtained in 2000 are above
100 Ωm, except the lowest frequency which is close
to 100 Ωm. This difference may be explained by the
actual field conditio ns existing during the survey.
The wet surface may have led to a lower resistivity
in 1999, whereas the dry top soil found in 2000 may
have increased the apparent resistivity for the high
frequency. In both years, the apparent resistivities
for the lowest frequencies have almost similar
values, indicating a variation in conductivity for
shallow depths.
In this case study can be seen that
electromagnetic methods can be used to investigate
the contaminated area and also to monitor what is
happening with the contamination plume in the area
durin g different periods of time and weather
conditions.

b) Hydrocarbons contaminated soil investigated
using GPR and electrical methods

As already mentioned before GPR method is
similar in principle to seismic reflecti on and sonar
techniques . With this method can be fulfilled
various objectives of investigations at sites
containing contamination cause d by hazardous
materials [12], including the determination of the
presence of contaminant plumes, their source(s),
and ge ometry; an d the assessment of associated
hydrogeologic conditions [6].
Surface resistivity measurements have long been
used in the study of contaminated soil especially of
hydrocarbon contaminant [17]. The technique is
very popular in detecting oil -contamin ated soil due
to its decrease in soil conductivity when
contaminated with oil. For this technique, basically
the current is injected into the ground through a pair
of electrodes and another pair of electrode will be
used to measure the potential existing d ue to the
passage of the current [7].
In this case study the area is about 2 0 km
southwest of Kuala Lumpur, Malaysia and is
basically consists of Quaternary alluvium overlying
the of weathered metasedimentary rock type.
The survey discussed in this paper w as carried
out using a RAMAC SYSTEM consisting of a
Model PR – 8304 profiling recorder with automatic
gain ranging, graphic, magnetic tape data recording,
and a copper -foil dipole antenna having a center
operating frequency of 100 MHz. Data collected
were p rocessed using software to produce 2D
radargram in time scale [14].
In the resistivity survey, measurements were
made along 9 traverse lines in NE -SW and NW -SE
directions as shown in Figure 4. The length of each
survey lines were between 50 to 100 metres.
Expected maximum depth of current penetration
into the soil is approximately about 10 to 20 metres.
ABEM Terrameter model SAS 1000 instrument was
used for the data collection. Sclumberger array
electrode confuguration was used to provide a good
vertical re solution.

Figure 5. A 2D radargram section of GPR line 16.

Figure 6. Inversed resistivity model for line 3 and
line 5 .

An example of 2D radargram section of Line 16
is presented in Figure 5. T he section can be divided
into three particular reflectio n pattern representing
different soil types. At depth 0 to 0.5 m, the
reflections are flat and showing high amplitudes
where as at depth of 0.5 until 1.5 m it shows
discontinuous and chaotic pattern.
Layer below than 1.2 m consists of soft grey clay.
This high conductivity soft clay absorbs the GPR
reflection to produce much weaker reflection zone
as compared with the above layer. The strong reflector at about 1.3 m depth in the GPR section is
interpreted as top of clay layer.
2D electrical resistivity meas urements using
Schlumberger array were conducted along 9 lines in
the study area. An example of inversed resistivity
model representing line 3 shows a top thin layer
with low apparent resistivities ranging from 8 Ωm
to 40 Ωm associated with water saturated layer from
top to about 0.5 m depth. For this line, the survey
was conducted during wet season where the water
saturated top layer shows low resistivity. Lying
immediately b elow this layer is a zone of higher
resistivity at depth between 0.5 to 1.5 m which
coincides with the oil -contaminated zone. The
underlying low resistivity zone corresponds to the
thick grey soft clay layer. Similar pattern of
resistivity distribution is also shown in line 5 where
the contaminant plumes are located on top of low
resistivity thick clay .

This survey show how can be used two different
geophysical methods in the same are a in order to
obtain better data. Using GPR results the resistivity
data can be correlated in a better way with the
geological environment and can be identified very
easy the contamination plume.

IV. CONCLUSIONS

This review was created in order to list the
major geophysical methods used in case studies
about soil contamin ation and their applicability
on this environmental problem. Soil it’s an
important part of our daily life , even we are
speaking about the plants that give us food or the
plants producing life. Because the soil is the first
layer of the Earth that is in c ontact with humans
is directly affected by pollution.
Speaking about soil pollution , it’s really
important to investigate the contaminated area in
order to see how the environment can be helped
and to monitor it along the time.
For this task geop hysical methods is playing
an important role. Because these methods are

non-invasive and really easy to use a lot of
scientists preffered to use them in surveying
contaminated areas.
Most used geophysical metyhods for
contaminated soils are electrical and
electromagnetic methods, GPR and magnetic
methods. Depending on various factors can be
used just one of these methods or can be used
more than two in order to obtain better data.
Two case studies were presented in this review
to show the applicabi lity of geophysics in soil
contaminated studies. In the first survey it was
used electromagnetical method to show that you
can investigate and monitor the contamination
plume in an area at Petrobrazi Rafinery in
Romania. In the second survey were used two
methods: GPR and electrical method. The
contamination plume was investigated in an area
of Kuala Lumpur in Malaysia. It showed how can
be correlate d the results from both methods to
have the best results.
Geophysical methods can be used
successfully in in vestigation of contaminated
soils. Depending on the studied area can be used
most of them in different ways.

.

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