Application of Geophysical Methods in the study of [616326]

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 determination of resistivity distribution of
the entire volume of soil [11]. In practice a rtificially
generated electric currents are generated to the so il
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
between 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
distributions 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
representing the length of the entire 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 fol lows:
(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
electr odes’’), and two other electrodes 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 resi stivity of soil is a function of a
number of different properties, including the nature
of the solid constituents ( e.g distribution of particles
of different sizes and mineralogy), arrangement of
voids ( e.g porosity, distribution of pores by sizes
and conn ectivity between pores ), degree of water
saturation , resistivity values of the fluid containted
in pores and the temperature [2]. If the medium is
an insulator (i.e. infinitively resistive), the n water
resistivity measured in solutions is a function based
on ionic concentration, the resistivity of the solid
grains being then related to the density of electrical
charges at constituents surfaces. . These parameters
affect the electrical resistivity, but in different ways
and to different extents [6]. Electric al 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 transmitt ers
operating in a frequency range between 10 kHz and
300 kHz. The radiomagnetotelluric 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 tho se
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 ratio 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 conductivity 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 considered an
useful geophysical method for a variety of surveys .
It can be used in general to study the contaminant
type that polluted the soil and groun dwater,
undergroun d cavities (natural or anthropologic
causes) and systems of fault s in subsurface area ,
these examples being the most frequent causes to
soil contamination [12].

The G PR method uses similar principle s like in
seismic reflection and sonar techniques. Ground
Penetrating Radar sys tem send short pulses of high frequen cy (10 -1000MHz) electromag netic energy
into the gro und using the transmitting antenna from
GPR system [8]. The propagation of the signal s
depends on the electrical properties of the ground
and need to generate a n elect romagnetic frequency
contrast [6]. Electrical conductivity of the soil along
the propagation is used to limit the depth of
penetration and to receive data about earth
formations. Electrical conductivity of the soil is
primarily dependent on the moisture c ontent 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 ref lected back to
the system and part or the energy with amplitude
AT is transmitted into the medium and po ssibly
through it [12]. In Figure 1 can be seen the basic
components of GPR system and a simplified mode
to use it . When measurement are made using GPR
system is important to tak e in consideration

electrical properties changes of geological materials
that are influenced by many factors. Most important
factors are water content and dissolved minerals
found in geological environment , also clay and
heavy mineral content that can go to drastic changes
of electric al properties [14]. Regarding the technical
aspects , usually reflected signals are a mplified
when are transmitted through the medium , recorded
on the system , processed by the operator , and
displayed in order to be interpreted later. Using t he
recorded display different features (e.g. soil/ soil,
soil/rock and unsaturated/saturated interfaces ) can
be identified in order to investigate the pollution of
contaminated soils [13].

D. Magnetic method

Magnetic susceptibility is a measur e 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 hav e been applied
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 mea sured rapidly. 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 typ e 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 magnetic
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
contami nation by 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 w ere 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 t he 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 v olume 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 hydrocar bons
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 contaminant s underground 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 th e groundwater 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.
Close to the Brazi Refinery in Romania
radioma gnetotelluric meas urements were carried
out on a contaminated test area. The plan was to
detect and to monitor an oil layer with a 1 m
thickness expected to be located at 5 m depth. For
this study were selected r adio transmitters using a
frequency range from 10 kHz to 300 kHz and the
target was to observe the apparent resistivity [16].
Before the study started there was assumed a
direc tion of the contamination plume and the
profiles were located parallel and perpendicular to
that direction. Based on the data resulted from t he
measurements and using a 2D inv ersion technique
conductivity structure of the area was deri ved [16].
The 2D inversion models showed that in the
contaminated area exist a poor -conductivity zone
above the groundwater level that was associated
with contami nation area.
Radiomagnetotelluric measurements were made
in two phases: first on the contaminated area during
a wet period 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 othe r. 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 resistivity
for the frequency 198 kHz (TE -mode). The markings
indicate the location of the radiomagnetotelluric stations.
The data were observed dur ing a wet period in 1999 .

Figure 3. Spatial distribution of the phase 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.

Approximately on e 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 obtai ned 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 clos e
to 100 Ωm. This difference may be explained by the
actual field conditions 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 investi gate
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 alrea dy mentioned before GPR method is
using similar principle s like seismic reflection and
the sonar techniques . Using this method can be
targeted different objectives of investigations of a
contamin ated area caused by hazardous materials
[12], including the investigation of contaminant
plumes and their characteristics, their source , also
can determine the geometry and the assessment of
associated hydrogeologic conditions [6].
Resistivity measurements have been used since
long time ago in the study of contaminat ed soil,
especially in the case of hydrocarbon s contaminant s
[17]. This method is very useful to detect oil-
contaminated soil s because the soils have low –
conductivity when are polluted with hydrocarbons .
Using this method , it’s needed usually two pairs of
electrodes, one pair is used to inject current to the
ground, the other pair being used to measure the
potential existing in the ground [7].
In this case study the area is located at 20 km
southwest of Kuala Lumpur, Malaysia and is
basically consists of Q uaternary alluvium overlying
the of weathered metasedimentary rock type.
The study made in the selected area was carried
out using a RAMAC SYSTEM consisting of a
Model PR – 8304 . This system is a profiling recorder
that is operating with a frequency of 100 MHz. Data
collected with this model were processed and in the
end it was obtained a 2D radargram [14].
In the resistivity survey, measurements were
made along 9 traverse lines in NE -SW and NW -SE
directions as shown in Figure 6 . For this study were
used line s with a length between 50 and 100 meters
and the expected maximum depth of penetration
into the so il is between 10 and 20 metres. For data
collection was used a ABEM Terrameter model
SAS 1000 instrument . In this case a Sclumberger
array electrode confi guration was used to obtain the
best vertical resolution of results .

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

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

In Figure 5 is presented a 2D radargram section
of Line 16 . In t he section can be seen three
particular reflection pattern r epresenting different
types of soil . Between 0 and 0.5 m the reflections
have high amplitudes and have a flat form. A t depth
of 0.5 until 1.5 m it shows discontinuous and
chaotic pattern.
Layer below than 1 .2 m have been interpreted to
be grey clay s. This lithological type have high
conductivity and absorbs the GPR reflection and
have as result a weaker reflection zone compared to lithological layer from above . At 1.3 m depth in the
2D radargram the layer wi th higher reflectance was
interpreted as top of clay layer.
Later in the study with 9 lines were made
electrical resistivity measurements using
Schlumberger array . Using data collected with line
3 was created an inversed resistivity model
representing line 3 and during the interpretetaion
was identified 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. In this case the survey was conducted during
wet season and because of that the top layer had
lower resistivity than usual . The undernea th layer
was interpreted as a zone of higher resistivity
situated at depth s between 0.5 to 1.5 m, these depths
being previously interpreted as the oil -contaminated
zone. The underlying low resistivity zone
corresponds to the thick grey soft clay layer. Sim ilar
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 d ata. 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 meth ods used in case studies
about soil contamination 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 contact 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 monito r it along the time.
For this task geophysical 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 me tyhods 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 pr esented in this review
to show the applicability 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.
Geophy sical methods can be used
successfully in investigation of contaminated
soils. Depending on the studied area can be used
most of them in different ways.

.

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