Download Document

Resistivity method
without groundings
B.G. Sapozhnikov
Institute of Environmental Geology of
the Russian Academy of Sciences, St.
Petersburg Division
Here is the list of key points
of the report:
1. Is there an electric field in the air at works
by a resistivity method?
2. Is it possible to measure it?
3. Is it possible to study subsurface objects
by electric field measurements in the air?
4. Technology and equipment.
5. Case history of the field and city works.
6. Conclusions.
1.1. The resistivity method. Electric field in the ground


r 
I
E A    g  A3 
2
rA



r 
I
E B    g  B3 
2
rB 



E AB  E A  E B 


(1)
Shown in Fig.1 is the pattern of the field caused by two grounded current
electrodes «A» and «B». This figure is commonly used to explain the
theoretical basis of the resistivity method. The formulae describe the d.c.
electric field in the ground.
The field pattern, however, is not complete. It does not show electric field
lines in the air. The air may be considered as an ideal insulator, which
prevents the galvanic current flow. Therefore, the current density lines cannot
describe the electric field in the air. Is this an evidence of nonexistence of the
electric field in the air?
1.2. Electric field in the ground and in the air



I 1  2 rA
EA  
 3

2  1   2 r

A



I 1  2 rB

EB  
 3

2  1   2 r

B




E AB  E A  E B



rA 
I
lim 1  E A    g  3
2
rA 
(2)
In order to give the answer to this question let us consider Fig.2. The
electric field pattern and the known equations describe the general case of
field calculations where a interface divides two homogeneous media with
resistivities «1» and «2».
Calculating the limit of the expression (2) it is easy to derive the equations
(1) which now describe electric fields in the air and in the ground.
The pattern of the static electric field in the air (equipotential lines and lines
of force of the field intensity) turned out to be a mirror image of the pattern
of the stationary electric field in the ground, as shown in Fig.2.
1.3. Electric field in the ground and in the air



I 1  2 rA
EA  
 3

2  1   2 r

A



I 1  2 rB

EB  
 3

2  1   2 r

B




E AB  E A  E B



rA 
I
lim 1  E A    g  3
2
rA 
(2)
As can be seen from Fig.2, the horizontal component of the electric field at the
central part of the «AB» line decreases equally slow as the observation point
moves away in both directions from the earth-air interface.
It means that the height dependence of this field component is extremely small
and can be neglected when observations are made at heights comprising 1-5 %
of length of the «AB» line.
The vertical component of the electric field in the air at the central part of the
«AB» line is much smaller than the horizontal component.
1.4. Electric field in the ground and in the air



I 1  2 rA
EA  
 3

2  1   2 r

A



I 1  2 rB

EB  


2  1   2 r 3

B




E AB  E A  E B



r 
I
lim 1  E A    g  A3 
2
rA 
(2)
Thus, when the electric field is measured in the air near the earth-air
interface, the obvious physical basis for the proposed improvement is the
known equality of the tangential components of the electric field on both sides
of the interface.
This allows to substitute contact measurements of the electric field using
galvanically grounded measuring electrodes by contactless measurements
with the use of receiving electric antennae (electrodes), which either have no
galvanic contact with the ground at all or the contact is extremely poor.
2. The measurements of the electric field in the ground
Fig.1 (slide 3) shows a conventional
grounded «MN» line. Fig.3 shows the
same line on an enlarged scale.
The equivalent electrical circuit of
the
receiving line (Fig.3) includes
e.m.f. «U»
(of the potential
difference
of
the
measuring
electrodes) and the microvoltmeter
input resistance
«Rin» and the
ground resistances of the measuring
electrodes «RM» and «RN».
From the equation in Fig.3 it is inferred that the input voltage «Uin»,
measured by the microvoltmeter, is almost equal to the potential difference
«U» at favorable grounding conditions when the sum of resistances (RM+RN)
is sufficiently small compared to the microvoltmeter input resistance.
From the measured value «U» and known electrode separation «a» the
intensity «Ex» of the electric field horizontal component can be easily
calculated: Ex = U /a.
3.1. The measurements of the electric field in the air
The non-grounded receiving line
made up of two similar line
electrodes «M» and «N» is shown
in Fig.2 (slide 6) and Fig.4 (on an
enlarged scale).
The electrode separation «a» is
here also the distance between the
electrode centers. A distance «a» is the effective length of an
antenna.
The line «MN» made up of two lengths of insulated wire may be either laid out
directly on the ground surface or raise to a considerable height. The receiving
line electrodes acquire the potentials of the equipotential lines through the
centers of the electrodes.
The equivalent electrical circuit of the line «MN» (Fig.4) includes the e.m.f.
«U» and the capacitance voltage divider formed by the microvoltmeter-input
impedance and self-capacitances «CM» and «CN» of the measuring electrodes.
3.2. The measurements of the electric field in the air
In order to select an operating
frequency let us consider the
equivalent circuit of the array in
Fig.4.
At d.c. the desired signal «UIN» is
zero. As the frequency rises the
circuit
transforms
into
a
frequency-independent
voltage
divider.
At a sufficiently small input capacitance «CIN» the desired signal is practically
equal to the e.m.f. value "U" being measured.
Calculations of the electric field intensity can be made from the equation, which
was given before for the grounded receiving line. When selecting the optimal
operating frequency, conflicting consideration must be given to the following
discrepancies.
3.3. The measurements of the electric field in the air
On the one hand, lower
frequencies
provide
greater
depths of investigation of the
resistivity method.
On the other hand, higher
frequencies enable a more
effective rejection of vibrational
interferences, which are specific
for the array in question.
These interferences are due to the e.m.f. of electrization of the ungrounded
line insulators.
The interferences are considerable in magnitude and have an essentially lowfrequency spectrum.
The investigations revealed that optimal frequencies range from 20 to 3000
Hz. A basic frequency selected for developed system "ERA" is 625 Hz.
4.1. The anomalous electric fields in the air
Let us focus on the nature of anomalous
electric fields in the air to be measured by
the resistivity method.
Fig.5 shows a vertical high-resistivity layer
a limited depth, which is a local causative
body of anomaly. In a normal electric field
with horizontal polarization the layer is
polarized and its vertical sides acquire
stationary electric charges.
This is the cause of anomalous electric currents and corresponding electric
fields. Since galvanic currents must not penetrate into the insulator (air), static
electric charges appear on the surface preventing the current flow from the
ground into the air.
The distribution of these charges is fully determined by the current flow pattern
in the ground, and hence, the electric field of these charges in the air
reflects this pattern.
4.2. The anomalous electric fields in the air
Fig.5 shows the curves of the
horizontal «Ex» and vertical «Ez»
components of the anomalous
electric field in the air in the vicinity
of the earth-air interface.
It may be noted that these curves are similar to the curves of the «Hx»
and «Hz» components of an anomalous magnetic field over a vertical
layer magnetized in the Earth's magnetic field in proximity to the equator.
5.1. Technology and equipment
Depending
on
the
spatial
resolution required and the type of
array, antennae of different
designs (Fig.6) can be used.
For example, an asymmetrical
antenna (b) made up of insulated
wires of up to 100 m in length
creeping over the ground (a
«snake»-like antenna) or a
telescopic symmetrical air antenna
(d) with an effective length of up
to 1.5 m.
5.2. Technology and equipment
In some types of arrays the operator caring a microvoltmeter may serve
himself as one of measuring electrodes («N») with the other electrode
(«M») being non-grounded («snake» line).
5.3. Technology and equipment
Fig.8 shows the transmitting lines for the gradient array: a – usual
galvanically grounded line; b, c – non-grounded lines.
b – «capacitive» line (unclosed loop), c - «inductive» line (closed loop).
5.4. Technology and equipment
Fig.9 shows the dipole - dipole array: a – non-grounded array, b – usual
galvanically grounded array.
A’A - transmitting line; MN – receiving line.
5.5. Technology and equipment
Fig.10
Transmitter
Receiver
A electric survey system «ERA» (Electrical Research Apparatus) has been
developed at the Research & Production Enterprise «ERA» (St. Petersburg) under
the supervision of the author.
The «ERA» system is provided with unique active potential electrodes (input
capacitance 0.1 pF; input resistance 20 GOhm). The transmitter producing output
voltage stepped up to 1-1.5 kV. A basic frequency 625 Hz.
Fig.10 shows the last version «ERA» system- «ERA-MAX» developed by L.I.
Dukarevich («RPE», 2002).
6.1. Case history of the field and city works
• The resistivity method without groundings proposed the author in 1963
has been successfully used in USSR and Russia in geological
prospecting, hydrogeology, engineering investigations, environmental and
archaeological studies.
• In 1981 the technique was approved for application in USSR territory of
the Ministry of geology of the USSR.
• The new technology essentially expands opportunities of resistivity and
miss-a-la-masse methods. It is applied in variants of profiling (gradient and
dipole-dipole arrays), sounding, 3D-vector air measurements with rotary
electric field, etc.
The advantages of technology are:
• increasing the efficiency of the resistivity method by extending its
application into the areas with unfavorable conditions of groundings;
• cutting down labor expenditures by reducing the size of field crews;
• widening the scope of functional potentialities of the method in particular
by virtue of 3D-vector measurements of the electric field in the air.
6.2. Case history of the field works
Fig.11 shows the apparent-resistivity graphs of the dipole-dipole arrays:
a – usual galvanically grounded array, b – non-grounded array.
Sayani, 1975. The size of the crew was reduced to 2 persons.
1, 3, 4 - metamorphic rocks, 2 - tectonic discontinuities, 5 – limestone.
6.3. Case history of the field works
Fig.12. Winter and summertime measurements with
gradient array.
The «AB» line length is 600 m.
The receiver air telescopic
antenna with effective length
1 m.
Winter-time electric survey
was carried out on the ice.
•Central Kareliya, 1980.
NEW VERTICAL ELECTRICAL SOUNDING SET
for small and average depths (1-2000 m)
Journal “Geoecology, engineering geology, hydrogeology, geocryology ”, 2005, № 5, p.p.454-462
Standard interpretation program IPI2WIN (Moskva State University, Bobachev A.A.)
Features
•
high degree of protection of measurements from induction and capacitor distortions
•
the small sizes and the big resolution of survey (length of new set is
almost twice less then usual set)
•
increase of productivity survey (only one the receiving electrode “M” moves,
but all others one are grounded only once)
•
small number of a field staff (1-2 persons instead of 3)
•
possibility of "contactless" measurements (in bad grounding conditions)
a
A
M
1
O
B
N
5м
2
3
3
б
M
O
N
2
Interpretation VES (new set)
Spain, El Saltador
а – cross-section “ρa”; b – geoelectrical cross-section;
1 – dry overburden, 1500 Ohm▪m;
2 – aeration zone conglomerates, 300 Ohm▪m;
3 – clay, 20 Ohm▪m;
4 – saturated conglomerates, 50÷60 Ohm▪m;
5 – marl (waterproof stratum), 5÷7 Ohm▪m.
A
3
B
1
VES set schemes
a – usual (symmetrical), b – new (with the transmitter
in line MN), 1 – receiver, 2 – transmitter, 3 – the coil
with a wire
Induction effect with new
and usual VES set
Experimental (a, b) and theoretical (c) VES
curves: a, c – new set ( the effect is absent );
b – usual (symmetricаl) set (the interpretation
is distort by the effect ); c – the new set VES
theoretical curve; d – parameters of the
two-layer geoelectrical cross-section; 1, 2 –
dry (1) and damp (2) alluvium; 3 – marl
(waterproof stratum)
6.4. Case history of the field works
Fig.13. Grounded and non-grounded VES arrays for winter-time measurements
on the river Zeya ice: a - comparison of VES curves for different frequencies and
arrays; b - VES profile with geological
cross-section. Amur region, 1996.
1- ice, 2- water, 3 – alluvium, 4 – eluvium, 5 – limestone.
6.5. Case history of the city works
Fig.14 shows the ground electric
resistivity map of a road cloth of
the city prospectus in a zone of
the emergency leakage of the
high pressure mains water
supply pipeline (1000 mm).
The main unloading of the
leakage happened here on ferroconcrete box of the adjacent
central heating pipeline.
However, the resistivity method has shown, that a part of the water flow
percolated under asphalt pavement as two sleeves of a water saturated
ground (zone 1 and 2). Saint-Petersburg, 1999.
1- water saturated ground zones; 2 – water supply pipeline with a leakage
location; 3 – sewerage with a man-hole; 4 – central heating pipeline;
5 – curbstone.
6.6. Case history of the city works
Fig.15. On the electrical survey
data the extensive zone of the
water
saturated
ground
caused
by
leaks
of the
sewage disposal plant channel
was established.
The said zone is marked on the
ground electric resistivity map
(Fig.15а) by area of low
resistivity 0.5-15 Ohm.m.
Fig.15
The leakage location of the
western «1» and east «2»
channel walls was executed
with the help of VES.
The number of the electric resistivity VES sections for outside (profile 58) and
inside (profiles 20, 18, 16) of the leakage area is shown on Fig.15b.
The proposed innovative technology is an adequate competitor to the widely used
ground penetrating radar technology.
7. Conclusion
The resistivity method without
groundings is a reality!
It lives and works in Russia
already more than 30 years.