research article investigation on a novel capacitive electrode for geophysical...

Research ArticleInvestigation on a Novel Capacitive Electrode forGeophysical Surveys

Zhiyu Wang,1,2 Shun Wang,1 Guangyou Fang,3 and Qunying Zhang1

1Key Laboratory of Electromagnetic Radiation and Sensing Technology, Chinese Academy of Sciences, Beijing 100090, China2University of Chinese Academy of Sciences, Beijing 100149, China3Chinese Academy of Sciences, Beijing 100090, China

Correspondence should be addressed to Zhiyu Wang; [email protected]

Received 6 December 2015; Revised 29 May 2016; Accepted 31 May 2016

Academic Editor: Alberto J. Palma

Copyright © 2016 Zhiyu Wang et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Nonpolarizable electrodes are applied widely in the electric field measurement for geophysical surveys. However, there are twomajor problems: (1) systematic errors caused by poor electrical contact in the high resistive terrains and (2) environmental damageassociated with using nonpolarizable electrodes. A new alternative structure of capacitive electrode, which is capable of sensingsurface potential through weak capacitive coupling, is presented to solve the above problems. A technique is introduced toneutralize distributed capacitance and input capacitance of the detection circuit. With the capacitance neutralization technique,the transmission coefficient of capacitive electrode remains stable when environmental conditions change. The simulation andfield test results indicate that the new capacitive electrode has an operating bandwidth range from 0.1Hz to 1 kHz. The capacitiveelectrodes have a good prospect of the applications in geophysical prospecting, especially in resistive terrains.

1. Introduction

Most electrical or electromagnetic geophysicalmethods, suchas magnetotellurics, induced polarization, direct current(DC) resistivity imaging, and other new similar electromag-netic geophysical methods, need to acquire the direct currentor alternating current electrical field signal when the systemtakes the measurement [1–5]. The measurement instrumentalways uses two separated grounded electrodes to detect theelectric potentials which are different from them.The electricfield intensity is calculated as electric potentials differencedivided by the distance. In order to avoid the nature self-polarization of the electrode while it is grounded or con-tracted to other conducted media, nonpolarizable electrodeis applied widely in most electrical and electromagneticgeophysical instruments for measuring the electric fieldaccurately [4]. The current typical nonpolarizable electrodesare Pb-PbCl

2electrodes and Ag-AgCl electrodes, and Pb-

PbCl2electrodes are applied widely in onshore exploration.

However, poor electrical contact is achieved in soil, under dryclimatic conditions, or on rocky ground, which may lead to

systematic errors [6]. Another problem of grounded elec-trode is that it is difficult to maintain the same contractedimpedance between the electrode and the ground while thesurrounded media take variety conductivity from point topoint. So it affects the potential measurement accuracy andconsistency. The damage and poison to the ground also arethe problems of grounded electrodes.

In the 1970s, Timofeev et al. began to apply capacitivecoupling to facilitate resistivity measurements on the frozenground for the purpose of mapping permafrost [7, 8]. Theyused the cables as transmitting electrode and receivingelectrode. The displacement current for a certain frequencygoverned the energy transmission from the transmitter elec-trode to the ground, while the receiver electrode measuredthe surface potential [9]. As there is no galvanic current pathbetween the cable and the ground, themethod is named non-contacting capacitively coupled resistivity (CCR) or capaci-tive resistivity (CR). Correspondingly, the electrode is calledcapacitive electrode.

Since the 1990s, a mass of empirical, technical, andtheoretical information had been researched, which was

Hindawi Publishing CorporationJournal of SensorsVolume 2016, Article ID 4209850, 9 pageshttp://dx.doi.org/10.1155/2016/4209850

2 Journal of Sensors

described by Kuras [10]. Kuras conducted a thorough studyon the fundamental theory [8], giving an applicable fre-quency band for capacitive coupled resistivity:

𝐼

2𝜋𝐶𝑈𝑇

< 𝑓 <𝜌

𝜋𝜇0𝐿2, (1)

where 𝑓 is the applicable frequency, 𝐼 is the introduced elec-tric current,𝐶 is the system capacitance,𝑈

𝑇is the transmitter

voltage, 𝜌 is the subsurface bulk resistivity, 𝜇0is the magnetic

permeability, and 𝐿 is the transmitter-receiver separation.Formula (1) indicates that the low frequency is limited by thepower output of the current systems and system capacitance.

In commercial applications, Geometrics Inc. and IRISInstrument Inc. designed their instruments based oncapacitive coupled resistivity: OhmMapper and CORIM.OhmMapper, which works at frequency of 16.5 kHz, is acapacitive coupled resistivity meter with line electrodes [11],while CORIM uses the plate-wire combination to couple tothe ground, which works at frequency of 12 kHz [12]. Thesesystems are pulled along the ground either by a single personor by being attached to an all-terrain vehicle.

Compared with nonpolarizable electrodes, the utilizationof the line electrodes or plate-wire electrodes speeds up themeasurement and displays good applicability on highly resis-tive ground for near-surface investigations. However, thereare some drawbacks. Firstly, the applicable frequency bandlimits the investigation depth [9]. Typically, investigationdepth is in the range of 10m to 20m. Secondly, the geometricfactor of the CR method is approximated by the traditionalfour-point dipole-dipole geometry, but line electrode data donot fulfill the point source assumption of DC resistivity the-ory. In order tomatch themeasuredDC resistivity data, someCR models are developed to correct the geometrical factorand measurement biases [9, 13–16].

In the 2000s, GroundMetrics Inc. and QUASAR FederalSystems Inc. had developed a capacitive electrode namedeQube, which operates at the frequency down to 0.1 Hz [17,18]. Similar to line electrode, eQube couples to the electricfield via capacitive coupling directly. However, little data hasbeen published on its performance in practical application.

The aim of this paper was to develop a new alternativecapacitive electrode, which could be used for measuring thelow-frequency geoelectric field accurately via weak capacitivecoupling. A new structure of capacitive electrode, which isdifferent from line electrode or plate-wire electrode, wasdeveloped. A metal plate is used for accumulating chargesvia capacitive coupling to the ground. The detection circuitis put closely to the plate, which converts charge into voltageand then outputs voltage signal. Because of the low outputimpedance, the signal can be transmitted for a long distancewithout the influences of the cable from the electrode tothe data acquisition equipment. The main difference fromline electrode is that the new capacitive electrode can beseen as point electrode, which is directly applied in the samegeophysical measurement system without correcting effec-tive dipole length. A technique is introduced to neutralizedistributed capacitance and input capacitance of the detec-tion circuit. With the capacitance neutralization technique,

the transmission coefficient of capacitive electrode remainsstable when environmental conditions change. The field testresults indicate that the new capacitive electrodes have anoperating bandwidth range from 0.1Hz to 1 kHz and theycould measure the low-frequency geoelectric field accurately.The capacitive electrodes are well suited for current geophys-ical prospecting systems, such as transient electromagneticsystem and audio magnetotelluric system. They have a goodprospect of the applications in geophysical prospecting,especially in resistive terrains.

2. Theory and Basic Principles

The generic electrical field measurement system can be mod-eled as electrical circuit, where 𝑍P1 and 𝑍P2 are the contactimpedance values of the electrodes and 𝑍Earth is the groundimpedance (see Figure 1(a)). There is a true current pathfor exchanging ions between the electrode and the solution,so the nonpolarizable electrodes or metal electrodes canbe taken as resistance 𝑅P (see Figure 1(b)). The capacitiveelectrodes, which couple to the electric field via capacitivecoupling, can be taken as capacitance 𝐶P as shown inFigure 1(c).

The paper assumes that electric potentials in the pointsof two electrodes are 𝜑

1and 𝜑

2, and input impedance of the

detection circuit is 𝑍in. The input voltage of the detectioncircuit with resistive electrodes is

Δ𝑉𝑅=

𝑍in𝑅P1 + 𝑅P2 + 𝑍in

⋅ (𝜑1− 𝜑2) . (2a)

As to capacitive electrodes, the input voltage of the detec-tion circuit is

Δ𝑉𝐶=

𝑍in1/𝑗𝜔𝐶P1 + 1/𝑗𝜔𝐶P2 + 𝑍in

⋅ (𝜑1− 𝜑2) . (2b)

Here, 𝜔 is angular frequency. When the operating condi-tions𝑍in ≫ 1/𝑗𝜔𝐶P and𝑍in ≫ 𝑅P are satisfied,Δ𝑉𝐶 ≐ Δ𝑉

𝑅is

set up. In contrast with nonpolarizable electrodes, capacitiveelectrodes can measure the geoelectric field more conve-niently, which do not rely on exchanging ions between theelectrode and the solution.This is important in the high resis-tive terrains, where it is hard to get good electrical contact.However, the value of Δ𝑉

𝐶is related to angular frequency 𝜔,

and the impedance of capacitive electrode enlarges with thedecrease of angular frequency. Because input impedance ofthe detection circuit is a finite value, there is a limitation ofthe lower frequency of capacitive electrode.

The capacitive electrode has two realization scenarios:the line electrode type and the plate-wire electrode type [8].The line electrode type adopts a cable as electrode, whichcapacitively couples to the ground as shown in Figure 2(a). Inthe latter type, not only can the cable forma capacitor, but alsothe plate couples capacitively to the ground. Its capacitance isequal to the parallel combination values of the cable capac-itance and the plate capacitance (Figure 2(b)). However, theeffective dipole of those types of capacitive electrodes is notthe geometric distance between the electrodes. But the effec-tive dipole of the resistive electrodes is the geometric distance

Journal of Sensors 3

ΔV

ZP1ZP2

𝜑1

𝜑1𝜑1

𝜑2

𝜑2𝜑2

ZEarth

ZEarthZEarth

ΔVR ΔVC

∼

∼ ∼

RP1 RP2 CP1 CP2

(c)(b)

(a)

Figure 1: Equivalent electrical circuit models based on a modified form of Kuras et al. [8]. (a) A generic receiver system, (b) nonpolarizableelectrodes, and (c) capacitive electrodes.

CwireCwire

∼

(a)

CwireCwire CplateCplate

∼

(b)

CplateCplate

∼

(c)

Figure 2: The conceptual model of (a) line electrode, (b) plate-wire electrode, and (c) new capacitive electrode.

between the electrodes. It will result in slight deviationbetween CR data and DC resistivity data. If the geometricdistance increases to 100m or more, which is common in DCresistivity method, the deviation is hard to be corrected.

As shown in Figure 2(c), the new capacitive electrodetakes a novel structure: a metal plate is used as a receiver,

which introduces charges via capacitive coupling to theground. The detection circuit inside the capacitive electrodeconverts the charges into voltage, and then the voltage signalis transmitted through the cable. At last, it is acquired bydata acquisition equipment.The equivalentmodel of the frontdetection circuit is shown in Figure 3, where 𝑉

𝑠is surface


OP1

+

Cplate Vo

−

Vs

Cg Cin Rin

∼

Figure 3: The equivalent model of front detection circuit.

potential,𝐶plate is coupling capacitance between the electrodeand the ground, 𝑅inU𝐶in are input impedance of the oper-ational amplifier OP1, and 𝐶

𝑔is the distributed capacitance

from electrode to front detection circuit. The output voltageof OP1 can be expressed as

𝑉𝑜=

𝑠𝐶plate𝑅in

1 + 𝑠 (𝐶plate + 𝐶𝑔 + 𝐶in) 𝑅in⋅ 𝑉𝑠, (3)

where 𝑠 represents 𝑗𝜔. In the regime where 𝑠𝐶plate𝑅in ≫ 1,(3) can be simplified to the form of

𝑉𝑜≐

𝐶plate

𝐶plate + 𝐶𝑔 + 𝐶in⋅ 𝑉𝑠. (4)

For a rectangular plate of finite thickness suspended overthe ground plane, an approximate functional expression ofthe capacitance is given as follows:

𝐶plate = 𝜀0𝜀𝑟⋅ [1.15

𝑤 ⋅ 𝑙

ℎ+ 1.40 (

𝑡

ℎ)

0.222

⋅ (2𝑤 + 2𝑙)

+ 4.12 (𝑡

ℎ)

0.728

⋅ ℎ] .

(5)

Here, 𝑤, 𝑙, and 𝑡 are the width, length, and thickness ofthe plate, respectively. ℎ is the height of the bottom face abovethe ground surface, 𝜀

0is absolute dielectric constant, and 𝜀

𝑟is

dielectric constant between the plate and the ground surface[8].

When 𝐶plate ≫ 𝐶𝑔+ 𝐶in, the output voltage 𝑉𝑜 is equal

to the surface potential 𝑉𝑠. If the input impedance of the

front detection circuit is large enough, the result is completelyanalogous to that of the nonpolarizable electrode. Comparedwith line electrode, the new capacitive electrode’s couplingcapacitance between the output cable and the ground doesnot contribute to the system capacitance any more. In otherwords, the new capacitive electrode can be viewed as a point

electrode as nonpolarizable electrode. So it is suitable forcurrent geophysical prospecting systems, such as transientelectromagnetic system and audio magnetotelluric system.Of course, there is a limitation of bandwidth, which will bediscussed in the next section.

3. Design of Capacitive Electrode

In a practice application, the dielectric constant of soil is nota constant with the changes of temperature and moisturecontent [19], which may lead to the change of couplingcapacitance as shown in (5). Furthermore, the height from theground surface to the bottom face of the plate and the flatnessof the contact surfacemay also cause the coupling capacitancechanging. In light of these problems, the charge amplifier,which is commonly used as the preamplifier of capacitivesensors such as pressure sensors and photoelectric sensors, isnot suitable for capacitive electrode. Also, the circuit shownin Figure 3 is unpractical.There is no bias current path, whichwill result in the saturation of the amplifier and will make itunable to cope with the change of the coupling capacitance.Given these problems, a utility detection circuit is developed.

3.1. Design. To avoid the influence of the changes of thecoupling capacitance, a front detection circuit is designed (seeFigure 4). Here, 𝑅in and 𝐶in are input resistance and inputcapacitance of op-amp OP1. 𝑉

𝑠, 𝐶plate, and 𝐶

𝑔are surface

potential, coupling capacitance between the electrode and theground, and distributed capacitance, respectively. 𝐶

𝑛, nega-

tive capacitance, is used to eliminate effects of input capaci-tance𝐶in and distributed capacitance𝐶𝑔. Parallel capacitance𝐶1and feedback capacitance 𝐶

𝑓are used to adjust the cutoff

frequency. In order to avoid the input bias current chargingup the coupling capacitance and resulting in the saturation ofthe amplifier or producing aDC shift, a path for the input biascurrent to the amplifier is supplied by 𝑅

1and 𝑅

2. The gain 𝐴

of operational amplifier OP2 is set by the ratio of 𝑅3and 𝑅

4,

and 𝐴 = 1 + 𝑅3/𝑅4. The transfer function from the source

to the output of the front detection circuit is expressed as

TF (𝑠) =𝑠𝑅in𝑛num + 𝑠

2𝑅in𝑚num

𝑅in + 𝑅1 + 𝑅2 + 𝑠 [𝐶1𝑅2 (𝑅in + 𝑅2) + 𝐶𝑓𝑅1𝑅2] + 𝑠𝑅in𝑛den + 𝑠2𝑅in𝑚den

, (6a)


OP1

+

+OP2

Vo

−

−

∼

Cplate

CfCn

R1 Cg Cin Rin

R2

R3 R4

Vs

C1

Figure 4: Practical front detection circuit.

where𝑚num = (𝐶1+𝐶𝑓)𝐶plate𝑅1𝑅2, 𝑚den = (𝐶

1+𝐶𝑓)[𝐶plate+

𝐶𝑔+ 𝐶in + (1 − 𝐴)𝐶

𝑛]𝑅1𝑅2, 𝑛num = 𝐶plate(𝑅1 + 𝑅

2), and

𝑛den = [𝐶plate + 𝐶𝑔 + 𝐶in + (1 − 𝐴)𝐶𝑛](𝑅1 + 𝑅2).In the regime where 𝑅in ≫ 𝑅

1, 𝑅in ≫ 𝑅

2, and 𝐶

𝑔+ 𝐶in =

−(1 − 𝐴)𝐶𝑛, 𝑚num = 𝑚den and 𝑛num = 𝑛den, and (6a) can be

simplified to

TF (𝑠)

=𝑠𝑛num + 𝑠

2𝑚num

1 + 𝑠 [𝐶1𝑅2+ 𝐶𝑓𝑅1𝑅2/𝑅in + 𝑛num] + 𝑠

2𝑚num.

(6b)

The transfer function indicates that the capacitive elec-trode can be seen as a combination of high pass filter (HPF)and band pass filter (BPF), which has a unit gain in thepassband. Denote the center frequency of BPF by 𝑓

0, quality

factor by 𝑄, and upper cutoff frequency and lower cutoff fre-quency by 𝑓

𝐻and𝑓𝐿, respectively; then𝑓

0,𝑄,𝑓𝐻, and𝑓

𝐿can

be expressed as

𝑓0=

1

2𝜋√(𝐶1+ 𝐶𝑓) 𝐶plate𝑅1𝑅2

,

𝑄 =

√(𝐶1+ 𝐶𝑓) 𝐶plate𝑅1𝑅2

[𝐶1𝑅2+ 𝐶𝑓𝑅1𝑅2/𝑅in + 𝐶plate (𝑅1 + 𝑅2)]

,

𝑓𝐻=𝑓0

2(√4 +

1

𝑄2+1

𝑄) ,

𝑓𝐿=𝑓0

2(√4 +

1

𝑄2−1

𝑄) .

(7)

The cutoff frequency of HPF is the same as the centerfrequency of BPF. Hence, the cutoff frequency of the filteris dependent on coupling capacitance, feedback capacitance,and correlative resistances. It is easy to set the lower cutoff fre-quency by adjusting the feedback capacitance and resistances.

In (6a), the coefficient of quadratic term in the denomi-nator is related to the input capacitance 𝐶in and distributedcapacitance 𝐶

𝑔. There will be an error in the output voltage if

input capacitance and distributed capacitance change. Setting𝐶𝑛, 𝐶𝑔+ 𝐶in = −(1 − 𝐴)𝐶

𝑛, the coefficient of quadratic term

in the denominator will be irrelevant to 𝐶in and 𝐶𝑔, referringto (6b). The coefficients of quadratic term in the numeratorand denominator are the same, which means that the trans-mission coefficient is one even though the condition 𝐶plate ≫𝐶𝑔+ 𝐶in is not satisfied. It is fairly practical to deal with the

variation of the coupling capacitance. However, input capaci-tance and distributed capacitancemay not be constant duringthe life of capacitive electrode. Some steps should be taken toreduce the changes of the input capacitance and distributedcapacitance, such as shortening the lead from the electrodeto the input of front detect circuit as much as possibleand fixing the lead to avoid it loosing.

The resistor 𝑅1can be replaced by diode or FET, which

provides a low leakage current when PN junction is reversebiased. Two series diodes connected in opposite directionwillserve the purpose of a large resistancewhen the voltage acrossthe diodes swings within the forward voltage and breakdownvoltage [20]. Another high pass filter can be added after theop-amp OP1 to remove the low-frequency noise and drift.

3.2. Simulation. Considering practical applications, thewidth 𝑤, length 𝑙, and thickness 𝑡 of a plate are 20 cm, 20 cm,and 0.1 cm, respectively. Parameters of the detective circuit


Table 1: Transmission coefficient changes with coupling capacitance at frequency of 0.1 Hz.

Height (cm) Coupling capacitance (pF) Transmission coefficient(Δ(𝐶in + 𝐶𝑔) = 0% 𝐶

𝑛)

Transmission coefficient(Δ(𝐶in +𝐶𝑔) = 10% 𝐶

𝑛)

Transmission coefficient(Δ(𝐶in +𝐶𝑔) = 20% 𝐶

𝑛)

0.25 171.06 1.000 (0%) 0.997 (0.3%) 0.994 (0.6%)1 46.75 1.000 (0%) 0.989 (1.1%) 0.979 (2.1%)2.5 21.23 1.000 (0%) 0.977 (2.3%) 0.955 (4.5%)5 12.41 1.000 (0%) 0.961 (3.9%) 0.925 (7.5%)10 7.77 0.999 (0.1%) 0.939 (6.1%) 0.885 (11.5%)

are chosen according to the transfer function in Section 3.1.Input resistance 𝑅in and input capacitance 𝐶in of the op-ampOP1 are 1014Ω and 1 pF, respectively. Resistances 𝑅

1and 𝑅

2,

respectively, are 20GΩ and 10GΩ, parallel capacitance 𝐶1

is 50 nF, and feedback capacitance 𝐶𝑓is 10 𝜇F. Distributed

capacitance 𝐶𝑔can be calculated by (4) given a different

coupling capacitance 𝐶plate and the ratio of source voltageand output voltage can be tested. Distributed capacitance 𝐶

𝑔

is 4 pF. Accordingly, feedback capacitance is set as 5 pF. Toobserve the influences of variations of coupling capacitanceand distributed capacitance on the transmission coefficientat frequency of 0.1 Hz, the height ℎ of the plane above theground surface and the residual unneutralized capacitanceΔ(𝐶in + 𝐶𝑔) are changed, respectively.

As shown in Table 1, the value ranges of the couplingcapacitance are from about 50 pF to 8 pF when the height ℎ,respectively, is 0.25, 1, 2.5, 5, and 10 cm. With the help of thenegative capacitance, the transmission coefficient of capaci-tive electrode changes by less than 0.1% when input capaci-tance and distributed capacitance are neutralized completely.However, the effect of negative capacitance drops downwhendistributed capacitance and input capacitance deviate fromthe original values.The transmission coefficient bias enlargeswith the increasing of the residual unneutralized capacitanceand the height. Compared with the change of the gap height,the change of the unneutralized capacitance has a greaterimpact on the transmission coefficient. For getting a reliablemeasurement result, the gap height should be less than 5 cmand the residual unneutralized capacitance should be notmore than twenty percent of the negative capacitance, whichare not hard to achieve in practical application.

The transmission coefficient is held constantwhich equalsthe ratio of 𝑚num and 𝑚den at higher frequency more than0.1Hz. At lower frequency, the measurement result willdeteriorate further to be not credible.The cutoff frequency ofhigh pass filter (HPF) is set to about 0.03Hz. After HPF, theoperating bandwidth ranges from 0.1Hz to 1 kHz for obtain-ing accuracy of more than 90% (see Figure 5).

4. Experiment Details

4.1. GroundConditions. Theexperimentwas carried out fromOctober 10 to October 15, 2014, in the area of Wuerhe, Kara-may, Xinjiang Autonomous Region, China. This region waschosen for its dry soil and hostile environment. The top layerof the soil was gravel with thickness of about 3 cm to 5 cm, thesecond layer was sandy soil, and the lower layer was sandy soil

mixed with salinity. Because the experiment compared thedifferences between capacitive electrodes signal and resistiveelectrodes signal, it was not concerned with the geologicalsituation of the entire region. The ground surface was notflat because of the gravel. And the measurement result couldverify the influences of the contact surface flatness.

4.2. Experiment Arrangement. The experiment is conductedwith BGP Inc. exploration. BGP explored the area using time-frequency electromagnetic method (TFEM), which adoptedthe arrangement as in Figure 6. The perpendicular distancebetween current dipole line and survey line was 8 km. Therewere 24 survey points evenly distributed along the survey lineand the center distance of two adjacent points was 400m.Each point contained two nonpolarizable electrodes and onemagnetometer. Only two pairs of capacitive electrodes weredeployed close to the nonpolarizable Pb-PbCl

2electrodes

coming from BGP Inc., and both of their dipoles were spaced100m apart. If capacitive electrodes got the same measure-ment waveforms at the same point, we will consider thatcapacitive electrodes have the ability to substitute nonpolar-izable electrodes directly with no correction of measurementdata.

For reducing connection resistance, nonpolarizable elec-trodes were buried about 20 cm under the ground andwatered with enough brine. The connection resistance wasless than 200Ω during the experiments. The new capacitiveelectrodes were put on the ground surface directly as shownin Figure 7.

4.3. Experimental Results and Discussion. Figure 8 showstime domain waveforms and power spectral density curves ofthe measurement data of capacitive electrodes and nonpolar-izable electrodes when the transmitted frequency is 2Hz. Asa premise of discussion, the data of nonpolarizable electrodesis assumed to be accurate and correct. The time domainwaveforms of the capacitive electrodes and nonpolarizableelectrodes are almost coincident. Through more test datacomparisons, the ability of capacitive electrodes can be tested.Referring to Figure 9, magnitude ratios and phase differencesof capacitive electrodes and nonpolarizable electrodes arearound one with fluctuation of less than 4% and less than20 degrees when the frequency ranges from 0.1Hz to 1 kHz,respectively. The largest bias of the magnitude appears at fre-quency of 0.125Hz.Themagnitude ratio is 0.967, and the biasis 3.3%,which is allowed in the geoelectric fieldmeasurement.At lower frequency ranges, the amplitude bias is increased


Mag

nitu

de (a

bs)

Bode diagram

1

0.9

0.8

0.7

Frequency (rad/s)10−1 100 101 102 103

(a)

0

30

60

Phas

e (de

g)

Frequency (rad/s)10−1 100 101 102 103

Bode diagram

(b)

Figure 5: The frequency response curve of the new capacitive electrode.

Current dipole

SeparationSurvey line

Magnetic

sensor

Electrodes line

apart

CE lineNPE line

8km1m

Figure 6: The experiment arrangement.

Capacitive electrode

Power supply

Nonpolarizable electrode

20 cm

Figure 7: The electrodes at test site.

by about 25% at the roll-off frequency of 0.03Hz, which isless than the simulation results in Figure 5. It is consideredthat capacitive electrodes are capable of accurately measuringgeoelectric field when the frequency ranges from 0.1Hz to1 kHz.

The background noises of nonpolarizable electrodes andcapacitive electrodes, which include the intrinsic noise ofelectrodes and the noise of natural electric field, are about0.5 𝜇V/√Hz and 1.5 𝜇V/√Hz, respectively, at the frequencyof 10Hz (refer to Figure 8(b)). Because the electrodes wereplaced at the same point and the measurement data wererecorded by the same equipment, the background noise

differences of those two kinds of electrodes are caused by theelectrode itself. The intrinsic noise of capacitive electrodesis about 1.5 𝜇V/√Hz at the frequency of 10Hz. The noiseis almost inversely proportional to frequency at lower fre-quency. At higher frequency range, the performances ofcapacitive electrodes and nonpolarizable electrodes are simi-lar.

The new capacitive electrode combines the advantagesof both the line electrode and the nonpolarizable electrode.Compared with the line electrode, the new capacitanceelectrode can measure the geoelectric field precisely down to0.1 Hz, which means that the system could have larger inves-tigation depth. In contrast with nonpolarizable electrode,capacitive electrode canmeasure the geoelectric field withoutreliance on exchanging ions between the electrode and thesolution.This is important in the high resistive terrains, whereit is hard to get good electrical contact. The new capacitiveelectrode has the same measurement results as the nonpolar-izable electrode with the same dipoles spacing.This indicatesthat the new capacitive electrode can be suitable for cur-rent geophysical system without correction of measurementbiases.

5. Conclusion

This paper analyzes the theory of the new capacitive electrodeand gives a practical detection circuit. Due to the variationsof the dielectric constant of the soil and flatness betweenthe electrode plate and ground surface or other factors,


0 0.5 1 1.5 2 2.5 3 3.5 4Time (s)

Am

plitu

de (m

V)

0

0.2

0.4

Nonpolarizable electrodeCapacitance electrode

−0.2

−0.4

(a)

Frequency (Hz)

Nonpolarizable electrodeCapacitance electrode

10−1 100 101 102 103

10−3

10−4

10−5

10−6

10−7

PSD(V

rms/√

Hz)

(b)

Figure 8: (a) Time domain waveform and (b) power spectral density curves of capacitive electrodes (red lines) and nonpolarizable electrodes(blue lines). The industry frequency (50Hz) and its odd harmonics have been filtered out.

Mag

nitu

de (a

bs)

0.7

0.8

0.9

1

Frequency (Hz)10−2 10−1 100 101 102 103

(a)

Phas

e (de

g)

Frequency (Hz)

0

20

40

60

10−2 10−1 100 101 102 103

(b)

Figure 9: (a) Magnitude ratios and (b) phase differences of capacitive electrodes and nonpolarizable electrodes.

the coupling capacitance will change, which results in inac-curate data. The negative capacitance technique is applied toneutralize the input capacitance and stray capacitance, whichis helpful for eliminating the deviation caused by this prob-lem. In order to avoid the saturation in a pure capacitivemea-surement, a feedback loop is designed to provide a bias cur-rent path to ground without reducing the input impedanceof the amplifier.The operating bandwidth ranges from 0.1Hzto 1 kHz via parameter matching between capacitive elec-trode and detection circuit. The power consumption of thedetection circuit is less than 10mW.

Thenew capacitive electrodes responddirectly to the elec-tric field via capacitive coupling. Compared with the conven-tional nonpolarizable electrodes and other metal electrodes,they are easy to be used especially in the very resistive ter-rains, such as desert and permafrost. Unlike the conventionalcapacitive electrodes used in CR systems, the new capacitiveelectrodes can operate in a lower frequency range, whichmeets the requirements of most electrical or electromagneticgeophysical surveys methods. Furthermore, the new capaci-tive electrodes can be seen as point electrodes which can beapplied directly in the existing electrical resistivity surveysystem with no correction in effective dipole length.

Competing Interests

The authors declare no competing interests regarding thepublication of this paper.

Acknowledgments

This research was supported by R&D of Key Instrumentsand Technologies for Deep Resources Prospecting, Grantno. ZDY2012-1-05. The authors are grateful to the help ofBGP Inc., China National Petroleum Corporation, in theverification experiments in the area of Wuerhe in XinjiangAutonomous Region.

References

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[7] V. M. Timofeev, A. W. Rogozinski, and J. A. Hunter, “A newground resistivity method for engineering and environmentalgeophysics,” in Proceedings of the Symposium on the Applica-tion of Geophysics to Engineering and Environmental Problems(SAGEEP ’94), pp. 701–715, 1994.

[8] O. Kuras, D. Beamish, P. I. Meldrum, and R. D. Ogilvy, “Funda-mentals of the capacitive resistivity technique,” Geophysics, vol.71, no. 3, pp. G135–G152, 2006.

[9] M. Neukirch and N. Klitzsch, “Inverting capacitive resistivity(line electrode) measurements with direct current inversionprograms,”Vadose Zone Journal, vol. 9, no. 4, pp. 882–892, 2010.

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[13] D. Groom, “Common misconceptions about capacitively-coupled resistivity (CCR) what it is and how it works,” inProceedings of the 21st EEGS Symposium on the Applicationof Geophysics to Engineering and Environmental Problems(SAGEEP ’08), Philadelphia, Pa, USA, April 2008.

[14] G. A. Oldenborger and A.-M. LeBlanc, “Capacitive resistivityinversion using effective dipole lengths for line antennas,”Journal of Applied Geophysics, vol. 98, pp. 229–236, 2013.

[15] Q. Niu and Y.-H. Wang, “Theoretical and experimental exam-inations of the capacitively coupled resistivity (line antenna)method,” Geophysics, vol. 78, no. 4, pp. E189–E199, 2013.

[16] Q.Niu, Y.-H.Wang, andK. Zhao, “Evaluation of the capacitivelycoupled resistivity (line antenna) method for the characteriza-tion of vadose zone dynamics,” Journal of Applied Geophysics,vol. 106, pp. 119–127, 2014.

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[18] A. F. Marsala, A. D. Hibbs, T. R. Petrov, and J. M. Pendleton,“Six-component tensor of the surface electromagnetic field pro-duced by a borehole source recorded by innovative capacitivesensors,” in Proceedings of the SEG Annual Meeting, pp. 825–829, Society of Exploration Geophysicists, Houston, Tex, USA,September 2013.

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