sensors Article

An Optimized Air-Core Coil Sensor with a Magnetic Flux Compensation Structure Suitable to the Helicopter TEM System Chen Chen 1,2,† , Fei Liu 1,2,† , Jun Lin 1,2 , Kaiguang Zhu 1,2 and Yanzhang Wang 1,2, * 1

2

* †

Key Laboratory of Geo-exploration Instruments, Ministry of Education of China, Changchun 130061, China; [email protected] (C.C.); [email protected] (F.L.); [email protected] (J.L.); [email protected] (K.Z.) College of Instrumentation and Electrical Engineering, Jilin University, Changchun 130061, China Correspondence: [email protected]; Tel.: +86-431-8850-2125 These authors contributed equally to this work.

Academic Editor: Wilmar Hernandez Received: 29 January 2016; Accepted: 6 April 2016; Published: 12 April 2016

Abstract: The air-core coil sensor (ACS) is widely used as a transducer to measure the variation in magnetic fields of a helicopter transient electromagnetic (TEM) system. A high periodic emitting current induces the magnetic field signal of the underground medium. However, such current also generates a high primary field signal that can affect the received signal of the ACS and even damage the receiver. To increase the dynamic range of the received signal and to protect the receiver when emitting current rises/falls, the combination of ACS with magnetic flux compensation structure (bucking coil) is necessary. Moreover, the optimized ACS, which is composed of an air-core coil and a differential pre-amplifier circuit, must be investigated to meet the requirements of the helicopter TEM system suited to rapid surveying for shallow buried metal mine in rough topography. Accordingly, two ACSs are fabricated in this study, and their performance is verified and compared inside a magnetic shielding room. Using the designed ACSs, field experiments are conducted in Baoqing County. The field experimental data show that the primary field response can be compensated when the bucking coil is placed at an appropriate point in the range of allowed shift distance beyond the center of the transmitting coil and that the damage to the receiver induced by the over-statured signal can be solved. In conclusion, a more suitable ACS is adopted and is shown to have better performance, with a mass of 2.5 kg, resultant effective area of 11.6 m2 (i.e., diameter of 0.496 m), 3 dB bandwidth of 66 kHz, signal-to-noise ratio of 4 (i.e., varying magnetic field strength of 0.2 nT/s), and normalized equivalent input noise of 3.62 nV/m2 . Keywords: helicopter TEM system; ACS; magnetic flux compensation structure; optimization of the ACS

1. Introduction The helicopter transient electromagnetic (TEM) system has been widely used in the field of exploration over the last decade. A number of helicopter TEM systems have been developed, including AeroTEM, SkyTEM, HoisTEM, VTEM, and others [1–4]. A helicopter TEM system is composed of a transmitter and receiver; the former is utilized to emit a large bipolar periodic current that can induce the magnetic field signal of the underground medium, and the latter obtains the signal based on Faraday1 s law and then stores the received data [5,6]. The air-core coil sensor (ACS) is the most important component of the receiver to obtain the magnetic field signal [7,8]. To achieve a superior detection performance of the helicopter TEM system suited to rapid surveying for shallow buried metal mine in rough topography, many stringent requirements for ACS must be met. First, the minimum Sensors 2016, 16, 508; doi:10.3390/s16040508

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signal-to-noise ratio (SNR) must meet the given requirement to reduce the deviation of the detection depth caused by ACS [9]. Second, the bandwidth of the ACS should be no less than 50 kHz, which can provide sufficient bandwidth to accurately characterize the shallow part of the crust [10]. Third, the signal acquired by the ACS is needed to avoid the interference from the helicopter in long distance transmissions [11]. Fourth, the selected material to fabricate the frame of the air-core coil should not exhibit magnetism to avoid magnetic interference. In addition, the mass of the ACS should be determined according to the actual situation to maintain the balance of the entire suspension frame. Finally, the induced signal will be larger than the allowable maximum value of the pre-amplifier if no bucking coil is used when the emitting current rises/falls. Electrical damage may occur in the pre-amplifier circuit of the receiver when the pre-amplifier works under an over-saturation state. To prevent damage to the receiver and increase the dynamic range of the secondary field while the emitting current turns off, a magnetic flux compensation structure must be designed [3,12,13]. Thus, the customized design and fabrication of an ACS with optimized performance is necessary. Currently, many commercial ACSs are applied in helicopter TEM systems, such as the MTEM-AL MulTEM Loop and 3D-3LF sensor. The MTEM-AL MulTEM Loop sensor designed by Phoenix Geophysics (Canada) has excellent performance in terms of SNR, with an equivalent area of 100 m2 and a diameter of 1.1 m [13]. However, the matched magnetic flux compensation structure should be oversized for it to be applicable in the helicopter TEM system. The 3D-3LF sensor with the size of 0.6 ˆ 0.6 ˆ 0.2 m3 and a differential structure to transfer signals in long distance can meet the majority of requirements in helicopter TEM systems. Nevertheless, the 3 dB bandwidth of the 3D-3LF sensor is only 28 kHz, which is too narrow for mentioned helicopter TEM systems [14]. As a consequence, an optimized differential ACS with the required SNR, 3 dB bandwidth, and magnetic flux compensation structure to compensate for the above shortcomings should be developed for mentioned helicopter TEM systems. This manuscript: (1) Analyzes the theory of magnetic flux compensation of the helicopter TEM system; (2) designs a magnetic flux compensation structure (bucking coil) suitable to helicopter TEM systems to compensate for the primary field when the emitting current rises/falls; (3) presents an ACS model with a differential structure to reduce the common-mode noise induced in cables during long distance transmission; (4) investigates the optimization of ACS in terms of size, 3 dB bandwidth, SNR, and normalized equivalent input noise; and (5) presents field experiment data to verify the performance of two optimized ACSs with bucking coil. The rest of this paper is structured as follows: Section 2 presents the ACS matched with a bucking coil suitable to the helicopter TEM system. Section 3 illustrates the optimization of ACS based on the theory proposed in Section 2. In Section 4, two physical ACSs are fabricated according to the aforementioned optimized method. To verify the designed ACS, Sections 5 and 6 describe the performances of two ACSs, which are conducted in a magnetic shielding room and in Baoqing County, China, respectively. The final section provides the conclusions and recommends directions for future research. 2. ACS Matched with Bucking Coil The configuration of the helicopter TEM system consists of a transmitting coil, bucking coil, and ACS, which is composed of an air-core coil and pre-amplifier, as shown in Figure 1. A transmitting coil with a large diameter emits a bipolar trapezoidal wave to stimulate the magnetic field of the underground medium. The bucking coil is coincident-coplanar in geometry with the transmitting coil. In addition, the current in the bucking and transmitting coils is reversed to fulfill the magnetic flux compensation for ACS. Moreover, the ACS should be placed in the middle of the bucking coil. The air-core coil receives the magnetic field signal of the underground medium based on Faraday1 s law. Meanwhile, the magnetic field signal is amplified by the differential pre-amplifier and is transmitted in long distance at a high common-mode rejection ratio.

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Figure 1. Helicopter transient electromagnetic (TEM) system configuration. Figure 1. transient electromagnetic (TEM) system configuration. Figure 1. Helicopter Helicopter transient electromagnetic (TEM) system configuration.

Analysis of the Eccentric Bucking 2.1.2.1. Analysis of the Eccentric Bucking CoilCoil 2.1. Analysis of the Eccentric Bucking Coil view suspension bracket is shown Figure transmitting (black TheThe toptop view of of thethe suspension bracket is shown in in Figure 2a.2a. TheThe transmitting coilcoil (black Theand top view of the suspension bracket iscoincident-coplanar shown in Figure 2a.inThe transmitting coil (black circle) circle) and the bucking (red circle) coincident-coplanar in geometry with reversed circle) the bucking coilcoil (red circle) areare geometry with thethe reversed and the bucking coil (red circle) are coincident-coplanar in geometry with the reversed currents I and TThe currents IT and B. These coils have T and B turns with radii T and B, respectively. currents IT and IB. IThese coils have NT Nand NB Nturns with thethe radii RT Rand RB,Rrespectively. The IBtransmitting . These coilscoil have NBaturns with the radii RT and RBwhen , respectively. Thecurrent transmitting coil can T and transmitting coil can generate a large primary magnetic field when emitting current rises/falls, canN generate large primary magnetic field thethe emitting rises/falls, generate a large primary magnetic field when the emitting current rises/falls, which overwhelms the which overwhelms available magnetic field signal or even destroys receiver. bucking which overwhelms thethe available magnetic field signal or even destroys thethe receiver. TheThe bucking coilcoil available magnetic field signal or even destroys the receiver. The bucking coil is utilized to cancel the utilized cancel large primary magnetic field based theory magnetic flux is is utilized to to cancel thethe large primary magnetic field based on on thethe theory of of magnetic flux large primary magnetic field based on the theory of magnetic flux compensation. compensation. compensation.

dlPdlP

dlQdlQ (a) (a)

(b)(b)

Figure 2. Top view of the suspension bracket and the magnetic field intensity. view of the Figure 2. Top Top view of bracket and thethe magnetic field intensity. (a) (a) Top view of the Figure 2. view of the thesuspension suspension bracket and magnetic field intensity. (a)Top Top view of the suspension bracket; (b) The magnetic field intensity of the suspension bracket. suspension bracket; (b) The magnetic field intensity of the suspension bracket. suspension bracket; (b) The magnetic field intensity of the suspension bracket.

seen in Figure according to the right-hand rule, magnetic field intensity M generated As As seen in Figure 2b,2b, according to the right-hand rule, thethe magnetic field intensity BM Bgenerated Asthe seen in Figure 2b, according right-hand rule, the magnetic field intensity Bintensity M generated transmitting current points out of the plane. Correspondingly, magnetic field by by the transmitting current points outto ofthe the plane. Correspondingly, thethe magnetic field intensity BA BA by the transmitting current points out ofcoil the plane. Correspondingly, the magnetic field generated current bucking coil points into plane. total magnetic flux of generated by by thethe current in in thethe bucking points into thethe plane. TheThe total magnetic flux ofintensity thethe Bbucking byinside thecan current in the bucking coil points into the plane. The total two magnetic flux of A generated bucking can obtained difference between aforementioned two magnetic coilcoil inside be be obtained by by thethe difference between thethe aforementioned magnetic the bucking coil inside can be obtained by the difference between the aforementioned two magnetic field intensities. field intensities. field intensities. First, analyzing magnetic field intensity at point M, the z-axis between transmitting First, analyzing thethe magnetic field intensity at point MM BM,Bthe z-axis between thethe transmitting First, analyzing the magnetic field at point M BM , the z-axis between transmitting bucking coils is not considered tointensity be geometrically coincident-coplanar. Thus, the rectangular andand bucking coils is not considered to be geometrically coincident-coplanar. Thus, thethe rectangular and bucking coils is not considered to be geometrically coincident-coplanar. Thus, the rectangular

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coordinate system xT , yT is established in Figure 2b. At point M, the magnetic field intensity BM can be obtained as ¿ dlQ ˆ RQM px T , y T q µ0 ¨ IT ¨ (1) B M px T , y T q “ NT ¨ 4π RQM 3 CT where µ0 “ 4π ¨ 10´7 N{A2 is the permeability of the vacuum, lQ is an infinitesimal segment at point Q, RQM (xT , yT ) is a vector from Q to M with the length RQM , and CT is the path of the line integral of the transmitting coil, described as x2 T ` y2 T “ R T 2 . The magnetic flux φBS∆R of the infinitesimal area is only a part of the magnetic flux of the infinitesimal annulus S∆ R , which can be calculated as ` ˘ φBS∆R “ B M ¨ k ¨ S∆R “ B M ¨ k ¨ π Ri`1 2 ´ Ri 2 µ ¨ N ¨ I ¨ k ¨p R 2 ´ R i 2 q u dl ˆ R px ,y q “ 0 T T 4 i`1 ¨ CT Q rQA 3 T T

(2)

QM

where k “ πθ . The magnetic flux φTr caused by the transmitting coil through the circle with the radius r can be calculated as ż DTR `r ¿ RQM px T , y T q ˆ dlQ µ0 ¨ NT ¨ IT dR (3) φTr “ ¨ k ¨ R¨ 2 RQM 3 DTR ´r CT Second, with respect to the magnetic field intensity BA , another coordinate xB , yB is established. The value of BA can be calculated by Biot Savart law, as shown in Equation (4): B A px B , y B q “ NB ¨

µ0 ¨ IB ¨ 4π

¿ CB

dl P ˆ rPA px B , y B q r PA 3

(4)

where lP is an infinitesimal segment at point P, rPA px B , y B q is a vector from P to A with the length r PA , and CB is the path of the line integral of the bucking coil , described as x2 B ` y2 B “ R B 2 . The value of the magnetic field intensity at the circle ring of ri is equal to BA because of its geometric symmetry. Thus, the magnetic flux φBS∆r through the infinitesimal annulus S∆ r can be computed as ´

2

φBS∆r “ B A ¨ S∆r “ B A ¨ π ri`1 ´ ri

2

¯

` ˘ ¿ µ0 ¨ NB ¨ IB ¨ ri`1 2 ´ ri 2 dl P ˆ rPA px B , y B q ¨ “ 4 r PA 3 CB

(5)

The magnetic flux φBr of the circle area with the radius r is the sum of φBS∆r , which can be calculated as żr ¿ µ0 ¨ NB ¨ IB rPA px B , y B q ˆ dl P ¨ r¨ dr (6) φBr “ 2 r PA 3 0 CB As a consequence, the total magnetic flux φr through the circle with the radius r can be described as φr

“ φBr ´ φTr “

µ0 ¨ NB ¨ IB 2

¨

rr 0

r¨

u

CB

rPA px B ,y B qˆdl P µ ¨N ¨ I dr ´ 0 2T T r PA 3

¨

r DTR `r DTR ´r

k ¨ R¨

u

CT

R QM px T ,y T qˆdlQ dR R QM 3

(7)

Hence, the bucking coil with the reversed current can compensate for the magnetic flux completely under certain specifications. The total magnetic field of ACS is greatly neutralized, and the ACS has better performance and reliability in geophysical exploration. In addition, an approximation method to calculate ΦTr is available if the area of the air-core coil is considerably smaller than that of transmitter loop. The magnetic field intensity BM (xT ,yT ) in the area of radius r is considered as a constant value BOB , which is the magnetic field intensity at the center point. Thus, the magnetic flux ΦTr generated by the transmitting current can be calculated as φTr « BOB ¨ π ¨ r2 , which can replace Equation (3). The error between the approximation result and the accurate result depends on the area ratio of the receiver to the transmitter loop.

the ACS has betteran performance and method reliability geophysical In addition, approximation to in calculate ΦTr isexploration. available if the area of the air-core coil In addition, smaller an approximation method to calculate Tr ismagnetic availablefield if theintensity area of the is considerably than that of transmitter loop. Φ The BMair-core (xT,yT) incoil the isarea considerably than that transmitter loop. magnetic intensity M(xT,yT) in of radius rsmaller is considered as of a constant value BOBThe , which is the field magnetic fieldBintensity atthe the area of point. radiusThus, r is considered as aflux constant value BOB is the magnetic at theas center the magnetic ΦTr generated by, which the transmitting currentfield can intensity be calculated 2 , Thus, center point. the magnetic flux Φ Tr generated by the transmitting current can be calculated as which can replace Equation (3). The error between the approximation result and the Tr  BOB   r Sensors 2016, 216, 508 5 of 17 , which can replace Equation (3). The error between the approximation result and the Taccurate r  BOB    rresult depends on the area ratio of the receiver to the transmitter loop. accurate result depends on the area ratio of the receiver to the transmitter loop. 2.2. 2.2.Equivalent EquivalentElectrical ElectricalModel Modelofofan anAir-core Air-coreCoil Coil 2.2. Equivalent Electrical Model of an Air-core Coil An Anair-core air-corecoil coilwith withaadifferential differentialstructure structureisisdesigned designedtotoreduce reducethe thecommon-mode common-modenoise noise An3).air-core coil with aFigure differential structure designed towound thea common-mode noise (Figure AsAs seen from Figure 3, several turns of wireisof are wound areduce nonmagnetic circular framework (Figure 3). seen from 3, several turns wire areon on nonmagnetic circular (Figure As seen from Figure 3, several turns of wire are wound on a nonmagnetic circular with four3).sections, which can decrease the distributed framework with four sections, which can decrease thecapacitance. distributed capacitance. framework with four sections, which can decrease the distributed capacitance.

(a) (a)

(b) (b)

Figure3.3.(a) (a)An Anair-core air-core coil with a differential structure; (b)typical its typical design (Racc—radius Figure coil with a differential structure; andand (b) its design (Racc —radius of theof Figure 3. (a) An air-core coil with a differential structure; and (b) its typical design (R acc—radius of the air-core coil, N lay—number of layers). air-core coil, Nlay —number of layers). the air-core coil, Nlay—number of layers).

The air-core coil is used to measure the variation of the magnetic field based on Faraday′s law. 1 s law. The air-core air-core coil The coil is is used used to to measure measure the the variation variation of of the the magnetic magnetic field field based based on on Faraday Faraday′s law. d dB 2 dB    n  A dB    n    R acc  dB V   ndφ (8) d dt dB 22 dB dt dt “ VV“´n  n¨  “´n  n¨ AA ¨  ´n  n ¨ π ¨ RRacc acc ¨ (8)(8) dtdt dt dt dt dt where Φ is the magnetic flux through the air-core coil, A and Racc are the area and radius of the where Φ magnetic flux through theturns air-core coil, coil, A and area radius the air-core Φis isthe the magnetic flux through the air-core A Rand Raccthe are theand area and of radius of the acc are air-core coil, respectively, and n is the of wire. coil, respectively, and n is the turns of wire. air-core coil, respectively, n isair-core the turns The electrical modeland of the coilofiswire. composed of the resistor, inductor, and capacitor, as electrical model of of the the air-core air-core coil coil is is composed composed of of the the resistor, resistor, inductor, and capacitor, as Theinelectrical shown Figure 4.model shown in Figure 4.

Figure 4. Electrical model of the air-core coil. Figure 4. Electrical model of the air-core coil. Figure 4. Electrical model of the air-core coil.

where r, L, and C are the resistance, inductance and capacitance of the air-core coil, respectively. whereThe r, L,self-thermal and C are the resistance, inductance and theimpacts air-coreits coil, respectively. noise of the air-core coil is thecapacitance only factor of that resolution according where r,electrical L, and Cmodel. are the resistance, inductance capacitance ofthe the air-coreitscoil, respectively. The self-thermal noise of the air-core coil is only factorbythat impacts resolution according to the Its self-thermal noise Vand t the is generated resistance and is affected by the The self-thermal noise of the air-core coil is the only factor that impacts its resolution according totemperature the electrical asmodel. well. Its self-thermal noise Vt is generated by the resistance and is affected by the to the electrical model. Its self-thermal noise Vt is generated by the resistance and is affected by the temperature as well. temperature as well. g a f a 32 ¨ k B ¨ T ¨ ρr ¨ n ¨ R acc n ¨ π ¨ 2 ¨ R acc f Vt “ 4 ¨ k B ¨ T ¨ r “ e4 ¨ k B ¨ T ¨ ρr ¨ ´ ¯2 “ d π ¨ d2

(9)

where the Boltzman factor k B “ 1.38 ¨ 10´23 W ¨ s{K, T is Kelvin temperature, ρr is the electrical resistivity of the wire, and d is the diameter of the wire.

Vt  4  k B  T  r  4  k B  T   r 

d     2

acc 2



d

23 where the Boltzman factor k B  1.38  10 W  s / K , T is Kelvin temperature,

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resistivity of the wire, and d is the diameter of the wire. Thus, the SNR of the air-core coil is calculated as Thus, the SNR of the air-core coil is calculated as

(9)

r

is the electrical 6 of 17

3 dB V  d   n  Racc 2  3 dt VVt dB  k¨B d T  r ? 32 π 2

SNR 

(10) ¨ n ¨ R acc ¨ “ ´a (10) Vt dt 32 ¨ k B ¨ T ¨ ρr It can be derived from Equation (10) that the SNR of the air-core coil is proportional to the turns of wire with power radius (10) Racc that the power ofof1.5. the requirement It can be half derived fromand Equation the SNR theConsidering air-core coil is proportional to of themagnetic turns of flux compensation and theradius effectRofaccthe ACS on the deviation in detection theflux Racc wire with half power and theself-noise power ofof 1.5. Considering the requirement of depth, magnetic and n of the air-core coil should be limited certain value. requirement details are compensation and the effect of the self-noise of within ACS onathe deviation in The detection depth, the Racc and in Sections 3.1 and In addition, original value V produced by the air-core coil ndiscussed of the air-core coil should be 3.2. limited within a the certain value. The of requirement details are discussed cannot be transmitted distance that a value suitable is the necessary. in Sections 3.1 and 3.2. in Inlong addition, the so original of pre-amplifier V produced by air-coreThe coilmentioned cannot be electrical parameters of the air-core such as self-resistance, inductanceThe andmentioned capacitance, are the transmitted in long distance so that coil, a suitable pre-amplifier is necessary. electrical key factors of that determine the selection of the operational amplifier for the pre-amplifier parameters thecan air-core coil, such as self-resistance, inductance and capacitance, are the key circuit. factors Thedetermine rotation of sensorofinthe Earth′s magnetic field for canthe cause motion noise within a wide that can thethe selection operational amplifier pre-amplifier circuit. 1 s magnetic frequency range of while flying.inThis aimsfield to analyze themotion electrical noise of athe air-core coil. The rotation the sensor Earthsection can cause noise within wide frequency Thus, while the motion is regarded of noise noise and not considered when calculating the range flying.noise This section aimsas to another analyze type the electrical of the air-core coil. Thus, the motion parameter SNR. as another type of noise and not considered when calculating the parameter SNR. noise is regarded SNR “

2.3. 2.3. Equivalent Equivalent Schematic Schematic of of ACS ACS with with aa Differential Differential Pre-Amplifier Pre-AmplifierCircuit Circuit The byby thethe air-core coil iscoil toois low to low transmit in long distance. an equivalent The signal signalproduced produced air-core too to transmit in long Thus, distance. Thus, an schematic of an ACS with a pre-amplifier circuit is suitable to the air-core coil, as shown in equivalent schematic of an ACS with a pre-amplifier circuit is suitable to the air-core coil, as shown Figure 5 [15–17]. The pre-amplifier circuitcircuit is connected to the air-core coil withcoil a differential structure in Figure 5 [15–17]. The pre-amplifier is connected to the air-core with a differential to decrease the common-mode noise. Moreover, the equivalent input noise of ACS is required meet structure to decrease the common-mode noise. Moreover, the equivalent input noise of to ACS is the requirement therequirement mentioned helicopter TEM system. required to meetofthe of the mentioned helicopter TEM system.

Figure 5. 5. Equivalent Equivalent schematic schematic of of ACS ACS with with pre-amplifier pre-amplifier circuit. circuit. Figure

As seen in Figure 5, the integrated operational amplifiers U1, U2, and U3 connected with the As seen in Figure 5, the integrated operational amplifiers U1 , U2 , and U3 connected with the resistors R4, R5, R6, R7, R8, R9, and R10 can regulate an appropriate gain to amplify the low-voltage resistors R4 , R5 , R6 , R7 , R8 , R9 , and R10 can regulate an appropriate gain to amplify the low-voltage signal produced by the air-core coil in the order of a few mV to the operational range for further signal produced by the air-core coil in the order of a few mV to the operational range for further processing in several voltages. R11 and R22 are matched resistors that adjust the working state of the air-core coil in accordance with the requirement of the mentioned helicopter TEM system. The thermal noise of all the resistors and the input voltage noise and input current noise of the integrated operational amplifiersa U1 , U2 and U3 are all illustrated in Figure 5. The thermal noise of the resistor can be described as i R “ 4k B TR, where R is the resistance of the resistor. The en1 , en2 , and en3 and in1 , in2 and in3 are the input voltage noise and input current noise, respectively, of the integrated

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operational amplifiers U1 , U2 , and U3 . The equivalent input noise of ACS at point A or B can then be calculated as b b 2 ` V2 “ 2 ` V2 ` V2 |VAn | “ |VBn | “ VAn Ven (11) Bn Rn in Vn |VAn | “ |VBn | “ ? given the symmetrically differential structure, and Vn is the total equivalent 2 input noise, and Ven is the sum of the voltage noises en1 , en2 , and en3 . Vin is the sum of the current noises in1 , in2 , and in3 . VRn is the total thermal noise of all the resistors. Analyzing point A as an example, the Ven generated by en1 and en3 is expressed as

d ˆ Ven “

2

pen1 q `

en3 G1

˙2 (12)

where G1 is the gain of the integrated operational amplifier U1 . The input current noise Vin produced by in1 and in3 can be described as d ˘2 ` pin1 Zq2 ` in1 Req1 `

Vin “

ˆ

in3 Req2 G1

˙2 (13)

where Z, Req1 , and Req2 can be expressed as Z “ pr1 ` jωL1 q ||

1 r1 ` jωL1 ||R11 “ jωC1 1 ´ ω 2 L1 C1 ` Rr1 ` jωpC1 r1 ` 11

L1 R11 q

(14)

where ω is the angular frequency, ω = 2πf. Furthermore, Req1 “ R4 ||

R5 R4 R5 “ 2 R5 ` 2R4

Req2 “ R7 ||R9 “

R7 R9 R7 ` R9

(15) (16)

Finally, the thermal noise VRn yielded by all the resistors can be obtained as c VRn “ “

` ˘2 ` ˘2 ´ i R ¯2 ´ i R ¯2 pir1 Zq2 ` pi R11 Zq2 ` i R4 Req1 ` i R5 Req1 ` R7G eq2 ` R9G eq2 1 1 d ˙ ˆ 4kT

r1 R11 Z2 r1 ` R11

`

p R4 ` R5 q Req1 2 R4 R5

`

p R7 ` R9 q Req2 2 R7 R9 G12

(17)

? ? The total equivalent input noise Vn can be calculated as Vn “ 2 ¨ VAn “ 2 ¨ VBn , according to the complete symmetrical structure of the pre-amplifier circuit. Thus, the noise contributions from each component can guide the optimization of the circuit performance. In summary, the above analysis presents all these aspects. The magnetic flux compensation structure (bucking coil) is designed to create a zero-primary field area for ACS. In addition, the specifications of the air-core coil and the noise sources of ACS both significantly impact the exploration results of the helicopter TEM system. Thus, the optimization of the aforementioned aspects of ACS is of great importance. 3. Specifications for the Optimization of the ACS 3.1. Optimization of the Geometric Location of the Bucking Coil To avoid covering the available magnetic field signal or even destroying the receiver, the radius of the air-core coil must meet the helicopter TEM system1 s requirement that the bucking coil can compensate for the magnetic flux completely, Φr = 0. A helicopter TEM suspension bracket is given

exploration results of the helicopter TEM system. Thus, the optimization of the aforementioned aspects of ACS is of great importance. 3. Specifications for the Optimization of the ACS of the Geometric Location of the Bucking Coil Sensors 2016,3.1. 16, Optimization 508 8 of 17 To avoid covering the available magnetic field signal or even destroying the receiver, the radius of the air-core coil must meet the helicopter TEM system′s requirement that the bucking coil can compensatecoil for the magnetic r = 0. AN helicopter TEM bracket given RB = 0.6 with a transmitting (i.e., radiusflux RTcompletely, = 6 m andΦturns a suspension bucking coil (i.e.,is radius T = 5) and with a transmitting coil (i.e., radius RT = 6 m and turns NT = 5) and a bucking coil (i.e., radius RB = 0.6 m m and turns NB = 1). The bucking coil can be shifted at a distance (DTR ) from 4.62 m to 4.7 m beyond and turns NB = 1). The bucking coil can be shifted at a distance (DTR) from 4.62 m to 4.7 m beyond the the center center of theoftransmitting the transmittingcoil. coil. Thus, theThus, magnetic flux Φ the coilis is computed according to Equation (7) by the magnetic flux Φr inside thebucking bucking coil computed according to Equation (7) by r inside changing D TR, as shown in Figure 6. changing D , as shown in Figure 6. TR

Figure 6. Contour map of the magnetic flux Φr versus DTR and the circle radius Racc.

Figure 6. Contour map of the magnetic flux Φr versus DTR and the circle radius Racc . In Figure 6, the value of Φr inside the bucking coil can be separated into two components, where one is6, Φthe r > 0 to illustrate that the magnetic flux caused by the bucking coil overwhelms the magnetic In Figure value of Φr inside the bucking coil can be separated into two components, where flux induced by the transmitting coil, and the other is reversed. The red line represents that bucking one is Φr >coil 0 to illustrate that the magnetic flux caused by the bucking coil overwhelms the magnetic compensating for the magnetic flux exactly, where Φr = 0. In consideration of the actual flux induced by the transmitting coil, andfrom the points other A is to reversed. The red represents bucking coil requirement of DTR, only the values B are available. As aline consequence, thethat radius R acc of the air-core coil can be determined as compensating for the magnetic flux exactly, where Φr = 0. In consideration of the actual requirement of DTR , only the values from points to B are available. Asmaconsequence, of the air-core subject to RA 4.62 DTR  4.7 m} the radius Racc (18) acc { values r  0, coil can be determined as where 0.15 m ≤ Racc ≤ 0.25 m according to Figure 6.

subject to ofRthe P t values acc Air-Core 3.2. Geometric Optimization Coil φr

“ 0,

4.62 m ď DTR ď 4.7 mu

(18)

The impact of the self-noise of ACS on the deviation of the detection depth should not exceed

where 0.152% mwhen ď Racc ď 0.25 m according to Figure 6. the half-space earth electrical conductivity is set at 0.01 S/m. Accordingly, the SNR of the

air-core coil should be larger than 4 when the varying magnetic field is 0.2 nT/s. Thus, the optimum

3.2. Geometric Optimization the Air-Core Coil (10) are as follows: values of n and Racc of according to Equation

The impact of the self-noise of ACS on the deviation dB of the detection depth should not exceed subject to SNR(n, Racc )  4，when  0.2 nT / s (19) dt 2% when the half-space earth electrical conductivity is set at 0.01 S/m. Accordingly, the SNR of the air-core coil should be map larger than the varying 0.2 Thus, , isnT/s. shown in Figurethe 7. optimum A contour of the SNR4ofwhen the air-core coil, whichmagnetic depends onfield n andisRacc values of n and Racc according to Equation (10) are as follows: subject to SNRpn, R acc q ě 4, when

dB “ 0.2 nT{s dt

(19)

A contour map of the SNR of the air-core coil, which depends on n and Racc , is shown in Figure 7. As can be seen in Figure 7, the red line is an equipotential line where SNR = 4. The values above the equipotential line are larger than 4, whereas the values below are smaller than 4. Certainly, the values in the upper area are available. In consideration of the preceding discussion on the magnetic flux compensation of the bucking coil, there is an adaptive region A for Racc from 0.15 m to 0.25 m, which is marked in the above figure. Thus, the values of n and Racc in region A are the matched parameters of the air-core coil suited to the helicopter TEM system.

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Figure 7. Contour map of signal-to-noise ratio (SNR) versus Racc and n.

As can be seen in Figure 7, the red line is an equipotential line where SNR = 4. The values above the equipotential line are larger than 4, whereas the values below are smaller than 4. Certainly, the values in the upper area are available. In consideration of the preceding discussion on the magnetic flux compensation of the bucking coil, there is an adaptive region A for Racc from 0.15 m to 0.25 m, which is marked in the above figure. Thus, the values of n and Racc in region A are the matched Figure 7. Contour map of signal-to-noise ratio (SNR) versus Racc and n. Figure 7. coil Contour map signal-to-noise ratiosystem. (SNR) versus Racc and n. parameters of the air-core suited to of the helicopter TEM Moreover, the in output voltage induced ACS should line not where be lessSNR than=1430 nVvalues because the As can be seen Figure 7, the red line isfrom an equipotential 4. The above Moreover, the output voltage induced from ACS should not be less than 1430 nV because the receiver has a 24-bit (ADC) with the voltage rangethan of ±12 V. When the the the equipotential lineanalog-to-digital are larger than 4,converter whereas the values below are smaller 4. Certainly, receiver has a 24-bit analog-to-digital converter (ADC)factor withisthe voltage range of ˘12 V. When the magnetic field varies at 0.2 nT/s, and the amplification 620, the resultant equivalent area S of values in the upper area are available. In consideration of the preceding discussion on the magnetic 2. The contour 2, as magnetic field varies at 0.2 nT/s, and the amplification factorblue is 620, the resultant equivalent area S the air-core coil should be larger than 11.6 m line is denoted by S = 11.6 m flux compensation of the bucking coil, there is an adaptive region A for Racc from 0.15 m to 0.25 m, 2 . The contour blue line is denoted by S = 11.6 m2 , of the air-core coil should be larger than 11.6 m shownisinmarked following which is a two-dimensional Figure 7. region A are the matched which in figure, the above figure. Thus, the valuesview of nof and Racc in 2, is in as shown in following figure, of which is a two-dimensional view of Figure 7. the available region A. At In Figure 8, the segment the blue line from M to N, S = 11.6 m parameters of the air-core coil suited to the helicopter TEM system. 2 Figure 8, the segment ofthe theradius blue line M m, to N, S =Racc 11.6 m , ism inat the available region A. pointIn M, the air-core coil has R acc from = 0.152 and = 0.248 point N. because The chosen Moreover, the output voltage induced from ACS should not be less than 1430 nV the At M, the aair-core coil has the Racc =available 0.152 m,value and R0.15 =m, 0.248 mthe at selected point N.RThe Raccpoint = 0.152 littleanalog-to-digital larger than theradius minimum and = chosen 0.248 is receiver hasmais24-bit converter (ADC) with theaccvoltage range of ±12 V.accWhen the Raacc = 0.152 m is a0.25 littlem larger than the minimum available value 0.15are m, and the selected of Racc =Notably, 0.248 is little less than with number of turns 160 and 60, which both multiples 4. magnetic field varies at 0.2 nT/s, and the amplification factor is 620, the resultant equivalent area S of apoints little less 0.25are m with number of turns 160 and 60, which are both multiples of of 4. Notably, points M than and extreme which meetblue the requirements helicopter the air-core coil N should the be larger thanconditions, 11.6 m2. The contour line is denoted by Sthe = 11.6 m2, as M andsystem. N are the extreme conditions, which meet the requirements of the helicopter TEM system. TEM shown in following figure, which is a two-dimensional view of Figure 7. In Figure 8, the segment of the blue line from M to N, S = 11.6 m2, is in the available region A. At point M, the air-core coil has the radius Racc = 0.152 m, and Racc = 0.248 m at point N. The chosen Racc = 0.152 m is a little larger than the minimum available value 0.15 m, and the selected Racc = 0.248 is a little less than 0.25 m with number of turns 160 and 60, which are both multiples of 4. Notably, points M and N are the extreme conditions, which meet the requirements of the helicopter TEM system.

Figure 8. 8. Two-dimensional Two-dimensional image image of of Figure Figure 7. 7. Figure

According to the preceding discussion, the air-core coil with the minimum Racc = 0.152 m, turns According to the preceding discussion, the air-core coil with the minimum Racc = 0.152 m, turns n = 160, and maximum Racc = 0.248 m, n = 60 can fulfill the magnetic flux compensation exactly. n = 160, and maximum Racc = 0.248 m, n = 60 can fulfill the magnetic flux compensation exactly. Comparative experiments are conducted in the latter part of the section to demonstrate the Comparative experiments are conducted in the latter part of the section to demonstrate the difference difference in their performances. in their performances. Figure 8. Two-dimensional image of Figure 7.

3.3. Optimization of the Pre-Amplifier Circuit 3.3. Optimization of the Pre-Amplifier Circuit To decrease equivalentdiscussion, input noise the ACS, the low-noise pre-amplifier is According to the preceding theof air-core coil with the minimum Racc = 0.152 circuit m, turns To decrease the equivalent input noise of the ACS, the low-noise pre-amplifier circuit is required. Twomaximum kinds of operational for magnetic comparison, lowest-voltage noise nrequired. = 160, and Racc = 0.248 amplifiers m, n = 60 are canutilized fulfill the fluxthe compensation exactly. Two kinds of operational amplifiers are utilized for comparison, the lowest-voltage noise operational Comparative experiments are conducted in the latter part of the section to demonstrate the amplifier and the lowest-current noise operational amplifier among the commercial operational difference in their performances. amplifiers. Correspondingly, the lowest-voltage noise operational amplifier AD797 (Analog Devices company) and the lowest-current noise operational amplifier AD795 (Analog Devices company) ? 3.3. Optimization of the Pre-Amplifier Circuit are adopted, where AD797 has the input voltage noise en “ 0.9 nV/ Hz and input current noise To decrease the equivalent input noise of the ACS, the low-noise pre-amplifier circuit is required. Two kinds of operational amplifiers are utilized for comparison, the lowest-voltage noise

operational amplifier and the lowest-current noise operational amplifier among the commercial operational amplifiers. Correspondingly, the lowest-voltage noise operational amplifier AD797 (Analog Devices company) and the lowest-current noise operational amplifier AD795 (Analog Devices are adopted, where AD797 has the input voltage noise en  0.9 nV/ Hz Sensors 2016,company) 16, 508 10 ofand 17 n

input current noise inn  2.0 pA/ Hz , and AD795 has enn  11 nV/ Hz and inn  0.6 fA/ Hz . ? ? ? inMoreover, “ 2.0 pA/theHz, and AD795 en “ 11 nV/ andpre-amplifier in “ 0.6 fA/ circuit, Hz. Moreover, resistors used tohas regulate the gainHz of the especiallythe theresistors resistors R44, Rto55, regulate and R66, should have a high resistance reduce the simulation the used the gainnot of the pre-amplifier circuit,toespecially thethermal resistorsnoise. R4 , R5The , and R6 , shouldofnot have a high resistance to(EIN) reduce noise. simulation of the equivalent noise (EIN) equivalent input noise ofthe thethermal ACS using theThe component parameters in Table input 2 is demonstrated ofinthe ACS9.using the component parameters in Section 4.2 is demonstrated in Figure 9. Figure

Figure 9. 9. Equivalent Equivalent input input noise noise calculated calculated using using AD797 AD797 and and AD745 AD745 Figure Figure 9. Equivalent input noise calculated using AD797 and AD745. .

Obviously, the ACS using AD797 has a lower equivalent input noise than that using AD795. Obviously, the ACS using AD797 has a lower equivalent input noise than that using AD795. Both of them have 1/f noise when the frequency is lower than the corner frequency, 100 Hz. At the Both of them have 1/f noise when the frequency is lower than the corner frequency, 100 Hz. At the frequency bandwidth from 100 Hz to 100 kHz, the equivalent input noise of ACS using AD797 is frequency bandwidth from 100 Hz to 100 kHz, the equivalent input noise of ACS using AD797 is smaller than that using AD795, which are 1.84 nV/? Hz and 11.4 nV/? Hz , respectively. Given the smaller than that using AD795, which are 1.84 nV/ Hz and 11.4 nV/ Hz, respectively. Given the small distribution parameters of the air-core coil (i.e., resistance, inductance, and capacitance), their small distribution parameters of the air-core coil (i.e., resistance, inductance, and capacitance), their contributiontotothe theequivalent equivalentinput inputnoise noisecan canbe beneglected neglectedwhen whenthe thefrequency frequencyisislower lowerthan than10 10kHz. kHz. contribution However, the equivalent input noise slightly rises, along with an increase in the frequency larger However, the equivalent input noise slightly rises, along with an increase in the frequency larger than kHz,indicates which indicates that the distribution of air-core coil affect the 10than kHz,10which that the distribution parametersparameters of air-core coil can affect thecan specification specification of ACS. of ACS. To investigate the noise contributions from the three sources, the input voltage noise, input To investigate the noise contributions from the three sources, the input voltage noise, input current current noise,noise, thermal and equivalent noise of AD797 ACS using AD797 is disassembled noise, thermal andnoise, equivalent input noiseinput of ACS using is disassembled in Figure 10. in Figure 10.

Figure10. 10.Noise Noisesources sourcesofof ofair-core air-corecoil coilsensor sensor(ACS) (ACS)using usingAD797. AD797. Figure 10. Noise sources air-core coil sensor (ACS) using AD797. Figure Rn of the pre-amplifier circuit is almost constant during the whole frequency The thermal noise VRn The thermal noise VRn of the pre-amplifier circuit is almost constant during the whole frequency bandwidth. The input voltage noise Ven en has a similar contribution as the thermal noise VRn Rn, apart bandwidth. The input voltage noise Ven has a similar contribution as the thermal noise VRn , apart from Rn. Moreover, the input from in 1/f noise region, where it is even larger than the thermal noise VRn in 1/f noise region, where it is even larger than the thermal noise VRn . Moreover, the input current current noise Vinin can be neglected when the frequency is lower than 10 kHz, and it rises with an noise Vin can be neglected when the frequency is lower than 10 kHz, and it rises with an increase in the increase in the frequency in the high-frequency bandwidth (>10 kHz). Thus, the thermal noise VRn Rn frequency in the high-frequency bandwidth (>10 kHz). Thus, the thermal noise VRn and input voltage noise Ven are the two major factors that impact the total equivalent input noise Vn . The low-voltage noise operational amplifier AD797 and low-resistance resistor are available approaches to reduce the input voltage noise and thermal noise in the mentioned helicopter TEM system.

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and input voltage noise Ven are the two major factors that impact the total equivalent input noise Vn. The low-voltage noise operational amplifier AD797 and low-resistance resistor are available Sensors 2016, 16, 508 11 of 17 approaches to reduce the input voltage noise and thermal noise in the mentioned helicopter TEM system. From From the the above above discussion, discussion, two two ACSs ACSs with with optimized optimized physical physical and and electrical electrical specifications specifications are are designed designed to to meet meet the the special special requirements requirements of of the the magnetic magnetic flux flux compensation compensation of of the the helicopter helicopter TEM TEM system. system. The The fabrication fabrication of of the the two two ACSs ACSs is is demonstrated demonstrated below. below. 4. Fabrication FabricationofofACS ACS 4. According to to the the requirements requirements of of the the helicopter helicopter TEM TEM system, system, two two ACSs ACSs are are fabricated fabricated in in this this According section. The The ACS ACS is is divided divided into into two twoparts: parts:the theair-core air-corecoil coiland andthe thedifferential differentialpre-amplifier pre-amplifiercircuit, circuit, section. which are represented separately below. below. 4.1. Fabrication Fabrication of of the the Air-Core Air-Core Coil Coil 4.1. The optimization optimization methods methods discussed discussed above above are are used used to to fabricate fabricate two two air-core air-core coils, coils, shown shown in in The Figure 11. Their frame materials are nylon and wood without magnetism, air-core coils A and Figure 11. Their frame materials are nylon and wood without magnetism, air-core coils A and B, B, respectively. respectively.

Figure Figure 11. 11. Two Two air-core air-core coils. coils.

The resultant effective area of air-core coil A (nylon material) is 11.6 m22, and its mass is 4.1 kg. The resultant effective area ofarea air-core A of (nylon material) is 11.6 m , and itsare mass kg. 2 and By contrast, the resultant effective and coil mass air-core coil B (wood material) 11.6ism4.1 2 and By contrast, the resultant effective area and mass of air-core coil B (wood material) are 11.6 m 2.5 kg, respectively. The detailed parameters of the two air-core coils are demonstrated in Table 1. 2.5 kg, respectively. The detailed parameters of the two air-core coils are demonstrated in Table 1. Table 1. Parameters of the two air-core coils. Table 1. Parameters of the two air-core coils.

Parameters Parameters Radius

Air-Core Coil A Air-Core 0.152 mCoil A

Radius Segments Segments Number turns Number ofof turns Diameter of of wire Diameter wire Resistance of air-core coilcoil Resistance of air-core Inductance of air-core coil Inductanceofofair-core air-core Capacitance coilcoil Response frequency Capacitance of air-core coil

0.152 4 m 4 160 160 mm 0.50.5mm 10.46 10.46 ΩΩ 4.05 mH 4.05 mH 773 pF 90 pF kHz 773

Response frequency

90 kHz

Air-Core Coil B Air-Core 0.248 m Coil B 40.248 m 4 60 60 0.5 mm 0.5 mm 3.883.88 Ω Ω 1.14 mH 1.14 mH 283 pF 283280 pF kHz 280 kHz

Both of the air-core coils use the same wire with four segments of frame structure. Air-core coil A of resistance, the air-corecapacitance, coils use theinductance, same wire and withlower four segments of frame structure. coil has aBoth larger response frequency given thatAir-core it has more A has a larger resistance, capacitance, inductance, and lower response frequency given that it has turns than air-core coil B. more turns than air-core coil B. 4.2. Fabrication of the Pre-Amplifier Circuit 4.2. Fabrication of the Pre-Amplifier Circuit A differential pre-amplifier circuit matched with both the two air-core coils is designed. The components utilized in the pre-amplifier circuit are both giventhe in Table 2. A differential pre-amplifier circuit matched with two air-core coils is designed. The components utilized in the pre-amplifier circuit are given in Table 2.

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Table 2. Components of the pre-amplifier circuit. Table 2. Components of the pre-amplifier circuit.

Parameters

Components Components R11 = R22

Parameters 1.3 kΩ

R11 =RR 4 = 22R6 R4 = RR65 R5 7 = R8 R7 =RR 8 9 = R9 =RR 10R10 G G U1 ,U2 U1,U2 U3

1.3 kΩ 1.5 kΩ 1.5 kΩ 100 Ω 100 Ω 1 kΩ1 kΩ 20 kΩ 20 kΩ 620 620 AD797 AD797 THS4131

U3

THS4131

The selected circuitwere werecarefully carefullymatched, matched, which guarantees The selectedcomponents componentsof ofthe the pre-amplifier pre-amplifier circuit which guarantees thethe differential structure. The resistance of R and R are chosen to make the step response 22are chosen to make the step response of of differential structure. The resistance of R11 11 and R22 thethe air-core coil under R55,, and andRR66have havelow lowresistance resistance several hundred air-core coil underthe thecritical criticaldamping dampingstate. state. R R44,, R inin several hundred ohms to reduce the thermal noise. Given that R , R , R and R have a small contribution ohms to reduce the thermal noise. Given that R7 88, R99and R1010have a small contribution to to thethe equivalent input their resistance resistancecan canbebeininkilo-ohms kilo-ohms increase equivalent inputnoise, noise,asasshown shownin inEquation Equation (17), their to to increase thethe input impedance the secondstage stageofofthe thepre-amplifier pre-amplifiercircuit. circuit. The The gain gain of the pre-amplifier input impedance ofof the second pre-amplifier circuit circuit G cancalculate be calculate canGbe as as

 R  R R R  22 44  ¨ 99 GG“ 1 1` R R R55  77  ˆ

˙

(20) (20)

The electrical performances of ACSs, such as frequency response and equivalent input noise, are The electrical performances of ACSs, such as frequency response and equivalent input noise, shown in Section 5. are shown in Section 5. 5. Electrical Performances of ACSs 5. Electrical Performances of ACSs To compare the electrical performances of the fabricated ACSs, experiments are conducted in a To compare the electrical performances of the fabricated ACSs, experiments are conducted in a magnetic shielding room. The experiments include the comparison of the frequency response and magnetic shielding room. The experiments include the comparison of the frequency response and equivalent input demonstratedbelow. below. equivalent inputnoise noiseofofthe theACSs. ACSs.The The results results are are demonstrated 5.1. Comparison of the Frequency Response of ACSs 5.1. Comparison of the Frequency Response of ACSs The 3 dB meet the the requirement requirementofofnonoless lessthan than kHz The 3 dBbandwidth bandwidthofofthe theACS ACS needs needs to to meet 5050 kHz in in thethe helicopter TEM performancesofofthe thetwo twomanufactured manufactured ACSs helicopter TEMexploration. exploration.The The 33 dB dB bandwidth bandwidth performances ACSs areare verified in the shielding room, as shown in Figure 12. verified in the shielding room, as shown in Figure 12. InInFigure ACSs are are capable capableofofmeeting meetingthe the3 3dBdBbandwidth bandwidth Figure12, 12,although although both both of of the the two two ACSs requirement the ACS ACSwith withair-core air-corecoil coilAAhas has a narrower 3 dB requirementofofthe thehelicopter helicopterTEM TEM exploration, exploration, the a narrower 3 dB bandwidth ofof 5353kHz, B has hasaawider wider33dB dBbandwidth bandwidthofof kHz. bandwidth kHz,whereas whereasthe theACS ACSwith with air-core air-core coil B 6666 kHz.

Figure 12. 3 dB bandwidth of the two ACSs. Figure 12. 3 dB bandwidth of the two ACSs.

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5.2. Comparison of the Equivalent Input Noise of ACSs equivalent input noise of the ACSs is illustrated and simulated simulated in Section 3. To To verify the The equivalent simulation results, the equivalent input noise of the ACSs is measured in the magnetic shielding room. results, the equivalent input noise of the ACSs is measured in the magnetic shielding Using ACS with air-core coil B as coil an example, test result compared simulation of the room. Using ACS with air-core B as an the example, the is test result iswith compared withresult simulation equivalent noise, as shown in Figure 13. in Figure 13. result of theinput equivalent input noise, as shown

Figure Figure 13. 13. Equivalent Equivalent input input noise noise of of ACS ACS with with air-core air-core coil coil B. B.

Figure 13 illustrates that the test result of the equivalent input noise of ACS has a good Figure 13 illustrates that the test result of the equivalent input noise of ACS has a good coincidence coincidence with the simulation result. However, the test performance is approximately 2.02 with the simulation result. However, the test performance is approximately 2.02 nV/Hz1/2 , which is 1/2 nV/Hz , which is slightly larger than the simulation result because of the influence of residual slightly larger than the simulation result because of the influence of residual magnetism noise in the magnetism noise in the shielding room. The noise performance of ACS with air-core coil A is 2.86 shielding room. The noise performance of ACS with air-core coil A is 2.86 nV/Hz1/2 , which is higher nV/Hz1/2, which is higher than ACS with air-core coil B, given its larger electrical specifications (i.e., than ACS with air-core coil B, given its larger electrical specifications (i.e., resistance, capacitance, and resistance, capacitance, and inductance). The normalized equivalent input noise (EINnor) of ACS can inductance). The normalized equivalent input noise (EINnor ) of ACS can then be calculated as then be calculated as b EINr2 EI N 2  B (21) B EIN EInorN (21) nor “ S S where B is the 3 dB bandwidth of the ACS and S is the resultant equivalent area of the air-core coil. where B isvalues the 3 dB bandwidth of the ACS and input S is thenoise resultant equivalent area of thewhere air-core The of the normalized equivalent of ACS can be acquired, thecoil. ACS The values of the normalized equivalent input noise of ACS can be acquired, where the ACS with 2 2 with air-core coil B is 2.84 nV/m and that with air-core coil A is equal to 3.74 nV/m . 2 and that with air-core coil A is equal to 3.74 nV/m2 . air-core coil B is 2.84 nV/m The performance of the 3 dB bandwidth and equivalent input noise of ACSs both meet the The performance of the 3TEM dB bandwidth equivalent noisefor of shallow ACSs both meet the requirements of the helicopter explorationand suited to rapid input surveying buried metal requirements of the helicopter TEM exploration suited to rapid surveying for shallow buried metal mine in rough topography. The fabricated ACS with air-core coil B has a wider 3 dB bandwidth and mine inequivalent rough topography. Thethan fabricated ACSair-core with air-core has a wider dB bandwidth and lower input noise that with coil A.coil TheB results show3 that the ACS with lower equivalent noisefor than that within air-core coil A. The results show that the that ACSwith with air-core air-core air-core B is moreinput suitable application the helicopter TEM exploration than B is more suitable for application in the helicopter TEM exploration than that with air-core A. The A. The comparison of field experiments using the two ACSs is performed in Section 6, which verifies comparison of field conclusion. experiments using the two ACSs is performed in Section 6, which verifies the the aforementioned aforementioned conclusion. 6. Field Experiments in Baoqing County 6. Field Experiments in Baoqing County Using the designed ACSs, which are the critical parts in the helicopter TEM system, field Using the designed ACSs, which are the critical parts in the helicopter TEM system, field experiments were conducted in Baoqing County, Heilongjiang Province, China in November 2015. experiments were conducted in Baoqing County, Heilongjiang Province, China in November 2015. The helicopter TEM system and magnetic flux compensation structure are illustrated in Figure 14. The helicopter TEM system and magnetic flux compensation structure are illustrated in Figure 14. In Figure 14, the bucking coil can shift a distance of 0.08 m (4.62 m to 4.7 m) along the shift In Figure 14, the bucking coil can shift a distance of 0.08 m (4.62 m to 4.7 m) along the shift direction. The ground experiments are operated to verify the feasibility of using the magnetic flux direction. The ground experiments are operated to verify the feasibility of using the magnetic flux compensation structure (bucking coil). Moreover, two flights of the helicopter TEM exploration with compensation structure (bucking coil). Moreover, two flights of the helicopter TEM exploration with ACSs are conducted to compare the specifications of the two ACSs by estimating the received ACSs are conducted to compare the specifications of the two ACSs by estimating the received field data. field data.

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Figure 14. Helicopter TEM system and magnetic flux compensation structure. Figure 14. Helicopter TEM system and magnetic flux compensation structure.

6.1. Ground Experiment Using the Magnetic Flux Compensation Structure 6.1. Ground Experiment Using the Magnetic Flux Compensation Structure Structure Air-core coil B is placed at the center of the bucking coil, and the bucking coil can shift at a Air-core coil coilBBisisplaced placed at the center of bucking the bucking coil,the and the bucking at a Air-core at the center of the coil, and bucking coil cancoil shiftcan at ashift distance distance (DTR) of only 0.08 m from 4.62 m to 4.7 m beyond the center of the transmitting coil. The distance (DTR)0.08 of only 0.084.62 m from to 4.7 m center of the transmitting coil. The (D m from m to 4.62 4.7 mmbeyond thebeyond center the of the transmitting coil. The waveform TR ) of only waveform captured from the helicopter TEM system when the center of the bucking coil is shifted to waveformfrom captured from theTEM helicopter system whenofthe of the coil shifted captured the helicopter systemTEM when the center thecenter bucking coilbucking is shifted to is the point to at the point at DTR = 4.685 m is demonstrated in Figure 15. the point at D TR = 4.685 m is demonstrated in Figure 15. DTR = 4.685 m is demonstrated in Figure 15.

Figure 15. Waveform captured from the helicopter TEM system. Figure 15. Waveform captured from the helicopter TEM system. Figure 15. Waveform captured from the helicopter TEM system.

There are three periods of the received waveform. First, period ① indicates the time when the There are three periods of the received waveform. First, period ① indicates the time when the emitting current rises and stays The receiving data are unavailable because they the are 1 indicates the There are three periods of theconstant. received waveform. First, period time when emitting current rises and stays constant. The receiving data are unavailable because they are merged into the rises signal induced by the modulated emitting current. In addition, period ② starts at emitting current and stays constant. The receiving data are unavailable because they are merged merged into the signal induced by the modulated emitting current. In addition, period ② starts at the point wheninduced the emitting begins to decline and at the time when emitting into the signal by the current modulated emitting current. In stops addition, period startsthe at the point the point when the emitting current begins to decline and stops at the time 2when the emitting currenttheis emitting cut off. current The voltage is to equal to 7.5 V,stops which is induced by the the emitting emitting current current.isThe when begins decline and at the time when cut current is cut off. The voltage is equal to 7.5 V, which is induced by the emitting current. The induced high voltage will decrease the dynamic range of signal or even destroy the receiver under off. The voltage is equal to decrease 7.5 V, which inducedrange by theofemitting The induced high voltage induced high voltage will the is dynamic signal orcurrent. even destroy the receiver under somedecrease severe conditions. Finally, the waveform atdestroy periodthe ③receiver is the magnetic fieldsevere response of the will the dynamic range of signal or even under some conditions. some severe conditions. Finally, the waveform at period ③ is the magnetic field response of the underground mediumat when no emitting current exists. 3 is the magnetic Finally, the waveform period field response of the underground medium when underground medium when no emitting current exists. By contrast, theexists. bucking coil is adjusted at the appropriate point, DTR = 4.693 m. The result of the no emitting current By contrast, the bucking coil is adjusted at the appropriate point, DTR = 4.693 m. The result of the received waveform isbucking shown in Figure 16. at the appropriate point, DTR = 4.693 m. The result of the By contrast, the is adjusted received waveform is shown coil in Figure 16. received waveform is shown in Figure 16.

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Figure 16. Waveform captured from the helicopter TEM system.

Figure 16. Waveform captured from the helicopter TEM system.

The primary field response caused by the emitting current canTEM be compensated for nearly up to Figure 16. Waveform captured from the helicopter system.

The field response caused by the emitting current canbe beused compensated for nearly up to zero.primary Under the actual condition, the secondary field signal cannot for data interpretation because of the position deviation between the bucking coil and the transmitter loop when on The primary field response caused by the emitting current can be compensated for nearly upthe to zero. Under the actual condition, the secondary field signal cannot be used for data interpretation ground and during flight. zero. Under the actual condition, the secondary field signal cannot be used for data interpretation because of the position deviation between the bucking coil and the transmitter loop when on the Theduring comparison shows that the primary response whenwhen the bucking because of the position deviation between thefield bucking coil can andbe thecompensated transmitter loop on the ground and flight. coil is placed at an appropriate point. This procedure results in the following benefits of using the ground and during flight.that the primary field response can be compensated when the bucking The comparison shows coil bucking coil. First, the over-saturated induced signal can be reduced to protect the receiver. Second, The comparison shows that the primary field response can be compensated when the bucking is placed at an appropriate point. This procedure results in the following benefits of using the bucking the high magnification factor ofpoint. the pre-amplifier canresults be utilized of benefits the reduction in the coil is placed at an appropriate This procedure in thebecause following of using coil. bucking First, the over-saturated induced signal can be reduced to protect the receiver. Second, the high magnitude the primary field signal.induced This reduction increases the dynamic range of the secondary coil.ofFirst, the over-saturated signal can be reduced to protect the receiver. Second, magnification factor of the factor pre-amplifier can be utilized because the reduction in the magnitude of fieldhigh signal. the magnification of the pre-amplifier can be utilizedofbecause of the reduction in the the primary field signal. This reduction increases the dynamic range of the secondary field signal. magnitude of the primary field signal. This reduction increases the dynamic range of the secondary 6.2. Field Experiments Using Two ACSs field signal.

6.2. Field Experiments Using Two ACSs

Field experiments were conducted in Baoqing County. The suspension bracket is about 40 m 6.2. Field Experiments Using Two ACSs above experiments the ground, which drawn by in a helicopter with a flyThe speed of 40 knots. To compare the 40 m Field were isconducted Baoqing County. suspension bracket is about specifications of which the twowere the by helicopter flight is conducted theofsame surveyisline of about experiments conducted Baoqing County. suspension about 40 m the above theField ground, isACSs, drawn a in helicopter with a The fly over speed 40bracket knots. To compare twice the km. normalization result of the received data given below. above the10 ground, which is drawn by a helicopter a flyisspeed of 40 knots. To compare theabout specifications of the The two ACSs, the helicopter flight iswith conducted over the same survey line of A total of 16 sampling channels in theflight normalization results are shownsurvey in Figure 17.about The specifications of the two ACSs, the helicopter is conducted over the same line of twice the 10 km. The normalization result of the received data is given below. channels with a The smallnormalization number correspond the data data received earlier, which indicate the signal twice the 10 km. result ofwith the received is given below. A total of 16 sampling channels in the normalization results are shown in Figure 17. The channels intensity of the underground medium. in The ones represent the are datashown received A total of 16 sampling channels thelarge normalization results in later, Figureespecially 17. The with the a small number correspond with the datathe received earlier, which indicate the signal intensity of last three which are deemed as background noise.earlier, which indicate the signal channels withchannels, a small number correspond with the data received the underground The medium. large ones represent the data received especially the last three intensity of themedium. underground The large ones represent the datalater, received later, especially channels, which are deemed as the background noise. the last three channels, which are deemed as the background noise.

Figure 17. Normalization result of the received data with air-core coil B. Figure 17. Normalization result of the received data with air-core coil B.

Figure 17. Normalization result of the received data with air-core coil B.

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The normalization results of received data using air-core coil B are as follows: maximum signal of The normalization results of received data using air-core coil B are as follows: maximum signal 3600 of nT/s, noisenoise of 32ofnT/s, andand ratio of maximum signal toto background 3600background nT/s, background 32 nT/s, ratio of maximum signal backgroundnoise noise of of112.5. Comparatively, the normalization results results using air-core coil Acoil areAgiven in Figure 18.18. 112.5. Comparatively, the normalization using air-core are given in Figure

Figure 18. Flight line with air-core coil A.

Figure 18. Flight line with air-core coil A.

As can be seen from Figures 17 and 18, the profiles of the two figures are similar and can be used to interpret information on17 theand underground medium. Thetwo normalization using As can be seen from Figures 18 the profiles of the figures areresults similar andair-core can be used A are asinformation follows: the larger maximum signal is 3800 nT/s, thenormalization larger background noiseusing is 38 nT/s, andA are to interpret on the underground medium. The results air-core the ratio of the maximum signal to background noise is 100. In addition, air-core coil B with a larger as follows: the larger maximum signal is 3800 nT/s, the larger background noise is 38 nT/s, and the can obtain a higher-quality signal of the underground in air-core helicopter TEM exploration. ratioSNR of the maximum signal to background noise is 100. Inmedium addition, coil B with a larger SNR From the above discussion, the magnetic flux compensation structure with ACS can can obtain a higher-quality signal of the underground medium in helicopter TEM exploration. compensate for the primary field response as expected, can increase the dynamic range, and protect From the above discussion, the magnetic flux compensation structure with ACS can compensate the receiver in helicopter TEM exploration. Moreover, field experiments are performed using the for the field response as meet expected, can increase dynamic range, and protect theACS receiver twoprimary optimized ACSs that both the requirements of the helicopter TEM exploration, and the in helicopter TEM exploration. Moreover, field experiments are performed using the two optimized with a higher SNR provides a higher-quality signal. Thus, the ACS with a higher SNR incorporating ACSsthe that both meet requirements of helicopter TEM exploration, and the ACS with a higher SNR magnetic flux the compensation structure is available for helicopter TEM exploration.

provides a higher-quality signal. Thus, the ACS with a higher SNR incorporating the magnetic flux 7. Conclusions and Prospects compensation structure is available for helicopter TEM exploration. For this study, ACSs combined with a magnetic flux compensation structure (bucking coil) are

7. Conclusions and Prospects designed, fabricated, and tested to remove the primary field response when the emitting current

rises/falls in helicopter exploration. physicalflux andcompensation electrical models of the air-core coil are For this study, ACSs TEM combined with aThe magnetic structure (bucking coil) are introduced, and its parameters meet the requirements of the helicopter TEM system suited to rapid designed, fabricated, and tested to remove the primary field response when the emitting current surveying for shallow buried metal mine in rough topography. A differential pre-amplifier circuit rises/falls in helicopter TEM exploration. The physical and electrical models of the air-core coil are suitable to the air-core coil transfers the signal in a long distance. The two ACSs are fabricated with introduced, and its parameters meetincluding the requirements of effective the helicopter system suited to rapid optimized electrical performances, the resultant area, 3 TEM dB bandwidth, SNR, and surveying for shallow buried rough differential pre-amplifier circuit normalized equivalent inputmetal noise,mine whichinare testedtopography. in a magneticAshielding room. Moreover, the suitable the air-core transfers the signal in a longshow distance. The two ACSs are fabricated fieldto experiment datacoil implemented in Baoqing Country that the primary field response can be with compensated for absolutely when including the buckingthe coilresultant is placed at an appropriate and the damage optimized electrical performances, effective area, 3 point, dB bandwidth, SNR, and to the receiver induced by the over-statured signal can be Eventually, a more suitable ACS the normalized equivalent input noise, which are tested in removed. a magnetic shielding room. Moreover, adopted anddata is shown to have better performance, withshow a massthat of 2.5 resultantfield effective area ofcan be field isexperiment implemented in Baoqing Country thekg,primary response 11.6 m2 (i.e., of 0.496 m),the 3 dB bandwidth ofplaced 66 kHz,atsignal-to-noise ratio of 4 (i.e., compensated fordiameter absolutely when bucking coil is an appropriate point, andvarying the damage magnetic field strength of 0.2 nT/s), and normalized equivalent input noise of 3.62 nV/m2. to the receiver induced by the over-statured signal can be removed. Eventually, a more suitable ACS is All of the parameters of the ACS, such as mass, resultant effective area, 3 dB bandwidth, SNR, adopted and is shown to have better performance, with a mass of 2.5 kg, resultant effective area of and normalized equivalent input noise, can be changed according to the requirements of the 2 (i.e., diameter of 0.496 m), 3 dB bandwidth of 66 kHz, signal-to-noise ratio of 4 (i.e., varying 11.6 m mentioned helicopter TEM exploration based on the proposed optimized procedure. Furthermore, 2. magnetic field strength of 0.2 nT/s), and normalized equivalent of 3.62the nV/m to improve the quality of the received signal, the ability of theinput ACS noise to suppress pendulum All of the parameters ACS, such as mass, effective area, dB bandwidth, SNR, and motion noise caused byof thethe oscillation of the towed resultant bird should be studied in3future work.

normalized equivalent input noise, can be changed according to the requirements of the mentioned helicopter TEM exploration based on the proposed optimized procedure. Furthermore, to improve the

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quality of the received signal, the ability of the ACS to suppress the pendulum motion noise caused by the oscillation of the towed bird should be studied in future work. Acknowledgments: This research was supported by the National High Technology Research and Development Program of China (No. 2013AA063904), the National Natural Science Foundation of China (NO. 61403160), the China Postdoctoral Science Foundation (No. 2014M551194), and Science and Technology Office, Jilin Province of China (Nos. 20140203015GX, 20140520118JH, and 20150414052GH). Author Contributions: C.C., Y.W., J.L. and K.Z. conceived and designed the research. C.C. and F.L. performed the research. C.C., Y.W. and F.L. wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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