mineral prospect identification using inducedpolarization

Adera Nurul et al.

ARTIKEL RISET

Mineral Prospect Identification UsingInducedpolarization And Very Low Frequency-EMMethods At Sangon, Kalirejo Village, KokapSub-District, Kulonprogo Regency, SpecialRegion Of Yogyakarta, IndonesiaAdera Nurul, Arga Kunang, Arief Khoiruddin, Arif Fikri, Benedicta Nathania, Dobrak Tirani, EnricoDiofano, Haryo Diofano, Haryo Satrio, Hesti Nilamprasasti, Ihsan Median, Indah Sasmita, Ivona Annisa,Jum Satriani, Laila A, Mariana Kambey, M. Syahdan, Oktavianus Eko, Rahmat Hidayat, Ramadhani,Satrio P, Siti Suci, Zeni A, Zukhruf Delva and Budi Arjo*

Abstract

A research of induced polarization and very low frequency method was carried out at Sangon Village, Kulonprogoto identify mineralization zone. The result of these methods were able to be used to support each other.Mineralization zone is shown by high resistivity and high chargeability area of induced polarization method, andis also shown by high electric current density area of very low frequency method. Interpretation of resistivity andchargeability model shows that mineralization zone is mostly located at the depth below 10 m from surface. Onthe other hand, interpretation of electric current density models at depth 10 m and 20 m show that mineralizationzone is not well distributed respect to alteration zone, which is the character of low sulfidation epithermal type,mostly at north-south direction.

Keywords: mineralization; Induced polarization; VLF-EM; low sulfidation

I. IntroductionThe research area is located at Sangon Village,about 20 km from Yogyakarta. According to itsphysiography, the research area is included inthe Kulon Progo Mountains physiography,whereindications of alteration and gold mineralization arefound. It has proven, there are many people thatoperate the tradisional mine. Gold mineral depositionoccurs in fractures form hydrothermal vein containsPyrite, Chalcopyrite, Spaleritic, Galena associatedwith Gold.

Mineralization processes of this area occured ingold-copper system of low sulfidation epithermal type[1]. Control of structures and lithologies dominatedby intrusive and volcanic rocks cross by sinistralstrike slip fault and normal fault. The Deformation

*Correspondence: [email protected]

Geophysics Sub-Department, Physics Department, Faculty of Mathematics

and Natural Sciences, Universitas Gadjah Mada, Yogyakarta, Indonesia

Full list of author information is available at the end of the article†Equal contributor

resulted fracture filled by hydrothermal fluid, occurslow sulfidation epithermal mineralization process. Thealteration formed is Propyliticand Argillic.

The use of geophysical methods for exploration ofmetallic ores is quite common. This research combinesVery Low Frequency-EM (VLF-EM) using NWCtransmitter and Time Domain Induced Polarization.The VLF-EM mapping have reflected conductiveanomalies that are sulphide zone and alterationto confirmed from existing geology data. VLF-EMline section would be prime areas to look for goldmineralization. In order to identify and exploredisseminated metal and sulfide mineralization zones,induced polarization(IP) measurement conductedusing dipole–dipole configuration. Inversion of IPdata enables to visualize of the subsurface. InducedPolarization measurement were conducted at 2different location, Western Block and East Block.Based on VLF-EM and IP method brought integratedmodeling to confirm each other on the identification ofgold mineralization zone within 2D model.

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II. Basic TheoryA. Induced Polarization

Resistivity method basically aims to determinethe subsurface resistivity distribution by makingmeasurements on the ground surface. This methodutilizes DC current which is injected through twometallic electrodes at the surface and potentialdifference as a result of the current flow in thesubsurface is measured by two potential electrodes(∆V ) at the surface. However, by using InducedPolarization (IP) method, we are also able to detectconductive minerals of very low concentrations thatmight be missed by resistivity or EM surveys.

The IP method deals with chargeability whichindicates the strength of polarization effects causedby ions in the vicnity of metallic grains rock. Themeasurement used in time-domain where decayingvoltage measured after the current is switched off asshown in Figure 1. Chargeability is defined as the areabeneath the decay curvey over a certain time intervalnormalized by the steady-state potential difference.

M =A

∆V=

1

∆V

∫ t2

t1

v(t) dt (1)

Figure 1: Chargeability in time domainIP measurement[2]

Apparent chargeability shows the duration of thepolarization effect shortly after the current is switchedoff. If delay time takes a long time, chargeability valuewill be great so that could be indicated as counductivemineral.

Dipole-dipole array was used to minimize theelectromagnetic coupling with geometry factor as

K = πan(n+ 1)(n+ 2) (2)

K is a geometric factor that depends on thearrangement of the four electrodes, a is a spacebetween two electrodes, and n is space ratio betweencurrent electrode and potential electrode.

B. Very Low Frequency

Very Low Frequency (VLF) is one of geophysicalmethods which uses electromagnetic wave originatedfrom low frequency-high power radio wave transmitterall around the world ranging 15-30 kHz. Thoseconditions make VLF a passive method. Its principaldepends on the propagation of electromagnetic wavedefined by Maxwell equation as follows:

∆ D = ρf (3)

∆ B = 0 (4)

∇×E = −∂B∂t

(5)

∇×H = J +∂D

∂t(6)

(7)

with E→ being the electric field vector (V/m),ρ electric charge density (C/m3), B→ magneticinduction vector (Tesla atau Weber/m2), D→

electrical displacement (C/m2), H→ magnetic field(A/m), J electric current density (A/m2).

Electromagnetic field measured by VLF-EM deviceis total complex field comprises of real component(inphase), imaginary (quadrature), total-field, andtilt-angle. The measured value of those fourcomponents are highly depend on the conductivity ofsubsurface target. Primary magnetic field componentcan be considered as a wave that horizontallypropagated. If there is a conductive subsurface target,the magnetic field component from electromagneticwave will induce that medium resulting inductioncurrent (Eddy Current), the bigger Eddy Current is,the bigger the charge density. Primary electromagneticfield of a certain VLF-EM radio transmitter hasvertical electric field component and perpendicularmagnetic field respective to the propagating directionat X axis [3]. The wave’s propagation is shown in thefollowing illustration 2.

The direction of the propagating wave hasperpendicular against target’s strike, with ±45

tolerance of deviating direction that will beautomatically corrected by the device.

III. MethodologyInduced Polarization

Two different instruments were used in this research;Syscal Jr and Ares Resistivitymeter. The acquisitiontake place on the quartz vein. The electrodeand porous pot were arranged in Dipole-Dipoleconfiguration. The measurement used n=6 and a=20meter. The measurement were done by injectingcurrent to the ground via two current electrodes


Figure 2: Electromagnetic wave propagationfrom transmitter generating Hp. Hp fieldgenerates Hs field when interacting withconductive material. From Hp and Hs value, tiltangle values is obtained to interpret conductivezone [4].

(A and B) and measure the potential differencevia two porous pots (M and N), seconder voltagewhen the current stopped, time needed for thevoltage decay. The result from Syscal Jr are averagevoltage (V) measured, chargeability (M), current(I), and Self Potential. Meanwhile the result fromAres Resistivitymeter are chargeability value fromeach window (before averaging). In this research, thenumber of windows used is 3.

Physical parameters that used in the processingare resistivity and chargeability values obtained oneach line. For the next, we did inversion for dataacquisition used Res2dinV. This inversion proposedto derive a subsurface model through measurementdata (observation). The distribution of the value ofchargeability in 2D is assumed as subsurface lithology.For 3D modeling use correlation for each line usingOasis Montaj.

The study area is divided into 2 zones, IP1 Zoneconsisting of 2 lines along 320 and 340 meters and IP2Zone consisting of 6 lines with line lengths of 220-320meters.

Very Low Frequency

Data is obtained from a survey conducted at KokapDistrict, Kulonprogo, Special Region of Yogyakarta.VLF-EM measurement had been done in 14 linesspreads in several places at Kokap District, mainly atKalirejo Village which believed to hold a substantialamount of minerals. In the targeted area it is presumedto bear Argillic and Prophilic mineralization andthis survey’s objective is to map its presence andcontinuation overlaying andesite lithology serve as thebase lithology in the surveyed area.

Figure 3: Induced Polarization dataprocessing flowchart

Data was taken by two T-VLF console sets alongwith sensors made by IRIS Instruments. One devicepowered with battery and the other uses accumulator.Both devices deployed at different lines which then theresult combined and analyzed. Generally, the analysisstep is done by data smoothing and several filtering,as shown below.

Figure 4: Very Low Frequency dataprocessing flowchart

Quality control used in data measurement isHhor value is always bigger than Hver. Based on


the principal of the inducing electromagnetic wavepropagation, it should be bigger than induced fieldof Hver (Figure 3). Other thing that has to beconsidered is the direction of transmitter propagationshould always perpendicular against target with ±45

tolerance shown by compass although the deviationwill be automatically corrected by the device. Q barreading should at least at 60%. Besides, the besttransmitter used is the nearest one from survey areaso we can get strong signal. Transmitter used inthis research is located in Australia with transmittingfrequency of 19800 Hz and incoming azimuth of 3o’clock. Other significant consideration is the spacingof measurement points, if there’s blank data gapbecause of impossible terrain, then it has to beinterpolated since the input data has to be uniformlyspaced [5].

IV. Result and DiscussionFigure 7a and 8a show the measured data we gotfrom the field and input it into res2dinv. After thatwe did the inversion from the measured data and gotthe inversion model (Fig 7c & 8c). From the inversionmodel, we can calculate the data from the model. Thecalculated data (Fig 7b & 8b) from the model and themeasured data is then compared to each other. Whenthe calculated and the measured data is close enough,the rms error will give a small value. But when thecomparison is far, the rms error will give a big value.

Induced Polarization: Western Block

There are 2 lines of Induced Polarization measurementconducted at the western block. The measurementdone by using Syscal Jr. On the field there are miningborehole found at 160 m measurement. That mineused for the indication of mineralization zone. Thedata shows there are anomaly in the same locationor meter. That anomaly has resistivity value range130-900 ohm.m and chargeability value range 30-35 msat 25-34 m depth. Those value used as the referencefor interpreting the mineralization zone.

There are 3 other anomalies that can be interpretedas mineralization zone referring to the mine borehole.All of the three have relatively same depth withthe reference and chargeability value 30-46 ms. Theanomaly at the 255 and 210 meter have resistivityvalue for 130 ohm m, meanwhile anomaly at the 70m has resistivity value for 130-460 ohm.m.

There is a continuity from line 1 at line 2, butthe anomaly is smaller than line 1. There are 3mineralization zone interpreted refering to the line 1(Fig 5). The anomaly at 270 and 210 meter have theresistivity and chargeability relatively same about 130ohm.m and 30 ms. At 150 m, there are anomaly withresistivity 130-470 ohm m and chargeability 33-55 ms.

Induced Polarization : East Block

The argillic alteration zone generally has mediumto low resistivity features caused by the presence ofclay minerals associated with medium-high IP. Thepropylitic alteration zone generally has high resistivityfeatures due to the presence of chlorite mineralsassociated with medium to high IP.• Low Chargeability: < 30% (dark blue), medium:

31 − 50% (light blue-green), high: > 50%(yellow-purple)

• Low resistivity: < 20 Ωm, medium: 21-129 Ωm,high:> 130 Ωm

In the eastern block (IP2 zone), there are 3 linesinterpreted as the prospect zone, it was line 3, 4 and 5so this paper will be discussed about those line. Fromthe resistivity and chargeability section on line 3 (Fig.11), the mineralized zone indicated by medium value ofchargeability in range 30%−35% (1 msec = 0.1%). Themedium resistivity values (21-47 ohm.m) are found onthe surface at distance of 160-170 m and also foundat elevation 320 m and distance of 115 m. In addition,there is also an intrusion zone at distance of 220-240 mwith high resistivity and low chargeability as evidencedby the presence of andesite on the surface.

In the resistivity and chargeability section of line 4(Figure 12), anomaly is indicated with chargeabilityvalue above 75% and for resistivity above 531 ohm.mat elevation 290-300 m. This zone is interpreted asmineralization zone, but there is no evidence for thepresence of quartz veins on the surface. It may causedby the location of measurement line that near thealteration zone. In the cross-sectional resistivity andchargeability of line 5 (Fig. 13), the mineralizationzone have the same characteristics as line 3, indicatinga chargeability value.

From all IP measurements interpretation, wecan conclude that the mineralization zones aredisseminated and located below 10 meters fromsurface.

Very Low FrequencyVLF survey were conducted over alteration zone using15 lines (Figure 6). The length of each line is 500meters and data spacing is 10 meters. Instrument thatused in this method is Iris T-VLF and the signaltransmitter is NWC 19800 Hz located in Australia.Line X has same location with line 3 of inducedpolarization method.

Data processing shown by flowchart in Figure 4has been done for each line. Filter moving averageis used to smooth the field data, then filter Fraserand filter Karous-Hjelt are used to show conductivityanomaly of subsurface. The results of this process are


fraser graph and electrical current density profile. Thepeak of fraser graph indicates anomaly conductivitylocation and the higher value of electrical currentdensity indicates area which more conductive thanthe lower value. Thus, mineralization zone is indicatedby positive value of electrical current density. Line Xinterpretation (Figure 14) shows that mineralizationzone interpreted by this method has the same locationwith mineralization zone interpreted by inducedpolarization method (Figure 11). It means thesemethods can support each other.

According to interpretation of induced polarizationmethod, mineralization zone located mostly below10 meters from surface. In order to identifyhorizontal distribution of this zone, mineralizationzone interpreted by VLF method of each line is slicedat depth 10 meters and 20 meters. Then, these sliceddata are interpolated with Kriging method. The resultsof interpolated sliced data (Figure 15 and 16) showthat the mineralization zone is not well distributedrespect to alteration zone, which is the character oflow sulfidation epithermal type, mostly at north-southdirection.

V. ConclusionMineral prospecting purpose of the survey can begained by integration of two method run in thearea. The VLF-EM method gives high and lowanomaly. High anomaly means that the subsurfacelitology is conductive. This conductive materials existbeneath the earth is confirmed by some IP linesthat intersect the VLF-EM survey area. VLF-EMconductive anomaly identified by electrical currentdensity differs alteration that contains mineralizationand the one which is not. The high anomaly is thencompared with IP lines. The IP lines model tracedshows relatively similar to the value of VLF-EMthat sliced in certain depth. The result of inducedpolarization method shows that mineralization zone inKokap mostly located below 10 meters from surface,while VLF method shows that mineralization zone isnot well distributed (low sulfidation epithermal type)at north-south direction.

VI. AcknowledgementsWriters would like to say thanked for all parties thathelped us finished our paper. Also we would like tosay our gratitude towards all the lecturer who alreadytaught us a lot of lessons so that we can be here asfinal year students.

Author1 Adera Nurul

From :

(1) Geophysics Sub-Department, PhysicsDepartment, Faculty of Mathematics andNatural Sciences, Universitas Gadjah Mada

2 Arga KunangFrom :(1) Geophysics Sub-Department, PhysicsDepartment, Faculty of Mathematics andNatural Sciences, Universitas Gadjah Mada

3 Arief KhoiruddinFrom :(1) Geophysics Sub-Department, PhysicsDepartment, Faculty of Mathematics andNatural Sciences, Universitas Gadjah Mada

4 Arif FikriFrom :(1) Geophysics Sub-Department, PhysicsDepartment, Faculty of Mathematics andNatural Sciences, Universitas Gadjah Mada

5 Benedicta NathaniaFrom :(1) Geophysics Sub-Department, PhysicsDepartment, Faculty of Mathematics andNatural Sciences, Universitas Gadjah Mada

6 Dobrak TiraniFrom :(1) Geophysics Sub-Department, PhysicsDepartment, Faculty of Mathematics andNatural Sciences, Universitas Gadjah Mada

7 Enrico DiofanoFrom :(1) Geophysics Sub-Department, PhysicsDepartment, Faculty of Mathematics andNatural Sciences, Universitas Gadjah Mada

8 Haryo SatrioFrom :(1) Geophysics Sub-Department, PhysicsDepartment, Faculty of Mathematics andNatural Sciences, Universitas Gadjah Mada

9 Hesti NilamprasastiFrom :(1) Geophysics Sub-Department, PhysicsDepartment, Faculty of Mathematics andNatural Sciences, Universitas Gadjah Mada

10 Hesti NilamprasastiFrom :



11 Ihsan MedianFrom :(1) Geophysics Sub-Department, PhysicsDepartment, Faculty of Mathematics andNatural Sciences, Universitas Gadjah Mada

12 Indah SasmitaFrom :(1) Geophysics Sub-Department, PhysicsDepartment, Faculty of Mathematics andNatural Sciences, Universitas Gadjah Mada

13 Ivona AnnisaFrom :(1) Geophysics Sub-Department, PhysicsDepartment, Faculty of Mathematics andNatural Sciences, Universitas Gadjah Mada

14 Jum SatrianiFrom :(1) Geophysics Sub-Department, PhysicsDepartment, Faculty of Mathematics andNatural Sciences, Universitas Gadjah Mada

15 Laila AFrom :(1) Geophysics Sub-Department, PhysicsDepartment, Faculty of Mathematics andNatural Sciences, Universitas Gadjah Mada

16 Mariana KambeyFrom :(1) Geophysics Sub-Department, PhysicsDepartment, Faculty of Mathematics andNatural Sciences, Universitas Gadjah Mada

17 M. SyahdanFrom :(1) Geophysics Sub-Department, PhysicsDepartment, Faculty of Mathematics andNatural Sciences, Universitas Gadjah Mada

18 Oktavianus EkoFrom :(1) Geophysics Sub-Department, PhysicsDepartment, Faculty of Mathematics andNatural Sciences, Universitas Gadjah Mada

19 Rahmat HidayatFrom :


20 RamadhaniFrom :(1) Geophysics Sub-Department, PhysicsDepartment, Faculty of Mathematics andNatural Sciences, Universitas Gadjah Mada

21 Satrio PFrom :(1) Geophysics Sub-Department, PhysicsDepartment, Faculty of Mathematics andNatural Sciences, Universitas Gadjah Mada

22 Siti SuciFrom :(1) Geophysics Sub-Department, PhysicsDepartment, Faculty of Mathematics andNatural Sciences, Universitas Gadjah Mada

23 Zeni AFrom :(1) Geophysics Sub-Department, PhysicsDepartment, Faculty of Mathematics andNatural Sciences, Universitas Gadjah Mada

24 Zukhruf DelvaFrom :(1) Geophysics Sub-Department, PhysicsDepartment, Faculty of Mathematics andNatural Sciences, Universitas Gadjah Mada

25 Budi ArjoFrom :(1) Geophysics Sub-Department, PhysicsDepartment, Faculty of Mathematics andNatural Sciences, Universitas Gadjah Mada

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2. Reynolds. An Introduction to Applied and Enviromental Geophysics.

Ghidester, West Sussex: Willey-Blackwell; 2011.

3. Bosch FP, Muller I. Continuous Gradient VLF Measurements: A New

Possibility For High Resolutio Mapping of Karst Structures. Tech Rep

EAGE First Break. 2001;19(6):343–350.

4. Turberg P, Muller I. La Methode Inductive VLF-EM Pour La

Prospection Hydrogeologique en Continue do Milieu Fessure. 11th ed.

Annales cientifiques de I’Universite de Besancon; 1992.

5. Karous M, Hjelt SE. Linear Filtering of VLF Dip-Angle Measurements.

Geophys Prospect. 1983;31(5):782–794.

Some Figures Available on The Next Page


Figure 5: Survey line of IP Method

Figure 6: Survey line of VLF-EM Method


Figure 7: Resistivity Inversion of Line 3 (EasternBlock)

Figure 8: Chargeability Inversion of Line 3 (EasternBlock)


Figure 9: Line Section 1 Resistivity and ChargeabilityWestern Block

Figure 10: Line Section 2 Resistivity and ChargeabilityWestern Block


Figure 11: Line Section 3 Resistivity and ChargeabilityEastern Block

Figure 12: Line Section 4 Resistivity and ChargeabilityEastern Block


Figure 13: IP Line Section 5 Resistivity andChargeability Eastern Block

Figure 14: a) Fraser Graph b) VLF Line XCurrent Density Map. Note that mineralization zoneinterpreted using this method has the same locationwith mineralization zone in line 3 of IP method.


Figure 15: Mineralization zone interpreted by VLFmethod at depth 10 m below surface

Figure 16: Mineralization zone interpreted by VLFmethod at depth 20 m below surface

mineral prospect identification using inducedpolarization

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