


































Energy and Earth Science 
Vol. 6, No. 2, 2023 

www.scholink.org/ojs/index.php/ees 

ISSN 2578-1359 (Print)   ISSN 2578-1367 (Online) 

 

30 

 

Original Paper 

Geophysical Mapping by Electromagnetic Induction of Gold 

Occurrences in Birimian Formations of Liptako: Case of Sorbon 

Haoussa Sector (Souhwest Niger) 

Abdoulwahid Sani
1
 & Souley Baraou Idi

1*
 

1
 Department of Geology, University of Agadez, Agadez, Niger 

*
 Souley Baraou Idi, Department of Geology, University of Agadez, Agadez, Niger 

 

Received: May 5, 2023          Accepted: May 17, 2023         Online Published: June 5, 2023 

doi:10.22158/ees.v6n2p30                  URL: http://dx.doi.org/10.22158/ees.v6n2p30 

  

Abstract 

The survey zone corresponds to the Sirba greenstone belt, one of Liptako Birimian Formations (LBF) 

of Niger. Previous aeromagnetic and geochemical works reveal the significant gold occurrences. The 

present study integrates geophysical mapping of these occurrences by electromagnetic induction on 

field. A multi-frequencies system of 10 frequencies was used for depth and subsurface investigation of 

resistive anomalies. The interpretation of obtained results shown two types of resistive anomalies, 

corresponding to the conductive units: deep anomalies obtained with low frequencies and subsurface 

anomalies detected with high frequencies. The subsurface anomalies were detected around the 15 

meters and the deep anomalies were recorded beyond 80 meters. All of the detected resistive anomalies 

are in a lenticulars shapes oriented N-S or E-W. The analysis of combined resistivity map from all 

frequencies shown three zones of resistive anomaly presenting a high potential in conductive metals: 

The first zone Z1 (most important), located in Eastern studied zone covering, covering a surface of 3.5 

Km
2
, the second zone Z2 is located in West and covers an area of 2.9 Km

2
 and the third anomaly zone 

Z3 is located in South with a surface of 0.96 Km
2
. On field, these conductive units correspond to 

auriferous quartz veins and/or highly altered manganese schists, exploited by using the artisanal 

mining wells, up to 20 meters in depth. 

Keywords 

electromagnetic induction, resistive anomaly zone, gold occurrences, Sirba Greenstone Belt, Liptako 

Birimian formations, Niger 

 



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1. Introduction 

The studied area (Sorbon Haoussa) is located in northeastern part of the Sirba volcano-sedimentary belt, 

one of the three belts of Lower Proterozoic Birimian greenstone formations to the SW of Niger (Figure 

1a). The two main geological units are the granitic batholite to the SE and the volcano-sedimentary 

formations to the NW. The survey area covered a surface of 21 Km
2
 with the altitudes varying from 

220 to 264 meters (Figures. 1b, c). This zone is well known for its significant gold mineralization, 

generally contained in graphitic and or manganous schists (Project NER/88/023, 1991; JICA, 1993; 

Colin Brown, 1998; Project PADEM, 1995; Claude Jobin et al., 2010). It has significant gold 

occurrences (JICA, 1993; PADEM Project, 1995; Claude Jobin et al., 2010), which are exploited by 

using artisanal mining pits, systematically implemented. Previous works are limited to airbone 

geophysic (Project NER/88/023, 1991) and geochemical (JICA, 1993; Project PADEM, 1995; SEMS 

Exploration Services Ltd. 2009; Claude Jobin et al., 2010) prospection. This study aims to determine 

the subsurface and depth spatial distribution of these occurrences. To achieve this objective, 

geophysical method by multi-frequency electromagnetic induction was adopted. These surveys 

specifically allowed the mapping of conductive (mineralized zones) and resistive (host rocks) units as 

well as the directions, thicknesses and depths of the anomalies. 



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Figure 1. Locations of the Study Area within the Geological Map of Liptako Birimian 

Formations of Niger (a, b) (Machens, 1973; Dupuis et al., 1991; Abdou et al., 1998) (c): 

Topographic Map and E-W Electromagnetic Survey Grid 

 

 

 

 

 

 

 



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2. Method 

Multi-frequency electromagnetic surveys allow to detect and map mainly the variations of conductive 

units in the subsurface (Acdi, 1973; Grauch et al., 2006) conductive veins and fractures. The survey 

grid used in the case of this study is consisted of 21 parallel lines with E-W direction (Figure 1c). The 

measurement stations are equidistant of 50 meters and are marked by stakes set up and surveyed with 

Differential Global Positioning System (DGPS). The PROMIS-10 system used in this work is a new 

multi-frequency electromagnetic induction technology that can send up to 10 frequencies up to 14 080 

Hz, 28160 Hz and 56320 Hz for high frequencies. The other frequencies are 110 Hz, 220 Hz, 440 Hz, 

880 Hz (low frequencies) and 1760 Hz, 3520 Hz, 7040 Hz (medium frequencies). In this system low 

frequencies are the most penetrative while high frequencies allow the investigation of the sub surface. 

In addition, the PROMIS-10 can record secondary magnetic fields along three axes X, Y and Z. The 

measurements are made profile by profile. At each measuring station, the system passes automatically 

from one frequency to another, which makes it possible to carry out a vertical survey (in depth). The 

subsoil is thus characterized horizontally and vertically along each profile, which allows to map the 

conductive anomaly in depth and in subsurface.  

Geo-electric parameters such as resistivity, depth and thickness of each conductive level are also 

determined. The depth of investigation varies according to the transmission frequency, the resistivity of 

medium and transmitter-receiver distance. In general, it is equal to half of the transmitter-receiver 

distance but can reach 100 to 125% of this distance depending on the conductivity of the medium. 

The principle (from the general viewpoint) is described as follows: When an electromagnetic wave 

penetrates the underground, it induces eddy currents in any conductive body (Faraday’s Law of 

Induction) which then generates a disturbing magnetic field (Ampere’s Law) of the primary magnetic 

field of the transmitter. The primary field Hp and the secondary field Hs are recorded at the receiver 

(Figure 2).  

 



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Figure 2. Electromagnetic Induction Device with Transmitting (Transmitter Tx) and Receiving 

(Receiver Rx) Loops Showing the Process of Penetration of Electromagnetic Field into the 

Underground and the Generation of Electric Currents and Recorded Secondary Magnetic Fields 

 

3. Result 

3.1 Results of Electromagnetic Survey 

The results of the electromagnetic survey are distributed as follows (Appendix 1): Total number 21 

profiles carried out oriented East-West, spaced of 200 meters. Total length of the profiles: 101.000 m 

and 2037 drilling points spaced of 50 meters. The data obtained allowed on the one hand to determine 

the geo-electrical parameters, in particular the resistivity, the thickness and depth of the anomaly 

(Appendix 2) and on the other hand to map the resistive anomaly zones (low, medium and high 

frequencies) corresponding to the conducting levels.   

3.2 Analysis and Interpretation of Resistive Anomaly 

3.1.1 Anomaly Maps for Low Frequencies 

The resistivity anomaly maps, low frequencies (110 Hz, 220 Hz, 440 Hz; D), 880 Hz; Figure 3 shows a 

spatial variation in the electrical resistivity data. They highlight the existence of an E-W conductive 

structure located on profile 12. This structure is highlighted by the 110 and 220 Hz frequencies (Figure 

3 a, b) which are the most penetrative. A resistivity anomaly appears also in the eastern part of survey 

zone, marked by very low resistivity values revealing the high conductive units with varying depths. 

The presence of shallower resistive units that can be un-mineralized quartz veins (resistivity between 

300 and 500 Ohm.m) can be observed. 

 

 

 



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The resistivity anomaly map, from all low frequencies (Figure 4) shows that the high conductivity units 

have a lenticular shape (green, blue and red color) contained in a resistive unit (host rock). These 

conductive lenses show a N-S alignment in the East of the zone, which can be considered as a zone 

having a highly potential in conductive metals (gold anomaly). The scattered high resistive lenses are 

observed in the center and in South of the zone, forming a cluster oriented E-W (Figure 4). On field, 

the conductive lenses correspond to quartz veins and/or highly altered manganese schists, operated by 

using the mining wells up to 20 meters in depth (Figure 5).  

 

 

Figure 3. Resistivity Anomaly Maps for Low Frequencies: a) 110 Hz), b) 220 Hz, C) 440 Hz; D), d) 

880 Hz 

 

 



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Figure 4. Combined Resistivity Anomaly Map for All Low Frequencies, Showing the Spread 

High Conductive Units in Lenticular Shapes (Green, Blue and Red Color). Other Colors 

Represent Resistivity Units (Host Rocks) 

 

 

Figure 5. Photographs of Host Rock Outcrops for Gold Showings and Artisanal Mining Pits, 

Showing the Gold Bearing Outcrops 



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3.1.2 Anomaly Maps for Medium Frequencies 

Medium frequencies are less penetrative than low ones. Analysis of the resistivity anomaly maps for 

the mid-frequencies (Figure 6) confirms the existence of three anomalous zones oriented N-S 

highlighted by the low frequencies (Figure 6 a). In addition, the 3520 Hz and 7040 Hz frequency maps 

(Figure 6 b and c) show the highly resistive levels, oriented N-S that can correspond to mineralized 

quartz veins. 

 

 

Figure 6. Resistive Anomaly Maps for Medium Frequencies: a) 1760 Hz), b) 3520 Hz, c) 7040 Hz 

 

3.1.3 Anomaly Maps for High Frequencies 

The high frequencies are the least penetrative and their depth of investigation varies from 1 to 10 

meters for subsurface conductive units. The 14080 and 28060 Hz high frequency maps (Figure 7a, b) 

show the presence of subsurface conductive units in East, West and South. Whereas the 56320 Hz 

frequency map (Figure 7c), more superficial, shows resistive levels in South specifically on the first 6 

profiles, in extreme North and West. These resistive levels could be the acidic intrusive units 

(granodiorite/granite) and un-mineralized quartz veins (Claude Jobin et al., 2010). The abundance of 

resistive units in 56320 Hz frequency map (Figure 7c) reveals the high meteoric alteration of host rocks 

on the surface. In the field, the conductive units (gold mineralization) are either associated with 

alluvium from meteoric alteration of manganese schists, or generally associated with highly altered 

quartz veins with low sulphide content. They are identified in the artisanal mining pits of gold. 



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The resistivity map from all frequencies (Figure 8) shows the spatial distribution of high mineral 

potential anomalies in the study area. Three resistivity anomaly zones with a high potential in 

conductive metals can be referenced Z1, Z2 and Z3 (Figure 8):  

The first zone Z1 (most important), located in East of the area has a N-S trend and covers an area about 

3.5 Km2;  

The second zone Z2 is located in West with a N-S direction and covers an area of 2.9 Km2;  

The third anomaly zone Z3 is located in South with a surface of 0.96 Km2, oriented N-S. 

 

 

Figure 7. Resistive Anomaly Maps for High Frequencies: a) 14080 Hz, b) 28060 Hz, and c) 56320 

Hz. Rc: Resistivity Contour 

 



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Figure 8. Spatial Distribution Map of High Potential Zones in Conductive Minerals from All 

Frequencies Showing the Location of Drill Points and Associated Anomaly Depths 

 

3.3 Analysis of Geo-Electric Data 

Geo-electrical parameters (resistivity values, anomaly depth and thickness) of anomaly by profile are 

indicated in the appendix 2. Four types of anomaly according to resistivity values are observed: very 

high (1 to 7 Ohm.m), high (7 to 10 Ohm.m), medium (10 to 19 Ohm.m) and low (19 to 52 Ohm.m). 

The projection of anomaly depth (Figure 8) show that the high values of depth are concentrated in East 

corresponding to Z1 of the survey zone and relative low values of depth in Northwest corresponding to 

Z2 and in South Z3. These observations reveal that the anomaly zone Z1, located in upstream (Figure 

1c), corresponds to the deep anomaly (primary) and Z2 and Z3 subsurface anomalies (secondary). The 

secondary anomalies zone Z2 and Z3, located in downstream can be interpreted as result of upstream 

(Z1) high alteration. 

 

 

 

 

 

 

 

 



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4. Discussion 

Petrographically, the survey area consists of altered black schists crossed by dolerite, quartz and granite 

dykes (Claude Jobin et al., 2010). The observation on field shown that the conductive lenses 

highlighted by the resistivity anomaly map (Figure 4) correspond to auriferous quartz veins and/or 

highly altered manganese schists, operated by using the mining wells up to 20 meters in depth (Figure 

5). Petrographic observations carried out in the artisanal mining wells shown that the black schist 

formations and acidic intrusive units are the main host rocks of conductive lenses. The geochemical 

prospection carried out in these host rocks revealed the gold contents varying from 20 ppb to 300 ppb 

(JICA, 1993). According to Claude Jobin et al. (2010), these black schist formations represent a 

chemical trap favorable to the gold precipitation from mineralized fluids. According to the same 

authors, a structural control of gold mineralization is associated to the features oriented N60° 

(M’Banga, Tiringui cluster), N90° to N110° (Sefa Nangue), N130° to N140° (Samira, Libiri, Koma 

Bangou), N350° to N10° (Tera and Allareni clusters), all located in Southwest of survey area. Further 

in Southwest of studied zone, magnetic and aeromagnetic data from several auriferous sites of 

Burkina-Faso have showed the similar observations. Indeed, in these regions, gold mineralization is 

tectonically controlled by an irregular NE-SW (N60° to N90°) shear zone and hosted by small 

geophysical anomalies units corresponding to an altered and silicified green schist facies (Sawadogo et 

al., 2018; Aziz et al., 2016; Bilardello & Jackson, 2014).  

 

5. Conclusion 

This study focused on the geophysical mapping of gold occurrences in Sirba greenstone belt, 

specifically in the Sorbon Haoussa area. Using multi-electromagnetic induction methods allowed to 

obtain thematic maps of deep and subsurface resistive anomalies. Geo-electric parameters such as 

resistivity, depth and thickness of conductive level are also determined. The projection of anomaly 

depth on resistivity map (all frequencies) led to determine the zone of high potential in conductive 

metals. This study helps to understand the geometry of anomalous zones as well as the depth and 

thickness of anomalous units. It constitutes also a guide to improve mining operations, which are 

hitherto artisanal. 

 

Acknowledgments 

The authors are grateful to the OM Goldstone staff for their scientific collaboration and technical 

support, especially for electromagnetic data to carry out this work. 

 

 

 

 



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References  

Abdou, A., Bonnot, H., Bory, K. D., Chalamet, D., Saint, M. M., & Younfa, I. (1998). Explanatory 

note of geological maps of Liptako at 1/200 000. Ministry of mines and Geology. Niger Republic. 

Ministère des Mines et de la géologie, Rep. Niger. [Origin language: French] 

Acdi. (1973). Report on Aeromgnetic survey of Liptako Gourma. Ministry of Mines and Energy. Niger 

Republic. [Origin language: French] 

Aziz, O., Ousmane, B., Gbélé, O., Eric, G., & Saga, S. (2016). Caractérisations structurales des gîtes 

aurifères du corridor de Bouboulou-Bouda au Burkina Faso, Afrique de l’Ouest. Afrique Science, 

12(5), 89-104. [Origin language: French] 

Bilardello, D., & Jackson, M. (2014). A comparative study of magnetic anisotropy measurement 

techniques in relation to rock-magnetic properties. Tectonophysics, 629, 39-54. 

https://doi.org/10.1016/j.tecto.2014.01.026 

Claude, J. P., & Souleymane, C. (2010). Report on drilling Tialkam Deba permits sirba belt, Liptako, 

Niger part 2. 

Dupuis, D., Pons, J., & Prost, A. E. (1991). Emplacement of granitoids plutons and characterization of 

Birimian deformations of western Niger. Comptes Rendus de l’Académie des Sciences, Paris, 312, 

769-776. [Origin language: French] 

Grauch, V. J. S., Sawyer, D. A., Minor, S. A., Hudson, M. R., & Thompson, R. A. (2006). Gravity and 

Aeromagnetic Studies of the Santo Domingo Basin Area, New Mexico. In A. M. Scott (Ed.), U. S. 

Geological Survey, Denver (pp. 63-86).  

JICA. (1993). Mining prospection report in Sirba region (Vol. 20). Niger Républic. [Origin language: 

French] 

Machens, E. (1973). Contribution to the study of basement formations and sedimentary cover in 

Western Niger Republic: Mémo. BRGM (No. 82, p. 167). [Origin language: French] 

Project NER/88/023. (1991). Aeromagnetic and electromagnetic surveys in Liptako region. 

Geophysical General Campany. [Origin language: French] 

Project PADEM. (1995). Research of Gold occurrences in Liptako region. In Final report Klöckner (p. 

162). Ministry of Mines and Energy. Republic of Niger. [Origin language: French] 

Sâga, S., Séta, N., Hermann, I., Abraham, S. T., Samuel, N., & Martin, L. (2018). The Belahourou 

granite pluton (Djibo greenstone belt, Burkina Faso): Emplacement mechanism and implication 

for gold mineralization along a shear zone. Journal of African Earth Sciences.  

SEMS Exploration Services Ltd. (2009). Interim report covering gold exploration proposals on the 

Sirba project. SW Niger.  

 

 

 

https://doi.org/10.1016/j.tecto.2014.01.026


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Appendixes 

 

Appendix 1. Results of Electromagnetic Surveys 

Profile Number of 

Measurements by 

profile 

Profile length (m) 

Profile 1 56 2800 

Profile 2 65 4200 

Profile 3 75 3700 

Profile 4 80 4000 

Profile 5 86 4300 

Profile 6 90 4500 

Profile 7 97 4800 

Profile 8 102 5050 

Profile 9 107 5300 

Profile 10 113 5600 

Profile 11 115 5700 

Profile12 116 5750 

Profile 13 113 5600 

Profile 14 116 5750 

Profile 15 113 5600 

Profile 16 109 5400 

Profile 17 104 5150 

Profile 18 104 5150 

Profile 19 96 4750 

Profile 20 92 4550 

Profile 21 88 4350 

Total 2 037 101 000 

 

 

 

 

 

 

 

 



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Appendix 2. Geo-electrical Parameters of Anomalies by Profile 

Profile (P) anomal stations Ohm.m 
Thickness 

(meters) 
Depth (m) 

Depth 

corrected 

(m) 

RMS (%) Anomaly 

P2 

S1 7.22 10.58 19 36 6.04 High 

S2 8.77 6.02 9.55 23.46 5.64 high 

S3 22.39 20.00 21.23 33.03 3.62 low 

S4 5.01 3.62 12.84 16.80 7.35 Very high 

S5 14.40 10.60 13.51 15.45 2.65 medium 

P3 

S1 9.04 11.16 19.99 40.37 2.31 high 

S2 2.47 5.16 18.05 41.67 4.95 Very high 

S3 6.04 7.58 22.08 51.22 2.29 Very high 

S4 6.53 13.77 23.62 53.85 2.62 Very high 

P4 

S1 15.04 17.30 36.37 69.45 4.93 Medium 

S2 15.61 16.73 35.79 67.12 4.81 medium 

S3 5.22 11.74 18.23 45.49 6.00 Very high 

P5 

S1 2.48 5.56 15.59 21.98 3.569 Very high 

S2 3.35 8.74 17.88 27.46 4.127 Very high 

S3 6.62 19.11 26.26 36.75 2.3819 Very high 

S4 2.27 6.00 15.01 25.69 3.3145 Very high 

P6 

S1 19.41 17.81 23.49 26.38 7.14 medium 

S2 18.44 16.88 17.63 19.73 4.91 medium 

S3 40.92 25.90 87.16 111.27 1.69 Low 

S4 2.52 5.01 20.01 49.59 8.79 Very high 

S5 3.62 7.67 19.16 51.62 2.35 Very high 

S6 6.35 10.71 21.04 53.07 4.44 Very high 

P7 

S1 19.64 32.56 38.54 73.88 1.57 Low 

S2 5.13 11.34 22.66 56.98 8.33 Very high 

S3 5.55 11.88 22.79 55.70 8.07 Very high 

P8 

S2 7.39 12.02 30.94 74.03 3.53 high 

S3 21.36 52.09 64.01 80.76 4.94 Low 

P9 

S1 3.34 7.96 18.32 59.11 2.48 Very high 

S2 5.68 13.86 25.30 65.94 8.21 Very high 

S3 6.99 25.86 34.05 56.25 10.10 Very high 

S4 2.05 6.91 16.22 16.22 36.47 Very high 

S5 19.16 24.78 49.03 68.16 3.07 low 



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S6 13.00 23.14 33.50 49.01 8.59 medium 

S7 16.63 16.03 27.79 31.81 2.96 medium 

P10 

S1 8.81 17.27 38.14 52.81 8.64 High 

S2 6.11 34.85 37.78 57.74 12.85 Very high 

S3 2.35 7.20 16.95 39.24 3.30 Very High 

P11 

S1 52.02 18.79 89.90 89.86 3.37 low 

S2 6.98 7.62 46.41 59.54 3.80 Very High 

S3 7.16 12.04 17.46 21.11 3.16 high 

S4 22.82 11.61 13.05 16.33 3.29 low 

P12 

S1 5.48 10.20 12.77 13.66 11.99 Very High 

S2 10.58 17.32 27.00 27.86 5.31 medium 

S3 2.06 0.92 0.92 3.34 2.96 Very High 

S4 10.84 7.18 7.75 13.02 2.94 medium 

S5 19.56 14.98 25.22 28.85 10.38 low 

S6 6.40 18.79 44.50 48.39 8.47 Very High 

S7 13.95 14.55 27.34 54.27 7.39 medium 

S8 13.61 20.79 22.37 45.56 6.69 medium 

S9 38.88 25.79 37.97 60.63 8.39 low 

P13 

S1 9.44 14.33 22.23 31.65 9.53 high 

S2 21.39 22.15 22.15 45.45 1.49 low 

S3 23.85 22.08 22.08 41.48 6.55 low 

S4 6.05 26.07 26.72 45.61 13.76 Very high 

S5 27.63 1.55 28.27 47.16 13.76 low 

S6 12.47 0.16 0.16 20.45 12.02 high 

S7 6.82 33.11 33.26 53.55 12.02 Very high 

P15 

S1 11.96 15.75 19.04 20.45 3.36 medium 

S2 15.36 31.37 31.37 33.54 9.50 medium 

S3 14.30 26.01 27.89 30.31 6.21 medium 

S4 7.05 16.27 25.87 28.69 7.48 high 

S5 10.76 18.26 25.32 31.49 10.52 high 

S6 3.26 2.07 3.23 17.21 6.74 Very high 

S7 7.41 21.51 29.06 44.64 6.69 high 

S8 6.87 2.44 3.99 22.99 6.58 Very high 

S9 9.32 23.31 27.30 46.30 6.58 high 

P16 

S1 20.43 29.43 37.24 42.81 9.96 low 

S2 16.83 23.76 27.94 37.85 23.71 medium 



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S3 11.85 22.84 23.70 35.12 9.06 high 

S4 4.27 22.34 23.87 38.43 12.53 Very high 

S5 3.49 13.46 14.06 31.60 7.32 Very high 

P17 

S1 8.25 10.65 11.07 26.38 3.09 high 

S2 1.43 0.81 1.52 12.67 7.22 Very high 

P18 

S1 6.75 15.08 15.66 21.44 2.19 Very high 

S2 6.84 24.48 27.12 36.72 11.55 Very high 

S3 6.97 0.25 0.25 10.64 13.47 Very high 

S4 3.18 17.69 21.72 33.22 7.57 Very high 

S5 5.23 19.95 22.25 36.99 4.32 Very high 

P19 

S1 4.58 15.45 18.55 34.70 5.79 Very high 

S2 4.08 9.97 19.20 32.85 5.07 Very high 

S3 4.35 13.33 15.17 27.36 2.98 Very high 

S4 4.58 15.45 18.55 29.85 5.79 Very high 

 

 


