3D imaging of quarry materials using electrical ... - Springer Link

Euro-Mediterranean Journal for Environmental Integration (2018) 3:9 https://doi.org/10.1007/s41207-017-0047-4

ORIGINAL PAPER

3D imaging of quarry materials using electrical soundings Abderrahmane Bouguern1 · Karim Allek1 · Mohamed Khalifa2 · Djamal Boubaya3 Received: 2 January 2017 / Accepted: 10 November 2017 © Springer International Publishing AG, part of Springer Nature 2017

Abstract In order to satisfy a need for aggregates required to build a section of the Algeria East–West Highway, a geophysical study was carried out. Seventy-five soundings were conducted in three different sectors, with twenty-five soundings per sector. The goal of the investigation is to study the deep limestone. The chosen method is sensitive to the formation types that characterize the area under investigation made of high-resistivity limestone and more conductive marls. The study site is located near Didouche Mourad city (eastern Algeria). Measurements were achieved using a Shlumberger device whose AB line length is equal to 650 m. Erroneous values were eliminated using the median filter. To invert the electrical sounding, the “smoothed model” technique was used. Based on the electrical sounding interpretation, we ended up with several conclusions. They can be summarized as follows: The electrical sounding method seems to have solved the problem raised from the beginning. Indeed, the electrical resistivity contrast between the limestone and the surrounding marl formations was very useful in precisely determining their geometry and evaluating their extension, depth and thickness. Sector 1 has an appreciable limestone thickness in its central part. Sector 2 indicates the presence of a substantially uniform limestone layer over almost the entire site. The layer’s small depth (a few meters) and its relative important thickness (50 m on average) make this sector very promising. Sector 3 is the least favorable in terms of limestone presence. Aggregate reserves in the first two sectors seem interesting. Keywords Limestone · Electrical soundings · Schlumberger device · Resistivity

Introduction In order to satisfy a need for aggregates required to build a section of the Algeria East–West Highway, a surface geophysical study was undertaken (Fig. 1). Based on the results obtained, an electrical sounding geophysical campaign, using a Schlumberger device, was conducted. The goal of the investigation is to provide a first evaluation of limestone quantities in three sectors. Since the need is of temporary duration, the deposit size does not have to be substantial.

* Abderrahmane Bouguern [email protected] 1

Earth Physics Laboratory, University of Boumerdes, Avenue de l’indépendance, Boumerdes, Algeria

2

Institute of Electrical and Electronics Engineering, University of Boumerdes, Avenue de l’indépendance, Boumerdès, Algeria

3

Geology Department, University of Tebessa, Tebessa, Algeria

On the study site, seventy-five vertical electrical soundings have been carried out, with twenty-five soundings for each sector. Each sector covers an area of approximately four hectares (200 by 200 m). The investigation’s main objective is to determine the limestone aspect by estimating its geometry (extension, thickness and depth). The vertical electrical sounding method has been chosen because it provides two major advantages: first, the method is sensitive to the formation types that characterize the area under investigation (limestone having high resistivities and more conductive marls); the second advantage of the method is by using multiple sounding points, evenly distributed over the entire site, one can provide by interpolation a three-dimensional (3D) underground exploration.

Sector study locations The first sector is located 4 km south east of Didouche Mourad city in eastern Algeria. Its UMT coordinates are: 291120/033391. It consists of a relatively rugged slanted

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Euro-Mediterranean Journal for Environmental Integration (2018) 3:9

Fig. 1 Localization of the three sectors on a geological background Fig. 2 Illustration of sector 1 relief using the UTM system. The black dots represent electrical soundings

terrain with a sloping degree varying from 20 to 30% (Fig. 2 shows sector 1).

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Sector 2 is situated 1 km south west of sector 1. Its UMT coordinates are: 290140/4032965. It is comprised of

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Euro-Mediterranean Journal for Environmental Integration (2018) 3:9 Fig. 3 Illustration of sector 2 relief using the UTM system. The black dots represent electrical soundings

mound-like terrain and peaks around 940 m (Fig. 3). Limestones outcrop at the top of the sector’s mid-part. Sector 3 is located southwest of sector 1 at approximately 200 m. Among the three sectors, this is the least rugged one (Fig. 4). It exhibits a light sloping terrain southwardly oriented. The terrain’s western side slope is very slight (around 2%), while it exceeds 20% in the most eastern part of the sector. Limestones outcrop mainly in the north, while they are underground located in the south.

On‑site data measurements Vertical electrical soundings were achieved by using a Schlumberger device (Fig. 5). The measurement acquisition was carried out using a Scintrex brand system, composed of a TQS-3 model transmitter and a high-sensitivity IPR-10A receiver. To insure an exploration depth of at least 100 m, the length of the AB line was equal to 650 m (Roy and Apparao 1971; Roy 1972).

Fig. 4 Illustration of sector 3 relief using the UTM system. The black dots represent electrical soundings

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Fig. 5 Schlumberger device

The Schlumberger device (Barker 1989; Dahlin 1996) used in the investigation was a symmetrical and linear quadrupole centered at the point “O”, where the length MN is less than AB . For a ground vertical exploration, we carried 5 out at the same point a series of measurements by progressively increasing the AB length. The measures were taken for increasing AB , without changing MN, as long as the sig2 nal was good (i.e. the potential difference is large enough). As we increased the AB spacing, the potential difference 2 measured between M and N decreased, necessitating the increase of the MN distance for an AB length equal to 30, then 130 and finally 450 m. We then took two measures of MN for the same AB length. Finally, when all the desired points were measured, the resistivity curve was checked for smoothness. The electrical apparent resistivity is given by the formula

The general formula of the geometric coefficient is k = 1 1 2𝜋 1 1 = 2 2𝜋 2 . The device is symmetric

(1)

The general apparent resistivity formula is(given by )

𝜌a = k × R where k is the geometric coefficient and R =

ΔV(mV) I(mA)

AM

−

The apparent resistivity is defined as follows:

2𝜋 −

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1 BM

−

+

BN

AM

−

AN

to AN, which is equivalent to AM × AN ∼ OA2 =

1 AN

−

1 BN

×

V I

(3)

2

,

AB 2

where O is the middle of AB. It follows that k becomes

2𝜋

k=

2 AM

−

2 AN

=

𝜋AM ⋅ AN 𝜋OA2 = AM − AN MN

(4)

(Xmelevskou 1984). From Eq. (4), one can derive the following:

k=

𝜋AM ⋅ AN MN

ΔV . I

(5)

After simplifications, we get k =

( )2

𝜋 L2 −

𝜌a =

𝜋L2 E a

a 2

a ( )2

𝜋(L2 −

A current of electrical intensity I is injected into a homogeneous and isotropic subsoil using two emission electrodes AB = 2L and a potential difference ∆v is measured between two other electrodes MN = a. ∆v is given by the following formula: ) I𝜌 ( 1 1 1 1 − − + Δv = vM − vN = (2) 2𝜋 AM BM AN BN

1 AM

AN

where AB = 2L, and therefore, we have 𝜌a = After other simplifications we get at the end

Measurement principles

𝜌a =

−

which means that AB = BN and BM = AN. Also, MN is very small compared to AB. We take MN to be very ( close )

𝜌a = k × ΔU(mV) 𝜌a = k I(mA)

BM

a

a 2

)

×

,

ΔV . I

(6)

where E is the electrical field.( The ) only possible error comes 2

from the negligible quantity a2 , and the relative error ε is ( )2 a bounded by 2L , which gives the inequality

𝜀≤

(

a 2L

)2

(7)

Observe that the smaller MN is, the smaller the relative error. The recorded measurements are then used to draw the for each apparent resistivity 𝜌a according to the distance AB 2 sector. Figure 6a is the plot of apparent resistivities of sector 1, Fig. 6b the plot of apparent resistivities of sector 2 and Fig. 6c the plot of apparent resistivities of sector 3.


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Fig. 6 a Inversion model of sector 1. b Inversion model of sector 2. c Inversion model of sector 3

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Fig. 6 (continued)

In order to eliminate erroneous values due to poor contacts of electrodes with the ground or to heterogeneous soils, the measured electrical resistivities of some soundings have been processed by the median filter.

Data processing and inversion Data interpretation (Kuntz 1970; Zohdy 1989), also known as inversion, consists of giving the most likely geological model solution (the internal underground structure that is the closest to reality). Indeed, it is this internal structure that has generated the apparent electrical resistivities measured on the ground (Loke and Barker 1996), with the used electrical device. For one-dimensional (1D) inversion of the electrical soundings, we used the “smoothed model”, based on a continuous variation between resistivity and depth. Thereafter, for each smoothed model obtained, we found a layered model having three or four layers depending on the resistivity curve shape. To give an enhanced image of the obtained geological model and in order to make the result interpretation easier,

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we conducted all processing from the layered models; then we combined them having as a final result a 3D image of the resistivity model (Loke and Barker 1996).

Achieved results Normally, to establish an electrical resistivity scale, necessary to interpret the results, drilling should be carried out on the study area. Since no drilling was performed, we computed a resistivity statistical mean of the geological formations that flourish in the site. The results obtained are given below: • Marl resistivity below 80 Ω m • Marl limestone resistivity between 80 and 200 Ω m • Limestone resistivity larger than 200 Ω m

In this analysis of the result interpretation, special care has been given to resistant horizons whose values exceed 200 Ω m, as they reflect the presence of limestone layers.

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Sector 1 The twenty-five electrical soundings of sector 1 show three different materials:

a 3D model for resistivities greater than 200 Ω m to give a clearer picture of the electrically resistant horizon appearance corresponding to the limestone formations. The image obtained indicates the presence of this layer over the entire site with a more or less homogeneous thickness (Fig. 8).

• The first material has a resistivity smaller than 100 Ω m • The second material has an average resistivity larger than

Sector 2

To better illustrate the results of the vertical electric sounding (VES) modeling, we have constructed an electrical resistivity section along profile 1. This section indicates the presence of a resistant horizon (red color of Fig. 7) whose thickness is greater at SEV 1_1 with an electrical resistivity around 1088 Ω m. The thickness of this resistant layer, which corresponds to the limestone, appears to be thinning towards the sector’s northern and southern portions. The depth to the roof of this layer is only a few meters in the northern part while it is 10 m to the south. Finally, we have constructed

The vertical electrical sounding interpretation of this sector shows that the explored geo-electrical section is mainly made of three to four electrically differentiated horizons. The curves are basically of two types. The first type concerns the soundings in the study site’s central part (posts 1, 2 and 3 of the five profiles); more specifically, the soundings were carried out on the highest mound part, where limestone outcrops. These points have a bell shape, which means that the intermediate layer has a resistivity greater than those immediately above and below it, given the fact that the passage from the surface altered boulders to the underlying limestone formations is gradual. Further down, the curves indicate a less resistant bedrock, creating a transition zone between limestone and marl formations. The second type of electrical sounding shows curves similar to increasing step functions. This type is visible at the points in the sector periphery (poles 0 and 4 of the five profiles), were marls outcrop. The last ascending branch curve

200 Ω m. Resistivities in this material can vary from a few hundred Ω m to a few thousand Ω m, according to the soundings. Moreover, its average thickness is more than 40 m. This is precisely the layer that would better correspond to the limestone, which is the main concern of this investigation. • The third material’s resistivity varies from a few dozen to less than 200 Ω m.

Fig. 7 Electrical section along profile 1 of sector 1

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Fig. 8 3D resistivity model of sector 1 with values greater than 200 Ω m. X, Y altitudes are expressed in meters; moreover, X and Y are local coordinates that can be linked to the UTM system

indicates a strong substratum. The inversion calculations carried out for these particular points generally show electrical resistivity values for this bedrock less than 100 Ω m. This resistivity value might correspond to calcareous marl or more certainly to water-saturated limestones. However, it is rather unlikely that there exists a significant extension of pure limestone underneath. Interpretation results of the 3D modeling enable visualization of the resistant terrain evolution, from the sector’s top part to the deepest zones, which are assumed to represent rich limestone areas. Figure 9 shows a 3D view of the electrical resistivity’s distribution with values larger than 200 Ω m. This model indicates the presence of limestone only in the central part of the sector and the deposit extension depth seems limited.

Sector 3 The inversion calculations carried out for these particular points generally show electrical resistivity values for this bedrock are less than 100 Ω m. This resistivity value might correspond to calcareous marl or more certainly to watersaturated limestones. However, it is rather unlikely that there exists a significant extension of pure limestone underneath. Electrical resistivities recorded in this sector are very low, suggesting the domination of marl formations. As far as the color scale is concerned, the blue represents the more

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conductive geological formations (marls) and the red the more resistant formations (limestones). In order to provide an improved 3D image of resistant bodies (Mascot Laurent 2004), representing limestone blocks, the values smaller than 200 Ω m have been hidden (Fig. 10).

Conclusion Based on the vertical electrical sounding interpretation, several conclusions can be drawn: • In addition to being cheaper than mechanical drilling,

the vertical electrical sounding method seems to have solved the problem we had at hand. Indeed, the electrical resistivity contrast between the limestone, the object of our interest, and surrounding marl formations was very useful in precisely determining their geometry and evaluating their extension, depth and thickness. • Sector 1 has an appreciable limestone thickness in its central portion and limestones are absent in the northern and southern parts of the sector. • Sector 2 indicates the presence of a substantially uniform limestone layer over almost the entire site. Its shallow depth (a few meters) and its relatively important thickness (50 m on the average) makes this sector very promising.


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• Sector 3 is the least favorable in terms of limestone pres-

ence, with the exception of its northwestern part, where a resistant zone has been identified. The rest of the sector is mainly composed of marl formations. • Aggregate reserves in the first sectors (1 and 2) seem very interesting and probably meet the needs for aggregates.

Acknowledgements The authors would like to thank the colleagues of the Physics of Earth Laboratory for all the help they bring to the success of our research projects. The Physics of Earth Laboratory is affiliated with the University of Boumerdès, School of Hydrocarbons, Algeria.

Compliance with ethical standards Conflicts of interests On behalf of all authors, the corresponding author states that there is no conflict of interest.

References Barker RD (1989) Depth of investigation of collinear symmetrical fourelectrode arrays. Geophysics 54(8):1031–1037

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Dahlin T (1996) 2D resistivity surveying for environmental and engineering applications. First Break 14(7):275–283 Kuntz G et al (1970) Traitement automatique des sondages électriques. Geophys Prospect 18:157–198 Loke MH, Barker RD (1996) Practical techniques for 3D resistivity surveys and data inversion. Geophys Prospect 44:499–523 Mascot Laurent (2004) Modélisation directe et inverse en prospection électrique sur des structures 3D complexes par les éléments finis, thèse de doctorat en cotutelle de l’université de Nantes et de l’université de Lausanne Roy A (1972) Depth of investigation in Wienner three-electrode and dipole-dipole resistivity methods. Geophys Prospect 20:329–340 Roy A, Apparao A (1971) Depth of investigation in direct current methods. Geophysics 36:943–959 Xmelevskou BK (1984) Electrical prospection, pp 105–115 (in Russian) Zohdy AAR (1989) A new method for the automatic interpretation of Schlumberger and Wienner Sounding curve. Geophysics 54(2):245–253