Detection of illegal mine voids using electrical resistivity tomography: The case-study of Raniganj coalfield (India)

Engineering Geology 213 (2016) 120–132

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Detection of illegal mine voids using electrical resistivity tomography: The case-study of Raniganj coalfield (India) Abhay Kumar Bharti, S.K. Pal ⁎, Piyush Priyam, Vipin Kumar Pathak, Rajwardhan Kumar, Sunny Kumar Ranjan Department of Applied Geophysics, Indian School of Mines, Dhanbad 826004, India

a r t i c l e

i n f o

Article history: Received 23 March 2016 Received in revised form 6 September 2016 Accepted 12 September 2016 Available online 13 September 2016 Keywords: ERT Joint inversion Mine voids Illegal mining Raniganj coalfield India

a b s t r a c t Unauthorised coal mining activities may result in development of hidden hollows, rat holes, galleries, goafs, shafts etc., which pose great threats of land subsidence, fire, water flooding leading to severe environmental hazards, health problems and safety issues to the local people. Present study deals with delineation and mapping of unauthorised coal mine voids/galleries over an abandoned old mine around Khudia open cast mine, Nirsa, Raniganj coalfield, India. Electrical resistivity tomography (ERT) study comprising Wenner, Schlumberger, Dipole-Dipole, and Gradient arrays has been carried out along three parallel profiles over the affected area. Further joint inversion of all combined arrays has also been carried out using 2.5D resistivity inversion program, to combine the relative advantages of all the arrays, for producing superior results. 2D ERT sections have been generated for the filtered data sets with a constant quality factors and two different current thresholds. The best results have been obtained, from joint inversion of all combine arrays for the filtered data with higher current threshold. The observed resistivity anomalies are well correlated with the depth of coal seam occurrences as observed in the borehole litholog of nearby area. Finally, a geoelectric model of four unauthorised coal mine galleries has been established with their extension and orientation over the study area. The results demonstrate the suitability of the ERT technique through joint inversion of all combined arrays for characterization of illegal coal mine workings. © 2016 Published by Elsevier B.V.

1. Introduction India is one of the biggest coal producing countries in the world with an annual production of ~ 600 million tonnes. Initially, coal mining in India started over Raniganj coalfields during eighteenth century in a random manner and regular mining started in early nineteenth century. The collieries in Raniganj coalfield were owned by several companies and owners. These were nationalized in 1973. Prohibited mining of coal has been serious matter of concern for a long period of time. Generally, abandoned mines are the main source of prohibited coal mining activities. Subsequent to the economic coal extraction, the remaining coal in an abandoned mine is stolen by coal mafias and illegal miners which leads to roof falling, water flooding, poisonous gas leaking and further results in loss of land, property and life. Generally, illegal miners dig rat holes in abandoned mining areas. Over the time, the underground hollows are left unfilled and diggers enter inside with ease and excavates unscrupulously. The mining cavities collapse due to natural alteration processes in the course of time. About 200 miners died in Mahavir Collieries, Raniganj coalfield, India in 2001. Illegal mining may also take

⁎ Corresponding author. E-mail address: [email protected] (S.K. Pal).

http://dx.doi.org/10.1016/j.enggeo.2016.09.004 0013-7952/© 2016 Published by Elsevier B.V.

place on fresh land in small patches in haphazard manner which always keep on changing in different direction and depth. Sometimes miners burst small explosives after the first entry point which is called as foxhole. The illegal mining may hamper the legal mining activities as the illegal tunnels are made in a random and unscientific manner. Other concern that may arise is the damage of the foundation of buildings located in these mining areas. The local roads and railway tracks may also be severely damaged due to potholes, sinkholes and land subsidence which cause inconvenience to the transportation. Sometimes, during illegal mining oxygen seeps into the underground methane-charged coal seam which leads combustion of the coal and spread of coal fire in the vicinity (Prakash et al., 1995, 1997; Prakash and Gupta, 1998, 1999; Vaish and Pal, 2013, 2015a, 2015b, 2016; Pal and Vaish, 2014; Pal et al., 2016; Singh et al., 2015; Kumar et al., 2014, 2015a; Singh and Pal, 2015; Bharti et al., 2014, 2016; Srivardhan et al., 2016). Such kind of coal fire incidents occurred four times between December 2007 and February 2008 near Nimcha village, Raniganj coalfield. The burning of coal leads to the formation of voids due to reduction of volume by the transformation of coal into ashes (Bharti et al., 2016). The term subsurface cavity in coal mining area is used to denote all subsurface features, such as mining galleries, goafs, rat holes, foxholes, caves, caverns, voids, potholes and sinkholes etc. caused by different coal mining and coal fire activities. The delineation and mapping of these subsurface cavities are

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essential over Raniganj coalfield, India for safety of the local environment, agricultural land, ecology and health of the people. Different geophysical methods used for detection of subsurface cavity/cave/void/goaf/gallery, sinkhole, karst topography etc. are i) electrical resistivity tomography (Cardarelli et al., 2006; Pánek et al., 2010; Cardarelli et al., 2010; Gómez-Ortiz and Martín-Crespo, 2012; Martínez-Pagán et al., 2013; Metwaly and AlFouzan, 2013; Satitpittakul et al., 2013; Cardarelli et al., 2014; Kumar et al., 2015b; Bharti et al., 2016; Bhattacharya and Shalivahan, 2016 among others), ii) Vertical Electrical Soundings (Rodríguez Castillo and Reyes Gutierrez, 1992) iii) Induced Polarization Tomography (Brown et al., 2011; Martínez-Moreno et al., 2014), iv) Self-Potential (Lange, 1999), v) Ground Penetration Radar (Leucci and De Giorgi, 2010; Brown et al., 2011; Gómez-Ortiz and Martín-Crespo, 2012 among others), vi) Electromagnetic (Lange, 1999), vii) Seismic Refraction Tomography (Cardarelli et al., 2010, 2014), viii) Multichannel Analysis of Surface Waves (Debeglia et al., 2006), ix) Microgravity methods (Gambetta et al., 2011; Reynolds, 2011; Martínez-Moreno et al., 2014 among others), and x) Magnetic (Mochales et al., 2008). Among the various geophysical

methods, electrical resistivity tomography (ERT) method has been established to be a very effective tool for characterization of cavities (Van Schoor, 2005; Ezersky, 2008; Cardarelli et al., 2006, 2010, 2014; Metwaly and AlFouzan, 2013; Martínez-Pagán et al., 2013; Singh, 2013; Singh et al., 2016; Martínez-Moreno et al., 2014; Bharti et al., 2016). In recent years, ERT technique is becoming more popular as a key technique for environmental, mining engineering, civil engineering and shallow subsurface investigations (Morelli and LaBrecque, 1996; Cardarelli et al., 2006, 2010; Santarato et al., 2011 among others), including void/cave detection (Van Schoor, 2002; Zhou et al., 2004; Abu-Shariah, 2009; Ortega et al., 2010; Pánek et al., 2010; Ravbar and Kovačič, 2010; Martínez-Moreno et al., 2014; Satitpittakul et al., 2013; Bharti et al., 2016). This is mainly due to its cost effectiveness, simplicity in automated data-acquisition, efficient user-friendly inversion of acquired data with highly reliable geoelectric model of subsurface features (Van Schoor, 2005; Athanasiou et al., 2007; Loke et al., 2013; Revil et al., 2013; Martínez-Moreno et al., 2014; Singh et al., 2016; Bharti et al., 2016; Bhattacharya and Shalivahan, 2016). The subsurface resistivity distribution is determined by making measurements on the ground

880

240

8604

121

23040/

(c) 5.4 - 5.7m, L14 13.2 - 13.6m, L13 14.8 - 15.55m, L12 16.6 - 17.25m, L11 18.8 - 19.65m, L10 22.3 - 23.85m, L9 25.9 - 26.65m, L8 29.2 - 29.65m, L7

(a)

(b)

20m C/

20m

41.55 - 42.10m, L4 43.75 - 44.30m, L3

504 m

A/

59.25 - 59.70m, L1

189 m

Portal

252 m

Belchuri Road

52.3 - 52.95m, L2

B/

A

33.4 - 33.95m, L6 34.7 - 35.20m, L5

Khudia Open Cast Abandoned Mine

Unauth orized mining

66.2 - 83.4m, X-seam

98.1 - 98.4m, L II

B

109.3 - 109.75m, L I

C Dhanbad

NH 2

Durgapur

Soil Major coal seam Sandstone / shale Local coal seam

Fig. 1. (a) Map of the study area along with generalized geological map of Raniganj Coal field, showing the location and lengths of ERT sections (b) field photograph showing illegal mining galleries, (c) available borehole litholog of nearby area showing different coal seams (ECL, 1998).

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surface. From these measurements, the true resistivity distribution of the subsurface can be estimated. The ground resistivity is related to various geological parameters, such as mineral and fluid contents, porosity, degree of water saturation in the rock etc. 2D ERT resistivity prospecting yields information about both lateral and vertical distribution of the Earth's resistivity and therefore can be used in both qualitative and quantitative ways to identify the structures and features at shallow depths. 2. Study area The Raniganj coalfield is an important coalfield in Burdwan District of West Bengal, India, which lies within latitudes 23° 35°N to 23° 55°N

and longitudes 86° 45°E to 87° 20°E over Damodar valley. It is a type of synclinal basin where Gondwana sediments is lying unconformable over the basement. The southern boundary of the basin is marked by major faults. The Quaternary alluvial and residual soils with laterite capping blanket the older Gondwana rocks at the eastern part. Gondwana rocks are exposed at western and southern parts. The rocks of Precambrian to Quaternary ages are also exposed in some parts but the major portion of the Raniganj coalfield is occupied by Gondwana Group of rocks (Guha and Vinod, 2012). In the Raniganj formation, about 10 major coal horizons, designated as seam-I to seam-X in an ascending order from bottom to top (near surface) with thickness ranging from 1 m to 24 m, have been observed. In addition, numerous local seams are also observed in different parts of Raniganj coalfield. Available

A RD in m Depth (m)

A/

(a)

Ωm

A RD in m

A/

Depth (m)

(b)

Ωm

A RD in m

A/

Depth (m)

(c)

Depth (m)

Ωm

A RD in m

A/

(d)

Ωm

A RD in m

A/

Depth (m)

(e)

Ωm Fig. 2. 2D ERT section along AA/ (Fig. 1) using (a) Wenner, (b) Schlumberger, (c) Gradient, (d) Dipole-Dipole arrays and (e) joint inversion of all the arrays, for the filtered data of 95% confidence level with current threshold of 100 mA.

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Borehole log of nearby area is shown in Fig. 1c (ECL, 1998). The Raniganj formation of upper Permian age is comprised of fine grained feldspathic sandstones, shales with coal seams. Raniganj coal basin is also affected by igneous intrusion as a form of sills and dykes (Gupta, 1999). 3. Methodology Conventional resistivity soundings or resistivity profiling may be inadequate for resolving 2D anomalies and shallow effects, such as

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shallow cavities (Griffiths and Barker, 1993). Subsequent to these disadvantages, the ERT method was developed for the investigation of areas of complex geology (Sumanovac and Weisser, 2001; Ogilvy et al., 2002; Tejero et al., 2002; Chambers et al., 2006; Cardarelli et al., 2006, 2010). Presently, ERT data have been acquired using 64 electrodes (with 61 channels) FlashRES-Universal electrical resistivity tomography (ERT) system manufactured by ZZ Resistivity Imaging Pvt. Ltd., Australia (Zhe et al., 2007). The total 64 electrodes used for entire data acquisition system are i) 2 current electrodes (A and B) ii) 1 reference electrode (M)

A RD in m Depth (m)

A/

(a)

Ωm

A/

A RD in m

Depth (m)

(b)

Ωm

A RD in m

A/

Depth (m)

(c)

A/

Depth (m)

Ωm

A RD in m

(d)

Depth (m)

Ωm

A RD in m

A/

(e)

Ωm Fig. 3. 2D ERT section along AA/ (Fig. 1) using (a) Wenner, (b) Schlumberger, (c) Gradient, (d) Dipole-Dipole arrays and (e) joint inversion of all the arrays, for the filtered data of 95% confidence level with current threshold of 150 mA.

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and iii) 61 potential electrodes (VMN1-VMN61) for 61 simultaneous reading (i.e. 61 channel) with respect to reference electrode (M). ERT data have been acquired using four standard arrays i.e., Wenner array, Schlumberger array, Dipole-Dipole array and Gradient array (Loke, 2004; Dahlin and Zhou, 2004, 2006; Loke et al., 2013). Generally, the Wenner array can delineate horizontal features very well but it is poor for resolving vertical structures (Loke, 2004; Dahlin and Zhou, 2004, 2006). The standard Schlumberger array is moderately effective for resolving both horizontal and vertical structures. The Dipole-Dipole array is quite poor to resolve horizontal resistivity variations, however quite good to resolve vertical resistivity variation. Dahlin and Zhou

(2004, 2006) argued that the gradient array with multiple current-electrode combinations is most suitable among the different electrode arrays in terms of resolution of subsurface structures with complex geology, especially for vertical structures. The acquired data have been processed in Universal data checking program of FlashRES for filtering. The enhanced filtered data of a constant quality factor (i.e., data of a constant confidence level or above) with different current thresholds have been inverted in 2.5D resistivity inversion program (Zhou and Greenhalgh, 1999, 2000; Zhe et al., 2007) for generating 2D subsurface geoelectric model along the profiles. 2D ERT sections have been generated separately for each array. Further,

B/

Depth (m)

B RD in m

(a)

Ωμ

B/

Depth (m)

B RD in m

(b)

Ωμ

B/

Depth (m)

B RD in m

(c)

B/

Depth (m)

Ωμ

B RD in m

(d)

Ωμ

B/

Depth (m)

B RD in m

(e)

Ωμ

Fig. 4. 2D ERT section along BB/ (Fig. 1) using (a) Wenner, (b) Schlumberger, (c) Gradient, (d) Dipole-Dipole arrays and (e) joint inversion of all the arrays, for the filtered data of 95% confidence level with current threshold of 100 mA.

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joint inversion of all combine arrays has also been performed using the 2.5D ZZRESINV inversion program (Zhou and Greenhalgh, 1999, 2000; Zhe et al., 2007; ZZ Resistivity Imaging Pvt. Ltd., Australia) for improved inverted 2D ERT section (De La Vega et al., 2003; Stummer et al., 2004; Athanasiou et al., 2007). Different features have been delineated based on the distinct resistivity variation, their approximate locations, depths and dimensions in the 2D ERT sections. The colour scales for all 2D resistivity sections are given separately for enhancement of different local resistivity anomaly distribution corresponding to different subsurface features (Cardarelli et al., 2006; Song and Kuenzer, 2014; among others).

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4. Results and discussions Three ERT profiles (AA/, BB/ and CC/) have been selected parallel to the Belchuri road over a known abandoned old mine, which is affected by illegal mining around Khudia open cast mine, Nirsa, Raniganj coalfield, India. Schematic location map of the study area has been shown in Fig. 1. Initially, a signature of land subsidence was observed over an area near RD (reduced distance) 80 m to RD 90 m before ERT data acquisition along the profile BB/. In this regard, three parallel profiles have been considered keeping the zone of subsidence as an approximate center of all the profiles. The lengths of ERT profiles AA/, BB/ and CC/ are

B RD in m Depth (m)

B/

(a)

Depth (m)

Ωm

B RD in m

B/

(b)

B/

Depth (m)

Ωm

B RD in m

(c)

Ωm

B RD in m

B/

Depth (m)

(d)

Ωm

B/

(e)

Depth (m)

B RD in m

Ωm

Fig. 5. 2D ERT section along BB/ (Fig. 1) using (a) Wenner, (b) Schlumberger, (c) Gradient, (d) Dipole-Dipole arrays and (e) joint inversion of all the arrays, for the filtered data of 95% confidence level with current threshold of 150 mA.

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252 m, 189 m and 504 m with electrode spacings of 4 m, 3 m and 8 m; respectively. The site near profiles AA/ and BB/ is of rugged topography with dump material. Generally, the illegal mine galleries/voids are of variable dimensions, depths and orientations. Accordingly, three parallel ERT profiles with three different lengths and electrode spacings have been considered for delineation of voids of variable dimensions and depths. Figs. 2, and 3 demonstrate the 2D ERT sections of profile AA/ generated using (a) Wenner array, (b) Schlumberger array, (c) Gradient array (d) Dipole-Dipole array, and (e) joint inversion of the all combined arrays for the collected data sets with a constant quality factor (95% confidence level) and two different current thresholds i.e., i) 100 mA (Fig. 2); and ii) 150 mA (Fig. 3), respectively. Similarly, 2D ERT sections of profiles BB/ (Figs. 4 & 5) and CC/ (Figs. 6 & 7) have also been generated with the same combinations of current thresholds and a constant

quality factor (95% confidence level). The numbers of filtered data points, extracted using Flashdatacheck programm (ZZ Resistivity Imaging Pvt. Ltd., Australia) for the acquired field data of the three ERT profiles with constant quality factor and three different current thresholds (50 mA, 100 mA and 150 mA), are given in Table 1. It is observed from Table 1 that total numbers of data points used in the inversion process are reduced with increasing current thresholds. It may be occurred as the relatively noisy data are filtered out with increasing order of current thresholds. The outcome may change with different current injections depending on contact resistance of the electrodes. It may also vary, depending on spontaneous potential due to environmental noise. Out of three different current thresholds with the constant data quality factor (95% confidence level), the best result has been obtained for the current threshold of 150 mA (Figs. 3, 5, and 7). The root mean square (RMS) error variation plots for the inverted ERT sections of

C RD in m Depth (m)

C/

(a)

Ωμ

C RD in m Depth (m)

C/

(b)

Ωμ

RD in m

C/

Depth (m)

C

(c)

Ωμ

C RD in m Depth (m)

C/

(d)

Depth (m)

Ωμ

C RD in m

C/

(e)

Ωμ

Fig. 6. 2D ERT section along CC/ (Fig. 1) using (a) Wenner, (b) Schlumberger, (c) Gradient, (d) Dipole-Dipole arrays and (e) joint inversion of all the arrays, for the filtered data of 95% confidence level with current threshold of 100 mA.

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RD in m

C/

Depth (m)

C

127

(a)

Depth (m)

Ωμ

C

RD in m

C/

(b)

Ωμ

C RD in m Depth (m)

C/

(c)

Ωμ

RD in m

C/

Depth (m)

C

(d)

Ωμ

C/

RD in m

Depth (m)

C

(e)

Ωμ

Fig. 7. 2D ERT section along CC/ (Fig. 1) using (a) Wenner, (b) Schlumberger, (c) Gradient, (d) Dipole-Dipole arrays and (e) joint inversion of all the arrays, for the filtered data of 95% confidence level with current threshold of 150 mA. Table 1 The filtered data points used in the inversion of different arrays for different current thresholds with a constant confidence level. Array

Data points with 95% confidence level 50 mA

100 mA

150 mA

Profile AA/ Wenner Schlumberger Gradient Dipole-Dipole Joint inversion

585 nos. 2132 nos. 7095 4408 nos. 58,714 nos.

574 nos. 2102 nos. 5682 3423 nos. 56,680 nos.

485 nos. 2010 nos. 4878 3223 nos. 54,898 nos.

Profile BB/ Wenner Schlumberger Gradient Dipole-Dipole Joint inversion

651 nos. 2598 nos. 7982 4777 nos. 81,112 nos.

594 nos. 2476 nos. 6560 3450 nos. 80,710 nos.

571 nos. 2397 nos. 4054 3277 nos. 80,005 nos.

Profile CC/ Wenner Schlumberger Gradient Dipole-Dipole Joint inversion

646 nos. 2509 nos. 7343 4746 nos. 71,311 nos.

589 nos. 2464 nos. 6894 4270 nos. 68,364 nos.

549 nos. 2412 nos. 4576 3447 nos. 57,424 nos.

Wenner array, Schlumberger array, Dipole-Dipole array, Gradient array and joint inversion of combine arrays with three different current thresholds for each profiles (AA/, BB/, and CC/) are shown in Fig. 8. It is observed from Fig. 8 that RMS error reduces with the increase in current threshold for all the arrays in all the three profiles. It is also observed that among all the arrays, RMS error is minimum for the joint inversion of all combined arrays followed by Gradient, Schlumberger, Dipole-Dipole and Wenner arrays for the same quality factor. The characteristics of various features delineated from the inverted resistivity sections of different arrays for ERT profiles AA/, BB/ and CC/ are given in Tables 2, 3 and 4, respectively. The features have been delineated based on considerable resistivity contrast among matrix rocks i.e. mainly sandstone/clay stone/shale etc. and void space. Different possible coal seams (as observed in Fig. 1c) mined by illegal activities near ERT profiles AA/, BB/ and CC/ are shown in Tables 2, 3 and 4, respectively. The comprehensive studies of Figs. 2 to 7 and Tables 2 to 4 demonstrate that different resistivity anomalies corresponding to different coal seam occurrences and associated illegal mining (Fig. 1c and Fig. 9) have been properly delineated using joint inversion of combined data collected by all arrays (Figs. 2e, 3e, 4e, 5e, 6e and 7e) because, it provides 2D inverted

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RMS

(a)

RMS

(b)

RMS

(c)

Fig. 8. Root-mean-square (RMS) error for the inverted ERT sections of Wenner, Schlumberger, Dipole-Dipole, Gradient and joint inversion of combine arrays with the data of 95% confidence level and different current threshold for profiles (a) AA/, (b) BB/ and (c) CC/.

sections with minimum RMS error among all the individual arrays (Fig. 8). It also delineates maximum numbers of resistivity anomalies (subsurface features) corresponding to different possible illegal mine voids/galleries. The resistivity anomalies delineated by this technique could also be correlated well with all the individual arrays (Wenner, Schlumberger, Dipole-Dipole, Gradient) and borehole litholog of nearby area (Fig. 1c and Fig. 9). However, no specific array could delineate all the features together. Generally, there is no single optimum array which can always provide valid and useful results, independent of the target characteristics. Likewise, the geoelectrical models estimated by the inversion of different arrays over the same structure can be dissimilar (Athanasiou et al., 2007). Stummer et al. (2004) suggested that combined data sets coming from different arrays provide more information than any individual array. De La Vega et al. (2003) observed that the

joint inversion of combined data collected over a hydrocarbon contaminated site using Dipole–Dipole and Wenner arrays, provides a greater depth of investigation and improved lateral resolution compared to the inversion results generated from individual array, separately. Theoretically, joint inversion of combined data sets which are collected using different arrays over the same profile would support to integrate the relative advantages of all the arrays for estimation of better inverted resistivity section (Athanasiou et al., 2007). The present study also proves the suitability of the joint inversion of all combined arrays for investigation of illegal mining activities. A model (Fig. 9) of illegal coal mine voids/galleries over the study area has been established by correlating different equivalent anomalous resistivity zones among all the three profiles (AA/, BB/ and CC/) using 2D ERT sections generated from joint inversion of all combine arrays with

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Table 2 Details of features delineated in AA/ profile (Figs. 2 and 3) using Wenner array, Schlumberger array, Gradient array, Dipole-Dipole array and joint inversion of all the arrays corresponding to different current threshold and quality factor. 2D ERT section

Mine working features

Reduced distance (RD in m)

Approx. depth (m)

Inverted resistivity (Ω m)

Possible coal seam for cavity formation (Fig. 1c)

RMS error

Current threshold of 100 mA with 95% confidence level Wenner 1 25–160 Schlumberger 1 14–159 Gradient 1 18–33 2 74–92 3 130–136 4 170–200 Dipole-Dipole 1 61–111 Joint inversion 1 20–38 2 75–93 3 123–143 4 173–202

10–44 15–35 14–23 13–25 24–32 14–27 11–21 13–23 14–26 17–31 15–29

~126 ~179 ~120 ~130 ~115 ~125 ~125 ~179 ~190 ~141 ~150

L3–L13 L5–L12 L9–L12 L8–L13 L7–L8 L8–L12 L10–L13 L9–L13 L8–L12 L7–L10 L8–L12

0.125 0.072 0.014

Current threshold of 150 mA with 95% confidence level Wenner 1 0–38 2 71–138 3 172–195 Schlumberger 1 0–16 2 79–134 3 179–197 Gradient 1 11–44 2 67–99 3 130–137 4 180–198 Dipole-Dipole 1 28–128 Joint inversion 1 12–46 2 69–101 3 124–140 182–192 4

12–28 14–41 25–36 11–23 12–48 30–39 12–23 15–30 22–26 18–31 10–26 13–23 14–29 16–26 16–22

~160 ~190 ~153 ~189 ~211 ~153 ~120 ~135 ~120 ~120 ~116 ~215 ~255 ~178 ~174

L8–L13 L5–L12 L5–L8 L9–L13 L3–L13 L6–L7 L9–L13 L7–L12 L8–L9 L7–L10 L8–L13 L9–L13 L8–L12 L8–L11 L10–L11

data of 95% confidence level and 150 mA current threshold. Four lateral galleries (G1, G2, G3 and G4) have been approximated based on interpolations among corresponding anomalies in subsequent ERT profiles (AA/, BB/ and CC/). Imprints of four small voids/galleries (G1, G2, G3 and G4) with relatively high resistivity of ~ 174 Ω m to ~ 255 Ω m at depth of ~ 13 m to ~ 29 m have been delineated in profile AA/, which are associated with mining of local coal seam L8 to L13 (Fig. 9). Imprints of two small voids/galleries with relatively high resistivity of ~150 Ω m

0.034 0.010

0.121

0.071

0.013

0.034 0.011

to ~190 Ω m at depth of ~14 m to ~30 m have been delineated in profile BB/, which are associated with mining of local coal seam L7 to L12 (Fig. 9). In addition, a signature of small collapsed void/gallery has also been delineated with relatively low resistivity of ~52 Ω m at depth of ~13 m to ~18 m, which is possibly associated with mining of local coal seam L7 to L12 (Fig. 9) and subsequent collapse of coal seam L13. The land subsidence was prominently observed just over this specific area at RD (reduced distance) 80 m to RD 90 m during ERT data acquisition

Table 3 Details of distinct features delineated in BB/ profile (Figs. 4 and 5) using Wenner array, Schlumberger array, Gradient array, Dipole-Dipole array and joint inversion of all the arrays corresponding to different current threshold and quality factor. 2D ERT section

Mine working features

Reduced distance (RD in m)

Approx. depth (m)

Inverted resistivity (Ω m)

Possible coal seam for cavity formation (Fig. 1c)

RMS error

Current threshold of 100 mA with 95% confidence level Wenner 1 31–120 Schlumberger 1 37–117 Gradient 1 40–64 2 145–168 Dipole-Dipole 1 27–69 Joint inversion 1 4–62 2 133–187

16–31 21–37 20–29 15–34 10–19 13–27 15–34

~167 ~147 ~164 ~158 ~92 ~185 ~143

L7–L11 L5–L9 L7–L10 L6–L12 L10–L13 L8–L13 L6–L12

0.223 0.112 0.023

Current threshold of 150 mA with 95% confidence level Wenner 1 0–122 Schlumberger 1 0–125 Gradient 1 0–70 2 80–100 3 139–170 Dipole-Dipole 1 24–75 2 121–142 Joint inversion 1 10–50 2 79–92 3 145–170

12–42 12–42 20–30 10–18 24–32 18–33 20–30 14–29 12–18 16–30

~101 ~136 ~165 ~54 ~160 ~116 ~85 ~150 ~52 ~190

L4–L13 L4–L13 L7–L9 L11–L13 L7–L8 L7–L10 L7–L9 L8–L12 L11–L13 L7–L11

0.071 0.020

0.090 0.067 0.024

0.029 0.021

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Table 4 Details of distinct features delineated in CC/ (Figs. 6 and 7) profile using Wenner array, Schlumberger array, Gradient array, Dipole-Dipole array and joint inversion of all the arrays corresponding to different current threshold and quality factor. 2D ERT section

Mine working features

Reduced distance (RD in m)

Approx. depth (m)

Inverted resistivity (Ω m)

Possible coal seam for cavity formation (Fig. 1c)

RMS error

Current threshold of 100 mA with 95% confidence level Wenner 1 112–321 Schlumberger 1 116–326 Gradient 1 125–323 Dipole-Dipole 1 118–319 Joint inversion 1 130–305

11–41 13–43 16–48 12–45 16–42

~150 ~160 ~170 ~108 ~460

L5–L13 L4–L13 L3–L11 L3–L13 L4–L11

0.074 0.061 0.055 0.056 0.050

Current threshold of 150 mA with 95% confidence level Wenner 1 150–260 Schlumberger 1 124–308 Gradient 1 130–322 Dipole-Dipole 1 123–312 Joint inversion 1 125–312

11–39 11–38 20–49 11–45 16–45

~191 ~204 ~223 ~180 ~320

L5–L13 L5–L13 L3–L9 L3–L13 L3–L11

0.080 0.055 0.051 0.058 0.048

along the profile BB/. A single imprint of large horizontally extended void/gallery from RD 125 m to RD 312 m with relatively high resistivity of ~320 Ω m at depth of ~16 m to ~45 m has been delineated in profile CC/, which is possibly associated with mining of local coal seam L3 to L11 (Fig. 9). An illegal mine portal has been observed just behind the profile CC/ (Fig. 1b). The ERT profile CC/ could not differentiate all the four distinct resistivity signatures as observed in ERT profile AA/. It may be due to relatively large electrode spacing covering long profile length. For the fixed number of electrodes, an array of larger electrode spacing covering larger profile length gains more depth of penetration with less resolution while an array of smaller electrode spacing covering smaller profile length gains more resolution with less depth of investigation. The galleries in central part (G2 & G3, Fig. 9c) of the study area may be connected through open portal (Fig. 9d). The galleries of both sides (G1 & G4, Fig. 9c) may be interconnected with the portal through a goaf (Fig. 9d) which lead to generate a single horizontally extended high resistivity zone near profile CC/. There is no signature of additional portal in the abandoned mine around profile CC/. It is clear that the depths of the delineated voids/galleries are relatively deeper near profile CC/ and become gradually shallower towards BB/ and AA/. The illegal miners are ignorant regarding extension and orientation of the galleries during underground mining, which may lead to shallow rock cover at distant place away from the portal. The initial illegal mining galleries would have been smaller in size. Afterward, the size of the galleries may be extended due to fall of roofs and side walls. The roofs and side walls may be damaged due to natural wear and tear process in different seasonal variations. This leads to generation of lose rock matrix with several primary and secondary fractures in the surroundings, which results with the imprints of bigger galleries with relatively high resistivity contour. Thus, the dimensions of the galleries are marked in conservative manner, covering all the surrounding fractured rock-masses. The differences in resistivity values among the equivalent anomalies/features (cavities/galleries) may be possibly due to dissimilarities in complex subsurface resistivity environment of the old mine workings near the corresponding profile (AA/, BB/ and CC/) locations caused by natural wear and tear processes. Air filled voids/galleries exhibit extremely high resistivity values. Nevertheless, the voids/galleries in all the profiles AA/, BB/ and CC/ have been delineated with resistivity of ~ 150 Ω m to ~ 350 Ω m. Generally, illegal mine voids/galleries would be partially filled with humid air and moist rocks, moist coal debris fallen from roof and surrounding walls, which are resulted in resistivity of ~ 150 Ω m to ~350 Ω m. The synthetic model study of Ezersky (2008) and Bharti et al. (2016) with simulated field conditions for voids with varying resistivity through forward modeling of Gradient and Dipole-Dipole arrays prove that locations and dimensions of the voids have been resolved with sufficient accuracy except their resistivities. They argued that the considered resistive voids are observed to be

moderately conductive void due to complex subsurface heterogeneous environment. 5. Conclusions Study for delineation and mapping of illegal coal mine voids/galleries/goafs has been carried out using electrical resistivity tomography with three parallel ERT profiles of different lengths and electrode spacings over a known abandoned old mine around Khudia open cast mine, Nirsa, Raniganj coalfield, India. Different 2D ERT sections have been generated using Wenner array, Schlumberger array, Gradient array, DipoleDipole array and joint inversion of the all combined arrays for the collected data sets with 95% confidence level and two different current thresholds i.e., i) 100 mA; and ii) 150 mA. It is observed that total numbers of data points used in the inversion process are reduced with the increasing current thresholds, while the collected data are filtered with constant quality factor and varying current thresholds. Present study also reveals that RMS error reduces with the increase in current threshold for all the arrays in all the three profiles. Moreover, RMS error is minimum for the joint inversion of all combined arrays followed by Gradient, Schlumberger, Dipole-Dipole and Wenner arrays for the same quality factor. The resistivity imprints corresponding to all possible illegal mine voids/galleries have been delineated very well from joint inversion of all combine arrays with the data of 95% confidence level and 150 mA current threshold. These imprints are correlated well with all individual arrays. However, no specific array could delineate all the features together. The depths of the observed resistivity anomalies are well corroborated with the coal seam occurrences in the available borehole litholog of nearby area which confirms the illegal coal mining activities. Finally, a geoelectric model of illegal coal mine voids/galleries with their extensions and orientations over the study area has been established by correlating different equivalent resistivity signatures among all the three profiles (AA/, BB/ and CC/) using 2D ERT sections generated from joint inversion of all combine arrays. Four mine galleries (G1, G2, G3 and G4) have been identified, which are possibly interconnected through a goaf to a double galleried portal in the open cast mine. The portal is openly visible. The electrode spacing and the data acquisition methods used in this study have resulted in an optimal compromise between resolution and depth of penetration for successful mapping and delineation of illegal mine galleries/voids. Acknowledgements Authors are thankful to DST for funding a project (SB/S4/ES-640/ 2012) on geotechnical characterization of Jharia coal field area using Geophysical techniques. Authors are also thankful to Prof. J. Wasowski, CoEiC, and anonymous reviewers for their valuable suggestions. The

A.K. Bharti et al. / Engineering Geology 213 (2016) 120–132

Sandstone / shale Major coal seam

5.4 - 5.7m, L14

G4

G3

A/

(a)

29.2 - 29.65m, L7

Ωm

33.4 - 33.95m, L6 34.7 - 35.20m, L5

52.3 - 52.95m, L2 59.25 - 59.70m, L1

B G1

G2

RD in m

Depth (m)

41.55 - 42.10m, L4 43.75 - 44.30m, L3

Approx. distance 20m

22.3 - 23.85m, L9 25.9 - 26.65m, L8

G2

Depth (m)

13.2 - 13.6m, L13 14.8 - 15.55m, L12 16.6 - 17.25m, L11 18.8 - 19.65m, L10

G1

A RD in m

G3

B/

(b)

Approx. distance 20m

Soil Local coal seam

131

Overburden, dump material

66.2 - 83.4m, X -seam

Ωm

(e) C

G1/G2/G3/G4 C/

(c) G2 G3

Ωm

G1

G4

Approx. distance 25m

Depth (m)

RD in m

A/

A

B

B/

C/

C

I llegal mining galleries

(f) (d) Fig. 9. A model of illegal mine voids/galleries has been established from 2D ERT sections of profiles (a) AA/ (b) BB/ and (c) CC/, using joint inversion of all the arrays with data of 95% confidence level and 150 mA current threshold. The galleries in central part (G2&G3, c) are exposed through open (d) portal. The galleries of both sides (G1 & G4, c) may be interconnected with the portal (d) near profile AA/, as there is no signature of additional portal. (e) Borehole litholog from Fig. 1c. (f) Final proposed model.

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