societal application of geophysics as an aid to a rescue operation at jaipur

JOURNAL GEOLOGICAL SOCIETY OF INDIAVol.79, February 2012, pp.155-160

0016-7622/2012-79-2-155/$ 1.00 © GEOL. SOC. INDIA

Societal Application of Geophysics as an Aid toa Rescue Operation at Jaipur

SUBASH CHANDRA1, SURENDRA ATAL

1,2, MD. AHMEDUDDIN1 and SHAKEEL AHMED

1

1CSIR-National Geophysical Research Institute, Uppal Road, Hyderabad - 500 6062Geological Survey of India, Lucknow

Email: [email protected]

Abstract: Geophysical techniques are normally employed to explore the subsurface and determine the anomalies butthey are always much larger in dimension. However, a very precise measurement was needed to pin point a misalignedhorizontal tunnel made at 47 m depth to intercept an old bore well and rescue a four year child fallen accidently on 9th

November at Jagatpura village in Jaipur, India. Due to the greater depth and presence of iron casing, electric power, etc.,no equipment could work to determine the orientation of the tunnel that has lost its direction. The dimension of the siteand many other constrains didn’t allow conventional geophysical methods to be used. An innovative method was thendesigned and successfully applied, which determined 1.92 m deviation of the tunnel end point to the east from the oldbore well. The digging was redirected accordingly and the bore well was intercepted. This paper describes the theoreticalbackground, forward simulations and the field experiment of the newly designed geophysical method.

Keywords: Tunnel alignment, Bore well, Societal application, Geophysics, Buried Bipole Pole (BBP) method.

INTRODUCTION

It is well known fact that India is severely facinggroundwater crisis for past few years. Low rainfall andexcessive water demand towards irrigation, industrial anddomestic purposes is causing the over-exploitation ofgroundwater resources leading to the lowering of the watertable and increasing the number of defunct wells. Suchdefunct wells are normally left unprotected carelessly, whichcauses an adverse effect on the small innocent children.Almost 18 cases were reported of child falling in theopen defunct bore wells in different parts of India. One ofsuch incident occurred in the early morning of 9th November2009 at Jagatpura village of Jaipur district in Rajasthan,India, where a four year old child, Sahil, son of Mr. LakshmanSingh (Fig.1) fell upside down and was stuck at 47 m depthinto a ~85 m deep defunct irrigation bore well. Althoughrescue operation was started promptly, a steady drizzle ofsand slowly covered the child in the well.

As site falls in the alluvium composed of sand and clay(Das, 1988), the rescue team removed 9 m of loose soil coverto the south of the old well, and made a 47 m deep rescuedug well of 1 m diameter at 6.4 m to the south of the old well.A horizontal tunnel was dug from the rescue well to reachthe old well. The tunnel could not intercept the old well in

spite of being extended to 7 m length. Using a magneticcompass it was estimated that the tunnel might have gotdiverted to a direction of N13°W, and a secondary tunnelwas made perpendicular to the main tunnel at a position of6.20 m with the expectation to intercept the old well at adistance of 1.40 m. However, old well still could not beintercepted in spite of digging 2.4 m long secondary tunnel.Thus, the position of the far end of the tunnel with respectof the old well was completely unknown. The rescueoperation was also stopped for a few days due to rainfall.Twenty two days had passed at this point of time, and thelocal authorities decided to make use of scientificinvestigations to retrieve the body of the child. Geophysicaltechniques are normally used for larger dimension, but wereused first time for such specific cause with the objective ofvery precise measurement and help the engineers to interceptthe misaligned tunnel.

GEOPHYSICAL APPROACH AND THEORETICALBACKGROUND

Although the exploration of tunnel alignment problemsare well dealt with the seismic methods (Cardarelli et al. 2003;Lehmann et al. 2010), but it was not safe to the existing

JOUR.GEOL.SOC.INDIA, VOL.79, FEB. 2012

156 SUBASH CHANDRA AND OTHERS

problem as the seismic wave generation might cause furtherdrizzling of the sand particles into the well or well collapsing.Thus, non destructive and safe surface geophysical surveyssuch as electrical resistivity tomography and magneticprofiles were planned, but could not be applied due toseveral constraints such as: lack of sufficient space,presence of 11 kV electric power transformer, power linenetwork and power cables lying on the ground, metallicmaterial viz. iron pipes, iron casing in the rescue well. Undersuch circumstances a Mise-à-La-Masse (MLM)investigation was the only option left to be applied.

MLM was first used by Schlumberger in 1920 and laterutilized extensively to study mineralization zones andhydrogeological problems (Parasnis, 1967; Ketola-Matti,1972; Gupta et al., 1999; Bhattacharya et al., 2001; Nimmer &Osiensky, 2002; Chandra, 2006; Chandra et al., 2006a;Chandra et al., 2006b), where one current electrode is plantedin a conducting body intersecting a borehole as a source of

energization, while the other current electrode is kept atinfinity, and resultant potential distribution at the ground ismeasured with one potential electrode keeping the otherpotential electrode at infinity. This works on the principle ofOhm’s law, where potential generated at the ground surfacedue to buried current electrode in a homogeneous half spacecan be computed as:

(1)

where V, I, r and r are respectively potential (mV), current(mA), resistivity (&!m) and distance (m) of measurementpoint from the buried electrode. Since only one currentelectrode is active, current flows through the naturalconductive path and therefore the potential measured atthe surface maps the shape of the conductive body. Thus,the MLM method was deemed applicable in the presentfield condition. It was experimented by planting the active

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Jodhpur

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Jagatpura

70 75 80 85 90 95

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Fig.1. Location map of the Jagatpura village in Jaipur, India. (a) File photo of Sahil, (b) photograph shows old bore well (presentlycovered with wood sheet) , where incident of child fallen took place, and (c): Photograph shows of southern portion where rescuedug well of 1 m diameter made. The upper portion of the dugwell is cased with iron rings. The (b) and (c) portion are placed as perthe geographical location

r1

2I

Vπρ=


SOCIETAL APPLICATION OF GEOPHYSICS AS AN AID TO A RESCUE OPERATION AT JAIPUR 157

current electrode at one end point of the tunnel in galvaniccontact with the casing, and potential was measured at thegrid pattern around the old well with active potential electrodekeeping other current and potential electrodes at infinity.Although, all possible precautions were taken and repeatmeasurements were carried out increasing the amount ofcurrent injection, the map did not indicate significantsignature that could be interpreted as the presence of thetunnel.

To tackle the above problem, MLM method was modifiedby embedding both the current electrodes (A and B)underground (hereafter called as Buried Bipole Pole (BBP)method). Thus, the potential generated at the ground surfaceby the two buried electrodes (Fig. 2a) in a homogeneoushalf space can be computed as:

(2)

Where rA and rB are distance of measurement points fromburied electrodes A and B respectively. The electrode layoutand its equi-potential line pattern over an isotropic andhomogeneous subsurface are shown in figure 2.

Forward simulations of MLM and BBP methods werecarried out in absolute resistivity mode for an isotropic andhomogeneous half space of 50 Ohm.m resistivity with 1 mAcurrent injection through the buried electrode at 1 m depth.A 7 m horizontal iron tunnel is considered with resistivity1e-02 Ohm.m placed underground (i.e. 1 m depth) between6-13 m lateral scale. MLM potential response was computedat the ground from 0-20 m for the embedded current electrode(A) in contact with the tunnel terminal located at 6 m. BBPpotential response was also computed for its buried currentelectrodes A at 6 m and B at 13 m with no contact to the

tunnel. In both the cases the potential decreases movingaway from the source electrode. However in MLM method,as the buried electrode A was in galvanic contact with thetunnel, the potential decrease over tunnel is relatively highdue to low resistivity (Fig.3a). In MLM method intension isto look the conductive path with low potential. In ideal noisefree condition this may work depending on the dimensionof the tunnel/conductor. However, moving far from the buriedelectrode and in the presence of noise, it is most likely thatsignal may go below the noise level and making difficult totrace the conductive path. In addition, the presence ofconductive iron tunnel makes potential further down. InBBP method, intention is to look for two maxima, where aline joining the two peaks projects the direction of thecorresponding target.

MLM and BBP responses were further simulated withaddition of different degree of random noise. Figure 3 showsforward responses of BBP and MLM methods with normallydistributed random noise with zero mean and variance offive different magnitudes i.e. 0.0, 0.5, 1.0, 2.0 and 4.0respectively. The low potential away from the buriedelectrode position falls within the noise level. However, thepeaks are still traceable. Thus BBP due to two peaks/maximagot the advantage over MLM method in exploring thealignment of buried tunnel.

FIELD EXPERIMENT

The BBP method was experimented in the field duringDecember 2-7, 2009 at Jagatpura village in Jaipur. The currentelectrodes (A and B) were embedded 7 m apart on eitherside of the primary tunnel with B at the centre of the rescuewell and A at the opposite end. None of the electrodestouched the metal casing of the tunnel. Keeping the electrode

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Fig.2. (a) Electrode layout of BBP method and (b) Equi-potential line pattern at XY plane in isotropic and homogeneous half space

(a)(b)

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⎝

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−πρ=

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N at infinity, the potential was measured by M at points in agrid pattern surrounding the old well and also surroundingthe rescue well at the lower ground surface (i.e. z=-9 m). It isimportant to note that these two measurement groundsurfaces are not at the same elevation due to removal of theloose sand around the rescue well (Fig. 1).

A Syscal Jr Switch resistivity meter was used in theresistivity mode, where absolute potential were measured.Non-polarizable (porous pots) electrodes were used tomeasure potentials. The measured potentials werenormalized for 1A current injection. Due to elevationdifference between the measurement ground levels of theold and rescue well sites, a considerable difference in themeasured potential was observed. The normalized measuredpotential maxima were found 1.59 mV around the old welland 5.38 mV at the lower ground around the rescue well.Therefore, it was not possible to plot them directly andneeded elevation correction. In ideal condition at the sameelevation over an isotropic and homogenous subsurface,the maxima were expected to be of equal magnitude (Fig. 2b& 3). With this consideration the ratio factor between themaxima of upper and lower ground potential (i.e. 0.3) was

multiplied with the measured potential at rescue well sitesas elevation correction and then krigging was done to plotthe contour map of equi-potential lines (Fig. 4).

RESULTS AND DISCUSSIONS

The two maxima of the closed contours marked as A*and B* are the projected locations of the buried currentelectrode bipoles A and B (Fig. 4). A line joining the twomaxima gives the direction of the tunnel as N12°E. Theprojected tunnel line runs ~0.6 m to the east of the rescuewell, which may be taken as an indication of its non-verticality,i.e. a shift of the bottom of the rescue well. But since themeasurement points are not equally distributed in the closevicinity of the rescue well, it may not be very well justifiedto quantify the non verticality. However, the other potentialpeak near the old well is very well justified due to equallydistributed observation points in the vicinity.

Thus from the known point of the rescue well and theprojected direction of the tunnel, the end point of the tunnelwas calculated to 1.92 m east of the old well. Of course, amanual error towards making the pit for potential

0 2 4 6 8 10 12 14 16 18 20-20

-10

0

10

20

pote

ntia

l (m

V)

MLM response (with noise)

Va=0.0Va=0.5Va=1.0Va=2.0Va=4.0

0 2 4 6 8 10 12 14 16 18 20-20

-10

0

10

20

--- Distance (m)--->

pote

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l (m

V)

BBP response (with noise)

Va=0.0Va=0.5Va=1.0Va=2.0Va=4.0

0 2 4 6 8 10 12 14 16 18 20-5

0

5

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tent

ial (

mV

)

MLM and BBP response

MLMBBP

Underground tunnel Buried electrode

Fig.3. Synthetic simulations of MLM and BBP responses over the buried electrode in association with random noise with zeroexpectation and varying variances.


SOCIETAL APPLICATION OF GEOPHYSICS AS AN AID TO A RESCUE OPERATION AT JAIPUR 159

measurements could be taken as ±0.05 m.Although the subsurface is considered to be an isotropic

and homogeneous, but in general multilayered aquifer existsin the alluvium. In any case, whether it is single layered ormulti-layered, the projected tunnel alignment is expected tobe the same. However, the presence of underneath threedimensional geological bodies may affect the projection tosome extent.

The tunnel was diverted at 6.4 m in the west directionperpendicular to the primary tunnel as per the above results.While digging the tunnel CCTV camera shown gradualincreases vibrations in the old well indicating that the tunnelapproached the old well. Unfortunately, due to the deepdigging near the wall of the old well, the well collapsed andthe child’s location was completely lost. Figure 5 showsphotograph of the collapsed well, where buried cable ofthe CCTV camera can be seen. Although it is difficult tocomment, but safe excavation by skilled and experiencedlabour could have avoided the collapsing and made itpossible to retrieve the body. However, the BBP methodproved its capability in exploring the direction of misalignedburied tunnel.

As the current electrodes was placed relatively at finitedistance in BBP array compared to infinite distance in MLM,the strong directionality of the current flow lines becomeadvantageous. Instead of one maxima (in MLM), there aretwo maxima in BBP method, one at known point (i.e. in therescue well) and other at unknown point and hencefacilitates for comparative analysis of anomalies in order toexplore the direction. Since the peaks are still traceable even

in presence of moderate noise, the chances of the assessingthe direction by BBP method is relatively high than theMLM.

Due to the deep target and noisy field condition, onlystrongest signal measured at the ground as potential maximafound to be noticeable. Whereas, the weak signal wentbelow the background noise level. This is the reason whythe measured potential is decreasing, in general, towardsnorthwest. These noises are linked with the power supplydue to nearby power transformer, power cables, geologicalnoise, etc., which is random in nature.

These could be the probable reasons why MLM failedto assess the tunnel direction. Even BBP potential are alsodisturbed to the great extent, giving significant anomaly atfew corresponding buried bipoles only. However, movingthe infinity current electrode little far, anomaly pattern wereovertaken by the noise and hence disappeared.

CONCLUSIONS

This particular study has very well demonstrated thatthe geophysical techniques could be applied successfullyto purely societal purposes as well as in emergency cases,and in the course of these trials a new method has emergedby modifying the conventional MLM technique to BBPtechnique.

In the present days, space technologies using GPS etc.has been very commonly practiced for locating any objectbut in this case due to depth involved and object beingburied, such an application was next to impossible. Also the

Fig.4. Normalized potential map obtained by BBP method. Thick dotted line is the projection of the misaligned underground horizontaltunnel with N12°E shift. The blue coloured plus mark is the observation point used to measure the potential at ground surface.



formation being extremely loose, strong casing e.g., G.I. pipeswere necessary to be used and they have prevented the useof clinometers to determine the direction or orientation ofthe tunnel. In fact, before using geophysics, the efforts haveprovided a wrong estimation of the tunnel’s orientation inthe presence of a large number of metal objects creatingartificial magnetic field. Thus geophysical investigation wasthe only alternative to determine the orientation of thetunnel.

The finite positioning of the buried electrodes in BBPmethod makes the current flow strongly towards each otherand the two maxima each one at known and unknown pointsfacilitates the comparative analysis to map the unknownburied electrode projection at the ground reliably. In case ofhorizontal placed buried electrodes, the two maxima are of

equal magnitude and hence a line joining these maxima givesthe direction of the corresponding target.

The BBP method assessed the direction of the tunnel asN12°E. As per the geophysical results further digging wasunder taken at the left side of the primary tunnel end point.The CCTV camera lowered in the well has shown gradualincrease of the vibration while further digging the newbranch of tunnel. The guided digging could intercept themain well, but due to lose sandy formation the body of thechild sunk further down along with well collapsing and itwas almost impossible to locate the new position. The studyhowever, concludes that this new method is a successfulapplication of geophysics at micro scale and for a societalcause.

Acknowledgements: We thank, Director, NGRI, Hyderabadfor permitting to publish the work. Special thanks to Dr. V. P.Dimri, former Director NGRI to take interest in the societal workand deputing us on the task. This survey was carried out underimmense pressure and tough conditions. Under such circumstancesthe support and encouragement provided by Sri Kuldeep Ranka,District Collector, Jaipur, and his trust in our expertise, are highlyappreciated. Extraordinary support extended by the Jaipuradministration, particularly Mr. Bijendra Singh, Controller, CivilDefence, Mr Lohmarod, Engineer and village volunteers was muchappreciated. Finally we would like to express thank to anonymousreviewer and editor for their critical reviews and valuablesuggestions, which tremendously improved the paper.

Fig.5. Photograph showing the collapsed old bore well with buriedCCTV cables on 7th December, 2009.

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(Received: 1 January 2011; Revised form accepted: 16 May 2011)

societal application of geophysics as an aid to a rescue operation at jaipur

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