The 2D ERT and IP surveys were conducted at three pairs of success and failed wells in the khondalitic terrain. The success well and failed well pair has less than 150 m apart from each other and they are in similar hydrogeological setting. The resistivity values of the kaolinised layers are considered to be less than 25 Ωm. Layers having resistivities between 25–65 Ωm are identified as aquifer layers which are composed of moderately weathered/ fractured khondalite. Layers with resistivities greater than 65 Ωm are interpreted to have basement characteristics (granitic gneiss) (Venkateswara Rao et al. 2013). Various layers with different resistivity ranges have been identified from the resistivity images.
Site 1: Sivaram village-near the road
At the site 1, the success and failed wells are having 70 m apart and the drilled depth was 30 m. The layers with the low resistivity range of 7.71–22.6 Ωm ( up to 21 m depth) and 4.64–22.7 Ωm (up to 26.2 m depth) are categorised as highly weathered material or kaolinised zone formed at success and failed well locations (vertically downwards) respectively (Fig. 4a). Hence the depth of kaolinisation at failed well is nearly 5 m more compared to the success well (Fig. 4a). The relatively low resistivity values for the kaolinised layer obtained at the failed well than at the success well indicates that the formation is more kaolinised at the failed well location. Apart from the kaolinised depths, the high aquifer thickness (15.9 m) with the two favourable resistivity layers (32.4 to 46.3 Ωm) is found at success well while at the failed well, a thin single aquifer layer (38.6 Ωm) of thickness 7.8 m is found (Table 1). In addition to the less kaolinised depths and high aquifer thickness, deeper depth to basement is found at success well. In the case of failed well, the basement characteristics (> 65 Ωm) are formed immediately after thin aquifer layer. The failed well is characterized by higher kaolinisation thickness and also has less aquifer thickness as well as shallow basement.
The 2D IP images at success and failed well locations have shown the similar kind of characteristics in terms of chargeability as appeared in the 2D ERT images for delineating the kaolinised zones as well as aquifer formations. At the success well location, the clay mineral composition in the kaolinite is clearly observed as the low chargeability of 1.03 m-sec in the top surface (up to 21.5 m depth) and in the entire depth of the image at bottom right side of the image (Fig. 4b). The low chargeability zone is also appeared as low resistivity layers in the resistivity image at the success well location. From the vertical depth of 21 m down the success well, the chargeability values are varying between 1.03–3.58 m-sec whereas in the case of failed well, low chargeability value of 0.3 m-sec is maintained up to the depth of 31.3 m (Fig. 4b) due to presence of kaolin. More polarization in the subsurface is produced from the 21 m depth due to the higher interactions between clay minerals and groundwater at the success well location. Hence due to more polarization effect, the layers have produced moderate chargeability values of 3.58 m-sec which are interpreted as saturated weathered/fractured khondalitic formation. This moderate chargeability for the aquifer layers in the IP image is supported with the moderate resistivity values (32.4–46.3 Ωm) in the ERT image at the same depths of 21 m downwards. In the case of failed well, the higher concentration of clay mineral in the kaolin is appeared as low chargeability of 0.3 m-sec due to membrane polarization is responsible for the failure of the well. The chargeability values greater than 6 m-sec is the indication of basement characteristics (granitic gneiss) at both success and failed well locations due to the presence of highly conductive metal minerals in the bedrock (electrode polarization) at deeper depths. It is confirmed that the failed well is drilled in the low resistive and low chargeable formation as indicated by the both ERT and IP methods respectively. The thick aquifer layers and thin kaolinised formations are responsible for success well.
Site 2: Sivaram village-near the stream
At the site-2, the success and failed wells are located 100 m apart and have the drilled depth up to 40 m. The success well has very high yield (more than 8000 lph) while the failed well has no yield in spite of similar hydrogeological settings at both the wells (Fig. 5a). The kaolinisation (15.1 Ωm) is limited to shallow depths (up to 25 m) and below that, the favorable conditions arose for aquifer layers or fractured khondalite with the resistivity values of 36.3–56.3 Ωm up to the deeper depths (57.4 m) at the success well location. The thin kaolinised layer is existing on either side of the success well and the effect of kaolinisation is also very less, and hence the well yields are high. At the failed well, there is intense kaolinisation (7.17–26.6 Ωm) that has occupied the entire depth up to 57.4 m as such even drilling couldn’t be completed at this location (Fig. 5a). The kaolinisation up to deeper depths followed by the basement characteristics and absence of aquifer formation led to the failure of this well. In view of the fact that the resistivity of the kaolinised formations is less than 25 Ωm, the resistivity of the kaolinised formation is at the higher side (15.1–23.5 Ωm) at success well while the same are at lower side (7.16–26.6 Ωm) at failed well location. The resistivity images at success and failed well locations are showing the large difference in kaolinised depths within few tens of meters of distance (Fig. 5a).
The 2D IP images of success and failed wells at site-2 are shown in Fig. 5(b). The IP image of success well couldn’t be delineated the chargeability variations effectively due to the high RMS error (28.9%). The last electrode at success well location has poor contact to the ground surface due to the exposure of basement rock which may be the cause for noise in the subsurface and resulted in the form of high error in the chargeability image. Owing to this, the chargeability values have abnormally increased from 1.07 to 82.0 m-sec without proper variation (Fig. 5b). Hence this chargeability image is not adequate for the delineation of aquifer zones in terms of chargeability. Therefore the analysis has carried out at the success well location from less information. The low chargeability layer (< 1.07 m-sec) is appeared at right side and few pockets on top surface of the image (success well location) which may be due to the dominance of clay mineral content (membrane polarization). The high chargeability values at the bottom (from 35 m downwards) of success well are the indication of interactions between rock and water due to the electrode polarization.
At the failed well location, the IP response is more due to clay mineral content (membrane polarization) in the majority of chargeability image. The low chargeability value of 1.06 m-sec is maintained up to the depth of 36.9 m due to the thick clay mineral content in the subsurface which is supported by the resistivity image as well as the drilling results (large amount of thick kaolinized material obtained from the subsurface) at the failed well. This could be interpreted as, when the electric current is passed, the cation reactions between the kaolinite and its associated clay minerals with the water content developed at faster rate and the less strength of polarization (or chargeability) appears as low chargeability for the highly weathered/kaolinised zones. This indicates that, the magnitude of polarization decreased due to the higher clay mineral concentration up to the deeper depths. The chargeability value of 5.57 m-sec indicates the hard rock (khondalite) which is underlying the clay material. The high polarization values (10.1 m-sec) have appeared for the bedrock (in the bottom of the image) due to the absence of clay mineral with water content at deeper depths.
Site 3: Khondasambham village
The success and failed wells are located nearly 150 m distance and have the drilled depth up to 35 m and 25 m respectively at Site-3. Even in this case also the kaolinised layer thickness at failed well is much higher when compared to success well as observed in the other two sites. In the case of kaolinised layer resistivity, the formation is more kaolinised (9.96 to 17.4 Ωm) at failed well compared to the success well (16.9 to 23.9 Ωm). The well is failed due to the thick kaolinised layer starting from top surface to 25 m depth (Fig. 6a) that is correlated with the drilling results as the drilling bit couldn’t be penetrated to deeper depths due to the kaolinised formation. Except few pockets on top surface, the resistivity image of the failed well location has shown the kaolinisation problem up to the depth of 35 m. The higher depth of kaolinisation and the lesser aquifer thickness are encountered at failed well when compared to the success well. Broader range of aquifer resistivity is obtained at success well (33.8 to 67.9 Ωm) when compared to the failed well (30.2 to 52.7 Ωm). The kaolinised layer is also limited to the shallow depths (up to 15 m) at success well and hence the well yields are high.
The chargeability images at success and failed wells at the Site-3 are shown in Fig. 6 (b). The thick clay mineral content (kaolinisation) with the chargeability values of 0.43 m-sec has been accumulated up to 25 m depth on either side of the success well. These low chargeability zones are due to the presence of clay mineral composition (kaoline) which is correlated with the low resistivity values (16.9–23.9 Ωm) of success well location (Fig. 6a). The success well is further penetrated into the deeper depths up to 35 m where the well has contacted the weathered rock with water content. At these depths (21.5 to 43.1 m), the chargeability values are gradually increased from 1.53 to 3.73 m-sec. These moderate chargeabilities are due to higher induced polarization effect by the saturated weathered khondalite. Hence the saturated formations are delineated as aquifers with moderate chargeability values of 1.53–3.73 m-sec and with resistivity values of 47.9–67.9 Ωm at the success well location. The high chargeability value of 4.84 m-sec may be the indication of bed rock formation as appeared in the resistivity image (96–136 Ωm) at success well location. Except few pockets on top, the failed well location is faced with the problem of kaolinisation as indicated by the low chargeability value of 0.23 m-sec. These low chargeability values of entire IP image are correlated with the low resistivity values (9.96–17.4 Ωm) in the resistivity image up to 35 m depth which is also indicated in drilling results (the extension of kaolinization to the deeper depths) at failed well location. The rate of decay of the induced polarization potential is appeared as low (0.23 m-sec) due to the higher clay mineral concentration at the failed well location.
5.1 Discussion of Results
The ERT and IP images have provided the resistivity and chargeability values of kaolinised layer and aquifer formations at success and failed wells in the khondalitic terrain. The depth extension and thickness of both layers is clearly demarcated with ERT and IP methods. The greater thickness of kaolinised formation is the main difference in success and failed well. It is identified from the ERT images that more depth of kaolinisation and the less thickness of the aquifer are characteristic features at failed wells when compared to success wells (Table 1). That means, the failed wells are associated with thin or no aquifer thickness below the kaolinised layer. The thickness of the kaolinised material at the failed wells is extended from about 21–50 m while the same is at success wells is about 10–26 m. This extended deeper kaolinisation of the aquifer is responsible for failure of wells. The kaolinisation is not only deeper at failed wells but also followed by the basement characteristics either with very thin or no aquifer layer immediately below the kaolinised layer. The kaolinised layer resistivities are at lower side of the range at the failed wells than success wells which indicates that the formations are more kaolinised at the failed wells compared to the formations at success wells. The aquifer thickness is varied nearly 15–31 m at success wells while the same is maintained only about 7–9 m at failed wells.
Table 1: Resistivity range, thickness and interpretation of subsurface layers at success and
failed well pair
Location | Success well | Failed well |
Resistivity range (Ωm) | Depth range, bgl (m) | Thickness (m) | Type of formation | Resistivity range (Ωm) | Depth range, bgl (m) | Thickness (m) | Type of formation |
Site-1 | 7.71 − 22.6 | 0–21 | 21 | Soil and Kaolinised formation | 4.64–22.7 | 0–26.2 | 26.2 | Soil and Kaolinised formation |
32.4 − 46.3 | 21–36.9 | 15.9 | Weathered/fractured khondalitic aquifer | 38.6 | 26.2–34 | 7.8 | Weathered/fractured khondalitic aquifer |
66.3–95 | 36.9 onwards | | Basement rock | 65.6–189 | 34 onwards | | Basement rock |
Site-2 | 15.1 − 23.5 | 0-26.2 | 26.2 | Soil and Kaolinised formation | 7.16–26.6 | 0–49.9 and more | 49.9 and more | Soil and Kaolinised formation |
36.3–56.3 | 26.2 onwards | 31.2 | Weathered/fractured khondalitic aquifer |
Site-3 | 16.9 − 23.9 | 0-9.94 | 9.94 | Soil and Kaolinised formation | 9.96–17.4 | 0–21.75 | 21.75 | Soil and Kaolinised formation |
33.8–67.9 | 9.94–26.2 | 16.26 | Weathered/fractured khondalitic aquifer | 30.2–52.7 | 21.75–31.55 | 9.8 | Weathered/fractured khondalitic aquifer |
96.2–193 | 26.2 onwards | | Basement rock | 91.8–485 | 31.55 onwards | | Basement rock |
bgl: below ground level |
In the IP method, depending upon pore geometry and degree of water saturation, the large difference in potential decay with time (induced polarization) is observed between the success and failed well pair. The chargeability in the subsurface is varied at success and failed well locations due to the difference of interactions between clay minerals and groundwater. The IP strength is increased at clay mineral concentration by the membrane polarization and it is higher at hard basement due to electrode polarization. The chargeability values of different subsurface formations at success and failed wells (vertically downwards) are shown in Table 2.
Table 2: Chargeability range, thickness and interpretation of subsurface layers at success and
failed well pair
Location | Success well | Failed well |
Chargea bility (m-sec) | Depth range, bgl (m) | Thick- ness in m | Formation | Chargeability (m-sec) | Depth range in m. | Thickness in m | Formation |
Site-1 | 1.03 | 0–20.0 | 20 | Soil and Kaolinised with associated clay minerals | 2.42 − 4.54 | 0–6.76 | 6.76 | Soil and Weathered formation |
3.58 | 20– 31.3 | 11.3 | Weathered rock saturated with water | 0.30 | 6.76–36.9 | 30.14 | Kaolinised formation with associated clay minerals |
3.58–6.13 | 31.3 onwards | | Basement rock | 2.42 | 36.9 onwards | | Basement rock |
Site-2 | 1.07 | 0–6.76 | 6.76 | Soil and Kaolinised formation with associated clay minerals | 1.06 | 0–36.9 | 36.9 | Soil and Kaolinised formation with associated clay minerals |
< 1.07 | 10–21.5 | 11.5 | Kaolinised formation with associated clay minerals | 5.57 | 36.9 onwards | | Basement rock |
>1.07 | 21.5 onwards | - | weathered rock saturated with water |
Site-3 | 1.53 | 0–11 | 11 | Soil and Clay with dry sand | 0.235 | 0–43.1 and more | 43.1 and more | Soil and Kaolinised formation with associated clay minerals |
0.33 | 11– 21.5 | 10.5 | Kaolinised formation with associated clay minerals |
1.53–2.63 | 21.5– 36.9 | 15.4 | Clay with dry sand |
2.63–3.73 | 36.9 onwards | 6.2 | Weathered rock saturated with water |
The formations with thickness range of 12–21 m at the deeper depths with the moderate chargeability values of 2.63–3.58 m-sec are identified as saturated aquifer formations due to saturated water content in the weathered rock. At success well locations, the kaolinisation is limited to the shallow depths with low chargeability values of 0.3–1.0 m-sec while in the cases of failed wells, the magnitude of chargeability decreased due to the higher clay mineral concentration. The membrane polarization is more pronounced due to the kaolin and its associated minerals. The lower chargeability range of 0.23–1.0 m-sec is characterized as kaolinised layers at failed wells up to the greater depths of 6.76–43 m. It is found that the weathered/fractured kondlaitic aquifer has moderate chargeability values giving high yields at the success wells. The kaolin and associated clay minerals exhibit the low chargeability values resulting in the failure or low yield of wells.
5.2 Results of newly drilled bore well
Based on resistivity and chargeability values obtained at above three well pair, a new bore well was drilled where the extent of kaolinisation is less. The location of drilled well site is presented in Fig. 7 and the newly drilled well is 50 m apart from the existing failed well at site-2 location. The resistivity image (57.4 m depth) and chargeability image (43.1 m depth) at newly drilled well site are shown in Fig. 8. A thick massive sticky clay content having low resistivity of 8.7 Ωm and relatively high chargeability anomaly of 5.82 m-sec is appeared in the left side of the image up to deeper depths (Fig. 8). The aquifer layers are overlain with the low resistive zone (kaolinised) (8.7–18.7 Ωm) up to 17 m depth range in the middle top of the resistivity image. This low resistivity zone is appeared as very low chargeability zone in the IP image. The entire right bottom of the resistivity image is occupied by massive hard rock. Below the drilled well, the kaolinised layer is occupied up to 15 m depth followed by the aquifer layer (weathered/fractured khondalite) with the moderate resistivity (27.3–58.6 Ωm) and chargeability (2.11 m-sec) values. During the drilling process, a thick kaolinised layer (sticky clay) is encountered up to the depth of 20 m underlined by weathered khondalitic aquifer which is followed by hard granitic basement. The moderate yield (the yield not so high) is obtained from the drilled well which may be due to the fact that the pore geometry of the aquifer is dominated by overlain kaolinised formation. However, subsurface data from more drilled wells could further confirm the aquifer’s resistivity and chargeability values in this khondalitic region.