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Fracture pattern and electrical resistivity studies for groundwater exploration N. Janardhana Raju 7 T. V. K. Reddy

Abstract The occurrence, movement and control of groundwater, particularly in hard-rock areas, are governed by different factors such as topography, lithology, structures like fractures, faults and nature of weathering. An attempt is made in the present study to investigate the extent of the influence of structures such as fractures and thereby delineate the nature of subsurface lithology with the help of an electrical resistivity method. For this study, the Upper Gunjanaeru River basin, Cuddapah district Andhra Pradesh was chosen to determine groundwater potentials. In order to understand the significance of the fracture pattern, geological, hydrogeomorphological and lineament maps were prepared based on the field data and also from the LANDSAT TM imagery. Further, electrical resistivity surveys were conducted to determine the subsurface lithology and also to confirm the studies of LANDSAT imagery. The isoresistivity contour map has been prepared based on the 45 VES conducted to determine the resistivity variations in the study area. The isoresistivity contours obtained were found to conform to the structural trends obtained by geological studies and also confirm the relationship between the structure and secondary porosity present in the rocks. The lineaments in the area have two preferred directions. One set is a NE-SW direction (N 307–707 E; S 307–707 W) and another is a NW-SE direction (N 07–307 W; S 07–307 E and N 607–807 W; S 607–807 E). The water-table contour map shows that the direction of groundwater flow is south to north. Key words Fracture pattern 7 Electrical resistivity 7 Lineament density 7 Groundwater

Received: 3 March 1997 7 Accepted: 17 June 1997 N. Janardhana Raju (Y) 7 T. V. K. Reddy Department of Geology, Sri Venkateswara University, Tirupati - 517 502, India

Introduction The Upper Gunjanaeru River basin is situated in drought-prone areas of Rayalaseema region (Fig. 1). Groundwater has become an important source of water and has played an important role in developing industry, agriculture and domestic purpose. The groundwater condition in a hard-rock terrain is multivariate because of the heterogeneity of the aquifer, due to the varying composition, compaction, degree of weathering and density of fracturing. Hence, surveys of a multidisciplinary nature are necessary for understanding the part played by these parameters. As a result, exploration of groundwater in a hard-rock terrain has proved to be a complex phenomena. However, the presence of a vast hard-rock terrain cannot be neglected as an unfavourable zone. Modern technology in the form of remote-sensing techniques and geophysical surveys is highly helpful for the exploration of groundwater in hard-rock terrains (Ambazhagan 1993). The fracture pattern study can elucidate the problem for a large extent of the area. The study area comprises sedimentary rocks such as shales, which have primary and secondary porosities. Shales are generally porous, and because of the presence of the fine-grained texture, the pores are very minute. Shales are thus considered as hard rock. Due to the presence of the fine-grained nature, groundwater potential mainly depends on secondary porosity, i.e. fracture, faults, lineaments and joints. Deep-seated geological controls like weathering, fissures, fractures and joints are very important for holding and transmitting substantial quantities of groundwater. The present study attempts to investigate the extent of the influence of the fracture pattern from remote sensing and thereby delineate the nature of subsurface lithology with the help of an electrical resistivity survey for exploration of groundwater. In order to understand the significance of the fracture pattern, geological, hydrogeomorphological and lineament maps have been prepared with the help of LANDSAT TM imagery (Janardhana Raju 1991). An isoresistivity map is prepared by joining the points of equal apparent resistivity (AB/2p30 m) and then comparing with a lineament map to identify the extent of correlation. Ambazhagan (1993) has studied in depth the structural elements, particularly the lineaments in relation to geomorphology and the electrical resistivity

Environmental Geology 34 (2/3) May 1998 7 Q Springer-Verlag

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Fig. 1 Location map of the Upper Gunjanaeru River Basin

method in a part of the Dharmapuri district, Tamilnadu. Usha and others (1989) have developed fracture pattern modelling in groundwater exploration in the North Arcot district of Tamilnadu, which exposes a complexly folded and fractured crystalline terrain, using the modern remotely sensed data. Lineaments are longer in distance, the weathering is wider and deeper. The relationship between geological and hydrogeomorphological units is studied. The Upper Gunjanaeru River basin has a length of about 29 km. It rises in the Palakonda and Seshachalam hill ranges and flows in a north-east direction until it joins the Cheyyair River. Its basin has a drainage area of 390 km 2. The study area is bounded by 137 43b 45n– 147 1b 44n north latitudes and 797 15b 16n–797 27b 38n east longitudes. The study area is generally occupied by hill ranges of varying relief of 200–1000 m above mean sea level. The general slope throughout the area is mainly in a north-eastly direction. The river basin includes the subbasins of Gundala Vanka, Kalleru Nala and Mustieru. Quartzite hills are observed in the west and south-east, and shale hills are observed in the eastern side of the study area.

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Geology The geological succession of the formations in the study area is as follows: Recent alluvium

Boulders, cobbles, pebble, gravel, clay and silt. Nallamalai series Birenkonda quartzites

Cuddapah Super group Cheyyair series

Pullumpet shales Nagari quartzites

The Nagari quartzites are occupied over the granites in the hilly area of southern and south-western portions of the study area (Fig. 2). It is characterized by bedding joints and master joints which trend vertically. The quartzites occur as flat or gentle dipping due north. The escarpment has an EW trend. These quartzites are called ortho-quartzites because of their high quartz content. The major portion of the study area is occupied by Pullumpet shales. The shales are trending N 157–207 W; S 157–207 E and dipping from 127–167 NE. The shale is mainly composed of silt and clay with calcareous and ar-

Cases and solutions

formed over the shales associated with folding and found along the NE parts of the study area. The depositional units are comprised of valley fills, piedmont zone and piedmont plains (Table 1). The valley fills are the narrow linear depressions filled with colluvium/ alluvium along the fractures and lineaments. The piedmont zones are formed along the foot of the hills by the boulders, cobbles, pebbles, gravel with little clay and silt. The area which appears in the form of a V shape is covered by piedmont plains of different overburden thicknesses. The piedmont plains are classified into three types based on thickness of the overburden ranges from 5 to 20 m. The piedmont plain is a gentle sloping plain formed by the accumulation of boulders, cobbles, pebbles and gravel rolled from the hills to the plains with little clay and silt brought by the hill streams.

Lineaments Linear structural features such as faults and fractures were studied in the field and a lineament map was prepared with the help of LANDSAT TM data (Fig. 3). Faults can be distinguished from the fractures by the observa-

Fig. 2 Hydrogeomorphology map of the study area

gillaceous cementing material. At some places the shales show intercalating folding. Recent alluvium deposits are formed over the shales from south to north, NW and NE portions of the area and also along the foot of the hills. The alluvium consists of boulders, cobbles, pebbles and gravel. The thickness of the alluvium deposit ranges from 5 to 20 m.

Hydrogeomorphology Hydrogeomorphological studies have been carried out with a view to delineating the various geomorphological forms for groundwater development (Fig. 2). LANDSAT data using 1 : 250000 made available by the NRSA has been studied in conjuction with the field studies. The area is a sedimentary terrain which may be in the form of a ’V’ shape trending in the direction of SE-NW. The relief gradually decreases from SE to NW, SW to NE and NE to SW. It consists of a number of valley fills confined to a quartzites terrain which is resting over the granites at the SW part of the study area. The structural units, like strike ridges, are developed on quartzites along Fig. 3 the SW and SE parts. Another set of strike ridges is Map showing fracture pattern of the study area


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Table 1 Hydrogeomorphological classification of study area map symbol

geomorphic unit

lithostratigraphy

structure

description

groundwater prospects

VF

filled in valleys

colluvium/alluvium

lineaments/fractures

narrow Linear depressions filled with colluvium/alluvium

moderate to good yields are expected

PPZ

piedmont zone

constitutes boulders cobbles, pebbles, general silt and clay

-do-

sloping zone formed at the foot hills by the boulders cobbles, pebbles, gravel with negligible silt and clay and overburden thickness varying from 0 to 5 m

poor

PPL-S

piedmont plain with shallow overburden

-do-

-do-

a gently sloping plain formed by boulders, cobbles pebbles, gravel rolled from the hills to the plains with little silt and clay brought by the streams varying from 1 to 5 m overburden

poor to moderate; moderate yields are expected along lineaments/fractures

PPL-M

piedmont plain with moderate overburden

-do-

-do-

a gently sloping plain formed by boulders, cobbles, pebbles, gravel rolled from the hills to the plains with little silt and clay brought by the streams varying from 5 to 10 m overburden

moderate to good; good yields are expected along the lineaments/fractures

PPL-D

piedmont plain with deep overburden

-do-

-do-

a gently sloping plain formed by boulders, cobbles, pebbles, gravel rolled from the hills to the plains with little silt and clay brought by the streams varying from 10 to 20 m overburden

good to very good; very good yields are expected along the lineaments/fractures

SH(Q)

structural hills over quartzites

quartzites

-do-

linear to arcuate hills showing definite trend lines

poor

SH(S)

structural hills over shales

shales

-do-

linear to arcuate hills showing definite trend lines associated with folding

poor

tions of the ground truth, where there is more recent origin and topographical expression of faulting along the river courses at few places. The lineaments in the area have two preferred directions and lineament directions are presented in Table 2. One set is a NE-SW direction (N 307–707 E; S 307–707 W) and another is a NW-SE direction (N 07–307 W; S 07–307 E and N 607–807 W; S 607– 807 E). The preferred orientations of these lineaments which represent deep-seated fracture openings extending 178


for several kilometers in the hilly regions act as underground conduits for groundwater movement. These preferred orientations of deep-seated fractures are responsible for the groundwater potential zones in the study area. The lineament system was subjected to further analytical treatment and a lineament density map has been prepared to identify the fracture concentration (Fig. 4). This was generated by gridding the whole area into 1-km 2 cells and counting the length of the lineament in each

Cases and solutions

Table 2 Lineament directions in the study area range in degrees

NE

NW

0–10 10–20 20–30 30–40 40–50 50–60 60–70 70–80 80–90

11 12 30 64 59 46 39 18 46

32 55 29 12 6 3 20 36 22

NS

EW

11

0

cell and counting these values. An integrated survey involving location of lineaments by remote sensing and resistivity surveys for location of fracture openings has indicated that in some areas both have shown a correlation. Such areas/locations are found to be highly productive for groundwater development. A good quantity (`500 lpm) of groundwater potentials has been observed in the high density of the lineament areas, and was thus indicated by a comparatively low apparent resistivity val-

Fig. 4 Lineament density map of the study area

ue as well as by more alluvium followed by weathered thickness encountered along high-density lineament zones. High-density lineament areas are positioned at higher altitudes which are exposed to the surface. Two or more interconnecting deep-seated fractures are present in highdensity lineament areas, which act as groundwater channels, and some of those interconnecting deep-seated fractures are responsible for the formation of groundwater potential zones in the study area, where the density of lineaments is found to be between 0.5 and 1.0. These zones may also have the continuity of the lineaments extending from high to low altitudes which may be buried under the transported deposits (20 m). This is in conformity with the good yields of the wells (`500 lpm). The lineament density study has been very helpful in the preparation of groundwater potential zones of the study area.

Relationship of groundwater to lineaments In order to prepare the groundwater-table contour map, 70 observation wells are identified to observe the watertable fluctuations for seasonal variations spreading in an area of 140 km 2 excluding hilly and reserved forest areas. The areal density of the wells is estimated as 2 km 2. The isolined area has been demarcated from the hilly and reserved forest areas as per the topographic map prepared by the Survey of India. Groundwater may be present in the hilly and forest areas; since stringent legislation and more restriction, no drilling/production is possible. These are considered as areas of recharge for groundwater development in the study area. The average ground elevation in the isolined area is 230 m and the average depth of water table is 210 m above mean sea level. The groundwater-table contour map above mean sea level (Fig. 5) has been prepared to determine the groundwater flow directions of the study area. From Fig. 5 it may be seen that groundwater generally flows from south to north. The trend of the water-table contours reveals that a few contours are convex and some are concave towards flow directions. The contours of a convex nature indicate the saturated zone losing the groundwater to the streams. This situation prevails in the south, central, NW and NE parts of the study area. Most of the study area comes under this category because the study area is a bazada plane, where the unconsolidated materials range in depth from 5 to 20 m. At two places – east and northern parts of the study area – contours are concave towards the flow direction, implying that the saturation zone is gaining water from the stream. The groundwater flows from the south, NW and NE pool/ accumulate between central and northern parts of the study area, which also indicates the higher lineament density zone of 2.0 km/km 2. From that region the groundwater flows are towards the northern side, indicat-


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malleswara Rao (1989) have applied electrical resistivity surveys for groundwater resources in Varaha River basin, A.P. Electrical resistivity surveys have been utilized for investigation of groundwater resources at selected levels in drought-prone area of Pahari block, U.P. (Yadav and Lal 1989). An electrical resistivity survey was carried out to determine the lithology, weathered, fractured pattern, depth to basement and resistivity variations in the study area. Forty-five vertical electrical soundings were taken at different locations within the study area, excluding the reserved forest area. These reserved forest areas are exempted from agricultural and industrial activity by the stringent rules of the government as regards environment protection and ecological balance. Hence, no groundwater exploration is possible in these areas. The variations in apparent resistivity in the area can often be related qualitatively to geological features (Schwartz and McClymont 1977). In the qualitative interpretation, the contour map of the apparent resistivity distribution for the separation AB/2p30 m is prepared to delineate high and low resistivity zones and shown in Fig. 6. Few resistivity soundings have been taken and correlated with lineament density zones. Resistivity soundings falling under high-density lineament zones provide favourable results when com-

Fig. 5 Watertable contour map (above MSL) of the study area

ing influent streams (concave contours). The majority of lineaments are in the directions of N 307–507 E and N 07– 307 W, which are the major sources of surface and groundwater flows. From the observation and comparison of the water-table map with the lineament density map, the study area has lineament densities ranging from 0.5 to 1.5 km/km 2. Most of the 2.0–2.5 km/km –2 lineament density contours are observed in the hilly areas, which are considered to be the potential recharge areas for groundwater.

Correlation of electrical resistivity with lineament Electrical resistivity depends on many factors; chief among these are the mineral content, texture, moisture content, salinity, fissures and fractures of geological formations. The resistivity values of rocks varies depending upon the presence of secondary porosity such as weathered, fractured and joints. Janardhana Raju and others (1996) have utilized electrical resistivity surveys for the delineation and exploration of groundwater potential zones in the Upper Gunjanaeru catchment, Cuddapah District, Andhra Pradesh (A.P.). Kshira Sagar and Naga180


Fig. 6 Apparent resistivity contour map for AB/2p30-m electrode separation

Cases and solutions

Table 3 Resistivity and yield potential in different zones of the study area village

soil & alluvium thickness (m)

weathered thickness (m)

fractured thickness (m)

apparent resistivity at depht of 30 m (Vm)

yield (lpm)

Koduru Ramapuram Dessitipalle Molakalapodu Billupativaripalle Buduguntapalle Konetiraju kandrika Yanadipalle Ballapalle Settigunta

0–15 0– 4 0–10 0–14 0–11 0–14 0– 9 0– 5 0– 2 0– 7

15–20 4–10 10–38 14–25 11–14 14–28 9–15 5–16 2–18 7–23

20–80 10–50 38–90 25–32 14–45 nil 15–70 nil nil 23–30

138 60 90 840 42 12 110 370 400 85

500 740 550 300 800 700 680 240 110 450

pared to soundings that fall under other zones. Table 3 shows the thickness of the different formations based on the resistivity values. The soil and alluvium layer is underlain by weathered shale and the weathered shale is underlain by fractured shale. Moderate-to-good (250–500 lpm) yields are tapping from weathered zones, where no fracture zones are present. In these areas the thickness of alluvium followed by weathered shales is greater, and the percolation of the groundwater in the unconsolidated material led to the formation of the moderate to good yields without the presence of fractured zones. In the case of Buduguntapalle, though the fracture zone is not present, yields (simple yield test) are high, maybe due to the presence of high alluvium thickness and the weathered zone, and also recharge from the adjacent stream. The areas of Balapalle and Yanadipalle are devoid of fractures and alluvium zones with low yields during rainy season (~250 lpm) and may dry up during summer season. The areas covered with high alluvium and more fractured zones are providing copious amounts of groundwater. Ranges of resistivities and thicknesses of different zones are presented in Table 4. It is not possible to distinguish clearly the weathered shale from the fractured shale through the resistivity survey due to overlapping of the resistivity values. Normally the layer overlying a high resistivity layer is taken to be either weathered shale or fractured shale, and this does not conform to the existing well log data in the field. In the study area the apparent

Table 4 Resistivities in different zones of shale formations litho-unit

range of resistivity (Vm)

range of depth (m)

weathered shale fractured shale hard shale

12–106 40–220 beyond 220

6–45 14–63

resistivity of shale for various lithounits varies from 12 to 840 ohm-m. The depths of weathering and fracturing of shales were delineated by the electrical resistivity method and are presented in Table 4. Compared to crystalline rocks these formations have low apparent resistivity values. Assuming that wide variations are not present within a few kilometers, groundwater potential zones have been delineated based on subsurface lithology, lineaments/fracture pattern from LANDSAT imagery and from electrical resistivity studies (Fig. 7).

Results and discussion Shales, the sedimentary rocks, are considered as aquicludes. Though shales possess primary porosity, groundwater occurrence is mainly due to the secondary porosity, i.e. weathering, joints, fissures and fracture/lineaments. The isoresistivity contour map (Fig. 6) depicts the horizontal variations in the subsurface lithology of the area. From Fig. 6 it is found that the low resistivity zones of less than 50 ohm-m occur from the north-central part of the study area. Most of the wells located in this zone yield a good quantity of water. The high resistivities are located over topographic high at the southern and northeastern portions of the study area. The wells located in high resistivity zones give relatively poor yields. The thickness of piedmont zones increases from southern to northern portions of the valley, where the apparent resistivity values are found to decrease in the same direction. The alluvium consists of boulders, cobbles, pebbles, gravel, sand, silt and clay, ranging in thickness from 5 to 20 m occurring from a southern to northern direction of the study area. Hydrogeomorphological studies indicate that the piedmont plains with thick alluvial deposits are considered good groundwater potential regions. Good quantity (`500 lpm) groundwater potentials have been identified in the high density of the fracture/lineament zones in Ko-


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eas which are correlated by comparatively low apparent resistivity values. 4. The groundwater pooling/accumulating between the central and northern parts of the study area is also in concurrence with the high-density lineament zone. 5. Moderate to good (250–500 lpm) yields are tapping from alluvium and weathered zones without the fractured zone underneath. 6. A relationship between ground measured resistivity value and the lineament data is observed for the identification of groundwater potential zones. Acknowledgements The study forms a part of the doctoral thesis of the first author (NJR); NJR is highly thankful to Alexander Von Humboldt Foundation, Germany for awarding the Post-Doctoral Research Fellowship. He is also very much thankful to Sri Venkateswara University for providing financial assistance. The authors are grateful to Prof. R. Jagadiswara Rao, Department of Geology, S. V. University, for his critical evaluation and suggestions.

References

Fig. 7 Different groundwater potential zones

dur, Ramapuram, Dessitipalle, Billupativaripalli, K. R. Kandrika and Settigunta. Low apparent resistivity values, as well as presence of thick alluvium underlain by deeply weathered bed rock, indicate good groundwater potentials in Billupativaripalli and Buduguntapalli. Groundwater flow directions indicate a south to north flow which follows the general trends of fracture/lineament and faults zones. Fracture/lineaments which are mostly present in hilly tracts are mainly responsible for the recharge and flow direction of the groundwater from hilly tracts to the plains.

Conclusions 1. The geological, geomorphological and geoelectrical studies indicate that the groundwater potentials are due to the presence of a secondary porosity, such as weathered, jointed and fractured/lineaments. 2. Hydrogeomorphological studies depicted the piedmont plain with moderate to high overburden alluvium thickness as groundwater potential zones. 3. A good quantity (`500 lpm) of groundwater potentials has been observed in high-density lineament ar182


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