the gravity method in groundwater exploration

Hydrogeology Journal (2002) 10:307–321 DOI 10.1007/s10040-001-0184-2

Abstract The application of variations in the earth’sgravity in groundwater exploration on a regional scale,especially in sedimentary basins, metamorphic terrains,valley fills, and for buried alluvial channels, is well es-tablished. However, its use in hard crystalline rocks islittle known. In granite, for example, the upper weath-ered layer is a potential primary aquifer, and the underly-ing fractured rock can form a secondary aquifer. Fractur-ing and weathering increases the porosity of a rock,thereby reducing the bulk density. Changes in gravityanomalies of 0.1–0.7 mGal for granites, due to weather-ing or variations in lithology, can be detected.

To test the use of gravity as a groundwater explorationtool for crystalline rocks, a gravity survey of the peninsu-lar shield granites underlying Osmania University Cam-pus, Hyderabad, India, was undertaken. At the site, gravi-ty anomalies reflect variations in the lithology and in thethickness of weathered zones. These anomalies also de-fine the position of intrusives and lineaments. Areas ofmore deeply weathered granite that contain wells of high-er groundwater yield are represented by negative gravityvalues. In the weathered zone, well yield has an inverserelation to the magnitudes of residual gravity. The studyconfirms the feasibility of gravity as a tool for groundwa-ter exploration in crystalline rocks.

Résumé L’application des variations de la gravité ter-restre à l’exploration des eaux souterraines à l’échelle ré-gionale, en particulier dans les bassins sédimentaires, lesterrains métamorphiques, les remplissages de vallées etles chenaux alluviaux sous couverture, est maintenant unfait établi. Toutefois, son utilisation dans les roches cris-tallines de socle est mal connue. Dans les granites, parexemple, l’horizon altéré supérieur est un aquifère pri-maire potentiel, et la roche fracturée sous-jacente peut

constituer un aquifère secondaire. La fracturation et l’al-tération accroissent la porosité d’une roche, réduisant enmême temps sa densité d’ensemble. On peut détecter deschangements dans les anomalies de gravité de0.1–0.7 mGal pour les granites, du fait de l’altération oude variations de lithologie.

Afin de tester l’utilisation de la gravité comme outild’exploration des eaux souterraines dans les roches cris-tallines, nous avons réalisé une campagne de gravité surles granites de bouclier de la péninsule du Campus del’Université Osmania, à Hyderabad (Inde). Sur ce site, lesanomalies de gravité reflètent les variations de la litho-logie et de l’épaisseur des zones altérées. Ces anomaliesdéfinissent aussi la position des intrusions et des linéa-ments. Des zones de granite plus profondément altéré quicontiennent des puits à fort rendement sont représentéespar des valeurs négatives de gravité. Dans la zone altérée,le rendement des puits est en relation inverse avec les ma-gnitudes de gravité résiduelle. Cette étude confirme quela gravité peut être utilisée comme outil d’exploration deseaux souterraines dans les roches cristallines.

Resumen La aplicación de las variaciones en la grave-dad terrestre a la exploración de aguas subterráneas a es-cala regional, sobre todo en cuencas sedimentarias, terre-nos metamórficos, rellenos de valle y canales aluvialesenterrados, es una técnica bien desarrollada. Sin embar-go, apenas se ha usado en rocas cristalinas duras. En gra-nitos, por ejemplo, la capa superior meteorizada se com-porta como un acuífero primario en potencia, mientrasque puede formarse otro, secundario, en la roca fractu-rada infrayacente. La fracturación y la meteorización in-crementan la porosidad de la roca, reduciendo su densi-dad total. Se puede llegar a detectar anomalías en la gra-vedad de 0.1–0.7 mGal para los granitos, causadas pormeteorización o variaciones de litología.

Se emprendió un estudio del escudo granítico penin-sular en el campus de la Universidad de Osmania (Hy-derabad, India), con el fin de ensayar el uso de la grave-dad como herramienta para explorar aguas subterráneasen rocas cristalinas. En este emplazamiento, las an-omalías gravitacionales reflejan cambios litológicos y deespesor de las capas meteorizadas. Estas anomalías tam-bién definen la posición de intrusiones y lineamientos.Las zonas de granitos meteorizados más profundos, quecontienen pozos de mayor rendimiento, están representa-

Received: 27 December 1997 / Accepted: 28 November 2001Published online: 12 March 2002

© Springer-Verlag 2002

B.V.S. Murty (✉ ) · V.K. RaghavanCenter of Exploration Geophysics, Department of Geophysics,Osmania University, Hyderabad 500 007, Indiae-mail: [email protected]

The gravity method in groundwater exploration in crystalline rocks: a study in the peninsular granitic region of Hyderabad, IndiaB.V.S. Murty · V.K. Raghavan

das por valores negativos de la gravedad. En la zona me-teorizada, el rendimiento de los pozos está inversamenterelacionado con la magnitud de la gravedad residual. Elestudio confirma el interés de la gravedad como herra-mienta para la exploración de aguas subterráneas en ro-cas cristalinas.

Keywords India · Crystalline rocks · Groundwater exploration · Geophysical methods

Introduction

Igneous and metamorphic rocks underlie about 20% ofthe land surface of the globe (cf. Bowen 1928). In mostof the shield areas, like in Africa, India, and Australia,surface-water systems are meager, and groundwater is animportant source for domestic use and irrigation. How-ever, compact, hard, crystalline rocks like granite havelow porosity and permeability and therefore are not verypromising groundwater sources (Davis and DeWiest1966). Thus, groundwater exploration in such lithologiesis a challenging task. Often a multifaceted explorationstrategy is required, involving geological, geohydrologi-cal, remote-sensing, surface, and subsurface geophysicaltechniques.

Among the surface geophysical techniques, the elec-trical resistivity method is widely used, mainly for localgroundwater problems, whereas gravity and seismic re-fraction methods are used for regional basin studies anddelineation of buried river channels and valley fills. Oth-er geophysical methods, such as magnetic-induced polar-ization and electromagnetic methods, are secondary anduseful to identify the presence of, for example, structuraldiscontinuities and intrusives. However, in crystallinerocks, specifically in granitic regions, exploration bythese methods is not highly successful. In this paper, theutility of the gravity method for groundwater prospectingin an area underlain by crystalline rocks is assessed byanalyzing gravity data in a typical granitic terrain inHyderabad, India.

Characteristics of Igneous Rocks Relevant to Aquifer Potential

Primary PorosityPlutonic igneous rocks, such as granite, generally havelow primary porosity (less than 7%) because they areformed by interlocking crystals (Davis and Turk 1969;Fetter 1988). Lithology, structure, and grain size controlprimary porosity. Available physical-property data (Kobranova 1989) for fresh igneous rocks (acidic to ultrabasic) indicate that primary porosity decreases with an increase in density. Based on the relationshipshown in Fig. 1, an approximate linear equation for density (σ) and porosity (φ) is

(1)

Acidic igneous rocks such as alaskite, granite, and quartzporphyry, whose density is about 2.60×103 kg/m3, thushave primary porosities of 4–6%, whereas ultrabasiceclogite, with a density of 3.35×103 kg/m3, has a porosi-ty of 0.8%. Primary porosity occurs over great depths;Fetter (1988) reports a porosity of 1.42% for granite at adepth of 1,600 m in a deep borehole in northern Illinois,USA.

Secondary PorosityBecause primary porosity of igneous rocks is very small,secondary porosity due to weathering and fracturingneeds to be evaluated in the search for groundwater inigneous crystalline rocks. Fracturing of crystalline rocksincreases porosity by 2–5%, and weathered plutonic andmetamorphic rocks have porosities of 30–40% (Fetter1988). The degree of weathering and fracturing dependsupon petrography and on factors such as depth, tectonicand structural deformation, and intrusive activity.

A large variety of granites with differing petrography,texture, and structural deformation underlies crystallineshield areas. Some are easily weathered, fractured, al-tered, and eroded, resulting in a variety of aquifer condi-tions within these rocks. The sheet-like structures ofsome (weathering) minerals, such as mica, increase po-rosity as they weather. Expansion cracks can form at thecrests of folds due to tectonic movement and intrusions.

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Fig. 1 Relation between density and porosity in igneous rocks (af-ter Kobranova 1989). 1 Eclogite, 2 pyroxenite, 3 dunite, 4 gabbro,5 peridotite, 6 syenite, 7 serpentinite, 8 alaskite, 9 labradorite, 10 quartz porphyry, 11 granite

Joint sets develop in crystalline rocks in three, mutuallyperpendicular directions. Classification of granites basedon their origin (Marmo 1971) or petrography (Balakrishna1961; Chapel and White 1974) is well established andhence their different weathering, fracturing, and aquifercharacteristics are also known (Landers and Turk 1973).

In weathered crystalline rocks, permeability is signifi-cant only along fractures. Weathering and fracturing thusenhance the porosity and also the permeability of hardrocks. Granites have the greatest permeability when theyare partially decomposed, and groundwater yields insuch weathered igneous and metamorphic rocks are re-ported to be 2,200–5,600 L/h (Davis and DeWiest 1966).Bhoumick and Renuka (1991) report an increase in yieldwith an increase in depth of weathering and fracture in-tensity in granites, based on results of geophysical log-ging of boreholes in the Ananthapur district of AndhraPradesh in the south Indian shield.

Variations in Weathering and Fracturing with Depth

The degree of weathering and fracturing in granites grad-ually decreases with depth; consequently, the decrease inporosity and permeability is also gradual (Landers andTurk 1973). The maximum depth of weathering in crys-talline rocks is generally about 30 m in tropical regionsand it is less than 20 m in semiarid regions. Groundwaterin weathered and fractured rocks is mainly within adepth of 50 m, occasionally down to 100 m, but rarely togreater depths (Karanth 1993). Campbell and Lehr(1973) report that in the north-central Australian shieldweathering occurs to a depth of 100 m, and the transitionzone from weathered to unweathered rock is about5–6 m thick. In granitic rocks, yields of wells are as highas 3,800–15,000 L/h in dug borewells (bores in openwells) and yields are 470–4,700 L/h (Ramakrishna et al.1983).

In summary, the primary characteristics of crystallinerocks that affect their aquifer potential are:

1. Primary porosity varies with lithology but is usuallyvery small (less than 7%);

2. Secondary porosity in weathered and fractured zonesis about 30–60%; and

3. Porosity and yield depend directly upon the degree ofweathering and fracturing, and these properties de-crease with depth.

Experience in shield areas throughout the world showsthat conventional methods of groundwater prospecting,including electrical resistivity and remote-sensing tech-niques, provide limited results in crystalline rocks(Lloyd 1981; Ballukraya et al. 1981). In this paper, theusefulness of the gravity method, which has been littleapplied in crystalline terrains, is tested in the granitic ter-rain of Hyderabad, India.

Hydrogeologic Factors Relevant to GroundwaterProspecting in Granites Using the Gravity Method

Various hydrogeologic factors need to be considered inthe context of prospecting for groundwater in granite us-ing the gravity method:

1. Variations in lithology, i.e., presence of different vari-eties of granites with small but different primary po-rosities (Fig. 1);

2. The thickness of the weathered and fractured zone,which varies with petrography and texture;

3. Intensity of fracturing; and4. Degree of saturation and quality of water filling the

pore spaces.

Each of these factors affects the effective density of therock. McCulloh (1967) gives the equation for the bulkdensity of rock as:

(2)

where σb=bulk density of the rock, which includes thetotal porosity and saturation with fluid, σg=grain or min-eral density of the rock, φ=porosity of the rock, σf=den-sity of fluid, and s=saturation percentage.

For example, consider a fresh granitic zone of densityσg=2.62×103 kg/m3 that is altered due to weathering andfracturing, resulting in an effective porosity of φ=20%,saturation s=80%, and groundwater densityσf=1.00×103 kg/m3. From Eq. (2), the bulk density of thealtered, water-bearing granite is σb=2.256×103 kg/m3.Furthermore, consider that an extensive, horizontal,sheet-like weathered and altered zone has a thicknessh=30 m. The Bouguer anomaly due to this layer relativeto fresh granite is estimated as

(3)

i.e., δg=–0.457 mGal, where G=6.67×10–8 cgs, is the uni-versal gravitational constant, and π=3.1416. Likewise, aweathered zone 10 m thick produces a gravity anomaly of–0.15 mGal. Thus, variations in porosity of crystallinerock result in variations in the gravity anomaly.

Using Eqs. (2) and (3), the bulk densities and expect-ed relative gravity anomalies have been calculated for aweathered crystalline-rock aquifer 30 m thick with po-rosities of 10, 20, 30, and 40%, saturation of 80%, andwater density of 1.00×103 kg/m3 in weathered hard rockof uniform lithology. The results are shown in Fig. 2.The gravity anomalies range from –0.2 mGal for 10%porosity in weathered rock located in a fresh-rock zoneof density 2.40×103 kg/m3, to –1.25 mGal for 40% po-rosity in weathered rock located in a fresh-rock zone ofdensity 3.30×103 kg/m3. This range of anomalies is simi-lar to that for buried alluvial channels and valley fills oflimited thickness, and the end values are well within thedetectable range for gravity meters available on the mar-ket. A sensitive microgal gravity meter is sensitive togravity variations even when the layers are thin and po-rosity values are small.

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Figure 3 shows a theoretical model incorporating twogranite bodies similar to the granites in Hyderabad, todemonstrate the expected nature and extent of the gravityanomalies and their vertical derivatives. In the model, a200-m-thick layer of gray granite (pyroxene granite witha density of 2.70×103 kg/m3) is intruded by a 300-m-wide pink granite (orthoclase K-feldspar and quartz-richgranite with a density of 2.62×103 kg/m3; Fig. 3a). Theweathered and exposed part of the pink granite is 150 mwide at the surface and it is 50 m thick; it tapers to a re-duced width of 75 m at depth. The weathered pink gran-ite is assumed to be saturated with water. The dry bulkdensity of the weathered pink granite is 2.31×103 kg/m3.The gravity response by the various parts of the model(Fig. 3b) relative to zero level for the gray granite (lev-el I) is a –0.45 mGal low for the intruded, fresh pinkgranite (level II), a –0.64 mGal low for the water-bear-ing, weathered pink granite (level III), and a –0.93 mGallow for the weathered but completely dry pink granite(level IV). The variations in the anomaly lows are dis-cernible and are large enough to be detected in ground-water exploration.

First and second vertical derivative profiles (Fig. 3c)were constructed, using the GEOSOFT program (North-west Geophysical Associates Inc., 600 SW WesternBlvd., Corvallis, Oregon 97333, USA), from the abovetheoretical gravity data for the weathered, saturated pinkgranite. The first vertical derivative, as expected, shows

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Fig. 2 Relation between relative gravity anomaly due to weather-ing and density of unaltered igneous rock. Assumed weathered-zone thickness 30 m and saturation of 80%, and fluid of density of1.00×103 kg/m3; φ Porosity

Fig. 3 Theoretical Bouguergravity and vertical gradientsfor typical lithologic variationsin granites

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Fig. 4 Map of Osmania University, Hyderabad, India, showing topography, cultural features, and well locations. Numbered wells arelisted in Table 1

the boundaries of the low-density (2.62) pink granite incontrast to the higher density (2.70) gray granite that hasvalues above the zero-derivative line. The shallow,weathered, water-saturated zone (effective density 2.50)in the pink granite produces a prominent negative anom-aly on the first-derivative profile. The boundaries of thisweathered zone, which are assumed to slope inwards, arenot well defined in the first vertical derivative profile.The second vertical derivative, which enhances the effectof shallow features, delineates the boundaries of theweathered zone with zero value. On the second-deriva-tive profile, positive values reflect higher density zones,and negative values lower density parts.

The theoretical model demonstrates that accurate andsystematic gravity surveys and proper data-processingtechniques, as employed in the detection of hydrocar-bons and in mineral exploration, may also prove usefulin groundwater prospecting in granitic regions.

Case Study

To test the usefulness of the gravity method for ground-water prospecting in granite, a field study at OsmaniaUniversity in the city of Hyderabad, India was undertak-en. Locations are shown in Fig. 4. Osmania Universitycovers an area of 10 km2 and it is underlain by granite.The general undulatory topography (Fig. 4) has a meanelevation of 520 m above mean sea level and maximumrelief of 40 m; the area slopes gently southward. Thedrainage pattern is mostly dendritic, but radial in places,and flow is southward to the Musi River, two kilometerssouth of the study area. Groundwater availability in thearea has been continually declining for the past fewyears, as a result of declining water levels.

The granitic rocks that underlie the area belong to thepeninsular gneissic complex and are usually mentionedin the geological nomenclature as “unclassified crystal-lines” (Geological map of India, published by the Geo-logical Survey of India, 1993). However, a two-fold clas-sification of these granites, based on their appearance, ismore common: (1) the pink granite of late- or post-kine-matic origin, with a high K-feldspar content, quartz andplagioclase; and (2) the gray granite of synkinematic ori-gin, pyroxene-rich. At places, biotite granites are alsopresent, and transition zones exist between pink and graygranites. In these zones, correct identification of thegranite is difficult due to weathering and alteration. Thearea is traversed by a few dolerite dikes and rarely bypegmatite and quartz veins (Narayanaswamy 1970; Kanungo et. al 1975; Satyanaryana 1983; Sarvothamanand Leelanandam 1987).

The geology of the area and lineament trends areshown in Fig. 5. The information shown consolidates datafrom earlier workers, including Raja (1959), Christopher(1963), Sitaramayya (1968, 1971), Subrahmanyam andVerma (1981), Subba Rao et al. (1983), Murty et al.(1984), and Gnaneshwar (1987). The study area is under-lain by a loose topsoil a few meters thick, which overlies

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weathered and semi-weathered (fractured) granite. Thefractured granite is as much as 40 m thick and gradesdownwards into hard granite.

Groundwater in the study area is available mostly inthe pink granite, in topographic depressions where moreextensive weathering occurs. Structural features such asfaults, joints, and dolerite dikes, form either conduits orbarriers to groundwater movement. A few open wells andmore than 20 borewells presently supply water for the var-ious offices, institutions, hostels, laboratories, and staffquarters; the yields of these wells are given in Table 1.Most high-yielding wells are located along three or fourmajor lineaments. The yields in general are reduced byhalf during summer as compared to the monsoon period;this indicates that recharge is mostly from precipitation.

Gravity Survey and AnalysisThe gravity survey was conducted in two stages. First,110 observations were established on 200- to 300-mspacings along roads and paths throughout the campus.Utilizing bench marks of known elevation for control,the elevation of each station was determined using anAmerican Paulin precision altimeter; then a contour mapwith a contour interval of 1.5 m (5 ft) was constructed.In addition, the elevations of stations on a 50-m spacing(not shown in Fig. 4) were determined by leveling alongtwo traverses – a north-trending traverse about 2,400 mlong, and an east-trending traverse about 2,000 m longthat intersects the first traverse near the middle. Traverselocations are shown in Fig. 6. Gravity measurementswith a W. Sodin gravity meter (sensitivity 0.024 mGal)

Table 1 Yields of wells on the Osmania University campus duringmonsoon period. Data from University Engineer

Well no. Type of well Yields (L/h)a

OW1 Open well 4,900OW2 2,367OW3 1,618OW4 3,157W1 Borewell 1,641W2 1,673W3 7,891W4 1,420W5 631W6 3,154W7 14,205W8 4,300W9 3,300W10 6,310W11 1,578W12 3,154W13 1,578W14 789W15 3,300W16 474W17 3,935W18 474W19 1,900

a Summer, dry-season well yields are approximately half. Data ob-tained in 1997

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were made at each observation station, and measure-ments with a Lacoste-Romberg gravimeter (sensitivity0.01 mGal) were made at each station along the travers-es. A Bouguer gravity-anomaly map (Fig. 6), utilizing adensity value of 2.67×103 kg/m 3 for the Bouguer correc-tion, was constructed with reference to a local base. Thereliability of the areal data is ±0.1 mGal and that alongthe traverse profiles is ±0.05 mGal.

A comparison of the distribution of gravity anomalies(Fig. 6) with the surface geology and structural features(Fig. 5) reveals:

1. The presence of various lineaments, including north-east-trending lineaments from the Arts college to

Tarnaka and from south of Jama-I-Osmania Railwaystation to IICT (Indian Institute of Chemical Technol-ogy), a northeast-trending lineament between the ob-servatory and HPS (Hyderabad Public School), and anorthwest-trending lineament just west of the live-stock farm. These are shown on the gravity map bythe contour alignments along the lineaments and bysteep gradients across them.

2. The area of local high topography and exposed gran-ite, generally in the south-central part of the campus,is represented by a broad gravity high. Topographical-ly low areas, generally in the southern half of thecampus, are represented as gravity lows, which are aresult of deeper weathering and a decrease in effec-tive density of granite.

3. The areas of higher gravity are mostly in gray granitewhich generally underlies the higher elevations. Theareas of pink granite are predominantly represented

Fig. 5 Geology of the Osmania University campus, showing rocktypes and structural features. (Modified after Christopher 1963and Raja 1959)

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Fig. 6 Distribution of relative Bouguer anomalies

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Fig. 7 Distribution of first vertical-derivative gravity

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Fig. 8 Distribution of second vertical-derivative gravity

by gravity lows. The pink granite is coarse grained,relatively friable, and easily weathered; therefore, itunderlies the lower elevations. Almost all the high-yielding borewells are located either in the pinkweathered granite or along lineaments in the area.

Vertical Derivatives of Gravity DataGradients and higher derivatives of geophysical data en-hance anomaly resolution by recording shallow andmore sensitive signatures at the expense of deep-seatedsources. The profiles of the derivatives model (Fig. 3c)show that near-vertical boundaries between differentlithologies can be more clearly delineated by the secondvertical derivative, whereas the first vertical derivativeenhances the geophysical signatures of shallow objects.Figure 7 is the first vertical-derivative map, derived fromthe relative Bouguer-anomaly map of the O.U. Campusarea. A broad gravity high in the south-central part de-fines the area of gray, relatively dense granite. In thenorthern half of the campus, the large number of closelyspaced, intense positive and negative anomalies suggeststhat the bedrock lithology varies in density and mass dueeither to changes in composition or degree of weather-ing. The alignment of these anomalies also suggests thepresence of lineaments, both faults and lithologic bound-aries, as is evidenced in the map of surface geology(Fig. 5).

Northwest-trending positive anomalies from thesouthern edge of the map west of long. 78°32′E to Jama-I-Osmania Railway station may mark a boundary be-tween gray and pink granite. Although a second vertical-derivative map generally enhances the resolution ofanomalies recorded in a first-derivative map, that is notthe case on the second-derivative map of this area,shown in Fig. 8. The dolerite dikes and quartz veins, ap-parently because of their narrow widths (1–10 m), do notrecord any signatures on the Bouguer-anomaly or deriva-tive maps.

Subsurface Density ConfigurationsUsing the radial spectra (Spector and Grant 1970; Hahnet al. 1974) of the gravity data from the 110 stations onthe Osmania University campus, the depths to density in-terface were estimated. Results are shown in Fig. 9. Thespectra show a three-layer subsurface configuration thatcorresponds to the known geological conditions. The up-per layer (40 m thick) may be a weathered zone, and thelayer below it (126 m thick) may represent a fracturedzone of relatively greater density and reduced porosity.The lowermost layer (360 m thick) may be a zone oftransition from pink to gray granite. Thickness values ofthe upper two layers nearly match those of layers inter-preted from a magnetic study by Rambabu et al. (1991).

Detailed Gravity ProfileDetailed gravity profiles were run along two traversesacross much of the campus (see Fig. 6 for location);however, the data from traverse 2 (east-trending tra-verse) were not suitable for density modeling. The dataalong the profile of traverse 1 were interpreted in a two-dimensional, gravity-modeling program; results are shown in Fig. 10. The minimum density of2.32×103 kg/m3 represents highly weathered pink gran-ite, and the dolerite intrusion has a density of2.80×103 kg/m3. Variations in the density values of themodels show a cumulative effect of variation in litho-logic composition as well as that of weathering. Lateralcontacts between various lithologic units are possiblejoint zones or faults, as is evidenced in surface litho-logic and structural maps. In the northern part of tra-verse 1, the maximum thickness of the weathered layer,presumably those areas with densities of 2.32 and2.53×103 kg/m3, is 35 m, which corresponds closelywith the available drilling data and the thickness report-ed by Rambabu et al. (1991).

Equivalent LithodensityTo refine the surface geological map, especially with re-gard to the distribution of different varieties of granite,an equivalent-lithodensity map, shown as Fig. 11, wasconstructed from the relative-gravity-anomaly map(Fig. 6). The lithodensity map reflects the effects ofweathering and fracturing and therefore may reveal po-tential aquifers in the granitic region.

By subdividing the Bouguer gravity (Fig. 6) intozones between selected anomalies, the total anomalous

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Fig. 9 Relation between log amplitude and radial frequency of ra-dial spectra of gravity data

mass in each zone was estimated using the Gauss theo-rem (Hammer 1945). Using the thickness obtained fromthe radial spectra (Fig. 9) and the estimated mass, theequivalent density of each zone was computed, as shownin Fig. 11. These density values, which are estimates ofthe entire rock column above the basement in each zone,are grouped into six units in which density values vary inthe range 2.62×103–2.72×103 kg/m3 at intervals of0.02×103 kg/m3. In a sense, the equivalent-density mapis a reflection of the relative Bouguer-anomaly map, butwith some modifications. The lower density zone mayhave a greater potential for the occurrence of aquifers, apossible indication of granite weathering, fracturing, sat-uration, and potable saturating fluid. Thus, these low-density zones, together with the lineaments, assumed tobe faults, are potential areas for groundwater exploita-tion. By contrast, the area of hard gray granite of rela-tively high density in the east-central part of the campushas a low potential.

Residual Gravity Versus Well YieldExcluding the borewells located on lineaments, all high-yield wells of the campus area are in zones of low gravi-ty. This relationship is demonstrated by Fig. 12, whichshows that yields of borewells have a linear inverse rela-

tion to residual gravity (δg), as y=0.6154×δg+3,200 L/h.This relation may be useful for assessing groundwateryield from the residual gravity of a hard-rock terrain, atleast under favorable conditions such as

1. Recharge conditions in the area are normal during theperiod of the investigation,

2. The permeability of the weathered layer is uniform,and

3. The weathered layer is horizontally stratified.

Inferences and ConclusionsThe distribution of relative Bouguer-anomaly valuesclosely reflects the geology of the area; gray granite hashigher gravity values than does the pink granite. At someplaces, the lateral transition zone between the pink andgray granites is marked by the zero-anomaly contour.The primary aquifers in this area are weathered zones,particularly in pink granite, which is more susceptible toweathering. Weathered pink granite has a lower densitythan unweathered pink granite, which is reflected by rel-atively low gravity values. Lineaments are abundant andmore recognizable in the gray granite than in the pinkgranite because gray granite has more foliation and min-eral lineations (Kanungo et al. 1975). Some of the linea-ments possibly are faults. The first and second vertical-derivative maps show the structural features in the area.Some high-yielding wells are in the lineaments. In thesouthwestern part of the area, the gray granite is proba-bly bounded by a northwest-trending fault.

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Fig. 10 Gravity profile and density model along traverse 1. Loca-tion of traverse 1 is shown in Fig. 6

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Fig. 11 Distribution of surface lithodensity, as inferred from gravity data

Conclusions from this study are as follows:

1. In this granitic area of gentle topography and smoothregional gravity, the gravity residuals reflect varia-tions in granitic composition as well as the thicknessof the weathered zones. The anomalies generally varyin the range 0.1–0.7 mGal, depending upon lithology,weathering, porosity, degree of saturation, and thick-ness of weathered zones.

2. Faults and extensive joints in the crystalline rocks maybe delineated by gravity gradients and on derivativemaps. Intrusives like dolerite dikes and quartz veins ofnarrow width cannot be identified from the gravity da-ta that were collected on the scale of this study.

3. With the presently available, sensitive gravity meters,small gravity signals (on the order of a few micro-Gals), resulting from differential weathering or varia-tions in structure or lithologic composition, can be re-liably mapped.

4. On the Osmania University campus, the gravity sur-vey helped to delineate the extent of pink and graygranites, to identify zones of weathering, and to moreclearly define lineaments that are potential aquifers. Acomparison of relative gravity anomalies with theyields of borewells in weathered zones reveals thatyield is directly proportional to the weathered thick-ness, and inversely proportional to the residual gravi-ty in weathered granitic regions.

5. The gravity method, either independently or in com-bination with other geophysical methods, is a usefultool for groundwater prospecting in hard igneousrocks.

Acknowledgments The authors are thankful to C.P. Gupta, Scien-tist ‘G’ (retired), National Geophysical Research Institute, Hyd-erabad, for useful discussions and valuable suggestions in finaliz-ing the paper. V.K. Raghavan is thankful to the University GrantsCommission, New Delhi, for awarding him a Junior Research Fel-lowship. The authors thank the Osmania University Engineer forproviding the borewell information, a topographic map, and eleva-tion bench-mark data of the campus. G. Rama Brahmam and K.Rama Mohan, Research Fellows in the Geophysics Department,assisted in the collection of field data. Thanks are due to B.N. Raofor the use of the GM Pac Spectra Software, and to H.V. Ramababu,Senior Scientist, National Geophysical Research Institute, for hishelp in processing the data using Geosoft programs. The authorsalso thank E. Klingele, Geophysics Institute, Honggerberg, Zurich,Switzerland, and David L. Campbell, US Geological Survey, forconstructive criticism and helpful suggestions. The illustrationswere prepared by P. Ravinder, College of Technology, OsmaniaUniversity.

References

Balakrishna S (1961) Granite tectonics of Hyderabad. Proc IndianAcad Sci 43:73–84

Ballukraya PN, Shaktivadivel R, Baratan R (1981) Inadequacies inthe technique of resistivity method for location of water wellsites in hard rock areas. Nord Hydrol 12:185–192

320


Fig. 12 Relation between residual gravity and yields ofborewells

Bhoumick AN, Renuka S (1991) Integrated geophysical and hy-drogeological field data analysis for groundwater explorationin hard rock terrain of Ananthapur dist. of A.P. India. In: ProcSem Exploration Geophysics, Hyderabad, November 1991, pp 25–30

Bowen NL (1928) The evolution of igneous rocks. Princeton Uni-versity Press, Princeton, New Jersey, USA, 334 pp

Campbell MD, Lehr JH (1973) Water well technology. McGrawHill, New York, 681 pp

Chapel BW, White AJ (1974) Two contrasting granite types. Pa-cific Geol 8:173–174

Christopher G (1963) Analysis of magnetic anomalies on granitesof Hyderabad. In: Proc Sem Geophysical Investigations in thePeninsular Shield, Osmania University, Hyderabad, 27–31May 1963, pp 87–92

Davis SN, DeWiest JM (1966) Hydrogeology. John Wiley, NewYork, 463 pp

Davis SN, Turk LJ (1969) Best well depth in crystalline rocks.Ground Water 2(2 April):6–11

Fetter CW (1988) Applied hydrogeology. Merrill, A Bell andHowell Information Co. Columbia, Toronto, Canada, 529 pp

Gnaneshwar P (1987) Litho-geochemical surveys in Hyderabadgranites. PhD Thesis, Osmania University, Hyderabad, 170 pp

Hahn A, Kind EG, Mishra DC (1976) Depth estimation of mag-netic sources by means of Fourier amplitude spectra. GeophysProsp 24:287–308

Hammer S (1945) Estimating ore masses in gravity prospecting.Geophysics 10(1):50–62

Kanungo DN, Rama Rao P, Murty DSN, Ramana Rao AV (1975)Structural features of the granites around Hyderabad, A.P.Geophys Res Bull 13(3, 4):55–72

Karanth KR (1993) Groundwater assessment, development andmanagement. McGraw Hill, New Delhi, 720 pp

Kobranova VN (1989) Petrophysics. Mir, Moscow, 375 ppLanders RA, Turk LJ (1973) Occurrence and quality of groundwa-

ter in crystalline rocks of the Lano area Texas, USA. GroundWater 2(1):5–10

Lloyd JW (ed) (1981) Case studies in groundwater resources evo-lution. Claredon Press, Oxford, UK

Marmo V (1971) Granite petrology and granite problem. Elsevier,Amsterdam, The Netherlands, 244 pp

McCulloh TH (1967) Mass properties of sedimentary rocks andgravimetric effects of petroleum and natural gas reservoir.USGS Prof Pap 528A, 50 pp

Murty BVS, Bhimasankaram VLS, Varaprasada Rao SM (1984)Structure and geology from Maneru valley from gravity andmagnetic surveys. Indian Acad Geosci 24:5–12

Narayanaswamy S (1970) Tectonic setting and manifestation ofthe upper mantle in the Precambrian rocks of south India. In:Proc 2nd Symp Upper Mantle Project, Hyderabad, December1970, pp 397–403

Raja N (1959) Pegmatites from Uppal (Hyderabad). Curr Science28:284–285

Ramakrishna BN, Murty BVS, Venkateshwara Rao M (1983)Geological and geophysical investigations in the granitic ter-rain for groundwater occurrence in Jeedimetla, Hyderabad,A.P. Geoviews 10:11–22

Rambabu HV, Kameshwara Rao N, Vijaya Kumar V (1991) Bed-rock topography from magnetic anomalies – an aid forgroundwater exploration in hard rock terrains. Geophysics56(7):1051–1054

Sarvothaman H, Leelanandam C (1987) Petrography and majoroxide chemistry of the Archaean granitic rocks of Medak area,A.P. J Geol Soc India 30:194–209

Satayanaryana B (1983) Petrological studies and their bearing onthe origin of Hyderabad granites. J Acad Geosci 26:1–8

Sitaramayya S (1968) Structure, petrology and geochemistry ofgranites of Ghatkesar, A.P. PhD Thesis, Osmania University,Hyderabad

Sitaramayya S (1971) The pyroxene bearing granodiorites andgranites of Hyderabad area (the Osmania granites). Q J GeolMining Metall Soc India 43:117–129

Spector A, Grant FS (1970) Statistical models for interpretingaeromagnetic data. Geophysics 15:293–302

Subba Rao DV, Sharma JRK, Krishna Brahmam N (1983) Densi-ties of geological formations of peninsular India. Geoviews10(11):371–405

Subrahmanyam C, Verma RK (1981) Densities and magnetic sus-ceptibilities of Precambrian rocks of different metamorphicgrade rocks (south India shield). J Geophysics 49:101–107

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