Gravity exploration for groundwater in the Bara basin, sudan

Geoexpploration, 19 (1981) 127-141 Elsevier Scientific Publishing Company, Amsterdam -Printed

GRAVITY EXPLORATION BASIN, SUDAN

FOR GROUNDWATER

in The Netherlands

127

IN THE BARA

H.O. AL1 and R.J. WHITELEY Rural Water Corporation, Khartoum (Sudan) School of Applied Geology, University of New South

Wales, Sydney

(Australia)

(Received January 21, 1981; accepted March 4, 1981)

ABSTRACT Ali, H.O. and Whiteley, R.J., 1981. Gravity exploration Basin, Sudan. Geoexploration, 19: 127-141.

for groundwater in the Bara

The gravity method may be used in the exploration and evaluation of groundwater resources in deep sedimentary basins. It allows the lateral and vertical extent of sedimentary fill to be determined and can provide estimates on the total water storage capacity of prospective basins. Provided some depth control is available, simple graphical techniques may be used to separate the regional and residual components of the measured gravity field. These compare favourably with results from computer trend surface analysis, even on relatively short lengths of profile. Both computer and manual gravity interpretation using dot charts provide similar results and can be employed to accurately determine basin shape within the limitations of available density information. The gravity method has been successfully applied to groundwater search in the Bara Basin, Sudan. Although the Bara Basin is not fully developed, the gravity interpretation together with laboratory measurements or samples and numerical simulation, have allowed the hydrogeological characteristics of the Basin to be evaluated.

INTRODUCTION

The water supply to Elobeid, the capital of Kordofan province in Sudan, is from rainwater ponded in artificial depress&s (known locally as hafirs), some 30 km to the south of the town. Elobeid supports a population of about 0.5 million and is regarded as the major market for Arabic gum, the second most important crop in Sudan. Due to poor rainfall in 1966 and 1973 the hafirs failed to store the required capacity to meet Elobeid’s demand, and as a result of these shortages, water rationing was imposed in the following dry seasons. To prevent future shortages and to provide permanent long term supply, the Rural Water Corporation and the Electricity and Water Corporation suggested the nearby Bara Basin as a primary target for a groundwater search, although little was known of its geology and hydrology. 0016-7142/81/0000-0000/$02.50

0 1981 Elsevier Scientific

Publishing Company

26

28

30

32

34

36

36

6

; UGANDA

Umm

Ruwaba

Red

$eo

Nubion

Formation

Basement

Fig.1.

Formation

Tertiary

Location

Complex

Marine

‘\

Plio-Pleistocene Deposits

Lower Cretoceous Pre-Combrlon

of the Bara Basin.

The Bara Basin is about 60 km north of Elobeid (Fig.1) and covers an area of about 6000 km’ of semi-desert terrain with sparse vegetation. Topographically the area is covered by white sands and sand dunes which form a rough surface with general elevations varying from 540 m (a.m.s.1.) in the south to 480 m (a.m.s.1.) in the north. Annual rainfall in the region is about 300 mm, reaching a peak in August. Geologically the basin is composed of relatively fine, unconsolidated sediments known as Umm Ruwaba Formation (Whiteman, 1971). These sediments are probably Tertiary to Pleistocene in age and may be related to the early Nile drainage system (Vail, 1978). They are enclosed to the west and the south by metamorphic and igneous rocks, and to the north by relatively thin sediments of the Nubian Sandstone Formation (Whiteman, 1971).

c

. .p . f

130

GRAVITY

SURVEY

The gravity method was selected for initial exploration of the Bara Basin because a large density contrast was expected between the basement and poorly consolidated prospective sediments. In addition, the large area (6,000 km*) of the basin could be explored in a short period of time at minimal cost with the gravity method. A total of about 800 gravity stations were occupied during the survey. These stations were spaced at intervals of between 0.5 km and 1 km, along six profiles oriented approximately perpendicular to the expected strike of the basin. To increase the speed of the survey, larger station spacings were used over areas of known shallow basement. Many of the special survey procedures suggested by Colley (1961) for operation in desert terrains were used in the gravity survey. Elevations were accurately determined by levelling and gravity measurements were carried out with a Worden Prospector Gravimeter. During the survey, maximum drifts of 0.2 mgal/h were recorded. The observed gravity values were corrected for drift, elevation and latitude. To apply Bouguer corrections, Nettleton’s (1940) profile method was used. This method provides bulk estimates for the bulk densities of ,the near surface geological materials and gave values of about 1.8 g/cm3 for the dune sands and 1.9 g/cm3 for the dry sediments of the Umm Ruwaba Formation. The resulting Bouguer gravity data were then computer contoured at 5 mgal intervals as shown in Fig.2. The location of the gravity stations occupied is also shown. The gravity measurements were also tied to an established gravity base station whose absolute value is 978J45.91 mgal, located at 13”ll”N and 30”40”E (Isaev and Mitwali, 1974). BOUGUER

GRAVITY

MAP

Physically, the Bouguer gravity map represents anomalies from all the vertical and lateral density variations within the earth and may be used to qualitatively deduce geological structure. As shown in Fig.2, a prominent gravity high occurs in the southern part of the surveyed area where basement is shallow or crops out. Small variations in gravity values in this region probably reflect density variations within the shallow basement caused by variations in weathering, especially in the south western portion of the area where graphitic schists occur (Strajexport, 1970); or are possibly due to the emplacement of lighter material, e.g., granite, within the crystalline schist basement. Fig.2 shows that the gravity low over the basin has an amplitude of 30 mgal and the general strike of the contours indicates that the Bara Basin is a narrow northwesterly--southeasterly elongated depression about 45 km wide formed into a semi-closed system with an outlet apparent only to the southeast.

131

Approximately 40 km east of Elobeid, Bouguer gravity values decrease rapidly at a rate of 4 mgal/km. This rapid decrease suggests a sharp contact between two lithological units; i.e., the basement complex and the lower density sedimentary fill within the Bara Basin. RESIDUAL

GRAVITY

ANOMALY

The Bouguer gravity map in Fig.2 may be thought of as a combination of large scale or regional anomalies and local or residual anomalies. The difference between Bouguer gravity and regional gravity is termed residual gravity related to the density contrasts of interest. In spite of all recent developments in gravity interpretation, unique separation of regional and residual anomalies can rarely be achieved and personal bias is inherent in the interpretation and plays an important role. The different approaches to this problem are discussed by Dobrin (1970) and Nettleton (1976). As a result of the steep Bouguer gravity gradients near the edges of the Bara Basin and the availability of some limited borehole information giving depth to the basement in this region, a graphical method modified from Ibrahim and Hinze (1972) was used to separate regional and residual anomalies. Each Bouguer gravity profile was assessed separately and regional trends were then adjusted for contouring. The regional background is not strong and apparently does not unduly influence Bouguer gravity data within the Basin. The graphically derived residual gravity map is shown in Fig.3. Attempts were made to quantify the regional field by fitting a first order polynomial trend surface to the Bouguer gravity data using a computer program by Davis (1973). This gave a resultant regional gradient of about 0.6 mgal/km with values decreasing in a southwesterly direction. This calculated regional gradient is probably inaccurate because of the limited length of the gravity profiles relative to the dimension of the Bara Basin gravity anomaly. However, its relatively low value suggests that little overall density contrast exists between the basement rocks on either side of the basin. This probably means that the Nubian Sandstone which crops out on the northeastern side of the Bara Basin is relatively thin. The residual gravity map of the Bara Basin area obtained by subtraction, from the Bouguer gravity, of a regional field defined by the first order surface is shown in Fig.4. This defines the Basin more clearly and qualitatively indicates the deepest part of the Basin, assuming that the low density sedimentary fill alone is responsible for the residual gravity anomaly. The limits of the basin are approximately indicated by the zero contour line in Fig.4. Comparison of Figs.3 and 4 indicates a general similarity between residual gravity obtained by graphical and computer methods over much of the area although there is some offset in the gravity low in the southeastern side of the area which may be significant. Other differences between the two maps

132

t t

P 6

133

id ts

134

i

c #

135

probably reflect personal bias inherent in the graphical method and the limited profile lengths. Each gravity profile was interpreted separately by assuming a two-dimensional model striking at right angles to the profile. The shape of the Basin was computed both graphically, using Morgan and Faersler’s (1972) dot chart method, and iteratively by computer, using the method described by Qureshi and Kumar (1976). A quantitative interpretation of one of the gravity profiles in the north of TABLE

I

Comparison between Graphical Bara-Elbaharya Profile Distance

(km) _~~_ 0.0 1.0 3.0 5.0 7.0 9.0 11.00 13.00 15.00 17.00 19.00 21.00 23.0 25.0 27.0 29.0 31.0 33.0 35.0 37.0 39.0 41.0 43.0 45.0 47.0 49.0 51.0 53.0 55.0 57.0 59.0 61.0 63.0

Observed g value (mgai) - 2.5 - 4.5 - 6.0 - 8.5 -12.5 -20.5 -21.2 -19.0 -20.0 -23.5 -28.0 -32.5 -34.0 -32.5 -31.0 -30.2 -29.0 -28.0 -27.0 -24.5 -19.5 -17.5 -17.0 -18.5 -20.5 -18.5 -17.0 -14.0 -10.0 - 5.5 - 2.5 - 3.00 - 3.00

Calculated

and Computer

Calculated

Sediment

(m)

Calculated

g value (mgal)

graphical

computer

-

4.00 5.00

-

8.00

400

100 100 300 400 500 700 800 900 900 900 1,000 1,400 1,500 1,400 1,300 1,200 1,100 1,000 900 800 700 700 700 700 600 600 500 400

170

200

-13.00 - 9.00 - 6.00

150

100

-

- 3.01 - 4.5 - 6.1 - 8.5 -12.41 -20.45 -21.21 -19.00 -20.00 -23.52 -28.01 -32.50 -34.00 -32.50 -31 .oo -30.19 -29.00 -28.00 -27.00 -24.52 -19.49 -17.48 -17.00 -18.50 -20.50 -18.50 -17.01 -14.00 -10.00 - 5.5 - 2.52 - 3.00

85.0 135 235 340 810 770 900 1,300 1,400 1,350 1,220 1,050 770 700 810 680

Elghaman-

___ depth

computer

graphical

Depths

-20.00 -20.00 -24.00 -32.00 -32.00 -30.00 -27.5 -24.00 -17.00 -19.00 -18.50

4.00

Location

Elghaman

Bara

Elbaharya

136

137

the area is shown in Fig.5. This profile extends in a northeasterly direction from Elghaman to Elbaharya (Fig.4). The regional gravity profile obtained graphically, Bouguer gravity, residual gravity and both the graphical and computer interpretations, are shown on Fig.5. Also shown are basin cross-sections resulting from both interpretations based on a two-dimensional model. It can be seen that these agree closely and that the maximum thickness of sediments occurs in the vicinity of Bara. Table I list a comparison between the graphical and iterative computer interpretations which shows that small differences of up to + 3% occur in the central part of the profile and up to + 10% at the edges. Similar results were obtained on the other profiles and in most cases the graphical and iterative computer solutions agreed to within about f 10%. It therefore appears that, within the limitations of the two-dimensional model and available density information, either interpretation method may be used. Depths calculated along each profile were contoured to give a total thickness map of prospective sediments (Fig.6). Also shown are the limits of shallow basement and the locations of shallow drillholes. This map shows that about 1,400 m of sediments occur near the central part of the Basin. To the north and south, the Basin is bounded by steep sides with an average gradient of 10% inwards, while along strike, basement gradients of about 1% are present. The low gradient along strike may indicate that the basement surface is relatively smooth and uncomplicated. The steep slopes of the other walls suggest that the Basin has resulted from normal faulting, possibly related to the latest stage in the development of the western branch of the African Rift System. Nearby earthquake incidents (Qureshi and Sadig, 1967) also suggest that the area is still tectonically active. ESTIMATION

OF TOTAL

WATER

STORAGE

IN THE BARA

BASIN

Based on Gauss’s Theorem, anomalous mass can be determined uniquely from residual gravity values (Lafehr, 1965; West and Summer, 1972). For practical purposes, anomalous mass (M), in tonnes is given by:

M=KL:GA where K is a constant equal to 10 *',G is the residual gravity value in mgal, A is the area enclosing the body in km2. In deep sedimentary basins, anomalous mass (M) may be divided into two parts: saturated and unsaturated. Alternatively, the total volume (V,)of the saturated sediments may be related to anomalous mass (M) by: M--P, -~i)hA v, = __ (P3 - P2)

138

Hence, the total storage volume (V) is given by: v= v,qJ where h = the average water level in the basin (50 m), A = surface area of the basin (4,500 km’), p3 = density of the basement, pz = density of the saturated sediments, p 1 = density of the dry sediments, 4 = bulk porosity. A density of 1.9 g/cm3 for the dry Umm Ruwaba sediments (p J was obtained using Nettleton’s (1940) profile method within the Basin. Densities for the saturated Umm Ruwaba Formation (p 2) and basement (p 3) were taken as 2.1 and 2.67 g/cm3 respectively based on work by Mitwali (1969) and Hunting (1973) in geologically similar areas. The difference in dry bulk density and assumed wet density for the Umm Ruwaba sediments allows an average total or bulk porosity to be calculated according to :

Q = (PZ

-

PIUP,

where p w = 1.0 g/cm3. This gave $ = 0.2. For the Bara Basin M was calculated at 3.6.lo’* tonne and the total storage volume (V,) was found to be 1O1*m3. It should be emphasised that these values are different (overestimated) from the storage permissible for abstraction (safe yield) for the following reasons: (1) The most important factor in sedimentary basins is the specific yield which is equal to the porosity (4) minus the specific retention. (2) The calculation from gravity values has encountered depths as great as 1,000 m, too deep for cost effective development of water supply. Thus, the calculated storage value provides only initial estimation of the total storage in the basin and cannot replace the hydrogeological methods for groundwater evaluation. HYDROGEOLOGICAL

CHARACTERISTICS

OF THE BARA

BASIN

Hydrogeologically, the Basin is not fully developed, though more than 40 shallow operating wells are present. These indicate that the bulk of the Basin is infilled with sediments of the Umm Ruwaba Formation and that aquifers, associated with gravels and sands have been found at depths greater than 60 m. In most cases the shallow Bara Basin sediments are poorly sorted clay beds and/or sandy clays in which vertical and lateral facies changes are common. Mechanical analysis tests have shown that the effective grain size is about 0.3 mm, and the uniformity coefficient varies from 3.2 to 5.7. The water level varies from 50 m to 75 m and is confined by relatively thick and fine layers of clays and sandy clays. Fig.7 shows a north-south geological section across the Basin derived from shallow drilling. This section extends from Zurigaelquizon to Eltayara (Fig.6). Regardless of recharge, numerical modelling (Ali, 1978) has predicted 0.5 m/y for the decline in water level. Estimation of hydraulic parameters by

SILTY

SAND

(FINE)

CLAY

HORIZONTAL

?,5,.

SAND

VERTICAL

“:

BASEMENT

Fig.7. Shallow narthsouth

,

100

Correlation

to

L

COMPLEX

geological cross-section across the Bara Basin.

200 I

1 300 0 EPTH

Rg.8.

IOkrn t

between salinitjr and depth.

(ml

=4882

BORE “OI_E

25

CLAY

I 400

%

fiATi

wan I

140

Salama (1977) has also indicated that the transmissivity of the Basin varies from 100 m2/day to 500 m2/day, whereas the storativity varies from lo-’ to 10-4. Chemically, the water quality varies between 500 ppm to 1,200 ppm TDS. The relatively high salinity may be due to low estimated velocity of the water (0.1-0.3 m/year), low infiltration rate, and high clay ratios in some places. Satisfactory correlation has also been found to exist between salinity (TDS), depth and clay ratio as shown in Fig.8. Based on these data a preliminary numerical model for the Bara Basin indicates that it can be pumped continuously for at least five years without recharge before water levels reach critical depths. However, further testing is required before more accurate predictions can be made. CONCLUSION

Gravity exploration for water resources in the Bara Basin has defined a basin which is elongate in shape widening to about 40-50 km in the northsouth direction, and which forms a semi-closed system with an outlet only on the southeastern side. Both graphical and computer interpretation using a two dimensional model show that sediment thicknesses decrease rapidly on the northern and southern sides of the Basin and attain a maximum thickness of about 1.4 km near the central part of the Basin. Based on residual gravity values, the total groundwater storage in the Basin is estimated at about 1012 m3. In order to reach a definite conclusion about the groundwater potential of the Bara Basin, further geophysical, i.e., geoelectrical and hydrogeological studies, need to be completed. Such studies should delineate clay and sand beds and allow determination of hydraulic parameters, as well as the annual recharge and the corresponding response in water levels. ACKNOWLEDGEMENT

The support of the Rural Water Corporation, Sudan is gratefully acknowledged in allowing this research work.

REFERENCES Ali, H.O., 1978. Gravity exploration and numerical simulation of groundwater resources in Bara Basin, Sudan. Thesis, Univ. New South Wales, Sydney. Colley, G.C., 1961. Gravity surveys in heavy sand dunes. Geophysics, 26(4): 490. Davis, J.C., 1973. Statistics and Data Analysis in Geology. Wiley, New York, N.Y., 550 pp. Dobrin, M., 1970. Introduction to Geophysical Prospecting. McGraw Hill, New York, N.Y., 405 pp. Ibrahim, A. and Hinze, W.J., 1972. Mapping buried bedrock topography with gravity. Groundwater, 10: 18-23. Isaev, E.N. and Mitwali, M.A., 1974. Gravity studies in Sudan, 1. Gravity bases. Bull. Sudan Geol. Min. Res. Dept., 26: 49 pp.

141 Lafehr, T.R., 1965. The estimation of the total amount of anomalous mass by Gauss’s theorem. J. Geophys. Res., 70: 110-118. Hunting Geology and Geophysics, 1973. Savanna Development Project, Geophysical Survey, Sudan. FAO/UNESCO, England. Mitwali, M.A., 1969. Interpretation of low gravity anomaly in northeast Kordofan, West Sudan. Bull. Geofis. Theoretica Appl., 11(41-42): 119-125. Morgan, N.A. and Faersler, C.N., 1972. A two and three dimension gravity dot chart. Geophys. Prospect., 20: 363-374. Nettleton, L.L., 1940. Geophysical Prospecting for Oil. McGraw Hill, New York, N.Y. Nettleton, L.L., 1976. Gravity and Magnetic in Oil Prospecting. McGraw Hill, New York, N.Y., 464 pp. Qureshi, I.R. and Kumar, A., 1976. Automatic interpretation of gravity anomalies associated with two dimensional mass distributions. Geophys. Prospect., 24: 660-669. Qureshi, I.R. and Sadig, A.A., 1967. Earthquakes and associated faulting in Central Sudan. Nature, 215(5098): 263-265. Salama, R.B., 1977, Groundwater resources of Sudan. Rural Water Corp. Rept., Sudan. Strajexport, 1970. Groundwater Research, northeastern part of Kordofan Province, Sudan. Rural Water Corp. Sudan, Final Rep. Vail, J.R., 1978. Outlines of the geology and mineral deposits of the Sudan and adjacent areas. Bull. Inst. Geol. Sci., London, 49. West, R.E. and Summer, J.S., 1972. Groundwater volumes from anomalous mass determinations for alluvial basins. Groundwater, 10: 24-32. Whiteman, A.J., 1971. Geology of Sudan. Oxford Press, Oxford.