q BULLETIN of the International Association of ENGINEERING GEOLOGY / de I'Association Internationale de GI~OLOGIE DE I'INGI-NIEUR Paris - No 54 - Octobre 1996 EVALUATION OF BOREHOLE SITES BASED ON AIRPHOTO DERIVED PARAMETERS EVALUATION DE SITES DE POMPAGE A L'AIDE DE LA PHOTO-INTERPRI~TATION EDET A.E.* Abstract Three parameters deri red from air photo interpretations namely: lithology, lineament index and drainage index have been employed to compute a ground-water site evaluation index (GWSEI). The GWSEI values are then used as a basis to evaluate the groundwater potentials of borehole sites. Results from the study show that high GWSEI values are consistent with high borehole yield values. R6sum6 Trois param~tres sont dEfinis "5. partir de ['interprdtation des photograpbies adriennes : la l ithologie, les lineaments, le drainage, caractErisds par des << indices ,,. Ces derniers servent de base ~.une 6valuation des nappes souterraines (indice GWSEI : ~,ground-water site evaluation index ,,). Les valeurs de cet indice permettent d'cstimer les potentialit0.s des sites de pompage. L'etude montre que les valeurs dlevdes du - GWSEI ,>correspondent effectivement b.des ddbits dlevds sur les sites. Introduction For the past few years the Federal and State Governments in Nigeria have been working towards the improvement of water supplies to the rural population for drinking, domes- tic and agricultural purposes. However, the search for un- derground water is being hampered by some problems. Some of these problems include lack of funds for detailed surveys, equipment purchase and maintenance of the exist- ing facilities. It is against this background that a simple, cheap and semi-quantitative method is being proposed as a prelimi- nary step in the exploration of groundwater. The proposed technique is intended to drastically reduce the cost of detailed survey as each site will be evaluated in order to minimise the number of geophysical sampling points. The paper therefore presents a groundwater site evaluation classification on the basis of air photo derived parameters. Study area The study area, Cross River State is located in the south eastern sector of Nigeria (Figure 1) and lies between Lati- tudes 4049 ' to 6056 ' North and Longitudes 7045 ' to 9028 ' East. It covers an estimated area of 23,000 km z. The area is characterised by wet and dry seasons of equa- torial climate with rainfall amount of more than 1 000 mm reaching about 3 000 mm around Calabar. The annual mean maximum and minimum temperatures are 30. I~ and 22.4~ respectively. Cross River State has an undulating type of topography with elevations reaching 2 000 m around the Obudu Pla- teau. Geologically the area is made up of the Precambrian Basement rocks comprising the Obudu plateau and the Oban massif. The major rock types include mostly migma- tite, gneiss, schist and phyllites. Cretaceous sediments con- sist mainly of sandstone, limestone, siltstone and shale while the southern part of the area is covered with alluvial deposits of Tertiary - Recent age. Intrusive rocks include pegmatites, quartz, syenites, dolerites, basalts, microgab- bro and microdiorites. The distribution of major units are shown in Fig. 1. Material and method Vertical black and white panchromatic aerial photographs at scales of I:10,000 and 1:5000 were used in this study. Interpretations were carried out stereoscopically using the WILD stereoscope (ST4.16472) and supported with topo- graphic and geological maps of the area and limited field surveys. * Department of Geology, University of Calabar, Calabar - Nigeria.