The tenfold increase in borehole siting in Northern Nigeria during the last few years has resulted in an increase in the use of electromagnetic (EM) methods and a corresponding decrease in the more traditional but less cost effective resistivity methods. The role of geophysics in exploration is optimally in combination with direct geological observation. Two types of aquifer which are amenable to geophysical investigation in the Nigerian environment are weathered, jointed crystalline rocks and alluvium. Location of joints and associated narrow zones of deep weathering in crystalline terrain is best achieved by EM techniques with resistivity backup where needed. Resistivity alone does not always reveal such features. Location of shallow aquifers in alluvium is amenable to a combination of EM and resistivity techniques. However, the complexities of alluvial and underlying Quaternary sedimentation necessitate extensive computer modelling of both sets of data in order to optimize the field techniques and interpretation methods. In particular the use of selected combinations of intercoil spacings to estimate maximum thicknesses of aquifers shows some promise.

Seneca and Cayuga Lakes have chloride levels 2-l 0 times higher than the other Finger Lakes. Approximately 170 x 1 O6 kg of salt appear to be added to Seneca Lake from within the basin each year. The region is underlain by Silurian beds of commercial-grade rock salt -450-600 m below the surface. It has been proposed that Seneca and Cayuga Lakes are saltier than the others because their basins intersect some of these beds.Fieldwork supports this hypothesis. A deep water mass up to 10% saltier than the rest of Seneca Lake was observed to expand and partially fill the hypolimnion from the bottom up during summer 199 1 and 1992. The saltier water was mixed into the rest of the lake in early winter, only to reappear after the thermocline was established. Sediment interstitial water profiles in both lakes reveal large regions of saline groundwater several meters below the sediment surface, and NaCl concentrations as high as 30?&0 have been found. New York's Finger Lakes occupy 11 north-south trending basins thought to be preglacial river valleys gouged by Pleistocene glaciation and dammed by moraines (von Engeln 196 1). The basins are extremely deep for their surface area (max depths: Seneca, 188 m; Cayuga, 132 m) and intersect a thick sequence of Paleozoic marine sedimentary rocks that dips gently to the south-southwest with little structural deformation.Seneca and Cayuga Lakes exhibit much higher concentrations of dissolved sodium and chloride than the smaller Finger Lakes do, in spite of similar patterns of land use throughout the region.Berg (1963) has noted that salt strata underlie the entire area and that beds of commercial-grade rock salt 450-600 m below ground level are mined at the southern end of both lakes. Because the mines are dry and lie well below the lake floors, he did not consider it probable that groundwater from these depths leaches upward into the lakes. He also considered and rejected the washings from the commercial salt processing plants themselves as the cause, because no point-sources for chlorides were indicated by the salinity distributions in the lakes, and mass balance considerations seemed to preclude it; a source for chlorides much greater than the mines could provide