An Investigation Of A Dry Water-Well Near Floriano In Northeast Brazil Using Combined Vestem-Emap Techniques

Combined dc resistivity (VES), transient electromagnetic (TEM) and high resolution<br>continuous electromagnetic array profiling (EMAP) techniques have been applied to<br>the determination of the subsurface structure across part of the Mato Gross0 Farm (a<br>small settlement located about 25km east of Floriano city in Piaui State of Brazil) in<br>order to ascertain any structural influence on the cessation in water supply when an<br>original 13m deep, water-bearing, hand-dug well was excavated to a depth of 23m in<br>1988. Droughts are frequent in this semi-arid region but nearby wells in the same<br>geological formation have good yields. TEM and VES measurements were made on<br>four sides of the dry well enabling a clear assessment of the aquifer geometry at the<br>survey site. Two orthogonal lines of continuous electromagnetic array profiling<br>(EMAP), with five contiguous electric field stations (each having a dipole length of<br>50m) for good lateral resolution, enabled the local structure to be tied into a preexisting<br>regional model and the rejection of cavernous structures as the possible cause<br>of water loss at the farm site. The results suggest that another uncompleted, hand-dug<br>lm-diameter shallow well, 50m east of the dry well, will not be productive and should<br>be aborted. It is shown that deep-lying regional aquifers underlie the survey site and<br>we suggest that these can be developed at the dry well location or further west by a<br>machine-drilled borehole.