new wells can be drilled a suitable flooding fluid e g fresh water mixed with a surfactant is pumped by pumps 20a and 20b into the injection wells 12a and 12b and expands radially therefrom through the formation 10 indicated by the arrows in fig1 driving the oil in the formation indicated in zones 10a and 10b towards the producing well 14 in addition to the residual oil there would normally be some indigenous water in the formation and the movement of the flood fronts 22a and 22b of the injected fluids causes a buildup of the formation water in oil water zones 10c and 10d between the flood fronts 22a and 22b and the driven oil in zones 10a and 10b the progress of the flood fronts 22a and 22b is detected in observation boreholes 18a and 18b respectively by means of a well logging sonde or tool 24 movement of the tool 24 through the boreholes 18a and 18b which as noted are preferably cased is accomplished by means of cables 25 connected in the usual manner to motor driven winches not shown as is conventional in well logging the cables 25 also carry power to the downhole tool 24 and convey the data bearing logging signals to the surface for processing and recording in a data van 26 in practice the flood.