Borehole temperatures in the Transvaal and Orange Free State

Abstract During the past few years numerous deep boreholes have been sunk in the search for the gold-bearing reefs of the Witwatersrand system, beneath thick covers of younger formations, in the southern Transvaal and adjoining parts of the Orange Free State. The opportunity for temperature determinations in these boreholes is being utilized by the Geological Survey of South Africa, but a considerable time will elapse before the investigations are completed and the results become available. It therefore seems desirable that the publication of Dr E. C. Bullard’s conductivity determinations of representative rock samples from the two deepest boreholes in which the temperatures have been measured should be accompanied by an account of these investigations. The two boreholes, on the farms Gerhard- minnebron No. 4 near Potchefstroom and Jacoba No. 878 near Bothaville, were selected for the conductivity determinations on account of their depth and also because of the exceptionally small amount of change in the stratigraphic columns. For the sake of comparison three other boreholes are included in the description. In this paper the term “geothermic step” is used to indicate the increase in vertical depth corresponding to a rise in temperature of 1° C.