oa Comparison of ground TEM and VTEM responses over kimberlites in the Kalahari of Botswana

One hundred meter square in-loop ground TEM soundings were an effective way to screen aeromagnetic, ground gravity, and Falcon® gravity gradiometer signatures caused by kimberlite intrusions overlain by 40-120 meters of transported Kalahari sedimentary cover in the Kokong kimberlite field of Botswana (Figure 1). Ground TEM’s effectiveness in identifying kimberlite pipes led to the flying of the VTEM aeroTEM system over selected areas at Kokong. Ten kimberlites previously covered by ground TEM surveys were over flown by the VTEM survey (Figure 2). A comparison of the ground TEM and VTEM responses show that the VTEM effectively drill-screened nine out of the ten kimberlite magnetic or gravity signatures, whereas the ground TEM systems effectively screened all 10 kimberlite signatures (Table 1). KS40, the largest kimberlite at Kokong and the one kimberlite less well mapped by the VTEM was, at 10-30 milliseconds (msec), a later-time conductor that best responded after the VTEM’s last channel 27 at 7.5 msec. KS40’s kimberlite crater was mapped as a good late-time conductor (later than 7.5 msec) by both ground TEM systems. The ground TEM systems employed were a Geonics TEM57 transmitter and PROTEM receiver, and two Zonge ZT-30 transmitters and GDP32 receivers; their specifications are compared with the VTEM in Table 2 below.