Kimberlite pipes contouring at a site of ALROSA
July 17, 2008
Members of the expedition
GPR survey of "Belaya" kimberlite pipe at "Belaya 2" site was held by Timer CJSC with the usage of GROT 12 GPR under an agreement with "ALROSA" Joint Stock Company.
The works took place in Arkhangelsk diamondiferous province, which is the second territory of the Russian Federation (the first one is Yakutia) in terms of proven reserves of primary diamond deposits. The expedition means of transportation were caterpillar rovers.
Arkhangelsk region has a distinctive geological structure of sedimentary rocks and volume of Quaternary sediments, which makes practically impossible the use of traditional methods of diamond prospecting (geochemical search and placer and small-volume testing).
The aim of the works was to conduct a GPR survey of an explosion pipe, detected with the use of the magnetic survey and drilling data, to discover specific search indicators, used for pipes GPR detection.
After drilling it was known that the pipe was a low contrast object in comparison with its host rock at the depth of 38-40 meters beneath Quaternary rocks:
Fig. Geoelectric model of the explosion pipe: a) the explosion pipe vertical cross-section; b) the explosion pipe line AA horizontal cross-section. Legend: 1 - Quaternary deposits; 2 - Devonian and Carboniferous deposits, 3 - kimberlite, 4 - changed zone with electrical resistivity anisotropy near the pipe.
Having taken into consideration the general watering of the area, we chose to use a GROT 12 GPR set, consisting of a low-frequency transmitter (with the voltage of 5 kV and a pulse length of 20 ns), the basic configuration of the receiver and 10 meters long antennas (with the centre frequency of 15 MHz). Additionally, a transmitter with the voltage of 15 kV was prepared, which would have provided the increased energy potential, if the basic base low-frequency transmitter pulse would not have passed through overburden rocks.
The area under study with GPR sensing tracks highlighted - a map of the area, based on the magnetic survey data:
The average scanning rate was 87 points per hour. It allowed us to conduct the GPR study of the territory with a high density of monitoring tracks.
Borehole B2 and borehole B1 were crossed during the scanning on the track 31 at distances of 98 and 132 meters from the beginning of the track accordingly. This information was used to easier compare the GPR scanning results to the drilling data.
The vertically located object anomaly was spotted and reflected as a change in the palette on all tracks.
It was obvious that in the overlying deposits over the fracture the conductivity correlation of linear layers is disrupted. From 90 m to 160 m marks of the track there was an increase of soil conductivity at the depths of above 30-35 m, which corresponds to the volume of the overlying deposits. This zone coincided exactly with the space area over the pipe.
The radargram "maksmin" version, used to highlight the boundaries of explosion pipe sub-vertical boundaries, allows locating the zones of the pulse reflections from both the horizontal and sub-vertical boundaries between the media:
In addition to the pipe, an anomaly, which was a fracture, characterized by the strong pulse absorption, was located. It took place with smaller time delays and before the pipe zone at the track.
The clearly seen on the radargram fracture anomaly can be considered as one of the possible pipe search indicators. The anomaly of the contact zone, as the main search indicator, allows locating such pipes with sufficient precision.
On the background of the fracture anomaly the host rock and the pipe contact anomaly stands out much less because of the low contrast between the pipe zone and the host rock. Nevertheless, due to the sufficient dynamic range of GROT 12 GPR, this anomaly was visible on the initial results of the GPR survey and was later confirmed after the areal survey analysis was complete.
Anomalous zones locations, obtained during the GPR scanning, have been put down on the map and compared with the data, already existing after the magnetic research. It allowed us to specify the boundaries of the kimberlite pipe and locate its centre.
The results of GPR scanning test usage at the pipe "Belaya" has proven that the GPR survey is a highly productive method of the research of the upper part of the geological section and that it can be successfully used in prospecting for diamond deposits.