Amakinskaya kimberlite pipe

Amakinskaya kimberlite pipe is located in northeastern Russia. It was surveyed in 2014 by multiple geophysical methods, including airborne TDEM (VTEM). There is notable AIP effect over the pipe, as well as light magnetic signature. VTEM data were inverted by Aarhus Geophysics using AarhusINV multiparametric Cole-Cole inversion for TDEM data and Mag3d for TMI data. The results were further compared with known petrophysical data showing good correlation. The data were interpolated over the same 3D volume and presented as depth slices and profile images. Introduction Amakinskaya kimberlite pipe is located in Yakutia (Figure 1). From the geological standpoint, the area surrounding It belongs to a sedimentary basin (Figure 2) with widespread outcrops of clays and alevrolites of Jurassic age (J1or), which unconformably overlay Cambrean limestone complex (Є3hl). Triassic basalts (T1kt) are also widespread in the area, especially to the north from the pipe. Amakinskaya Kimberlite pipe shows a great deal of anisotropy in the vertical direction, shifting from weathered, clayish upper facies, affected by permafrost to consolidated hard kimberlite below 30 m depth. Figure 1. Location of Amakinskaya kimberlite pipe (northeastern Russia); (a): VTEM flight lines shown over Landsat image. (b): IP effect measured over the kimberlite, shown in VTEM individual transients. This obviously has reflection in the physical properties of the kimberlite. Resistivity and chargeability changes with depth, showing lowering resistivity and increasing chargeability values in the upper facies of the kimberlite, while magnetic susceptibility increases with depth, as kimberlite consolidates (Bondarenko and Zinchuk, 2004). The airborne survey was flown in late 2014 using a VTEM system and the data were seriously affected by the IP effect (Figure 1b). It should be noted however, that IP effect over the kimberlite, although evident in the transients, does not result in negative voltage measurements and therefore may not be instantly obvious in the data space at a first glance. Figure 2. Geology of the area surrounding Amakinskaya kimberlite pipe. Inversion of TDEM data The TDEM data, collected over Amakinskaya pipe were inverted using the AarhusINV Spatially Constrained Inversion approach (SCI, Viezzoli et al., 2008). The code was modified as per Fiandaca et al. (2012) to accommodate Cole-Cole modelling to account for the IP effect (Cole and Cole, 1942). In addition to strong IP effect, which made conventional inversion challenging, there was a strong electrified industrial infrastructure situated just 200 meters east of the target, which seriously affected both EM and magnetic data. This cultural noise had to be removed from all data sets in order to proceed further with inverting the data. As a result of SCI inversion approach with IP modelling, all four ColeCole parameters were extracted. The target misfits of the inversions were achieved, producing distributions of Cole-Cole parameters, which were further interpolated to populate 3D volumes. The distributions of electrical resistivity and chargeability with depth are shown in Figure 3. Case studies: Amakinskaya kimberlite pipe Figure 3. 3D distribution of electrical resistivity and chargeability recovered by SCI inversion with IP modelling over Amakinskaya kimberlite pipe, shown as series of depth slices. Inversion of magnetic data The Total Magnetic Intensity (TMI) data were inverted using Mag3D code, developed at the University of British Columbia (Li and Oldenburg, 1996). Figure 4 shows comparison of observed TMI data vs data predicted by Mag3D inversion. Figure 4. Observed TMI data vs predicted TMI data. The results of the magnetic inversion are shown as series of depth slices in Figure 5. Despite presence of strong industrial artefacts and very subtle magnetic signature of the pipe, the 3D inversion of magnetic data successfully identified the location of Amakinskaya pipe. Furthermore, there are in fact two known shafts of the kimberlitic body, which is resembled in a 3D volume of magnetic susceptibility distribution (Figure 6). Figure 5. 3D distribution of magnetic susceptibility recovered by 3D magnetic inversion over Amakinskaya kimberlite pipe, shown as series of depth slices. Figure 6. 3D volume of magnetic susceptibility recovered over Amakinskaya kimberlite pipe suggesting two kimberlitic adjacent bodies.