Screening kimberlite magnetic anomalies in magnetically active areas

Abstract

Airborne and ground magnetic surveys are a primary exploration tool in the search for kimberlites. Interpretation of magnetic data provides direct information on possible pipe magnetic signatures and indirect information about the structural setting of the area. Magnetic anomaly screening in an area of highly active magnetic relief and magnetic texture such as an Archean shield area or volcanic terrain is very difficult. Conventional techniques such as stacked profile interpretation and modelling/inversion are almost impossible, as there is so much interference and overlap between adjacent anomalies. There is also the additional problem of distinguishing possible kimberlite sources from other sources with very similar magnetic signatures. However, the magnetic signature of kimberlite pipes is band limited so enhancing wavelengths of interest plays an important part in interpretation. Semi-automatic interpretation, using a combination of specialised filters and analytical techniques can provide objective information on anomaly attributes. These can be used in conjunction with the geological setting of the area to prioritise anomalies for follow-up. The application of separation or layer filters allows deconvolution of the causative sources around a particular mean depth. Plotting of gradient maxima/strike symbols maps circular or elliptical anomalies as closed clusters. 3D Euler deconvolution provides first-pass depth estimates and an indication of the nature of the source. Finally, matched filtering using a cylinder model can identify roughly circular anomalies over a broad range of amplitudes and wavelengths. Anomalies selected in the first phase are refined by automatic profile analysis and the total anomaly attributes compiled into a database. Finally targets are assessed in their geological context, using available geology and the magneto-tectonic interpretation of the area.