Kevitsa Ni-Cu-PGE deposit, North Finland ? A seismic case study

Abstract

A 3D seismic survey was designed, acquired and processed by HiSeis Pty. Ltd. in 2010 at the Kevitsa Ni-Cu-PGE deposit. The objectives of the survey were the definition of sub-vertical structures (knowledge of which could assist in the design and characterization of the slopes of the proposed open pit), and mapping out the general structural setting of the mafic intrusive. The 2010 processing of the Kevitsa 3D seismic data was accelerated to meet engineering design deadlines. Although this phase of work was restricted to processing sequences that were not amplitude consistent and to the post stack migration algorithm, never-the-less the resultant product achieved good resolution of the complex structural setting. The dataset was re-processed in 2014 with the goal of preserving relative signal amplitudes, in order that the volume could be inverted into an acoustic impedance cube. Another reason for re-processing was to improve imaging in shallow depth, by improving the static solution and velocity model used for imaging. Both of these processes are considered to be crucial steps in hard rock seismic data processing. Considerable improvement was achieved through the application of a pre-stack time migration (PSTM) algorithm. Conventional 3D deep-move out corrections (DMO), followed by a post-stack migration algorithm proved to be insufficient to handle the lateral changes of velocities. Consequently, pre-stack time imaging was attempted to aid in handling the highly complex velocity field. The goal was to derive a velocity model appropriate to the geologic environment in order to place events in their correct positions, to properly focus the energy, to avoid introduction of false structures and to flatten the image gathers. The Kevitsa 3D seismic dataset is considered as being of high quality and as the data volume contains a statistically significant number of log measurements, it is deemed suitable for the seismic inversion.