Reverse time migration (RTM) imaging of iron oxide deposits in the Ludvika mining area, Sweden

Abstract

Abstract. To discover or delineate mineral deposits and other geological features such as faults and lithological boundaries in their host rocks, seismic methods are preferred for imaging the targets at great depth. One major goal for seismic methods is to produce a reliable image of the reflectors underground given the typical discontinuous geology in crystalline environments with low signal-to-noise ratios. In this study, we investigate the usefulness of the reverse time migration (RTM) imaging algorithm in hardrock environments by applying it to a 2D dataset, which was acquired in the Ludvika mining area of central Sweden. We provide a how-to solution for applications of RTM in future and similar datasets. When using the RTM imaging technique properly, it is possible to obtain high-fidelity seismic images of the subsurface. Due to good amplitude preservation in the RTM image, the imaged reflectors provide indications to infer their geological origin. In order to obtain a reliable RTM image, we performed a detailed data pre-processing flow to deal with random noise, near-surface effects, and irregular receiver and source spacing, which can downgrade the final image if ignored. Exemplified with the Ludvika data, the resultant RTM image not only delineates the iron oxide deposits down to 1200 m depth as shown from previous studies, but also provides a better inferred ending of sheet-like mineralization. Additionally, the RTM image provides much-improved reflection of the dike and crosscutting features relative to the mineralized sheets when compared to the images produced by Kirchhoff migration in the previous studies. Two of the imaged crosscutting features are considered to be crucial when interpreting large-scale geological structures at the site and the likely disappearance of mineralization at depth. Using a field dataset acquired in hardrock environment, we demonstrate the usefulness of RTM imaging workflows for deep targeting mineral deposits.

Preface: State of the art in mineral exploration

“…In the last contribution to the imaging of the Ludvika Mines, Ding and Malehmir (2021) investigate the potential of reverse time migration to image the deep (1 km) subsurface in hard rock environments. To test this technique the authors apply reverse time migration to a 2.2 km long 2D section acquired in 2016 (Markovic et al, 2020), employing 451 seismic receivers and a 500 kg drop hammer as source.…”

3D high-resolution seismic imaging of the iron oxide deposits in Ludvika (Sweden) using full-waveform inversion and reverse time migration

“…In addition to the standard time-domain imaging, an advanced Kirchhoff-based PreSDM was also applied to the 2D dataset, which showed the extent of the mineralisation clearly down to 1000 m depth (Bräunig et al, 2020). Later, RTM was also applied along the same profile (Ding and Malehmir, 2021), which highlighted two sets of strong seismic reflectors dipping south-east which matched well with the known mineralisation. It also showcased two oppositely dipping reflectors intersecting the mineralisation and suggested the termination of extension of mineralisation further in depth.…”

Pre‐stack depth imaging techniques for the delineation of the Carosue Dam gold deposit, Western Australia

“…), imaging techniques such as reverse time migration (RTM) and inversions such as full-waveform inversion (FWI). This led to early investigations into the potential of FWI in hard rock environments (Adamczyk et al, 2014(Adamczyk et al, , 2015, while investigations into imaging techniques were documented by several authors (Brodic et al, 2021;Broüning et al, 2020;Ding & Malehmir, 2021;Hloušek et al, 2015;Singh et al, 2022).…”