SPECIAL TOPIC: LEGACY DATA Giving the legacy seismic data the attention they deserve Musa Manzi1*, Alireza Malehmir2 and Raymond Durrheim1 discuss the importance of retrieving, recovering, and reprocessing legacy seismic data for mineral exploration. Introduction retrieved, reprocessed, and interpreted using today’s standard Key minerals may soon be in short supply as shallow mineral techniques, they can be of significant value, particularly in the deposits are mined-out; therefore exploration for economically mining regions where no other data are available or the acqui- feasible deep-seated deposits to sustain a long-term global sition of new data is difficult and expensive. The development growth is a great challenge. New deposits are likely to be found of multitudes of processing algorithms and seismic attributes, in using reflection seismic surveys in combination with drilling, particular, make it worthwhile to reprocess and interpret legacy field geological mapping and other geophysical methods. data to enhance the detection of steeply dipping structures and Seismic methods have already have contributed significantly geological features below the conventional seismic resolution to the discovery of some of the world’s major mineral deposits limits (i.e., a quarter of the dominant wavelength), which was (Milkereit et al., 1996; Pretorius et al., 2000; Trickett et al., not possible with the tools that were available when the data 2004; Malehmir and Bellefleur, 2009; Malehmir et al., 2012). were originally acquired and processed. The new information However, use of the method is not widespread because it is obtained from the legacy data may benefit future mine planning deemed to be expensive. Although improvements in computing operations by discovering new ore deposits, providing a better capabilities have led to cost reductions, the costs are still estimation of the resources and information that will help to beyond exploration budgets of many companies. Thus, mining site and sink future shafts. Thus, any future mineral exploration companies have had little financial ability to acquire new project could also take the geological information obtained from reflection seismic data, and very little governmental support has the reprocessed and interpreted legacy seismic data into account been available to acquire research seismic surveys for mineral when planning new advanced seismic surveys (Manzi et al., exploration. 2018). The latest seismic algorithms are particularly interesting Over the last few years, there has been a proliferation of to South Africa’s deep mining industry because South Africa seismic solutions that employ various combinations of equip- has the world’s largest hard rock seismic database, which could ment, acquisition, and processing techniques, which can be benefit from new processing techniques and attributes analyses. applied in hard rock situations to improve the imaging resolution These techniques could be applied to legacy seismic data to (Denis et al., 2013). The best acquisition solutions to date have identify areas of interest, improve structural resolution and to come from the deployment of high-density receiver and source locate deeper ore deposits. Seismic attributes, in particular, could arrays which the extension of the seismic bandwidth to six be used to identify any subtle geological structures crosscutting octaves using broadband sources (Duval, 2012). Another area these deposits ahead of the mining face that could affect mine of seismic research has focused on surface seismic acquisition planning and safety. using three-component (3C) microelectro-mechanical (MEMS- based) seismic landstreamers (Brodic et al., 2015), coupled with Deep mining and legacy seismic surveys in South wireless seismic recorders, and surface-tunnel-seismic surveys Africa (Brodic et al., 2017). However, numerous difficulties have been Historically, South Africa has been a leading global supplier of encountered, even with these innovative acquisition seismic mineral production, and it retains an important role in mining approaches. Seismic surveys acquired in the mining regions and minerals. The Witwatersrand Basin of South Africa contains suffer from noise produced by the drilling, blasting and transport the world’s largest known accumulation of gold, and has yielded of rock and the crushing of ore. Furthermore, in some mining more than a third of the gold ever mined. The basin is known to regions the acquisition of new data is not permitted due to new host substantial resources that may become attractive to exploit environmental regulations. in the future, depending on the gold price and technological In such a fast evolving seismic technological era, legacy developments. Furthermore, the Bushveld Complex of South reflection seismic data are often regarded by mining companies Africa hosts more than 90% of the world’s platinum-group and geoscientists as inferior compared with the newly acquired metal resources, output has expanded tremendously in recent data. This paper demonstrates that if the legacy data are properly decades, and mines are already reaching mining depths beyond 1 University of the Witwatersrand, South Africa | 2 Uppsala University, Sweden * Corresponding author, E-mail: [email protected] FIRST BREAK I VOLUME 37 I AUGUST 2019 89 SPECIAL TOPIC: LEGACY DATA 2 km. As gold and platinum mining proceed, some of the ore architecture that controls gold and platinum mineralisation. In bodies are found to persist at greater depths (>2 km) beneath the addition, geological information from the reprocessed legacy data cover rocks, with significant challenges to continue exploration can be used to identify areas of interest and optimize acquisition and extraction. Geological problems encountered in mining can parameters for new advanced seismic surveys. include the loss of the deposit, unexpected dykes and faults, rock bursts, rock falls, and intersections of gas and water reservoirs. Reappraisal of legacy seismic data for gold Even with detailed advanced exploration, mining operations have exploration and mine safety been affected by many problems, such as gas outbursts, water Kloof-South Deep gold mine inundations, dangerous strata conditions, and severe operational The targets of Witwatersrand Basin exploration are the gold- and problems, that can result in injuries to personnel, as well as major uranium-bearing quartz pebble conglomerates or ‘reefs’, which losses of equipment and production. are mainly covered unconformably by the predominantly volcanic The challenge of an exploration method is to achieve ever Ventersdorp Supergroup; the dolostone, shale, lava, and quartzite greater resolution in increasingly complex environments and at formations of the Transvaal Supergroup; and the shale and sand- greater depths. A step in this direction was taken by field trials stone formations and coal seams of the Karoo Supergroup. The of high-resolution reflection seismic surveys by the Gold Divi- reefs are approximately 2 m-thick and their reflection coefficient sion of Anglo American Corporation (now AngloGold Ashanti relative to the enclosing quartzites is almost zero (with the excep- Ltd.), with the aim of reducing geological ‘surprises’ prior to tion of the Ventersdorp Contact Reef (VCR), which is interbedded mining and shaft sinking (Pretorius et al., 2003). Between the between quartzites and basic lavas). Fortunately, reflections arise 1980s and early 2000s, the Gold Division of Anglo American from marker horizons above and below the reefs, allowing the Corporation and various mining companies acquired a series of geological structure to be mapped (Pretorius et al., 2000; Manzi 2D seismic data (36,000 km in total length) and more than seven et al., 2018). 3D reflection seismic surveys in the Kaapvaal Craton for gold The 3D reflection seismic example from the Kloof and South and platinum exploration in South Africa, with special focus in Deep gold mines of the Witwatersrand Basin represents one of the the Witwatersrand Basin and the Bushveld Complex. This is one world’s first 3D seismic surveys acquired for mineral exploration. of the most extensive hard rock seismic exploration programmes The 1987 South Deep and 1994 Leeudoorn (now Kloof) seismic in the history of mineral exploration. Some of these legacy surveys were acquired, processed and merged to form a single data have been donated to Wits Seismic Research Centre of the seamless Kloof-South Deep 3D seismic cube by a contractor, School of Geosciences, University of the Witwatersrand, for which was presented to Gold Fields Ltd in a report. The 1987 academic and research purposes. The details of these exploration South Deep survey was the first 3D seismic survey to be acquired programmes, including the acquisition and processing of these for mineral exploration in South Africa (Campbell and Crotty, data, are reported by Pretorius et al. (2003). 1990). Both 3D seismic surveys were acquired over a known gold The quality of some of the legacy seismic data from South deposit (i.e., VCR – Ventersdorp Contact Reef, located between 2 Africa is poor when they only been preserved in published in and 3.5 km depth), covering a major fault system (i.e., WRF- West paper copies, and it is difficult to judge existing geological Rand Fault with 1.5 km maximum throw). Figure 1a shows a interpretations. Moreover, some of the original data are unre- 1994 seismic section, showing poor imaging of the fault zone and coverable due to tape deterioration. Therefore, recovering