First Break - Volume 20, Issue 7, 2002

This paper demonstrates the impact of a high-resolution 3D seismic survey on the development program of the Karee platinum mine in South Africa, and shows that reflection seismology can play an important role in positioning the structural position of thin, layered ore bodies. This case history shows that seismic methods are capable not only of directly detecting sub-metric layered platinum ore bodies at depths of 800 m, but they can also help the mining industry to optimize mine-planning in a cost effective manner. For such shallow targets, compared with the more traditional oilfields, the acquisition parameters were tuned to meet the survey’s objectives, including a bin size of 7.5  7.5 m2 and a fold of 28. This led to a very high density of shots per km2 of almost 900. The vibroseis source was well adapted to the surface conditions of the area. After full 3D processing, the seismic data provided a clear image of the structures of the Merensky and UG2 reef horizons and led to a significant increase in the level of confidence. In addition, effective application of new advanced seismic processing and visualization software used in the oil industry also made it possible to accurately highlight some geological features which disrupt the platinum reef horizons, such as slump structures (known as potholes), faults, pegmatoid bodies and dykes. These disturbances are of prime importance for implementing galleries, optimizing reef extraction, mining engineering and positioning the shafts. Their interpretation helped optimize mining in a significant, cost-effective way. This article is adapted from a paper presented at the SAGA conference in South Africa in October 2001 and was also presented in the ‘Mining and Best of Saga’ session at the 64th EAGE Conference in Florence.

Ground penetrating radar (GPR) is a suitable tool for the detection and location of fractures in resistive rocks, for example, limestone, salt and granite. It is commonly used in quarries and mine excavations (Dubois 1995; Grasmueck 1996; Dérobert & Abraham 2000; Halleux et al. 2000). The determination of the fracture (opening and filling material) cannot be directly deduced from the radar profiles. It requires a more detailed analysis of the radar signals. Recently, radar signal analysis has become an innovative research topic for the characterization of interfaces and layers (Fechner & Yaramanci 1996; Al-Qadi et al. 2000; Olhoeft 2000; Zeng et al. 1995). In order to characterize thin layers (thinner than the resolution), an inversion method based on the frequency content analysis of the radar reflection was developed (Grégoire, 2001a,b). This method is based on a comparison between the real reflection coefficient and a synthetic reflection coefficient. The reflection coefficients corresponding to field data are calculated using a reference signal. In this paper, some examples are given of the efficiency of GPR for the location of fractures of various scales (millimetric scale up to decimetric scale) in a limestone quarry and in an operating potash mine (K+S Group in Germany). In this potash mine, GPR investigations are regularly carried out to detect new fractures or the further extension of existing fractures by the exploitation itself (excavation of galleries and drifts, the use of explosives, etc.). Their location is of great importance for the safety of the miners and of the equipment. Safety measures, such as the installation of anchors, can be taken in time. The inversion method was tested by experiments with controlled fracture openings using a 1 GHz radar antenna. Then it was applied to radar data registered in the potash mine to estimate the openings of open fractures present in the roof of the gallery.

In VHR3D data, the positions of the source and receivers are required to decimetre accuracy in all three directions to ensure the correct processing of the data. However, current acquisition technology does not permit this level of accuracy in a cost-effective way. Any available instrumentation for the real-time measurement of source and receiver position is both very costly and designed for conventional seismics. During acquisition, movement of the boat and the streamers away from the nominal geometry can affect the recorded data and the quality of the processed results. In conventional seismic acquisition, these variations are small compared to the dimensions of the system and are generally not considered a major problem. In very high-resolution data however, where frequencies above 800 Hz and bin sizes of 1–2 m are common (Table 1), they can severely affect the results. Wave motion and tidal variations can produce degradation of the signal by destructive interference in the stack. Variations in x and y, if uncorrected, can cause traces to have an erroneous source-to-receiver offset distance or to be included in the wrong 3D bins. A schematic diagram of the possible movements that the acquisition system can be subjected to is shown in Fig. 1. The three main movements that are identified produce variations in the theoretical x, y and z positions and require different corrections in processing. Vertical variations due to wave motion and swell require static corrections (or time-shifts). Lateral variations, due to currents and changes in the ship’s heading, require dynamic corrections since they change the source-to-receiver offset. Tidal variations, on the other hand, require time shifts but at zero-offset, i.e. after application of NMO. The vertical variations are analogous to the near-surface problems in land data which require the application of residual static corrections. Algorithms exist in most processing packages to derive these residual static corrections using various correlation techniques. These are generally surface consistent with each shot and receiver having a consistent static. In the marine case, consistency is maintained for the shots but not for the receivers, since they move with the streamer. CDP consistent algorithms can be used to correct these problems and to improve the quality of the final stack section. However, they cannot take into account variations in the sea floor or provide reliable information for the relative positions of shot and receivers if pre-stack imaging or reflection tomography is envisaged.

In this rejoinder to our debate on creativity, initiated last year by Prof. Peter Hubral, Risto Jurvakainen enlarges on his earlier plea for all humanity, but geoscientists in particular, to seek out the spiritual in the creative process basing his argument upon the wisdom of the Ancients