Development of high-resolution seismic methods to locate deep-seated base-metal orebodies

A research project was undertaken with the aim of developing a three-dimens ional high-resolution seismic system suitable for mineral exploration and d evelopment. The final objective was a prototype system of hardware and soft ware able to define structures and possibly detect sulphide mineralization at depths up to 1000 m below surface. The starting point for the prototype design was the SEAMEX (Seismic Apparat us for Mining and Exploration) system used extensively in the coal industry . The SEAMEX system employs up to 120 remote units, each with two geophones , and feeds data from 140 channels to a central control unit. Prototype dev elopment involved the design and construction of high-resolution equipment and development of new signal acquisition and control software. The final v ersion of the prototype, named SUMMIT, has a data transmission rate ten tim es faster than SEAMEX, a large dynamic range with up to 1920 channels and s ignificantly improved data resolution. Four field surveys were carried out over part of the Navan zinc-lead orebod y in Ireland at a location where ore lenses occur at around 650 m below sur face in gently inclined limestone strata cut by major faults. The first sur vey used the SEAMEX system, whereas the following three surveys used progre ssively improved versions of the prototype SUMMIT system. To support the se ismic interpretation a total of 14 352.9 m of diamond drilling was complete d for the project, comprising 20 drill-holes to a maximum depth of 847.2 m, and downhole surveys were carried out to log rock density, resistivity, na tural gamma responses, sonic responses and conductivity in five deep holes to depths up to 800 m. The data from the SEAMEX survey were of low resolution, but there was a mar ked improvement when the prototype SUMMIT system was used and data interpre tation improved correspondingly. Deep drilling was used as a control throug hout the project and the final interpretations were based on a combination of seismic and drilling data. Significant geological structures, including faults and unconformities, were identified and in some cases plotted in thr ee dimensions. The information on faulting proved particularly useful for e xploration and development activities at Navan. The seismic data showed cle arly that both the major and minor faults that control ore disposition beha ve unexpectedly at depth, in some Eases splitting, curving and joining toge ther. This type of variable fault geometry cannot be detected by drilling a lone and is of considerable importance for the selection of drilling target s at depth. The zinc-lead overbodies in the areas surveyed proved too small and lateral ly impersistent for confident detection. There is, however, a strong possib ility that the Navan sulphide orebodies would have been identified if the s urveys had been carried out over the main part of the orebody. The results indicate that the method has potential for exploration in such areas as par ts of the Pyrite Belt in Spain and Portugal, where the mineralization is in massive bodies or sheets and there is no significant glacial overburden. Future research should concentrate on the improvement of seismic data proce ssing techniques to produce detailed interpretation at specific depths, on combining surface seismic surveys with refraction seismic techniques and do wnhole seismic surveys so as to maximize useful data generated by a single seismic field survey and on the creation of broad images of the geology at depth for regional mineral exploration in the way that the method is applie d in the hydrocarbons industry. The project was coordinated by Tara Prospecting, Ltd. (Ireland), and the pa rtners were Deutsche Montan Technologic GmbH (Germany) and TNO Institute fo r Applied Geophysics (Netherlands).