Joachim Place

Delineating structures controlling sandstone‐hosted base‐metal deposits using high‐resolution multicomponent seismic and radio‐magnetotelluric methods: a case study from Northern Sweden

Over the past few decades seismic methods have increasingly been used for the exploration of mineral, geothermal, and groundwater resources. Nevertheless, there have only been a few cases demonstrating the advantages of multicomponent seismic data for these purposes. To illustrate some of the benefits of three-component data, a test seismic survey, using 60 digital three-component sensors spaced between 2 m and 4 m and assembled in a 160 m-long prototype landstreamer, was carried out over shallow basement structures underlying mineralized horizons and over a magnetic lineament of unknown origin. Two different types of seismic sources, i.e., explosives and a sledgehammer, were used to survey an approximately 4 km-long seismic profile. Radio-magnetotelluric measurements were also carried out to provide constraints on the interpretation of the seismic data over a portion of the profile where explosive sources were used. Good quality seismic data were recorded on all three components, particularly when explosives were used as the seismic source. The vertical component data from the explosive sources image the top of the crystalline basement and its undulated/faulted surface at a depth of about 50 m–60 m. Supported by the radiomagnetotelluric results, however, shallower reflections are observed in the horizontal component data, one of them steeply dipping and associated with the magnetic lineament. The vertical component sledgehammer data also clearly image the crystalline basement and its undulations, but significant shear-wave signals are not present on the horizontal components. This study demonstrates that multicomponent seismic data can particularly be useful for providing information on shallow structures and in aiding mineral exploration where structural control on the mineralization is expected.

Using Supervirtual First Arrivals for Improving the Seismic Imaging of Deep Deposits - Well Worth the Effort

Different applications of seismic interferometry have arisen in the last decade, however the potential of this technique to improve reflection seismic processing in hardrock environments has not been regarded explicitly. Therefore, in this paper we investigate the potential of retrieving the first arrivals originally hindered by high noise level in the exploitation of controlled-source data acquired over the apatite-iron deposit at Grangesberg (Sweden) and its mining-induced structures. The supervirtual first arrivals generated using interferometry methodologies allowed first-breaks to be picked more extensively than in the original data. Revised static corrections significantly improved the linearity of the first arrivals and continuity of reflections in the source gathers. Especially, reflections considerably enhanced in the source gathers stacked constructively in the final seismic section. Comparison with geologic data, supported by reflection-traveltime forward modelling, indicates that these reflections represent the deep (> 700 m) and unmined part of the deposit. Other reflections at shallower depth are interpreted as anthropogenic faults possibly located at lithological contacts (pegmatites). Even though the potential of first-arrivals retrieval is likely case-dependent, this study illustrates that interferometry may substantially improve the accuracy of field static corrections and subsequent stack for hardrock imaging and deep mineral exploration.

Reflection Seismic Characterization of the Grängesberg Iron Deposit and Its Mining-induced Structures, Central Sweden

Reflection seismic investigation has been conducted on the Grangesberg apatite iron deposit. At the time of closure in 1989, the mine was operated at about 650 m below the surface. Mining activities might be resumed in the next years, which require better understanding of (1) the ore geometry and (2) the fault network which has developed up to the surface from excavated zones at depth. Two E-W oriented reflection lines with a total length of 3.5 km were acquired. The seismic lines intersect the Grangesberg ore body and open pit, as well as several of the mining-induced faults. A weight drop mounted on an hydraulic bobcat truck was used as a seismic source; both cabled and wireless receivers were used for the data recording. Preprocessing of the data first required the cable- and wireless- recorded datasets to be merged before stacking all data available at each shot point. The dataset exhibits several shallow reflections which are likely to occur on steep lithologic or tectonic structures. Other deeper reflections are recorded; careful processing will be carried out in order to preserve such events in final stacked sections and help with refining the geological model of the area.