ALBERT

Description: A 3-D reflection seismic trial survey was conducted at Collingwood Park, on the outskirts of Brisbane, Australia, where coal mining activity ceased in the 1980’s and two subsidence events occurred in 1988 and 2008. The objective of the survey was to demonstrate the feasibility of using seismic methods to locate subsurface structures and features such as faults and old mining workings. In spite of the strong contamination of refractions and surface waves, the survey was able to confirm the location of the previously known Waterline fault, as well as identify another fault. It accurately delineated the subsidence zone based on differences in the amplitude of seismic reflections from the surveyed area, and mapped the surface subsidence boundary, which correlated well to existing data from observations in the field. In addition, the seismic data were used to map the failure boundary at the seam level in relation to the subsidence zone. It was found that the mapped area at the seam level was larger than the surface subsidence boundary, with an estimated angle of ~21° between the subsidence failure surface and the vertical depth axis, depending on location. Post 3-D seismic drilling also confirmed that pillar failure extended beyond the Waterline fault. However, the survey failed to image the mine workings because of insufficient seismic resolution. To our knowledge, this is the first report on the use of 3-D seismic surveying to map deeply buried mining-related geotechnical failure boundaries.

Description: Carbon capture and storage (CCS) is vital to reduce CO2 emissions to the atmosphere, potentially providing 20% of the needed reductions in global emissions. Research and demonstration projects are important to increase scientific understanding of CCS, and making processes and results widely available helps to reduce public concerns, which may otherwise block this technology. The Otway Project has provided verification of the underlying science of CO2 storage in a depleted gas field, and shows that the support of all stakeholders can be earned and retained. Quantitative verification of long-term storage has been demonstrated. A direct measurement of storage efficiency has been made, confirming that CO2 storage in depleted gas fields can be safe and effective, and that these structures could store globally significant amounts of CO2.

Description: 〈span〉Oxygen and iron isotope variations have been investigated in three compositionally distinct garnet samples to assess natural variations and search for suitable reference material. We report 〈span〉in situ〈/span〉 major, trace element and O isotope analyses for mantle-derived garnet xenocrysts from Kakanui, New Zealand, as well as magmatic and hydrothermal garnets (skarn) from two different localities in Erongo, Namibia. The 〈span〉in situ〈/span〉 analyses are complemented by bulk mineral separate Fe isotope analyses for all samples and CO〈sub〉2〈/sub〉 laser fluorination oxygen isotope analysis for Kakanui garnet. Mantle-derived pyrope-rich garnet megacrysts from Kakanui are chemically homogeneous in major and trace elements, and in O isotopes, (δ〈sup〉18〈/sup〉O〈sub〉VSMOW〈/sub〉 = 5.67 ± 0.02‰). Magmatic garnet from Erongo, Namibia, is rich in Mn and Fe〈sup〉2+〈/sup〉 and very poor in Ca showing minor variations along the almandine–spessartine join [(Fe,Mn)〈sub〉3〈/sub〉Al〈sub〉2〈/sub〉Si〈sub〉3〈/sub〉O〈sub〉12〈/sub〉]. Although rare earth elements vary over one order of magnitude, no resolvable O isotope zoning is observed (δ〈sup〉18〈/sup〉O = 9.3 ± 0.3‰, 1σ). Hydrothermal garnet from Namibia is rich in Ca and Fe〈sup〉3+〈/sup〉 and shows strong zonation along the andradite–grossular join [Ca〈sub〉3〈/sub〉(Fe〈sup〉3+〈/sup〉,Al)〈sub〉2〈/sub〉Si〈sub〉3〈/sub〉O〈sub〉12〈/sub〉] with a considerable spread in trace-element contents, accompanied by a limited, but resolvable, spread in O isotopes values between cores (8.3 ± 0.3‰, 1σ) and rims (7.4 ± 0.3‰, 1σ). Iron isotopes (expressed as δ〈sup〉57〈/sup〉Fe〈sub〉IRMM-014〈/sub〉) within bulk garnet separates are heterogeneous in both crustal garnet from Erongo with a large spread ranging from −0.15 to +0.30‰ in igneous garnet and from +0.4 to +1.1‰ in hydrothermal garnet. Igneous garnet from Kakanui are homogeneous with an average δ〈sup〉57〈/sup〉Fe〈sub〉IRMM-014〈/sub〉 of +0.09 ± 0.01, 1σ. The Fe〈sup〉3+〈/sup〉-dominated andradite shows very heavy Fe isotopes, suggesting a link between preferential ferric iron incorporation into garnet and Fe isotope signatures. Combined O and Fe isotope analyses in garnet can provide potentially important insights into the nature of parental medium from which the garnet forms (based on O isotopes) and associated petrogenetic processes (〈span〉e.g.〈/span〉, redox conditions based on Fe isotopes), though more systematic studies are required to further assess these proxies in natural systems. Finally, we propose that Kakanui garnet might represent a suitable reference material for both, O and Fe isotope analyses.〈/span〉

Description: A common acquisition scenario in microseismic monitoring is the deployment of large areal receiver arrays at or near the surface. This recording geometry has the advantage of providing coverage of the source’s focal hemisphere as well as characterization of the arrival time moveout curve; however, the accuracy of many location techniques applied to these data sets depends on the accuracy of the depth velocity model provided prior to location. We have developed a simple oriented time-domain location technique so that full knowledge of the velocity model is not required a priori. The applicability of the technique is limited to horizontally layered models and also to models with dipping interfaces of small angles; however, this restriction is acceptable in many unconventional reservoirs. Implementation of the technique includes three steps: (1) smoothing of the observed time arrivals by fitting a hyperbolic moveout curve with a broad set of constraints, (2) updating and restricting the constraints using a local-slopes-based location workflow, and (3) estimation of the focal coordinates of passive sources using the updated constraints for the final least-squares fitting of the moveout curves. We have tested the performance of the proposed technique on several 2D examples and a 3D field data set. The results from synthetic examples suggest that, despite the assumption of the method that the arrival moveout can be modeled using a constant effective velocity, a reliable event location is achieved for layered models without considerable lateral heterogeneities. Our tests on the field data set find that the focal point coincides with a previously derived estimate of the source location. To assess the uncertainty of the proposed technique, bootstrap statistics was also used and applied to the field data set.

Description: A 3D reflection seismic survey was conducted over an area of about $$9\hbox{ \hspace{0.17em} }\hbox{ \hspace{0.17em} }{\mathrm{km}}^{2}$$ at the Kevitsa Ni-Cu-PGE (platinum group elements) orebody, northern Finland, where open-pit mining started in mid-2012. The principal objective of the survey was to image major fault and fracture zones at depth that may have an impact on the mine stability and safety. Mine planning would then take into account the geometry of these zones at Kevitsa. Processing results, using conventional prestack DMO and poststack migration methods, show gently dipping and steeply dipping reflections from depths of approximately 2 km to as shallow as 150–200 m. Many of the reflections are interpreted to originate from either fault systems or internal magmatic layering within the Kevitsa main intrusion. Further correlation between the surface seismic data and VSP data suggests that numerous faults are present in the imaged volume based upon time shifts or phase changes along horizontal to gently dipping reflections. Some of these faults cross the planned open-pit mine at depths of about 300–500 m, and are therefore critical for geotechnical planning. In terms of in-pit and near-mine exploration, the magmatic layering internal to the intrusion controls the distribution of the bulk of economic mineralization. The ability to image this magmatic layering could therefore guide future drilling, particularly by constraining the presumed lateral extents of the resource area. Exploration also will target discrete reflectors that potentially represent higher-grade sulfide mineralization.

Description: Due to high metal prices and increased difficulties in finding shallower deposits, the exploration for and exploitation of mineral resources is expected to move to greater depths. Consequently, seismic methods will become a more important tool to help unravel structures hosting mineral deposits at great depth for mine planning and exploration. These methods also can be used with varying degrees of success to directly target mineral deposits at depth. We review important contributions that have been made in developing these techniques for the mining industry with focus on four main regions: Australia, Europe, Canada, and South Africa. A wide range of case studies are covered, including some that are published in the special issue accompanying this article, from surface to borehole seismic methods, as well as petrophysical data and seismic modeling of mineral deposits. At present, high-resolution 2D surveys mostly are performed in mining areas, but there is a general increasing trend in the use of 3D seismic methods, especially in mature mining camps.

Description: We investigate the potential of using high-resolution seismic methods for rock characterization and for targeting of gold deposits at the St. Ives gold camp. The application of seismic methods in hard-rock environments is challenged by complex structures, intrinsically low signal-to-noise ratio, regolith distortions, and access restrictions. If these issues can be addressed, then the unparalleled resolving power of reflection seismic can be used for mineral exploration. Appropriate spatial sampling of the wavefield combined with a survey geometry design and rigorous data processing to incorporate high fold and long offsets are necessary for creation of high-quality seismic images. In the hard-rock environment of Western Australia, accurate static corrections and multiphase velocity analysis are essential processing steps. This is followed by a rigorous quality control following each processing step. In such a case, we show that the role of reflection seismic could be lifted from mere identification of first-order structures to refined lithological analyses. Five deep boreholes with sonic logs and core sample test data were used to calibrate 2D seismic images. Despite seismic images were produced with relatively robust scaling it was possible to achieve reasonably high seismic-log correlation across three of the tightly spaced boreholes using a single composite wavelet. Amplitude-versus-offset (AVO) analysis indicated that gold-bearing structures may be related to elevated AVO effect and increased reflectivity. Consequently, partial stack analysis and acoustic and elastic inversions were conducted. These results and impedance crossplots were then evaluated against known gold occurrences. While still in the preliminary stages, hard-rock seismic imaging, inversion, and the application of AVO techniques indicated significant potential for targeting mineral reserves.