ALBERT

Description: In this experiment, we measured acoustic reflected wavefield from water and z-cut alpha quartz interface in wide azimuthal and incidental directions. This unique dataset was collected at Experimental Geophysics Group at the University of Alberta, with the large transmitter and fine receiver with the resonant frequency of 0.7 MHz and the temporal sampling of 10ns, the incident angular sampling of 0.25 degrees and azimuthal sampling of 10 degrees. Measured acoustic reflectivity indicates double Schoch shift and null reflection at the Rayleigh angle of water-alpha quartz boundary. The double Schoch shift collapses into one where the azimuthal directions are parallel to the plane of anisotropic symmetry.

Description: In this contribution, we experimentally test the effects of azimuth and tilt angle on the acoustic reflectivity of a liquid- anisotropic solid interface. For this study, we are using a large source transducer, and acquired data for samples with different tilt angles. We use Phenolic CE material, which is known to have orthorhombic symmetry. Our results show that changes of the tilt angle produce important variations on the reflectivity that are larger as the tilt increases. The most remarkable feature is the change of the critical angle with the azimuth, which shows a larger spread for larger tilts. The spectral components of the acquired waveforms also show characteristic features linked to the location of the critical angle, we particularly observed a drop in the peak frequency. These observations suggest that care must be taken about the interpretation and inversion of observed incidence and azimuth dependent seismic reflectivities and critical angles in obtaining information on a formation's anisotropy. Zip archive contains four segy files: - LAB_TI00, is not tilted sample in contact with water, - LAB_TI30, is 30degrees tilted sample in contact with water, - LAB_TI45, is 45 degrees tilted sample in contact with water, - LAB_TI90, is 90 degrees tilted sample in contact with water.

Description: ABSTRACT The Kevitsa mafic–ultramafic intrusion in Northern Finland hosts a large, disseminated nickel–copper sulphide ore body. The Kevitsa intrusion is an active mining and exploration site, for which we have built a 3D model of the main lithological contacts and near-mine structures in the area. To build the 3D model, 2D and 3D reflection seismic data have been used together with borehole data and geological map of the area. The Kevitsa reflection seismic data reveal the internal architecture of the Kevitsa intrusion and the surrounding units. For example, the seismic data have uncovered a previously unknown, deeper continuation of the Kevitsa intrusion. Improved 3D knowledge of the basal contact of the intrusion provides an exploration target for contact-type mineralization. Within the intrusion, a limited area of strong reflections is observed in the data. This has been associated with discontinuous, smaller-scale magmatic layering that is thought to control the extent of the Kevitsa main mineralization. Thus, our 3D model of the extents of the internal reflectors can provide a framework for near-mine and deep exploration of the main type of mineralization in the area. In addition to exploration, the original purpose of the 3D seismic survey was geotechnical planning of the Kevitsa open-pit mine. Accordingly, the 3D seismic data were used to create a 3D model of the subsurface structures, with a focus on the vicinity of the mine. The interpreted structures reveal a complex pattern of fault and fracture zones, some of which will be important for slope stability and operational planning of the final stages of the mine.

Description: ABSTRACT Seismic methods are becoming an established choice for deep mineral exploration after being extensively tested and employed for the past two decades. To investigate whether the early European mineral exploration datasets had potential for seismic imaging that was overlooked, we recover a low‐fold legacy seismic dataset from the Neves‐Corvo mine site in the Iberian Pyrite Belt in southern Portugal. This dataset is comprised of six, 4–6 km long, profiles acquired in 1996 for deep targeting. Using today's processing algorithms, the world‐class, ca. 150 Mt, Lombador massive sulphide and other smaller deposits were better imaged. Additionally, we also reveal a number of shallow but steeply dipping reflections that were absent in the original processing results. This study highlights that legacy seismic data are valuable and should be revisited regularly to take advantage of new processing algorithms and the experiences gained from processing such data in hard‐rock environments elsewhere. Remembering that an initial processing job in hard‐rock should always aim to first obtain an overall image of the subsurface and make reflections visible, and then subsequent goals of the workflow could be set to, for example, understanding relative amplitude ratios. The imaging of the known mineralization implies that this survey could likely have been among one of the pioneer studies in the world that demonstrated the capacity of directly imaging massive sulphide deposits using the seismic method. This article is protected by copyright. All rights reserved

Description: ABSTRACT Reflection seismic data were acquired within two field campaigns in the Blötberget, Ludvika mining area of central Sweden, for deep imaging of iron‐oxide mineralization that were known to extend down to 800–850 m depth. The two surveys conducted in years 2015 and 2016, one employing a seismic landstreamer and geophones connected to wireless recorders, and another one using cabled geophones and wireless recorders, aimed to delineate the geometry and depth extent of the iron‐oxide mineralization for when mining commences in the area. Even with minimal and conventional processing approaches, the merged datasets provide encouraging information about the depth continuation of the mineralized horizons and the geological setting of the study area. Multiple sets of strong reflections represent a possible continuation of the known deposits that extend approximately 300 m further down‐dip than the known 850 m depth obtained from historical drilling. They show excellent correlation in shape and strength with those of the Blötberget deposits. Furthermore, several reflections in the footwall of the known mineralization can potentially be additional resources underlying the known ones. The results from these seismic surveys are encouraging for mineral exploration purposes given the good quality of the final section and fast seismic surveys employing a simple cost‐effective and easily available impact‐type seismic source.

Description: 〈span〉〈div〉ABSTRACT〈/div〉We deployed a newly developed 3C microelectromechanical system-based seismic land streamer over porous glacial sediments to delineate water table and bedrock in Southwestern Finland. The seismic source used was a 500 kg vertical impact drop hammer. We analyzed the SH-wave component and interpreted it together with previously analyzed P-wave component data. In addition to this, we examined the land streamer’s potential for multichannel analysis of surface waves and delineated the site’s stratigraphy with surface-wave-derived S-wave velocities and VP/VS ratios along the entire profile. These S-wave velocities and VP/VS ratios complement the interpretation conducted previously on P-wave stacked section. Peculiarly, although the seismic source used is of a vertical-type nature, the data inspection indicated clear bedrock reflection on the horizontal components, particularly the transverse component. This observation led us to scrutinize the horizontal component data through side-by-side inspection of the shot records of all the three components and particle motion analysis to confirm the S-wave nature of the reflection. Using the apparent moveout velocity of the reflection, as well as the known depth to bedrock based on drilling, we used finite-difference synthetic modeling to further verify its nature. Compared with the P-wave seismic section, bedrock is relatively well delineated on the transverse component S-wave section. Some structures connected to the kettle holes and other stratigraphic units imaged on the P-wave results were also notable on the S-wave section, and particularly on the surface-wave derived S-wave velocity model and VP/VS ratios. Our results indicate that P-, SV-, and SH-wave energy is generated simultaneously at the source location itself. This study demonstrates the potential of 3C seismic for characterization and delineation of the near-surface seismics.〈/span〉

Description: ABSTRACT 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 radio-magnetotelluric 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.

Description: ABSTRACT To provide a guide for future deep (〈1.5 km) seismic mineral exploration and to better understand the nature of reflections imaged by surface reflection seismic data in two mining camps and a carbonatite complex of Sweden, more than 50 rock and ore samples were collected and measured for their seismic velocities. The samples are geographically from the northern and central parts of Sweden, ranging from metallic ore deposits, meta-volcanic and meta-intrusive rocks to deformed and metamorphosed rocks. First, ultrasonic measurements of P- and S-wave velocities at both atmospheric and elevated pressures, using 0.5 MHz P- and S-wave transducers were conducted. The ultrasonic measurements suggest that most of the measured velocities show positive correlation with the density of the samples with an exception of a massive sulphide ore sample that shows significant low P- and S-wave velocities. The low P- and S-wave velocities are attributed to the mineral texture of the sample and partly lower pyrite content in comparison with a similar type sample obtained from Norway, which shows significantly higher P- and S-wave velocities. Later, an iron ore sample from the central part of Sweden was measured using a low-frequency (0.1–50 Hz) apparatus to provide comparison with the ultrasonic velocity measurements. The low-frequency measurements indicate that the iron ore sample has minimal dispersion and attenuation. The iron ore sample shows the highest acoustic impedance among our samples suggesting that these deposits are favourable targets for seismic methods. This is further demonstrated by a real seismic section acquired over an iron ore mine in the central part of Sweden. Finally, a laser-interferometer device was used to analyse elastic anisotropy of five rock samples taken from a major deformation zone in order to provide insights into the nature of reflections observed from the deformation zone. Up to 10% velocity-anisotropy is estimated and demonstrated to be present for the samples taken from the deformation zone using the laser-interferometery measurements. However, the origin of the reflections from the major deformation zone is attributed to a combination of anisotropy and amphibolite lenses within the deformation zone.

Description: The Bergslagen region is one of the most ore prospective districts in Sweden. Presented here are results from two nearly 25 km long reflection seismic profiles crossing this region in the Dannemora mining area. The interpretations are constrained by seismic wave velocity measurements on a series of rock samples, cross-dip analysis, prestack time migration, and swath 3-D imaging, as well as by other available geophysical and geological observations. A series of major fault zones is imaged by the seismic data, as is a large mafic intrusion. However, the most prominent feature is a package of east-dipping reflectors found east of the Dannemora area that extend down to at least 3 km depth. This package is associated with a polyphase, ductile-brittle deformation zone with the latest ductile movement showing east-side-up or reverse kinematics. Its total vertical displacement is estimated to be in the order of 2.5 km. Also clearly imaged in the seismic data is a steeply dipping reflector near the Dannemora mine that extends down to a depth of at least 2.2 km. The geological nature of this reflector is not known, but it could represent either a fluid-bearing fault zone or a deep-seated iron deposit, making it an important target for further detailed geophysical and geological investigations.

Description: ABSTRACT The development of cost‐effective and environmentally acceptable geophysical methods for the exploration of mineral resources is a challenging task. Seismic methods have the potential to delineate the mineral deposits at greater depths with sufficiently high resolution. In hardrock environments, which typically host the majority of metallic mineral deposits, seismic depth‐imaging workflows are challenged by steeply dipping structures, strong heterogeneity and the related wavefield scattering in the overburden as well as the often limited signal‐to‐noise ratio of the acquired data. In this study, we have developed a workflow for imaging a major iron‐oxide deposit at its accurate position in depth domain while simultaneously characterizing the near‐surface glacial overburden including surrounding structures like crossing faults at high resolution. Our workflow has successfully been showcased on a 2D surface seismic legacy dataset from the Ludvika mining area in central Sweden acquired in 2016. We applied focusing prestack depth‐imaging techniques to obtain a clear and well‐resolved image of the mineralization down to over 1000 m depth. In order to account for the shallow low‐velocity layer within the depth‐imaging algorithm, we carefully derived a migration velocity model through an integrative approach. This comprised the incorporation of the tomographic near‐surface model, the extension of the velocities down to the main reflectors based on borehole information and conventional semblance analysis. In the final step, the evaluation and update of the velocities by investigation of common image gathers for the main target reflectors were used. Although for our dataset the reflections from the mineralization show a strong coherency and continuity in the seismic section, reflective structures in a hardrock environment are typically less continuous. In order to image the internal structure of the mineralization and decipher the surrounding structures, we applied the concept of Reflection Image Spectroscopy (RIS) to the data, which allows the imaging of wavelength‐specific characteristics within the reflective body. As a result, conjugate crossing faults around the mineralization can directly be imaged in a low‐frequency band while the internal structure was obtained within the high‐frequency bands. This article is protected by copyright. All rights reserved