Sasha Ziramov

Physical property analysis and preserved relative amplitude processed seismic imaging of volcanogenic massive sulfides—a case study from Neves–Corvo, Portugal

Neves–Corvo is one of the biggest mining districts on the Portuguese side of the Iberian Pyrite Belt hosting six different lower Carboniferous copper, zinc, lead and tin orebodies including Lombador, Neves, Grac¸a, Corvo, Zambujal, and Semblana. During the past 50 years, geological, geochemical, and geophysical methods were utilized in the exploration of volcanogenic massive sulfide deposits at Neves–Corvo. Electromagnetic, earth resistivity, and principally gravimetry methods played major roles in the geophysical exploration of the area. However, in 2011, as the exploration depth for volcanogenic massive sulfide mineralization became ever deeper, the surface reflection seismic technique was trialled. Initially, elastic property measurements were employed on numerous core samples to determine the seismic properties of the major formations of Neves–Corvo. The contrast in acoustic impedance values derived from these measurements showed that there should be a significant difference in the seismic response of mineralization relative to the surrounding host rocks. Based on this, a high-resolution 3D seismic survey was acquired over the Neves–Corvo mine and its southeastern extension in order to image known deep volcanogenic massive sulfide mineralization to validate the seismic reflection technique and to potentially identify new mineralization targets. As a result, the Semblana and Lombador deposits were successfully imaged, along with key lithological contacts and geologic structures. Additionally, copper sulfide extensions south of Semblana were discovered. Unfortunately, all of the high-priority targets that were identified from the seismic data were subsequently drilled and many of them found to be non-economic. In order to overcome the non-uniqueness of the original seismic data, full-waveform sonic and pseudo-logs were used to model different interfaces and calibrate the seismic data. These results indicated that preserved relative amplitude processing might be of importance to help reduce the ambiguity in direct detection of volcanogenic massive sulfide based on seismic amplitude anomalies. The customized relative amplitude processing of a sub-dataset over the Semblana deposit was then performed. The newly obtained seismic cube was calibrated with existing drillholes, and a volumetric interpretation was performed by utilizing amplitude-based geobodies. Eventually, superior target zonation and precision for the subsequent deep drilling campaign was achieved with the revised interpretation, clearly showing that the high priority targets originally identified from the legacy data would not have passed the targeting criteria in the reprocessed data due to their relatively weak amplitude response. The results obtained from this study inspired the subsequent reprocessing of the full seismic dataset.

Subsurface Imaging Using Buried DAS and Geophone Arrays - Preliminary Results from CO2CRC Otway Project

A permanent geophone array along with a fibre optic distributed acoustic sensing (DAS) array were deployed at the CO2CRC Otway Project site in order to conduct seismic monitoring of a CO2 plume during a small-scale injection test. This study aims to assess the ability for a permanent geophone array to overcome issues related to different acquisition (receiver) designs, high ambient noise level and seasonal variations in the near surface, as well as to test the DAS system for performing cost-effective time lapse seismic measurements. The acquisition of 3D seismic data is performed for this purpose using ~3000 vibroseis source points. We show the preliminary results of seismic reflection imaging conducted using DAS data. We observe the differences in performance between a standard commercially available tactical fibre optic cable and a custom helically wound cable. The results of this study and the workflows established will be used for processing a complete 3D seismic dataset acquired with a DAS array before being compared to a conventional geophone array.

Kevitsa Ni-Cu-PGE deposit, North Finland - A seismic case study

A 3D seismic survey was designed, acquired and processed by HiSeis Pty. Ltd. in 2010 at the Kevitsa Ni-Cu-PGE deposit. The objectives of the survey were the definition of sub-vertical structures (knowledge of which could assist in the design and characterization of the slopes of the proposed open pit), and mapping out the general structural setting of the mafic intrusive. The 2010 processing of the Kevitsa 3D seismic data was accelerated to meet engineering design deadlines. Although this phase of work was restricted to processing sequences that were not amplitude consistent and to the post stack migration algorithm, never-the-less the resultant product achieved good resolution of the complex structural setting. The dataset was re-processed in 2014 with the goal of preserving relative signal amplitudes, in order that the volume could be inverted into an acoustic impedance cube. Another reason for re-processing was to improve imaging in shallow depth, by improving the static solution and velocity model used for imaging. Both of these processes are considered to be crucial steps in hard rock seismic data processing. Considerable improvement was achieved through the application of a pre-stack time migration (PSTM) algorithm. Conventional 3D deep-move out corrections (DMO), followed by a post-stack migration algorithm proved to be insufficient to handle the lateral changes of velocities. Consequently, pre-stack time imaging was attempted to aid in handling the highly complex velocity field. The goal was to derive a velocity model appropriate to the geologic environment in order to place events in their correct positions, to properly focus the energy, to avoid introduction of false structures and to flatten the image gathers. The Kevitsa 3D seismic dataset is considered as being of high quality and as the data volume contains a statistically significant number of log measurements, it is deemed suitable for the seismic inversion.

CO2 storage site characterisation at the location of harvey-3 well, harvey, Western Australia

The South West Hub is the first commercial scale CO2 capture and sequestration project in Australia. The project aims to capture CO2 from several significant polluters that are located around the town of Harvey, Western Australia. The potential CO2 reservoir is the prominent Lesueur sandstone formation while the Harvey-3 well is likely to be utilised for the first geo-sequestration test. Unfortunately, due to survey restrictions imposed in 2014, the area around the Harvey-3 well is void of seismic information. New developments have given rise to new seismic investigations which had to be designed in a unique manner due to limited access to the site. The data acquisition program is comprised of 2D surface and borehole surveys (550 OVSP points) and 3D surface and VSP surveys. Results will be discussed in detail.

The first experimental seismic investigation over prospective uranium deposits at Mulga rock, Western Australia

or investigating such geological setting was reflection seismic. Our experimental survey involved several high resolution seismic lines over future test pits. Reflection profiles were helped by refraction tomography and MASW. The last two are to be utilised for geotechnical information for open pit mining. Reflection data utilised a “minimalistic” approach, but the resultant images are pleasing. Test pits below the lines will allow for a unique calibration but also ultimate evaluation of the seismic method.

Seismic Exploration of Mineral Resources - An Australian Perspective

The only geophysical method that can image deep structures with the precision required for targeting and discovering new resources is reflection seismic. However, mineral prospecting with seismic methods is not straightforward. Lack of understanding of the seismic response, necessity to adapt the method to the specifics of each target and underestimating the complexity of mineral environments introduced complexities that have resulted in its sporadic rather than systematic application. Here we present and briefly discuss the results and the lessons learnt after more than a decade of dedicated investigations in different mineralised environments. We expect that seismic will become a standard geophysical method for exploration of most brown and then green fields.

Neves-Corvo 3D - A High-resolution Seismic Survey at a Mine Camp Scale

Four consecutive 3D seismic surveys were acquired across the Neves Corvo mining camp in Portugal over a period of two years. The first survey undertaken was in 2011 over the known Semblana deposit, which proved that the 3D seismic method was capable of producing a very clear image of a complex underground orebody at this site. The main objective of the subsequent seismic surveys was the definition of the main structures, which were controlling mineralisation. Initial processing of the seismic data showed great promise in resolving the complex structural environment and also hinted at the possibility for direct targeting from seismic data. However, the quality across four different surveys was highly variable due to different acquisition geometry and source parameters. Moreover, initial processing did not involve preserved relative amplitude processing, which was key for the direct targeting of ore shoots. After successful application of surface consistent amplitude compensation, it turned out that pre-conditioning in the offset planes, followed by exclusion of unsuccessfully imaged events prior to stacking, produced the highest quality fully merged PSTM seismic cube. This cube could then be used for direct targeting with much more certainty than with any other processing result produced before.

Seismic exploration for volcanogenic massive sulphides: The DeGrussa copper-gold mine, Western Australia

Traditional geophysical prospecting techniques used for mineral exploration rarely provide either the depth of penetration or resolution required to accurately target orebodies at depth. Based on this, the seismic reflection method was trialled over a known VMS orebody at the DeGrussa copper-gold mine, Western Australia, in the hope of providing a viable exploration tool for deeper depths of investigation. However, a structurally complex geologic setting and a thick, highly variable regolith caused significant challenges in the processing of the seismic data. An initial 3D survey was not successful in imaging the orebody, so a follow-up downhole and 2D survey was acquired to address the potential issues. After verifying the in-situ seismic properties of the orebody through zero-offset Vertical Seismic Profiling (VSP) and increasing the down-dip offset range in the follow-up 2D survey it was found that the target provided a clear and unambiguous seismic response. However, a deep and variable regolith continued to cause significant issues during the imaging phase. This was overcome by applying a tomography-derived velocity field to a Kirchhoff migration, which produced outstanding results. Numerous tests and extensive data analyses eventually verified the seismic technique as a viable exploration tool for the region, with the direct detection of the target orebody.

A Case for Regional Seismic Reflection Surveys in the Gawler Craton, South Australia.

The seismic reflection method provides the possibility for delineation of very complex geological and this method might be good for detecting the presence of Iron Oxide Copper-Gold (IOCG) deposits. Despite many technically superior attributes, no arguments for regional seismic exploration have been proposed; probably because a cost-benefit analysis has never been conducted at such a scale. In this study we analyse such a case by modelling a Hillside IOCG deposit scenario where 2D seismic with relatively sparse source-receiver geometry is used to detect the presence of a possible intrusive package near a deep fault. The modelling results show that seismic reflection using 20m geophones and 40m shot spacing as an exploration tool is feasible, and that with the spacing halved we can definitely recover reasonable images of the upper parts of the mineralisation. The presences of such intrusives are clearly detectable and with the seismic method are detectable from 100m to 1000m deep. Thus, we propose that using 2D seismic is viable for IOCG exploration as it can detect mineralised intrusive structures along known favourable corridors or structures.

Harmonising diverse 3D geometries in a hard rock environment for pre-stack imaging

Four 3D seismic surveys were acquired by HiSeis Pty Ltd over two years across an existing mining camp. The main objective of these seismic surveys was definition of structures which could assist characterization of mineralised zone that consists mainly of massive sulphides. Initial processing of seismic data started shortly after acquisition in 2011. Preliminary products have shown great promise in resolving complex structural environments and showed potential for direct targeting form seismic data. Our motive for reprocessing the dataset was in integration of all 2011 and 2012 seismic data in an amplitude consistent routine which could bring new value for amplitude based analysis of massive sulphide bodies. Conventional 3D deep-move out corrections (DMO), followed by a post-stack migration algorithm has not been successful in merging diverse datasets. Considerable improvement was achieved through the application of pre-stack time migration (PSTM) algorithm. This allowed us to use unique bin size for all merged seismic surveys. Successful imaging of merged datasets has been challenged. The main reason was that 3D seismic datasets had vastly diverse offset and azimuth distribution. Highly irregular migration fold coverage is an obvious problem which had to be overcome by exclusion of unsuccessful imaged events prior to stacking.