Jai Kinkela

Ground Roll Repeatability Analysis - CO2CRC Otway Project Case Study

Time-lapse (TL) seismic is an essential tool for the monitoring of changes in reservoir conditions induced by reservoir production, reservoir stimulation and more recently CO2 sequestration. The Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC) is currently conducting a carbon dioxide geo-sequestration pilot study in the Otway Basin, onshore Victoria, Australia. An extensive TL seismic monitoring is a part of monitoring and verification program of the project. Being a well-developed methodology for offshore applications, time-lapse seismic is also gaining popularity as an onshore reservoir monitoring technique. However its onshore use is limited by issues of seismic repeatability. This is primarily attributed to the higher near-surface variability as compared to marine seismic. Analysis of several repeated Otway 2D surveys show that different frequency bands have different repeatability. Low frequencies (< 30 Hz), contaminated by the ground roll noise, are characterised by particularly poor repeatability. In this paper we analyse three repeated 2D surveys obtained with same geometry but different sources and near-surface conditions in order to investigate ground roll repeatability by evaluation of surface waves phase velocity spectra.

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.

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.

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

Traditional geophysical prospecting techniques used for mineral exploration rarely provide the 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. This paper addresses these challenges and looks at strategies used to overcome them eventually leading to the direct imaging of the orebody.

Direct VMS Targeting through Preserved Relative Amplitude Processed Seismic Imaging at Neves Corvo, Portugal

Exploration of deep VMS deposits at the Neves Corvo mine on the Iberian Pyrite Belt was further advanced through a specialised application of surface seismic techniques. The contrast in impedances from elastic property measurements of core samples showed that there should be a significant difference in the seismic response of mineralisation compared to the surrounding host rocks. These results indicated that relative amplitude preservation processing may be of importance to help reduce the ambiguity in direct, seismic amplitude anomaly based targeting of Volcanogenic Massive Sulphides. However, such processing was not easy to implement due to an intrinsically low signal to noise ratio, complex 3D geology, high scattering level and often patchy and poor reflectivity. The newly obtained 3D seismic cube was calibrated with the existing boreholes to show superior ore zonation and precision required for the subsequent deep drilling campaign.

Seismic exploration for volcanogenic massive sulphides-The rosebery zinc, lead, copper mine, Tasmania

An experimental 3D seismic survey conducted in extreme topographic and weather conditions in Rosebery, Tasmania, Australia -a known VMS province - was aimed at validating the technique for this region as well as prospecting further down dip for additional mineralisation. Significant challenges were faced during the survey design stage stemming from trying to image dipping structures while taking into account extreme topographical variation of up to 400 m using a limited survey grid of approximately 1.5 km². Initial processing results were less than satisfactory with the majority of the target geology being migrated outside of the conventional 3D space. By expanding the geometry additional space for migration was provided, allowing seismic events to migrate to their true spatial position. This unambiguously imaged the controlling structures and achieved the major objectives of the trial survey.

Direct Targeting of VMS through Amplitude Consistent Seismic Imaging

Based on physical property measurements of core samples and the often observed difference in elastic properties from these there should be a significant difference in seismic amplitude between mineralisation and the surrounding host rocks. These results indicate that relative amplitude preservation processing may be of importance in the use of seismic data for the targeting of mineral resources, particularly in the case of massive ores. Such ‘true relative amplitude’ processing is not easy to achieve due to intrinsically low signal to noise ratio in hard rock environments, complex 3D geology, steeply dipping structures, high seismic velocities and often patchy and poor reflectivity. To help reduce the ambiguity in targeting and increase the likelihood of success we have worked on careful re-processing of 3D seismic data with the application of true amplitude preservation. We compare the anomalous amplitude zones that are related to massive sulphide bodies using a true amplitude seismic cube and a conventionally processed cube with the application of AGC (automatic gain control). A higher level comparison is conducted after seismic calibration with boreholes. The zonation and precision of targeting is discussed in this paper.