The initial appraisal of buried DAS system in CO2CRC Otway Project: the comparison of buried standard fibre-optic and helically wound cables using 2D imaging

This study aims to assess the ability of shallow distributed acoustic sensing (DAS) to serve as a cost-effective seismic sensor array for permanent monitoring applications. To this end, as part of the CO2CRC seismic monitoring program, a fibre-optic DAS array was deployed alongside a permanently buried geophone array at the Otway Project site (Victoria, Australia). The DAS array consisted of a standard commercially available tactical fibre-optic cable, which was deployed in 0.8 m deep trenches. A custom-designed helically wound (HW) cable was also deployed in one of the DAS trenches for comparison of the cable designs. Simultaneous acquisition of the seismic data was carried out using ~ 3000 vibroseis source points and geophones, DAS standard and HW cables. For initial assessment of the seismic images acquired with DAS and to compare different cable designs, preliminary 2D seismic reflection processing is conducted on both DAS cables and geophone data along a single 2D line. The geophone data processing guided processing of the DAS data. Several shallow structures (100–450 ms) and some important reflectors at 450–600 ms are observed on the final DAS images. Comparison of the two different DAS cable types demonstrated that seismic imaging would benefit DAS technology. However, the benefit of utilising HW cable is modest compared with the standard cable. The workflows and results of this study pave the way for processing of the 3D seismic data set acquired with the DAS array, as well as further detailed analysis of the DAS cables and the system itself

Analysis of subtle structures using different 3D survey geometries - CO2CRC Otway project case study

Three vastly different 3D seismic surveys acquired in the Otway basin since 2000 are analyzed and compared in terms of resolution, imaging accuracy and also field methodology. The main differences between the surveys were with respect to the data density, source parameters and the unit cell design. The first survey was executed in a conventional manner using arrays on both source and receiver side. Subsequent surveys utilised single geophone/single source approach. After unified processing was applied to all three surveys, the result revealed a striking difference in resolution and fault expression between the surveys. Of particular interest was the difference between the two latest surveys that utilised single source/single receiver approach. Significant improvement in resolution provided by the latest survey is attributed to very dense spatial sampling and the unit cell designed that assumed ÄRL/ÄR ratio of 5 rather than more common ratio of 10. Consequently this survey provided very accurate images of the shallow faults which existence was not previously known. Three surveys analyzed also witness the improvements in seismic acquisition technology over the period of nearly two decades that enabled new data acquisition trends to be developed and applied

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

Model-guided processing of time-lapse seismic for realtime monitoring of CO2 geosequestration - CO2CRC Otway Project case study

Carbon capture and storage is gaining acceptance around the world as one of the means for CO2 emission reduction. As the technology is just emerging, there is a necessity to address arising issues from technical details to geological hazards to public liaison. Otway Project is the first Australian attempt to address these issues. Stage 2C of the Otway Project is designed to identify the lower limit of CO2 detectable by seismic monitoring in saline aquifers. In this paper, we show that designing seismic processing workflows using a combination of baseline field data and a synthetic finite-difference 4D dataset speeds up time-lapse seismic processing to provide express time-lapse images of the plume with excellent signal-to-noise ratio and repeatability in less than a week upon completion of the monitor acquisition. Obtained time-lapse seismic images allow detection and tracking of the evolution of a small CO2-rich plume injected into a saline aquifer at 1500 m depth

CO2 Storage Site Characterisation using Combined Regional and Detailed Seismic Data: Harvey, Western Australia

Some 115 km 2 of regional 3D seismic data were acquired in the first quarter of 2014 near Harvey, Western Australia, for the needs of the South West CO2 Hub project. The survey proved to be of great importance for a regional characterisation of the reservoir, identification of the large structures and key geological interfaces. However, small to medium size structures of interest for the development of the static and dynamic models were poorly imaged in this survey as the recording geometry was adjusted for the greater depths, which was between 2 km and 3 km. To improve the imaging of the shallow structures, a high-resolution (nested) 3D survey centered at Harvey 4 well was undertaken in 2015 (Urosevic et al., 2015). This survey utilised a single geophone and single, 24 s long, broadband (6-150 Hz) sweep combined with high data density to improve signal to noise ratio that was initially lowered by not employing high-power sources and geophone arrays. The results of this high-resolution 3D survey demonstrate that high-density surveys are important even at the characterisation stage and are crucial for development of a detailed static model. For that purpose, both post and pre-stack inversions of these data were utilised to model distribution of paleosols, lenses of high clay content, which are assumed to serve as baffles for CO 2 upward migration. A good correlation was established between very low impedance values and increased percentage of paleosols and on the other end of the scale very high impedance values and low porosity sandstones. A pre-stack migrated high-resolution cube and the attribute derived from it, such as coherency and impedance, enabled improved structural and stratigraphic analysis around Harvey 4 well. The results shown were of a crucial importance for the containment studies, development of the dynamic model and establishment of the injection intervals

Geophysical characterization of landslides in Serbia and Bosnia and Hercegovina - A GWB project

Continuous, heavy rainfall commencing in May 2014 has resulted in extensive flooding in Serbia, Bosnia and Herzegovina (BiH). Thousands of landslides developed rapidly after several days of torrential rain. Shortly after a diverse group of geophysicists and geologists got together and set out a project to investigate these landslides. The main objective was to characterize and categorize landslides and provide results that could help devise an optimum mitigation program. Prediction of the reactivation potential of landslides was also of direct interest to the investigations. In the first phase of the project investigations included simultaneous acquisition of reflection, refraction, MASW and resistivity data along 17 profiles, distributed over six different localities. In addition, a mini 3D survey was successfully recorded along steep slope. These measurements were supported by sparse drilling, logging, coring, geotechnical analysis and the airborne laser scans. Very good agreement was achieved between different methods, despite vastly different geometry and composition of landslides. Joint analysis of resistivity images, reflection data, and P and S-velocity fields, obtained from refraction tomography and inversion of surface waves, provide new geological insights that are important for understanding the mechanism of a landslide. This will provide valuable input for a mitigation program

Distributed Acoustic Sensing Applied to 4D Seismic: Preliminary Results From the CO2CRC Otway Site Field Trials.

Carbon geosequestration requires the mapping and monitoring of the injected CO2 to assure the gas is safely stored in the formation. In this context, Distributed Acoustic Sensing (DAS) has the potential to reduce the costs of seismic operations and increase equipment survivability. Field demonstrations show that DAS can be used for reservoir monitoring in geosequestration applications but most of its studies are limited to borehole seismic surveys, while very little has been published towards DAS for surface seismic. To bridge this gap, capability for seismic monitoring using a 3D DAS array was tested during a test CO2 injection of the CO2CRC Otway Project (Victoria, Australia). DAS was deployed together with geophones along eleven receiver lines and data were acquired before and after the injection of 5kt, 10kt and 15kt of supercritical CO2. Processing of DAS data is very challenging due to strong angle dependence of its sensitivity and very large volumes of recorded data. Preliminary processing results show that we can identify the main reflectors but noise is still strong. The results also show strong azimuth dependence of DAS sensitivity

The CO2CRC Otway Shallow CO Controlled Release Experiment: Site Suitability Assessment, Energy Procedia

The CO2CRC is undertaking a feasibility study for a planned controlled release and monitoring experiment at a shallow fault at the CO2CRC Otway Project site in 2018. Interpretation of pre-2016 seismic data could trace the height of the fault to approximately 100 m below the ground surface, at which point the resolution of the existing seismic data was insufficient to delineate the fault any further. To better understand the shallow geology at the Otway Project site and to map the extent of the shallow fault, new geophysical surveys were acquired over the Otway site during 2016. This included a high resolution, shallow focused, 3D seismic survey to provide greater delineation of the newly identified fault in the Port Campbell Limestone on the Otway Project site, and a high resolution resistivity survey to map the vertical extent of the fault towards the ground surface. Aerial imagery and LIDAR data were also collected. The seismic survey data exhibit greatly improved vertical and lateral resolution compared to previous seismic surveys. Preliminary pre-stack time migration (PreSTM) processing of the data show that the target fault can be clearly imaged at 30 ms TWT and the fault tip can be mapped to within approximately 25 m of the surface. Approximately 5 m of throw is identified at approximately 140 m depth and the throw appears to decrease in magnitude as the fault extends towards the surface. This, plus an identified dip angle of ∼70° (east), suggests that it is most likely a normal fault. There is no evidence of topographical features associated with the surface expression of the shallow fault using LIDAR and aerial imagery. Electrical Resistivity Imaging (ERI) results indicate that there are 3 distinct layers in the shallow geology of the Otway site, including a higher resistivity, more clay influenced, 3-5 m thick layer at the surface. The resistivity is also surprisingly heterogeneous over the site, suggesting that the shallow geology is complex. Preliminary hydraulic conductivity measurements confirm that the Port Campbell Limestone is highly permeable in the vicinity of the Otway Project site. The target fault at the CO2CRC Otway Project site appears to be a suitable candidate for a shallow CO2 injection experiment

4D surface seismic tracks small supercritical CO2 injection into the subsurface: CO2CRC Otway Project

Time-lapse (4D) seismic monitoring of injected CO2 in geological formations is being increasingly employed as the principal method for ensuring containment of the CO2 and testing conformance of predicted plume behaviour. However, to bring further confidence in this method, the CO2 volume detection limit in the seismic monitoring and key factors controlling it need to be quantitatively understood. The CO2CRC Otway Project attempts to improve this understanding by exploring the capability of seismic reflection method to detect and monitor a 15,000 t injection of supercritical CO2/CH4 mixture in a saline aquifer at a depth of 1500 m. To increase the signal to noise ratio and to reduce the disruption to land users, seismic acquisition is performed using a buried geophone array. Seismic acquisition occurred at injection intervals of 5000, 10,000 and 15,000 t over a 5-month period. The seismic images clearly show the distribution and evolution of the stored CO2/CH4 plume. The analysis confirms that signal from pure CO2 would be of similar magnitude to the signal from CO2/CH4 mixture. The results demonstrate the potential of time-lapse reflection seismic to provide key information to both operators and regulators for confirming the security and behaviour of stored CO2 at very small volumes