Research in Seismic Processing and Imaging is currently ongoing in the following areas:
- Land and downhole time-lapse seismic monitoring
- Seismic attenuation from VSP and log data
- Seismic anisotropy from VSP and surface seismic data
- Sea-bed electromagnetics modelling, 3D visualisation and survey design
- Surface Orbital Vibrators
Land and downhole time-lapse seismic monitoring
In the framework of CRC for greenhouse gas technologies, CRGC researchers are involved in the first integrated land time-lapse seismic project in Australia.
The project involves monitoring of CO2 sequestration using 4D seismic data and time-lapse 3D VSP over Naylor field, Otway Basin, Victoria. The work involves research in the following topics:
- Analysis of land 2D and 3D seismic repeatability with field tests using surface and buried geophones
- Developing time-lapse processing flows and specialised algorithms for consistent processing of legacy land data shot with different sources and in different weather conditions
- Analysis and modelling of time-lapse noise; Rock physics modelling of changes of elastic properties of reservoir rocks from flow simulations; Seismic forward modelling of time-lapse seismic signal from flow simulations
- Assessment of feasibility of time-lapse monitoring using integration of geological modelling, flow simulations, rock physics, seismic forward modelling, and seismic noise estimation
- Data acquisition using novel technologies, including buried geophones, permanent sources, and distributed acoustic sensors.
Seismic attenuation from VSP and log data
Seismic attenuation is an important contributor to the overall amplitude decay of the seismic wave travelling through the medium.
Seismic attenuation anomalies in the overburden can both mask amplitude changes at the target horizons and carry important information about presence of free gas or overpressure. Vertical seismic profiling (VSP) is one of the standard tools to study seismic attenuation. Understanding magnitudes, causes and spatial distribution of attenuation is important for seismic imaging and reservoir characterization. Thin layering can cause both scattering attenuation and anisotropy. These phenomena can only be significant if there is a strong contrast in elastic properties between the layers.
We carried out a combined attenuation and anisotropy study driven by our multiple-well study using open file data, mainly from Carnarvon basin. Analysis of numerous wells in that area shows that intrinsic attenuation is the dominant factor, while large scattering attenuation was reported for some other areas. A modelling study was conducted using log data where strong scattering attenuation and anisotropy were observed. 1D scattering in the layer of carbonates present in the well can cause significant seismic attenuation (Qscat~55). This attenuation should have a substantial effect on surface seismic data quality, resulting in a loss of energy by the factor of 4 and ~10 Hz centroid frequency shift towards the low frequencies. The same stack of high-contrast layers can also cause significant anisotropy. This opens a possibility to use anisotropy parameters to predict anomalously high attenuation areas and vice-versa. A combined analysis of vertical seismic profiling and well log data can be used to estimate apparent attenuation and the relative contributions of intrinsic and scattering attenuation.
Seismic anisotropy from VSP and surface seismic data
Most sedimentary rocks are anisotropic, yet it is often difficult to accurately incorporate anisotropy into seismic workflows because analysis of anisotropy requires knowledge of a number of parameters that are difficult to estimate from standard seismic data.
We are providing a methodology to infer azimuthal P-wave anisotropy from S-wave anisotropy calculated from log or vertical seismic profile (VSP) data. This methodology involves a number of steps. First, we compute the azimuthal P-wave anisotropy in the dry medium as a function of the azimuthal S-wave anisotropy using a rock physics model, which accounts for the stress dependency of seismic wave velocities in dry isotropic elastic media subjected to triaxial compression. Once the P-wave anisotropy in the dry medium is known, we use the anisotropic Gassmann equations to estimate the anisotropy of the saturated medium.
We test this workflow on the log data acquired in the North West Shelf of Australia, where azimuthal anisotropy is likely caused by large differences between minimum and maximum horizontal stresses. The obtained results are compared to azimuthal P-wave anisotropy obtained via orthorhombic tomography in the same area. This methodology could be useful for building the initial anisotropic velocity model for imaging, which is to be refined through migration velocity analysis.
Sea-bed electromagnetics modelling, 3D visualisation and survey design
Sea Bed Electromagnetic methods are a relatively new and valuable technology for hydrocarbon exploration in deep water settings.
Modern surveying equipment provides the possibility of multi-offset, multi-frequency and more recently multi-azimuth data sets. The arrangement and orientation of EM sources and receivers relative to the geoelectrical nature of the target and host is a key part in determining the success of a Sea Bed Electromagnetic survey.
Further analysis of complex Sea Bed EM data sets requires the assistance of inversion. The CRGC research in Sea Bed EM is focused on: (a) integrated visualisation and survey planning; (b) interactive 2D/3D inversion of Sea Bed Electromagnetic data; (c) design of novel instrumentation; and, (d) understanding the impacts of electrical anisotropy on the Sea Bed EM response.
Surface Orbital Vibrators
Onshore seismic monitoring applications ideally involve the deployment seismic receiver arrays and mobile sources to image the subsurface.
TL surveys rely on accurate positioning source points and receivers to monitor the changes in the reservoir.
Common land access issues and the imprecise positioning of seismic equipment might result in significant and irreversible TL signal loss. Furthermore, reservoir monitoring requires significant labour, as a large amount of seismic equipment needs to be deployed, and then retrieved, on every survey. Permanent reservoir monitoring seeks to overcome the limitations of the conventional approach by fixing either the seismic receivers, sources or both.
To reduce the cost and land impact compared to vibroseis sources, permanent installation of Surface orbital Vibrators (SOVs) can be utilised in monitoring surveys. SOVs consist of common AC induction motors. They produce vibrations as an effect of the rotation of eccentric weights, which produces both a vertical and horizontal shear force. The force of the source increases as frequency squared. With their low production and operating cost and adequate force, SOVs can be a good alternative to common seismic source.
In the CO2CRC Otway project, we optimise permanent SOVs coupled up with conventional geophones as well as Distributed Acoustic Sensors to acquire high quality surface and borehole TL seismic data at relatively low cost and a minimum land impact.