Traditionally, site surveys for wind farms have been acquired using a grid of UHR 2D seismic lines. However, this approach presents several pitfalls when shallow geology is complex. High interpretation uncertainty makes it challenging to predict hazards confidently due to the limited spatial coverage of the 2D lines and the risk of false structure caused by out-of-plane reflections. Interpolation may be required to link seismic lines to geotechnical data for calibration. If hazards are identified and the turbine locations must be moved, this can necessitate a complete re-shoot of the 2D survey, increasing cost and development time.
These challenges are driving wind farm operators to consider UHR3D seismic as an alternative. By collecting a UHR3D survey in the first instance, the developers can be confident that they will meet the geophysical requirements for the development program. Although the cost of a 3D survey may be higher than 2D, the need for costly 2D re-shoots is avoided. Subsurface imaging is also much more robust, and planners can identify hazards more confidently. Geophysical and geotechnical measurements coincident in space allow better calibration and quantitative interpretation of subsurface properties, which opens the possibility of using inversion, rock physics, or machine learning approaches to derive the subsurface engineering properties of interest.
Finally, combining the acquisition of UHR3D P-cable with multiphysics measurements such as multibeam bathymetry, side-scan and synthetic-aperture sonar, and magnetic data can achieve a one-and-done site survey more efficiently and at a lower cost.
PGS Now Offers Ultra-High-Resolution 3D Seismic Acquisition, Imaging, and Interpretation
PGS now offers ultra-high-resolution 3D seismic, following the purchase of NCS Subsea, the operator of the P-cable system. Many of the advances pioneered by PGS in 3D marine seismic acquisition and imaging - such as wide-tow multi-sources for dense spatial sampling and near-offset coverage - are being applied to maximize efficiency and resolution.
Sverre Planke and Christian Berndt first conceived the P-cable as an efficient way of collecting ultra-high-resolution 3D seismic data. The National Oceanography Centre, Southampton, built and tested a prototype between 2001 and 2004. Commercial surveying began in 2009, and since then, surveys have been conducted globally. The technology has applications from hydrocarbon exploration, appraisal, and 4D reservoir monitoring, to high-resolution mapping of the shallow subsurface for windfarm development.
The P-cable system achieves UHR imaging of the subsurface by sampling the seismic wavefield at a high spatial and temporal rate. The eponymous P-cable is a cross-connecting cable that links several short recording streamers, typically 50-100 m long. Usually, the source is high frequency, and data are sampled at 0.125 – 0.25 msec in bin sizes of less than 1 m to about 6 m. The array is towed at shallow depths, typically 2-4 m, to emphasize the high-frequency components in the data.
Ultra-high-resolution 3D seismic data is best suited to detailed imaging on a smaller scale for targets such as complex, highly fractured, thin, or compartmentalized formations. P-Cable data typically focuses on the 50 – 150 m offset range for the highest spatial resolution.
Most people think of seismic resolution in terms of wedge models, wavelengths, and the ability to separate objects in the subsurface. All of this is, of course, germane. However, there is another critical consideration: without sufficient spatial resolution, it may be challenging or impossible to tell what you are looking at geologically. As a simple example, it is sometimes hard to tell what a low-resolution pixelated image represents. Increase the number of pixels, and the picture becomes clear. The image below illustrates the seismic analogy; it shows, on the left, a time slice from a conventional high-resolution 3D seismic survey with bin size 6 x 25 m, and on the right, the equivalent time slice from a UHR3D P-cable study in which the bin size was 3.125 x 3.125 m. Decreasing the bin size by a factor of two in the inline and eight in the crossline direction means that the P-cable data can resolve the complex polygonal fault system in the area. It is invisible in conventional 3D data.
Although the scale of a P-cable acquisition is small compared to high-capacity 3D surveys, the ultra-high-resolution spatial and temporal sampling mean data volumes that are larger or comparable to the 3D data used in oil and gas exploration. Extremely accurate positioning of both sources and receivers is required, and static corrections (among other things) must be undertaken with care. Regardless, planners need interpreted data products delivered within tight timescales, a challenge that PGS is ready and able to address with a combination of our organizational capability and technology.
P-Cable in New Energy
The oil and gas industry has used P-cable for exploration, reservoir appraisal, monitoring, geohazard mapping, and site survey. Today, wind farm development is an obvious application for its ultra-high-resolution understanding, but this is not the only new energy application for the technology.
UHR3D seismic data can help to address challenges related to subsurface carbon storage:
- Ensuring seal integrity | If the impermeable seal over a reservoir used for CO2 storage is compromised, there is the potential for CO2 to leak over time. High-resolution P-cable imaging of small-scale, steeply dipping overburden faulting addresses this challenge.
- Leakage detection (the worst case) | If the seal is compromised, the operator will want to detect leakage as quickly as possible, halt any ongoing injection, and consider remediation measures. The P-cable system has provided high-resolution images of gas chimneys, a proxy for CO2 leakage in the overburden. Of course, by correlating high-resolution images of leakage with fault systems, potential or actual leakage paths can be better understood.
- Injection monitoring | P-cable data has been successfully applied in 4D hydrocarbon reservoir studies with excellent repeatability between time-lapse surveys. We, therefore, expect that P-cable will also be relevant for high-resolution 4D monitoring of CO2 storage sites.
Poised to Provide High-quality, High-resolution Datasets
With the P-cable system within its portfolio, PGS is getting ready to offer UHR3D seismic acquisition, imaging, and quantitative interpretation to traditional and new-energy markets; for wind farms and carbon storage, as well as hydrocarbon exploration, appraisal, and monitoring.