Marine surveys are recorded under dynamic conditions, with every part of the system being in constant motion. In 4D, these variations from one vintage to another become important as differences due to acquisition may hide the 4D differences in seismic response over time due to changes in the reservoir. Therefore, at CGGVeritas great care is taken to ensure the monitor surveys are shot in as similar a manner to the baseline survey as possible.
There are many factors affecting the final quality of a 4D survey, including:
Survey planning
The ideal 4D monitor survey is an exact replica of the baseline acquisition. This involves repeating source & receiver positions and matching environmental conditions such as currents, seasonal variations in water temperature and salinity, etc. Shooting the same line in the same direction at the same tide state when possible, replicates the height of the water-column and therefore the periodicity of water bottom multiples.
Current prediction
Accurate knowledge of currents can be a valuable tool in minimising infill and maximising 4D repeatability. CGGVeritas uses sophisticated current models which include meteorological data and satellite observations, augmented by onboard Acoustic Doppler Current Profiling (ADCP).
Streamer steering
Once the gross positioning has been accounted for by careful planning and ADCP prediction of feathering, further fine-tuning can be achieved by steering the streamers dynamically. The Sercel Nautilus system combines streamer steering and acoustic positioning functions into a single device.
Multi-vessel operations Multi-vessel operations can play a range of important roles in the context of 4D acquisition. In cases of extreme feathering CGGVeritas has used multi-vessel operations to improve repeatability of long offsets. Where obstructions are present, two vessels can be used to achieve greater repeatability of azimuth or perform undershooting.
Survey over-specification
A denser, over-specified dataset can be recorded for a new baseline, or successive monitor surveys, by using more streamers to achieve wider spreads and closer streamer separation. Modern 4D processing techniques can then make good use of this higher-density data to select or interpolate the data for optimum repeatability with other vintages.
Recording equipment
CGGVeritas’ fleet is predominantly equipped with Sercel Sentinel solid streamers which offer clear benefits for 4D recording. Solid streamers are proven to be quieter, with a higher signal-to-noise ratio than fluid filled ones. Solid streamers offer more consistent ballast, improving depth control. In turn this improves the repeatability of receiver ghosts which can be crucial when looking for small timing or phase shifts.
