France: CGGVeritas’ BroadSeis One Year On
Since launching BroadSeis at Barcelona last year, CGGVeritas has recorded BroadSeis data in many locations around the world, in different water depths and over different geologies. In all cases the increases in bandwidth at both low and high frequencies (six octaves have been recorded in the Gulf of Mexico) have provided significant improvements in imaging and data quality.
Since the beginning of the year, CGGVeritas has four commercial BroadSeis projects awarded and is currently acquiring and processing a multi-client 3D Survey in Quad 29 of the Central North Sea. Initial results from this survey are causing considerable excitement.
Off North West Australia the extra low-frequency energy recorded allows far better differentiation of layers than conventional seismic, due to the lack of side lobes to the wavelet and the excellent phase control of the low frequencies. The broad bandwidth provides high resolution as well as stunning textural and stratigraphic detail and, perhaps for the first time, allows for direct discrimination of rock or fluid properties. These characteristics have also been particularly noticeable offshore Guyana.
Where penetration sub-salt or sub-basalt is an issue BroadSeis performs well. In these areas resolution is limited by a lack of high frequencies, which are mainly attenuated due to scattering and transmission losses, lack of illumination, multiple contamination and velocity model errors. Three octaves of signal are generally considered necessary for adequate seismic resolution. Therefore, in order to achieve sufficient bandwidth in these areas, it is necessary to extend the low frequencies. Conventional marine streamer acquisition lacks sufficient signal-to-noise ratio in the 2-7 Hz bandwidth due to streamer depth, tow noise and source array configuration. BroadSeis acquisition achieves usable frequencies down to 2.5 Hz, providing three octaves of data below 20 Hz, so allowing seismic resolution sub-salt and sub-basalt.
West of Shetlands considerable increases in signal-to-noise ratio and continuity have been observed sub-basalt on BroadSeis data. Clear definition of intra and base basalt layers has also been seen, including probable multiple basalt flows pinching out. In the Gulf of Mexico considerable improvement to the base salt illumination was achieved. On this survey six octaves of usable data was recorded, providing impressive resolution and texture in the near surface, as well as continuity at depth.
In a pilot 3D survey in the Central North Sea BroadSeis provided increased lateral resolution, with the timeslices in the near surface (see images) showing clear definition of channels and yielding detail similar to a topographic map. Despite being recorded with short streamers (4 km) and having a limited aperture, being only 7.5 km wide, the images of the Base Cretaceous Unconformity (BCU) provide considerable improvements in structural resolution and imaging over the reference high-quality conventional long-offset (6 km) data in the area.
The BroadSeis 3D data is less noisy and the additional low-frequency energy helps delineate deep structures, showing clear layer differentiation and local impedance contrasts and heterogeneities, without the confusion caused by wavelet side-lobes. The broader frequency content, especially at the low end, improves sedimentary package differentiation and delineation within the sub-BCU Upper Jurassic strata as well as in the Cretaceous above.
The pilot 3D was deliberately acquired over a known AVO anomaly. The BroadSeis deghosting algorithm is true-amplitude preserving and angle stacks over this anomaly showed the AVO effect more clearly than the conventional data. The improved low frequencies achieved using BroadSeis give better stability and accuracy to the inversion process and the prestack inversion of this dataset provided better correlation with the well data than inversion of the conventional dataset.
Interpreters find it much easier and faster to work with the BroadSeis 3D data. The subtlety, texture and continuity within the volume mean that auto-picking of horizons is far quicker and more reliable. The frequency bandwidth allows the auto-tracking tool to work optimally with greatly reduced intervention, allowing interpreters more time to focus on interpretation subtleties.
The many examples of BroadSeis data that CGGVeritas has now acquired from around the world all show considerable improvement in data quality, increasing both lateral and temporal resolution. BroadSeis decreases confusion caused by wavelet side lobes and so clarifies impedance contrasts. The extra low-frequency energy allows layers to be easily differentiated. The increased bandwidth and resolution of BroadSeis provides a step-change in seismic stratigaphy and allows us to directly infer lithology and fluid effects in the data. BroadSeis 3D data has more than lived up to the expectations of the earlier 2D data and CGGVeritas looks forward to further BroadSeis success later this year.
Source:CGGVeritas , May 27, 2011