SEISMIC SURVEYING
Fig. 2. Shallow timeslice (262ms) from UK CNS Quad 20 showing exceptional resolution, character and detail resembling a topographic map.
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 (4km) and having a limited aperture, being only 7.5km 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 (6km) data in the area. Te BroadSeis 3D data is less noisy and
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. Te 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.
11 IHSS
Tese 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. Tree octaves of signal are generally considered necessary for adequate seismic resolution. Terefore, 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 2-7Hz bandwidth due to streamer depth, tow noise and source array configuration. BroadSeis acquisition achieves usable frequencies down to 2.5Hz, providing three octaves of data below 20Hz, 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.
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. Te 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.
Te pilot 3D was deliberately acquired
over a known AVO anomaly. Te BroadSeis deghosting algorithm is true-amplitude preserving and angle stacks over this anomaly showed the AVO effect more clearly than the conventional data. Te 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. Te subtlety, texture and continuity within the volume mean that auto-picking of horizons is far quicker and more reliable. Te frequency bandwidth allows the auto-tracking tool to work optimally with greatly reduced intervention, allowing interpreters more time to focus on interpretation subtleties.
Jo Firth is with CGGVeritas, Crawley, UK.
www.cggveritas.com
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