Broadband seismic to support hydrocarbon exploration on the UK Continental Shelf

1. Broadband seismic to support hydrocarbon exploration
on the UK Continental Shelf
Gregor Duval 1
1
CGGVeritas Services UK Ltd, Crompton Way, Manor Royal Estate, Crawley, RH10 9QN, UK
Variable-depth streamer acquisition (BroadSeisTM
) is emerging as an effective technique for providing wide-bandwidth
seismic data (Soubaras and Dowle, 2010). This technology combines elements of improved equipment, acquisition and
processing to deliver broadband data. The use of solid streamers, reducing acquisition noise, allows the recording of
signal at the very low frequency range, down to 2Hz (Dowle 2006). The proprietary curved, variable-depth geometry
typically uses cable depths down to 50m, such that the data acquired tends to be less noisy due to the quieter recording
environment. The optimised variation in the receiver depth introduces receiver ghost diversity over different offsets,
which enables the receiver ghost to be fully removed by using a joint-deconvolution method (Soubaras 2010).
Combining all of these aspects together results in truly broadband data, giving the interpreter additional information in
the form of stunning, high resolution images of the subsurface. Details are enhanced thanks to the sharper wavelets
giving better resolution of important features such as thin beds and stratigraphic traps. The improved low frequencies
provide better penetration for deep targets, as well as better stability for seismic inversion.
In a mature basin such as the North Sea, such data provide significant enhancements in a range of environments:
exploring for new stratigraphic traps and subtle structural closures, and enhancing hydrocarbon recovery with more
information about local facies variations and reservoir compartmentalization. This paper takes a step-by-step approach
through the stratigraphy of the North Sea, demonstrating the benefits of broadband data from shallow to deep, illustrated
by examples from two 3D surveys from the North Sea, one in the region of UKCS Q20 and the other around UKCS Q29.
Need for a broad bandwidth
It is easy for seismic interpreters to understand the need for higher frequencies in seismic data since they provide more
detail about the geology, such as thin stratigraphic features and subtle rock structures. However, low frequencies are just
as important. They contribute to better imaging of deep targets and large scale facies variations as well as providing
better stability for pre- and post-stack seismic inversions.
Ideally, the seismic interpreter would like to see a seismic wavelet with as sharp a peak as possible and no side-lobes.
Adding in more high frequencies narrows down the wavelet central peak while introducing more low frequencies reduces
the amplitude of the side-lobes, making the wavelet look more like a single spike. Broadening the frequency spectrum at
both the high and low ends has a combined effect, thereby giving the genuine seismic signature of formation interfaces.
For instance, Figure 1 displays a comparison focused around the top of the Dornoch Formation, which includes several
thin and highly reflective coal layers, in the Outer Moray Firth area. Broadband data are less subject to tuning effects and
allow interpretation of pinch-outs (Zone 1 in Figure 1) and thin beds (Zone 2). Furthermore, the absence of side-lobe
interferences gives the genuine seismic signature of the Dornoch coal layer (Zone 3). Conventional data fails to resolve
any of these details.

2. Figure 1: Wavelet signature comparison at top Dornoch Formation level.
Shallow interpretation
Due to the steep slope of the near offsets when using curved variable-depth streamers, the benefits of broad bandwidth
are achieved even at very shallow depths, giving stunning images of recent deposits and formations lying directly below
the sea floor. Figure 2 shows a time slice comparison at 216ms TWT. The high frequency content is key in defining the
sharp edges of the channels and imaging the narrowest ones (less than 50m width). On the other hand, the low frequency
content helps differentiate the bed rock from channel fill deposits which appear darker gray in Figure 2. Such high
resolution seismic data are of great assistance for preliminary site surveys and identification of shallow hazards. Also,
these shallow channels produce rapid velocity variations that affect seismic ray path modelling. If not modelled properly,
this can result in severe artefacts on tomographic velocity models generated for Pre-Stack Depth Migration (PSDM)
work. The use of broadband data then becomes very important to carefully model these channels and get the best out of
the PSDM processing sequence.
Figure 2: Time slice comparison at 216ms showing shallow channels

3. Interpreting the Tertiary stratigraphy
Most of the Tertiary structural closures have been drilled in the North Sea basins. However, part of the remaining
potential lies within subtle stratigraphic traps and pinch-outs. This is where broadband technology has the potential to
play a major role. The higher frequency content pushes the limits of tuning effects further. It helps in resolving thin beds
and pinch-outs that have never been seen before. The low frequencies also play an important role by reducing side-lobe
interferences and helping in the interpretation of facies transitions.
Focusing on the prospective Tertiary turbidite systems of the North Sea, Figure 3 displays images of a thick, low acoustic
impedance shale sequence in the Paleocene Lista formation. The seismic correlates very well with well log data showing
top and base of these soft shales. The colour scale uses the following convention: white for a decrease of impedance and
black for an increase of impedance. At first glance, the broadband seismic data give the impression of a shaded relief 3D
image, but in fact this effect is due to the presence of strong amplitude low frequencies as they bring in the true envelope
signature of this formation revealing a major impedance contrast. Away from the well calibration point, in the central
part of the section, both the conventional and the broadband seismic data show a prominent relief structure corresponding
to a channel compactional feature. The presence of such a feature usually tells the geologist that the channel fill deposits
have a different lithology, i.e. that they are composed of less compactable sediments, including the desirable reservoir
channel sands. Figure 3 shows that the top of these sands can hardly be identified on conventional data while it appears
very clearly on broadband data. We can also see that channel levee sands, over bank deposits and sand body pinch-outs
are easier to identify. Even Top Chalk appears as a much more continuous horizon. Auto-picking tests have demonstrated
that it is five times faster to interpret this horizon on broadband data. This example emphasizes again the important role
that broadband seismic data can play in efficiently and accurately mapping facies distributions and ultimately describing
new potential stratigraphic play concepts in the UKCS area.
Figure 3: Seismic section comparison focusing on the turbidite complexes of the Paleocene Lista formation

4. Sub-BCU and sub-basalt imaging
Sub-BCU imaging can often be a challenge in the North Sea basins, especially for deep HP/HT targets in the Central
Graben or in the Viking Graben. The addition of low frequencies to the spectrum allows easy discrimination of the main
sedimentary packages and sequences found below the BCU. These low frequencies give an envelope to the seismic
signal that shapes the larger scale impedance variations corresponding to major lithology variations. This helps in
correlating seismic interpretation across major Jurassic rift faults, as well as better defining sub-BCU fault planes on
broadband data (Figure 4). In the West of Shetland area, imaging of deep sub-basalt features is enhanced thanks to the
deep penetration of low frequencies. Deep sub-basalt stratigraphy and volcanic intrusions can be interpreted with more
confidence.
Figure 4: Deep sub-BCU seismic section comparison
Figure 5: BCU amplitude map comparison

5. The benefits for sub-BCU interpretation are also demonstrated in Figure 5 which displays a comparison of BCU
amplitude maps for variable depth streamer and conventional data. This comparison illustrates the fact that broadband
data give a much better definition of fault polygons and their associated amplitude discontinuities. In the upper left
corner, the broadband amplitude map gives additional details of sub-cropping reflectors which are more difficult to
identify on conventional data. It is also worth noting that using the same technique to pick the BCU reflector on both the
broadband and conventional datasets (i.e. manual horizon picking every 10 inlines, no crossline interpretation, and 3D
auto-tracking using a 10ms correlation window), the auto-tracking was noticeably faster on the broadband dataset and the
final 3D BCU horizon was less noisy (i.e. fewer mis-picks and spikes).
Conclusions
The use of broadband seismic data is key to unlocking the remaining hydrocarbon potential of the UKCS. Variable-depth
streamer acquisition technology delivers a much broader frequency range to final seismic images, enabling the interpreter
to:
Accurately interpret rock stratigraphy, facies distribution and subtle structures – benefiting from the high frequency
content
Produce a clearer interpretation of deep targets: sub-salt, sub-basalt and sub-chalk HP/HT plays – benefiting from the
low frequency content
Extract the ‘true’ seismic signature of the geological formations by removing the wavelet side-lobes – benefiting
from both the low and high frequencies
Thanks to these benefits, such data save seismic interpreters precious time, a key benefit in the race for finding the
remaining hydrocarbon accumulations in the UK Continental Shelf area.
References
Dechun, L. et al. (2011). Optimizing the processing flow for variable-depth streamer data. First Break vol. 29,
September 2011, pp. 89-95.
Dowle, R. (2006). Solid streamer noise reduction principles. 75th SEG Annual Meeting, Expanded Abstracts 25, pp.
85-89.
Soubaras, R. (2010). Deghosting by joint deconvolution of a migration and a mirror migration. 80th SEG Annual
Meeting, Expanded Abstracts, pp. 3406-3410.
Soubaras, R. and R. Dowle (2010). Variable-depth streamer – a broadband marine solution. First Break vol. 28,
December 2010, pp. 89-96.