FACTOR 50: a simple way to ensure you don't get burned by ultra-high resolution seismic
RockWave offers an opportunity for geophysicists to transition from pure O&G seismic processing and imaging, to broaden their frequency ranges, to pull their head out of the sand and into… well, lots more sand and/or muds in the case of shallow geophysics for offshore wind!
Processing geophysicists are problem solvers at heart and many of you will be wondering how shallow geophysics could possibly compare to the exciting challenges presented by the world of deep exploration data.
We were there once too… read about our peoples’ backgrounds in O&G seismic here .
That is why we appropriated the FACTOR 50 method. A simple way to transform your thinking and switch perspective from conventional to ultra-high resolution seismic (UHRS).
So, if you are a geoscientist (or an engineer that wants a bit more understanding of what Geophysics really is) and interested in transferring your skillset into the renewables sector, read on!
This article is taken from work presented by Nick Woodburn and David Monk at EAGE/SUT Workshop 2024 in Boston, SEISMIC24 in Aberdeen, Houston Geophysical Soc (online) and EAGE NSG23 in Edinburgh.
The Grand Piano of Seismic Bandwidth
Here we have two sample portions of seismic. On the left we have a piece of conventional seismic used to image targets several kilometres deep, and on the right we have UHRS used to image targets down to tens of metres below the sea floor.
Plotting the useful bandwidth at target for these two data types on Nick’s “Grand Piano of Seismic Bandwidth” we notice something immediately.
4-64 Hz in conventional seismic occupies the same number of piano keys as the 200 Hz to 3.2 KHz in UHRS.
Why?
You need to think about octaves!
The useful bandwidth for an O&G target at 3 km depth has the same number of octaves as for UHRS imaging targets tens of metres below the seafloor.
To go between the two. All you need to do is scale by 50. Not claiming this scalar value is exact, but it’s a very helpful rule of thumb for:
Let’s explore each of these individually.
If your background is in O&G seismic, this FACTOR 50 is great for expanding your understanding into the world of UHRS.
Acquisition Setups
The table shows a set of typical NAZ parameters to provide about 4 octaves at targets several kilometres down, compared against scaling these all down by 50 to make them suitable for the 4 octaves typical for UHRS.
We’ve highlighted parameters of interest. Note the cable spacing of 3 m and channel spacing of 25 cm contribute to a natural bin size of 12.5 x 25 cm (CENTIMETRES!!). This is incredibly dense.
Such a set up would be inefficient for covering wind farm sites as it has a subsurface crossline footprint of only 16.5 m.
Cost-efficient 3D UHRS carpet coverage over wind farm sites therefore has to make compromises if we are to see this become common practice.
So, let’s look at what might be considered more practical….
Here is a quad source, 12 cable set up can provide a bin size of 1.56 x 0.78 m and the sail-lines can be spaced in the order of 90 m.
This seems exciting, it’s certainly do-able. We mentioned compromises, what are they?
This is where our FACTOR 50 method really shows us the challenges when processing UHRS data.
Let’s scale these dimensions up by 50 to make them comparable to their equivalent ‘conventional’ set up.
Some of the parameters become perhaps somewhat alarming…
Now we have a cable spacing of 625 m, and channel spacing of 78.13 m
No one in O&G exploration would be comfortable with a bin size of 39.1 x 78.1 m.
The processing geophysicist may be concerned with this magnitude of channel spacing as coherent linear noise generated in the water column will alias above only 10 Hz.
Furthermore, a cable depth of 2 m in a UHRS set up is equivalent to 100 m in the ‘conventional’ equivalent. Processing geophysicists reading these words will be acutely aware that deghosting with a cable depth of 100 m would be challenging due to the vast number of notches this imposes in the recorded frequency spectra.
In your own time you will probably be able to spot many other challenges associated with the spec shown, feel free to chip into the comments section.
What this clearly shows is the need to be mindful of such challenges when designing your 3D-UHRS configuration, which also needs to take into account budgets, vessel capability, expected geology etc. We can help with this.
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Environmental Conditions (Tide/Wave Heights)
In the conventional O&G world it has been reasonable to consider these as a negligible or minor issue. But let’s use FACTOR 50 to ‘blow our minds’…
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Consider two source-receiver pairs, each one from a neighbouring sail-line. These have been acquired during different moments in the tidal cycle leading to a difference in water column thickness of 2 m.
The time difference between these would approximately be a single 2 ms sample. Not too challenging in a conventional setup.
However, using FACTOR 50, this same 2 m difference in tidal range is quite a different beast in the UHRS world. We are dealing with a difference in water column thickness that would be equivalent to 100 m on a conventional 3D seismic setup!
The same scalar can be applied to wave motion at the sea surface. Acquiring data in 1m swell is considered a gloriously flat calm day on a 3D O&G seismic vessel. When processing conventional seismic bandwidth in these conditions, the geophysicist thinks nothing of it.
However, scaling this wave magnitude by FACTOR 50, we realise, in the UHRS world, we are dealing with wave heights that would be equivalent to 50 metres! Shooting seismic in these conditions is certainly NOT recommended!!
If you want to see how this variable wave height impacts our seismic data, take a look at these synthetic gathers and stacks...
On the left we have a conventional setup, on the right you see the UHRS equivalent. Note the non-flat seabed reflection (and other reflections) due to the amplitude of the sea surface motion, that is, essentially, 50 times bigger than what we are used to in conventional O&G acquisitions.
The processing geophysicist is therefore met with a number of challenges, such as:
Spatial and Temporal Dimensions of UHRS Volumes
Here is a sample section of conventional seismic acquired several decades ago for O&G exploration purposes.
In this display we see the top 1 km from sea-surface, and almost 8 km across.
Now let’s overlay the UHRS acquired almost over the same line. It provides much more detail about the top 150-200 m.
But if we zoom in at the real target zone for OWF, namely tens of metres below the seafloor, we see the incredible detail available with the 4 octaves of bandwidth between 200 and 3.2 KHz.
Or to put it another way that is in line with this entire article, for any given unit length of seismic transect, there is 50 times as much detail <that you could choose to> interpret on UHRS data!!
Conventional O&G processing geophysicists (and interpreters!) reading this article will be quaking in their boots at the prospect of having such a data volume to QC.
Just for fun, shall we show you a comparison between the area covered by a typical offshore wind farm site scaled up by FACTOR 50 to represent its equivalent size in the conventional 3D world?
Let’s take the Morven windfarm site Offshore Aberdeen, which is in early planning stage. This is about 54 x 16 km in size, with the aim of constructing 190 turbines producing 3 GW of power.
After applying the scale factor of 50, this site would have the equivalent size to that shown here, going from the Netherlands up to Svalbard.
At some point a ground model is going to be generated from UHRS covering this site, and it will be the equivalent to interpreting the structural geology over the whole of the North Sea, M?re, V?ring basins and the Barents Sea.
This will be a monumental feat. Give these interpreters the credit they deserve. ????????
Conclusions
In this article we have presented a handful of challenges that we experience when designing, acquiring, processing, QC’ing and interpreting ultra-high resolution seismic data for offshore wind farms.
FACTOR 50 serves as a useful tool for adjusting our perspective and get our heads around the scale of such challenges, in language that seismic processors from all backgrounds can understand (hopefully).
The offshore wind industry is growing rapidly and is it is incredibly exciting to be so involved as governments around the world scale up their commitments to renewable energy generation.
Within this growth, at RockWave we have experienced massive demand for our advanced high/ultra/ultra-ultra high-resolution seismic processing services, as our clients increasingly realise the need to underpin engineering ground models with reliable data.
There are other great companies in the space who are also transferring technology and expertise, developed within O&G exploration, for application in offshore wind. We are working with companies using pre-stack seismic inversion to obtain elastic and geotechnical soil properties in areas of sparse borehole (BH) and cone penetration test (CPT) coverage. Bringing greater flexibility to turbine placement and especially useful where there is 3D seismic coverage.
Working in the elastic domain places even greater demand on seismic processing geophysicists, who are required to evaluate the effects of each processing stage on amplitude variation with offset (AVO) properties and gather alignment that are critical for pre-stack seismic inversion. This is routine in O&G processing, but presents a greater challenge on UHRS.
The geoscientists at RockWave are #fascinated by all this. And we’ll need your expertise in future to help us continue driving innovation and grappling with the UHRS challenges presented in this article… and many more!
The global energy mix is transforming and the seismic landscape is changing with it.
Do you like the sound of broadening your frequency range too?