19th Feb, 2024

3 Common challenges faced when processing seismic data acquired in desert environments

Deserts, with their vast expanses of shifting sands and relentless sun, aren't just challenging for the wanderers who cross them; they present a unique set of obstacles for geophysicists seeking to map the world beneath their surfaces.

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Seismic surveys and the processing of seismic data acquired in desert environments are notoriously challenging. In this blog post, our Senior Processing Geophysicist, Eamonn Murray, delves into the unique challenges this type of data presents and how STRYDE is helping companies overcome these challenges…

Eamonn Murray

Challenge 1: Time distortions and Reverberating noise

“Desert environments with large sand dunes present significant obstacles when it comes to seismic data processing. A surface that’s on the move is never ideal. Sensors are routinely toppled over, subtly altering their location and elevation – changes the processor may never be aware of. However, it’s what’s below the dune’s surface that’s most troublesome. Its unique composition, broadly characterised by a low-velocity, loosely bound sand sitting above a solid, hard surface, is a significant barrier to the seismic method. Its complex internal structure slows the wave passing through it by varying amounts, distorting the picture of the layers beneath. Most standard approaches struggle to correct for these and so we usually turn to a bespoke additional treatment to fix it – variations on the “dune static” method. This seeks to, in effect, scrape away the dunes, as if the acquisition took place on the hard surface beneath.

But even if we’re successful in this we still have the problem of the “dune noise” - another equally significant problem that doesn't get as much attention. It’s quite easy when looking at a seismic section to pick out where the dunes are on the surface above. The picture beneath them degrades noticeably, as if a curtain of noise has been draped over it. Some nice work has been done recently in understanding the nature of this noise, describing how the dune body is adept at trapping and re-emitting the surface wave energy as it passes across it. This trapped energy bounces around within the dune between the dune top and the hard surface at its base affecting any sensor on it or close by. It appears on the recorded data as noise reverberating down the section, effectively masking the weaker, reflection events of interest beneath. This “scattered” noise is difficult to treat with conventional tools and tailored solutions, combining various methods, are currently the best ways to tackle it.

Excellent animation showing how the passing surface wave energy is captured by the barchan sand dune and re-emits as a secondary noise source Seismic Ground Roll Absorption and Reemission by Sand Dunes Arran, M. I.; Vriend, N. M.; Muyzert, E., Journal of Geophysical Research: Solid Earth, Volume 123, Issue 7, pp. 5675-5689

Challenge 2: Intricacies in the near surface in desert regions

“One defining feature of desert environments is the presence of heterogeneous carbonates close to the surface, which have endured atmospheric weathering or groundwater dissolution over time. These carbonates have a pockmarked texture and act as barriers to wave field propagation. It’s quite similar to the limestone pavements in places like Yorkshire - there are huge caverns in the first few hundred metres of the subsurface which can be the size of St. Paul’s Cathedral in some places.

“This topographical complexity not only causes time static distortions but also contributes to a small-scale scattering effect as you fire soundwaves through the subsurface, resulting in a web of surface wave contamination and imaging distortions.

“Additionally, as the higher frequencies of the source signal pass through these jumbled carbonate sequences, they are not only distorted but they also lose their energy both on their way down and upon reflection back to the surface sensors recording the data. This double hit severely diminishes the bandwidth of the source signal at greater depths, diminishing the imaging resolution at target areas and negatively impacting attribute extraction later in the processing sequence.

“Many of our clients want to look a couple of kilometres down, but the chaotic top few hundred meters obstruct and muddy the desired picture. Understanding and navigating this challenging subsurface environment is not only crucial for accurate data interpretation, but also essential for safe and efficient drilling operations. If a drilling team hits one of these huge caverns, they’ll suffer catastrophic loss of drilling fluid and possibly loss of equipment.

Challenge 3: Multiple contaminations and velocities in the subsurface

“Carbonate sequences found specifically in the desert regions of the Middle East are notorious for a phenomenon termed 'multiple contamination.' This is when reflected energy returning to the surface is reflected back down in the earth by certain rock layers – an extra “bounce”. This process can repeat several times, causing "multiples". While these repeated bounces tend to weaken with each reflection, the initial bounce can still interfere significantly with the primary imaging.

“This is an issue we see mainly in the desert environment of the Arabian Peninsula, and what makes this especially challenging is the alternating fast and slow rock layers. Unlike marine environments, for instance, where rock velocity typically increases steadily with depth, the Middle East features layers that oscillate between fast and slow velocities.

“Traditional methods to negate this interference, based on differing velocities or dips, often fall short in the Middle East as the contaminating layers often share similar properties with the target rocks. They may have similar or faster velocities and the same dip, so they can look very similar to the primary picture. This makes distinguishing between the primary image and the contaminating reflections quite difficult.”

STRYDE data processing

How STRYDE can resolve data processing challenges in deserts

“Traditional surveying practices often relied on cabled geophone intervals of up to 50 metres and had large gaps of 300-400 metres between lines due to the sheer weight, cost and effort required to deploy them. This configuration tends to create quite a distorted and inaccurate view of the near surface. Once it became evident that understanding the first few hundred metres is crucial for accurate imaging and risk avoidance, the demand for higher-resolution imaging grew, and the number of seismic sensors required to achieve this increased dramatically.

“One of the standout benefits of STRYDE's lightweight and affordable nodal technology is the ability to drastically reduce the spacing between seismic traces, enabled by the lower price-point of the seismic sensors. Instead of the conventional 50 or 25 metre intervals, STRYDE's system facilitates spacing as close as 5 to 10 metres. The advantages of this are twofold:

1. High-resolution imaging:

“A denser survey captures a more detailed representation of the near surface. This heightened granularity offers invaluable insights into the complicated subsurface that’s typical in desert environments, providing geologists and decision-makers with a better representation of its composition. This knowledge is crucial for modelling and subsequently countering the effects of surface disturbances on seismic waves, especially the higher frequencies that get absorbed.”

2. Efficient Noise Removal:

“The increased sensor density also empowers geophysicists to sample and understand noise patterns with more precision. This comprehensive noise data, in turn, facilitates efficient noise removal. In essence, the denser the survey, the easier it becomes to filter out unwanted disruptions.

“Furthermore, with high-density acquisition surfaces, novel methods such as the 'least squares demultiple' are proving instrumental. Such techniques leverage the detail-rich data to create images of the near surface, modelling the multiple wave field and eliminating it at the target to create image clarity that would be unthinkable with the sparser setups from two decades ago.

“Overall, STRYDE's nodal technology is revolutionising seismic surveys by delivering precision and clarity, which enables data processing teams to better navigate the challenges of desert landscapes and achieve superior imaging outcomes."

Learn more about STRYDE’s solutions for high-resolution seismic data in the Middle East