How much can geological maps substitute site-specific geotechnical surveys?
Our Principal Marine Geologist, Dr Aggie Georgiopoulou, shares her thoughts on the usefulness of geological maps in site investigations.
Early stage work on offshore projects
Early stage engineering works for offshore development commonly start with a desktop study, and if there are legacy or preliminary data available, a preliminary ground model is constructed. In this early stage of any offshore project, Developers want to avoid large expenses as much as possible so they will rely heavily on existing information instead of getting into an early, and perhaps perceived unnecessary, investment with site investigation data. Of course, there may be other drivers for delaying site investigation surveys, such as permitting/consenting, weather, vessel availability, budget restrictions etc which makes an early assessment of the site more important.
So what can we rely on for these early assessments?
A useful tool is knowing the sub-seabed stratigraphy well; this allows us to make fairly educated predictions (forecasts?) for what we should at least anticipate to encounter. For example, the Irish Sea sub-seafloor stratigraphy is well known, so any project in the Irish Sea should be able to make some high-level predictions for what soil conditions should be expected.
Subsurface stratigraphy is described in terms of geological formations that have been attributed ages. A formation is a fundamental unit and is defined as a mappable unit of sediment or rock layers with distinct, (relatively) uniform characteristics that distinguishes it from adjacent (overlying or underlying) layers. However, there can be a lot of sedimentological, and by extension geotechnical variability, within these formations, especially as one of the key criteria is that it is "mappable", therefore of an appropriate size that matches the scale of the work that is being produced. As an example, the Western Irish Sea Formation Mud Facies is described as very soft to firm sandy silty fine clay or sandy silt. This description suggests that the range of geotechnical parameters that will characterise this formation is very broad; the soil behaviour and engineering required will be substantially different if the soil in a project area is very soft CLAY compared to firm CLAY.
Mapping of offshore formations
Part of the reason is that offshore formations are mapped using geophysical tools and although acoustically a formation may appear relatively uniform laterally, lithologically it may contain a lot of lateral variability. Formations can represent entire sedimentary environments within which there can be many depositional processes operating adjacent to one another. Consider all the depositional processes that can be active in a coastal environment contemporaneously; there can be aeolian sands adjacent to shallow marine fine sand or silt, adjacent to a lagoonal mud, adjacent to a barrier island, all within a few hundreds of metres from one another which is within the mappable scale criterion to define a formation and which may be considered a "transgressive" formation. Equally extensive variability can be anticipated at the edge of a glacier; along with till in moraines, there can be glaciofluvial and glaciolacustrine sedimentation, all with distinctly different sedimentological and geotechnical characteristics, with sometimes similar acoustic character and within mappable scale distances from one another. Clearly then formations need to be used with extra caution, particularly where they describe such wide range of depositional processes.
Even mono-composition formations, such as the Cretaceous chalk and Eocene London Clay can have wide-ranging geotechnical properties due to a number of factors, such as small-scale fractures, alteration or diagenetic features, which may also be at different stages of maturity. Besides, the purpose of defining geological formations was not to address engineering characteristics. The main purpose of defining formations is two-fold; to create geological maps and to reconstruct palaeoenvironments. This means that no matter how well we understand the sub-seafloor geology from a stratigraphic point of view, it can never truly be a substitute for site-specific geotechnical data.
In places like the NE Atlantic (capturing here the Irish, Celtic and North Seas) there is a plethora of knowledge and data publicly available after decades of offshore exploration and research. One could argue that this part of the world is one of, if not the best-studied seafloors and sub-seafloor sequences in the world. There may be parts where closely spaced drilled wells can provide the kind of differentiation required in pre-FEED, but alas, often the top shallow-most stratigraphic interval was often bypassed and unlogged by the O&G industry.
Geological maps and site investigation planning can work very well in tandem. Let geological maps and geophysical mapping lead the determination of geotechnical sampling. Clever, informed geotechnical sampling plans rather than a grid of evenly spaced locations can save significant amounts from budgets and provide more relevant and therefore higher value information to a project. So, whilst stratigraphic formation maps and their broad descriptions can never replace physical sampling they can be invaluable in guiding and optimising survey planning.